بدون تصویر

aci302

302.1R-1

FOREWORD

The quality of a concrete floor or slab is highly dependent on achieving a

hard and durable surface that is flat, relatively free of cracks, and at the

proper grade and elevation. Properties of the surface are determined by the

mixture proportions and the quality of the concreting and jointing operations.

The timing of concreting operations—especially finishing and jointing—

is critical. Failure to address this issue can contribute to undesirable

characteristics in the wearing surface such as cracking, low resistance to

wear, dusting, scaling, high or low spots, and poor drainage, as well as

increasing the potential for curling.

Concrete floor slabs employing portland cement, regardless of slump, will

start to experience a reduction in volume as soon as they are placed. This

phenomenon will continue as long as any water or heat, or both, is being

released to the surroundings. Moreover, since the drying and cooling rates

at the top and bottom of the slab will never be the same, the shrinkage will

vary throughout the depth, causing the as-cast shape to be distorted, as

well as reduced in volume.

This guide contains recommendations for controlling random cracking and

edge curling caused by the concrete’s normal volume change. Application

of present technology permits only a reduction in cracking and curling, not

their elimination. Even with the best floor designs and proper construction,

it is unrealistic to expect completely crack-free and curl-free results. Consequently,

every owner should be advised by both the designer and contractor

that it is completely normal to expect some amount of cracking and

curling on every project, and that such occurrence does not necessarily

reflect adversely on either the competence of the floor’s design or the quality

of its construction.1,2 Refer to the latest edition of ACI 360 for a detailed

discussion of shrinkage and curling in slabs on ground. Refer to the latest

edition of ACI 224 for a detailed discussion of cracking in reinforced and

nonreinforced concrete slabs.

This guide describes how to produce high quality concrete slabs on ground

and suspended floors for various classes of service. It emphasizes such

aspects of construction as site preparation, concreting materials, concrete

mixture proportions, concreting workmanship, joint construction, load

transfer across joints, form stripping procedures, and curing. Finishing

methods, flatness/levelness requirements, and measurements are outlined.

A thorough preconstruction meeting is critical to facilitate communication

among key participants and to clearly establish expectations and procedures

that will be employed during construction. Adequate supervision and

inspection are required for job operations, particularly those of finishing.

Keywords: admixtures; aggregates; concrete construction; concrete durability;

concrete finishing (fresh concrete); concrete slabs; consolidation;

contract documents; cracking (fracturing); curing; curling; deflection; floor

toppings; floors; forms; form stripping; heavy-duty floors; inspection;

joints (junctions); mixture proportioning; placing; quality control; site

preparation; slab-on-ground construction; slump tests; specifications; standards;

suspended slabs.

CONTENTS

Chapter 1—Introduction, p. 302.1R-2

1.1—Purpose and scope

1.2—Work of other relevant committees

Chapter 2—Classes of floors, p. 302.1R-4

2.1—Classification of floors

ACI 302.1R-96

Guide for Concrete Floor and Slab

Construction

Reported by ACI Committee 302

Carl Bimel

Chairman

Eldon Tipping

Secretary

Robert B. Anderson Edward B. Finkel William S. Phelan

Charles M. Ault Barry E. Foreman Dennis W. Phillips

Charles M. Ayers Terry J. Fricks John W. Rohrer

Kenneth L. Beaudoin Eugene D. Hill, Jr. Moorman L. Scott

Michael G. Callas Jerry A. Holland Nandu K. Shah

Angelo E. Colasanti Arthur W. McKinney Peter C. Tatnall

Gregory Dobson John P. Munday R. Gregory Taylor

Robert A. Epifano Scott Niemitalo Miroslav F. Vejvoda

Samuel A. Face, III Robert W. Nussmeier Sam J. Vitale

William C. Panarese

ACI 302.1R-96 became effective October 22, 1996. This document supersedes ACI

302.1R-89.

Copyright © 1997, American Concrete Institute.

All rights reserved including rights of reproduction and use in any form or by any

means, including the making of copies by any photo process, or by electronic or

mechanical device, printed, written, or oral, or recording for sound or visual reproduction

or for use in any knowledge or retrieval system or device, unless permission in

writing is obtained from the copyright proprietors.

ACI Committee reports, guides, standard practices, design handbooks,

and commentaries are intended for guidance in planning, designing,

executing, and inspecting construction. This document is intended

for the use of individuals who are competent to evaluate the significance

and limitations of its content and recommendations and who will accept

responsibility for the application of the material it contains. The American

Concrete Institute disclaims any and all responsibility for the application

of the stated principles. The Institute shall not be liable for any

loss or damage arising therefrom.

Reference to this document shall not be made in contract documents.

If items found in this document are desired by the Architect/Engineer to

be part of the contract documents, they shall be restated in mandatory

language for incorporation by the Architect/Engineer.

302.1R-2 ACI COMMITTEE REPORT

2.2—Single-course monolithic floors: Classes 1, 2, 4, 5,

and 6

2.3—Two-course floors: Classes 3, 7, and 8

2.4—Class 9 floors

2.5—Special finish floors

Chapter 3—Design considerations, p. 302.1R-6

3.1—Scope

3.2—Slabs on ground

3.3—Suspended slabs

3.4—Miscellaneous details

Chapter 4—Site preparation and placing

environment, p. 302.1R-15

4.1—Soil support system preparation

4.2—Suspended slabs

4.3—Bulkheads

4.4—Setting of screed guides

4.5—Installation of auxiliary materials

4.6—Concrete placement conditions

Chapter 5—Materials, p. 302.1R-17

5.1— Introduction

5.2—Concrete

5.3—Portland cement

5.4—Aggregates

5.5—Water

5.6—Admixtures

5.7—Liquid surface treatments

5.8—Reinforcement

5.9—Curing materials

5.10—Evaporation reducers

5.11—Gloss-imparting waxes

5.12—Joint materials

5.13—Volatile organic compounds (VOC)

Chapter 6—Concrete properties and consistency,

p. 302.1R-23

6.1—Concrete properties

6.2—Recommended concrete mixture

Chapter 7—Batching, mixing, and transporting,

p. 302.1R-25

7.1—Batching

7.2—Mixing

7.3—Transporting

Chapter 8—Placing, consolidating, and finishing,

p. 302.1R-26

8.1—Placing operations

8.2—Tools for spreading, consolidating, and finishing

8.3—Spreading, consolidating, and finishing operations

8.4—Finishing Class 1, 2, and 3 floors (tile-covered, offices,

churches, schools, hospitals, ornamental, and garages)

8.5—Finishing Class 4 and 5 floors (light-duty industrial

and commercial)

8.6—Finishing Class 6 floors (industrial) and monolithicsurface

treatments for wear resistance

8.7—Finishing Class 7 floors (heavy-duty industrial)

8.8—Finishing Class 8 floors (two-course unbonded)

8.9—Finishing Class 9 floors (superflat or critical surface

tolerance required)

8.10—Toppings for precast floors

8.11—Finishing structural lightweight concrete

8.12—Nonslip floors

8.13—Decorative and nonslip treatments

8.14—Grinding as a repair procedure

8.15—Floor flatness and levelness

8.16—Treatment when bleeding is a problem

8.17—Delays in cold-weather finishing

Chapter 9—Curing, protection, and joint filling,

p. 302.1R-50

9.1—Purpose of curing

9.2—Methods of curing

9.3—Curing at joints

9.4—Curing of special concretes

9.5—Length of curing

9.6—Preventing plastic shrinkage cracking

9.7—Curing after grinding

9.8—Protection of slab during construction

9.9—Temperature drawdown in cold storage and freezer

rooms

9.10—Joint filling and sealing

Chapter 10—Quality control checklist, p. 302.1R-52

10.1—Introduction

10.2—Partial list of important items to be observed

Chapter 11—Causes of floor and slab surface

imperfections, p. 302.1R-53

11.1—Introduction

11.2—Cracking

11.3—Low resistance to wear

11.4—Dusting

11.5—Scaling

11.6—Popouts

11.7—Blisters

11.8—Spalling

11.9—Discoloration

11.10—Low spots and poor drainage

11.11—Curling

11.12—Analysis of surface imperfections

Chapter 12—Selected references, p. 302.1R-61

12.1—Specified and recommended references

12.2—Cited references

12.3—Additional references

Addendum—p. 302.1R-66

CHAPTER 1—INTRODUCTION

1.1—Purpose and scope

This guide presents state-of-the-art information relative to the

construction of slab-on-ground and suspended-slab floors for

industrial, commercial, and institutional buildings. It is

applicable to the construction of normal weight and structural

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-3

lightweight concrete floors and slabs made with conventional

portland and blended cements.

The design of slabs on ground should conform to the recommendations

of ACI 360R. Refer to ACI 223 for special

procedures recommended for the design and construction of

shrinkage-compensating concrete slabs on ground. The design

of suspended floors should conform to requirements of

ACI 318 and ACI 421.1R. See Section 1.2 for relevant work

by these and other committees.

This guide identifies the various classes of floors as to

•use,

•design details as they apply to construction,

•necessary site preparation, and

•type of concrete and related materials.

In general, the characteristics of the concrete slab surface

and the performance of joints have a powerful impact on the

serviceability of floors and other slabs. Since the eventual

success of a concrete floor installation is greatly dependent

upon the mixture proportions and floor finishing techniques

used, considerable attention is given to critical aspects of

achieving the desired finishes and the required floor surface

tolerances. This guide emphasizes choosing and proportioning

of materials, design details, proper construction methods,

and workmanship.

1.1.1 Prebid and preconstruction meetings—While this

guide does provide a reasonable overview of concrete floor

construction, it should be emphasized that every project is

unique; circumstances can dictate departures from the recommendations

contained here. Accordingly, contractors and

suppliers are urged to make a thorough formal review of contract

documents prior to bid preparation.

The best forum for such a review is the prebid meeting.

This meeting offers bidders an opportunity to ask questions

and to clarify their understanding of contract documents prior

to submitting their bids. A prebid meeting also provides

the owner and the owner’s designer an opportunity to clarify

intent where documents are unclear, and to respond to lastminute

questions in a manner that provides bidders an opportunity

to be equally responsive to the contract documents.

1.1.2 Preconstruction meeting—Construction of any slabon-

ground or suspended floor or slab involves the coordinated

efforts of many subcontractors and material suppliers. It

is strongly recommended that a preconstruction meeting be

held to establish and coordinate procedures that will enable

key participants to produce the best possible product under

the anticipated field conditions. This meeting should be attended

by responsible representatives of organizations and

material suppliers directly involved with either the design or

construction of floors.

The preconstruction meeting should confirm and document

the responsibilities and anticipated interaction of key

participants involved in floor slab construction. Following is

a list of agenda items appropriate for such a meeting; many

of the items are those for which responsibility should be

clearly established in the contract documents. The list is not

necessarily all-inclusive.

1. Site preparation

2. Grades for drainage, if any

3. Work associated with installation of auxiliary materials,

such as vapor barriers, vapor retarders, edge insulation, electrical

conduit, mechanical sleeves, drains, and embedded plates

4. Class of floor

5. Floor thickness

6. Reinforcement, when required

7. Construction tolerances: base (rough and fine grading),

forms, slab thickness, surface configuration, and floor flatness

and levelness requirements (including how and when measured)

8. Joints and load transfer mechanism

9. Materials: cements, fine aggregate, coarse aggregate,

water, and admixtures (usually by reference to applicable

ASTM standards)

10. Special aggregates, admixtures, or monolithic surface

treatments, where applicable

11. Concrete specifications, to include the following:

a. Compressive and/or flexural strength and finishability

(Section 6.2)

b. Minimum cementitious material content, if applicable

(Table 6.2.4)

c. Maximum size, grading, and type of coarse aggregate

d. Grading and type of fine aggregate

e. Air content of concrete, if applicable (Section 6.2.7)

f. Slump of concrete (Section 6.2.5)

g. Water-cement ratio or water-cementitious material

ratio

h. Preplacement soaking requirement for lightweight

aggregates

12. Measuring, mixing, and placing procedures (usually

by reference to specifications or recommended practices)

13. Strikeoff method

14. Recommended finishing methods and tools, where

required

15. Coordination of floor finish requirements with those

required for floor coverings such as vinyl, ceramic tile, or

wood that are to be applied directly to the floor

16. Curing procedures, including length of curing and time

prior to opening the slab to traffic (ACI 308)

17. Testing and inspection requirements

18. Acceptance criteria and remedial measures to be used,

if required

1.1.2.1 Additional issues specific to suspended slab construction

are as follows:

1. Form tolerances and preplacement quality assurance

survey procedures for cast-in-place construction

2. Erection tolerances and preplacement quality assurance

survey procedures for composite slab construction; see

ANSI/ASCE 3-91 and ANSI/ASCE 9-91 (Section 12.1).

3. Form stripping procedures, if applicable

4. Items listed in Section 3.3

1.1.3 Quality control—Adequate provision should be

made to ensure that the constructed product meets or exceeds

the requirements of the project documents. Toward this end,

quality control procedures should be established and maintained

throughout the entire construction process.

302.1R-4 ACI COMMITTEE REPORT

The quality of a completed concrete slab depends on the

skill of individuals who place, finish, and test the material.

As an aid to assuring a high-quality finished product, the

specifier or owner should consider requiring the use of

prequalified concrete contractors, testing laboratories, and

concrete finishers who have had their proficiency and experience

evaluated through an independent third-party certification

program. ACI has developed programs to train and to

certify concrete flatwork finishers and concrete testing technicians

throughout the United States and Canada.

1.2—Work of other relevant committees

1.2.1 ACI committees

117—Prepares and updates tolerance requirements for

concrete construction.

201—Reviews research and recommendations on durability

of concrete and reports recommendations for appropriate

materials and methods.

211—Develops recommendations for proportioning concrete

mixtures.

223—Develops and reports on the use of shrinkage-compensating

concrete.

224—Studies and formulates recommendations for the

prevention or control of cracking in concrete construction.

301—Develops and maintains standard specifications for

structural concrete for buildings.

308—Prepares guidelines for type and amount of curing

required to develop the desired properties in concrete.

309—Studies and reports on research and development in

consolidation of concrete.

318—Develops and updates building code requirements

for reinforced concrete and structural plain concrete, including

suspended slabs.

325—Reports on the structural design, construction,

maintenance, and rehabilitation of concrete pavements.

330—Reports on the design, construction, and maintenance

of concrete parking lots.

332—Gathers and reports on the use of concrete in residential

construction.

347—Gathers, correlates, and reports information and prepares

recommendations for formwork for concrete.

360—Develops and reports on criteria for design of slabs

on ground, except highway and airport pavements.

421—Develops and reports on criteria for suspended slab

design.

423—Develops and reports on technical status, research,

innovations, and recommendations for prestressed concrete.

503—Studies and reports information and recommendations

on the use of adhesives for structurally joining concrete,

providing a wearing surface, and other uses.

504—Studies and reports on materials, methods, and systems

used for sealing joints and cracks in concrete structures.

515—Prepares recommendations for selection and application

of protective systems for concrete surfaces.

544—Studies and reports information and recommendations

on the use of fiber reinforced concrete.

640—Develops, maintains, and updates programs for use

in certification of concrete construction craftspeople.

1.2.2 The American Society of Civil Engineers—Publishes

documents that can be helpful for floor and slab construction.

Two publications that deal with suspended slab construction

are the “ASCE Standard for the Structural Design

of Composite Slabs” (ANSI/ASCE 3-91) and “ASCE Standard

Practice for Construction and Inspection of Composite

Slabs” (ANSI/ASCE 9-91).

CHAPTER 2—CLASSES OF FLOORS

2.1—Classification of floors

Table 2.1 classifies floors on the basis of intended use, discusses

special considerations, and suggests finishing techniques

for each class of floor. Use requirements should be

considered when selecting concrete properties (Section 6.1),

and the step-by-step placing, consolidating, and finishing

procedures in Chapter 8 should be closely followed for different

classes and types of floors.

Wear resistance should also be considered. Currently,

there are no standard criteria for evaluating the wear resistance

of a floor, and it is not possible to specify concrete

quality in terms of ability to resist wear. Wear resistance is

directly related to the concrete-mixture proportions, types of

aggregates, and construction techniques used.

2.2—Single-course monolithic floors: Classes 1, 2,

4, 5, and 6

Five classes of floors are constructed with monolithic concrete;

each involves some variation in strength and finishing

techniques. If abrasion from grit or other materials will be

unusually severe, a higher-quality floor surface may be required

for satisfactory service.3 Under these conditions, a

higher-class floor, a special metallic or mineral aggregate

monolithic surface treatment, or a higher-strength concrete is

recommended.

2.3—Two-course floors: Classes 3, 7 and 8

2.3.1 Unbonded topping over base slab—The base courses

of Class 3 (unbonded, two course) floors and Class 8

floors can be either slabs-on-ground or suspended slabs, with

the finish to be coordinated with the type of topping. For

Class 3 floors, the concrete topping material is similar to the

base slab concrete. The top courses for Class 8 floors require

a hard-steel troweling, and usually have a higher strength

than the base course. Class 8 floors can also make use of an

embedded hard aggregate, or a premixed (dry-shake) mineral

aggregate or metallic hardener for addition to the surface

(Section 5.4.6).

Class 3 (with unbonded topping) and Class 8 floors are

used when it is preferable not to bond the topping to the

base course, so that the two courses can move independently

(for example, with precast members as a base), or

so that the top courses can be more easily replaced at a later

period. Two-course floors can be used when mechanical

and electrical equipment require special bases, and

when their use permits more expeditious construction

procedures. Two-course unbonded floors can also be used

to resurface worn or damaged floors when contamination

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-5

prevents complete bond, or when it is desirable to avoid

scarifying and chipping the base course and the resultant

higher floor elevation is compatible with adjoining floors.

Class 3 floors are used primarily for commercial or nonindustrial

applications, whereas Class 8 floors are primarily

for industrial-type applications.

Plastic sheeting, roofing felt, or a bond-breaking compound

are used to prevent bond to the base slab. Reinforcement

such as deformed bars, welded wire fabric, bar mats or

fibers may be placed in the topping to reduce the width of

shrinkage cracks. Unbonded toppings should have a minimum

thickness of 3 in. (75 mm). The concrete should be proportioned

to meet the requirements of Chapter 6. Joint

spacing in the topping must be coordinated with joint spacing

in the base slab.

2.3.2 Bonded topping over base slab—Class 3 (bonded

topping) and Class 7 floors employ a topping bonded to the

base slab. Class 3 (bonded topping) floors are used primarily

for commercial or nonindustrial applications; Class 7 floors

are used for heavy-duty, industrial-type applications subject

to heavy traffic and impact. The base slabs can be either a

conventional portland cement concrete mixture or shrinkage-

compensating concrete. The surface of the base slab

should have a rough, open pore finish and be free of any substances

that would interfere with the bond of the topping to

the base slab.

Table 2.1—Floor classifications

Class Anticipated type of traffic Use Special considerations Final finish

1 Single course Exposed surface—foot

traffic

Offices, churches, commercial,

institutional, multiunit residential

Decorative

Uniform finish, nonslip aggregate

in specific areas, curing

Colored mineral aggregate, color

pigment or exposed aggregate,

stamped or inlaid patterns, artistic

joint layout, curing

Normal steel-troweled finish,

nonslip finish where required

As required

2 Single course

Covered surface—foot

traffic

Offices, churches, commercial,

gymnasiums, multiunit residential,

institutional with floor coverings

Flat and level slabs suitable for

applied coverings, curing. Coordinate

joints with applied coverings

Light steel-troweled finish

3 Two course Exposed or covered

surface—foot traffic

Unbonded or bonded topping

over base slab for commercial or

non-industrial buildings where

construction type or schedule

dictates

Base slab —good, uniform, level

surface, curing

Unbonded topping—bondbreaker

on base slab, minimum

thickness 3 in. (75 mm) reinforced,

curing

Bonded topping—properly sized

aggregate, 3/4 in. (19 mm) minimum

thickness curing

Base slab —troweled finish

under unbonded topping; clean,

textured surface under bonded

topping

Topping—for exposed surface,

normal steel-troweled finish. For

covered surface, light steeltroweled

finish

4 Single course

Exposed or covered surface—

foot and light

vehicular traffic

Institutional and commercial

Level and flat slab suitable for

applied coverings, nonslip

aggregate for specific areas, curing.

Coordinate joints with

applied coverings

Normal steel-troweled finish

5 Single course

Exposed surface—industrial

vehicular traffic, that

is, pneumatic wheels, and

moderately soft solid

wheels

Industrial floors for manufacturing,

processing, and warehousing

Good uniform subgrade, joint

layout, abrasion resistance,

curing

Hard steel-troweled finish

6 Single course

Exposed surface—heavy

duty industrial vehicular

traffic, that is, hard wheels,

and heavy wheel loads

Industrial floors subject to heavy

traffic; may be subject to impact

loads

Good uniform subgrade, joint

layout, load transfer, abrasion

resistance, curing

Special metallic or mineral

aggregate surface hardener;

repeated hard steel-trowelling

7 Two course

Exposed surface—heavy

duty industrial vehicular

traffic, that is, hard wheels,

and heavy wheel loads

Bonded two-course floors subject

to heavy traffic and impact

Base slab —good, uniform subgrade,

reinforcement, joint layout,

level surface, curing

Topping—composed of wellgraded

all-mineral or all-metallic

aggregate. Minimum thickness

3/4 in. (19 mm). Metallic or mineral

aggregate surface hardener

applied to high-strength plain

topping to toughen, curing

Clean, textured base slab surface

suitable for subsequent bonded

topping. Special power floats for

topping are optional, hard steeltroweled

finish

8 Two course As in Class 4, 5, or 6

Unbonded toppings—on new or

old floors or where construction

sequence or schedule dictates

Bondbreaker on base slab, minimum

thickness 4 in. (100 mm),

abrasion resistance, curing

As in Class 4, 5, or 6

9 Single course or

topping

Exposed surface—superflat

or critical surface tolerance

required. Special

materials-handling vehicles

or robotics requiring

specific tolerances

Narrow-aisle, high-bay warehouses;

television studios, ice

rinks

Varying concrete quality requirements.

Shake-on hardeners cannot

be used unless special

application and great care are

employed. Ff50 to Ff125

(“superflat” floor). Curing

Strictly follow finishing techniques

as indicated in Section

8.9

302.1R-6 ACI COMMITTEE REPORT

The topping can be either a same-day installation (prior to

hardening of the base slab) or a deferred installation (after

the base slab has hardened). The topping for a Class 3 floor

is a concrete mixture similar to that used in Class 1 or 2

floors. The topping for a Class 7 floor requires a multiplepass,

hard-steel-trowel finish, and it usually has a higher

strength than the base course. A bonded topping can also

make use of an embedded hard aggregate or a premixed (dryshake)

mineral aggregate or metallic hardener for addition to

the surface (Section 5.4.6). Bonded toppings should have a

minimum thickness of 3/4 in. (19 mm). Joint spacing in the topping

must be coordinated with joint spacing in the base slab.

2.4—Class 9 floors

Certain materials-handling facilities (for example, highbay,

narrow-aisle warehouses) require extraordinarily level

and flat floors. The construction of such “superflat” floors

(Class 9) is discussed in Chapter 8. A superflat floor could be

constructed as a single-course floor, or it could be constructed

as a two-course floor with a topping, either bonded (similar

to a Class 7 topping) or unbonded (similar to a Class 8

topping).

2.5—Special finish floors

Floors with decorative finishes and those requiring skid resistance

or electrical conductivity are covered in appropriate

sections of Chapter 8

Floors exposed to mild acids, sulfates, or other chemicals

should receive special preparation or protection. ACI 201.2R

reports on means of increasing the resistance of concrete to

chemical attack. Where attack will be severe, wear-resistant

protection suitable for the exposure should be used. Such environments,

and the methods of protecting floors against

them, are discussed in ACI 515.1R.

In certain chemical and food processing plants, such as

slaughterhouses, exposed concrete floors are subject to slow

disintegration due to organic acids. In many instances it is

preferable to protect the floor with other materials such as

acid-resistant brick, tile, or resinous mortars (ACI 515.1R).

CHAPTER 3—DESIGN CONSIDERATIONS

3.1—Scope

This chapter addresses design of concrete floors as it relates

to their constructability. Components of a typical slab

on ground4 are shown in Fig. 3.1. Specific design requirements

for concrete floor construction are found in other documents:

ACI 360R for slabs on ground, ACI 223 for

shrinkage-compensating concrete floors, ACI 421.1R for

suspended floors, ANSI/ASCE 3-91 for structural design of

composite slabs, and ANSI/ASCE 9-91 for construction and

inspection of composite slabs. Refer to ACI 318 for requirements

relating to the building code.

3.2—Slabs on ground

3.2.1 Suggested design elements—The following items

should be specified in the contract documents prepared by

the engineer of record.

•Base and subbase materials, preparation requirements,

and vapor retarder, if required

•Concrete thickness5

•Concrete compressive, or flexural strength, or both

•Concrete mixture design requirements (ASTM C 94)

•Joint locations and details

•Reinforcement (type, size, and location), if required

•Surface treatment, if required

•Surface finish

•Tolerances (base, subbase, slab thickness, and surface)

•Curing

•Joint filling material and installation

•Special embedments

•Preconstruction meeting, quality assurance, and quality

control

3.2.2 Soil support system—The performance of a slab on

ground depends on the integrity of both the soil support system

and the slab, so specific attention should be given to the

site preparation requirements, including proof-rolling, discussed

in Section 4.1.1. In most cases, proof-rolling results

are far more indicative of the ability of the soil support system

to withstand loading than are the results from in-place

tests of moisture content or density. A thin layer of graded,

granular, compactible material is normally used as fine grading

material to better control the thickness of the concrete

and to minimize friction between the base material and the

slab.

3.2.3 Vapor retarder—Proper moisture protection is desirable

for any slab on ground where the floor will be covered by

tile, wood, carpet, impermeable floor coatings (urethane, epoxy,

or acrylic terrazzo), or where the floor will be in contact

with any moisture-sensitive equipment or product.

Vapor retarders are often incorrectly referred to as “vapor

barriers.” A vapor retarder is a material that will effectively

minimize the transmission of water vapor from the soil support

system through the slab, but is not 100 percent effective

in preventing its passage. Although no specific national standard

has been established for the effectiveness of these products,

it is generally recognized that a vapor retarder is one

with a permeance of less than 0.3 US perms (0.2 metric

perms) as determined by ASTM E 96.

Although polyethylene film with a thickness of as little as

6 mils (0.15 mm) has been satisfactory as a vapor retarder,

the committee strongly recommends that a thickness of not

less than 10 mils (0.25 mm) be used. The increase in thickness

offers increased resistance to moisture transmission

Fig. 3.1—Typical slab on grade

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-7

while providing more durability during and after its installation.

A number of products, such as laminated kraft paper with

glass fiber reinforcement and reinforced polyethylene film,

have previously been incorrectly used as vapor barriers. True

vapor barriers are products, such as rugged multiple-reinforced

membranes, that have water transmission ratings of

0.00 perms per square foot per hour when tested in accordance

with ASTM E 96. Proper performance of a vapor barrier

requires that laps in the material be sealed. Refer to

manufacturer’s recommendations.

Concrete placed in direct contact with a vapor barrier or

vapor retarder exhibits significantly larger longitudinal dimensional

changes in the first hour after casting than does

concrete placed on a granular base6; there is also more vertical

settlement. Where reinforcing steel is present, settlement

cracking over the steel is more likely because of the increased

vertical settlement resulting from a longer bleeding

period. If the concrete is restrained by connecting members,

base friction, or reinforcement, shrinkage cracking is more

likely because the concrete placed directly on a vapor barrier

or vapor retarder retains more mixing water and thus shrinks

more. In one study, high-slump concrete placed directly on

plastic sheets exhibited significantly more cracking than

concrete placed on a granular base.7

Surface crusting is also more likely for slabs placed directly

on a vapor barrier or vapor retarder. Concrete that doesn’t

lose water to the base won’t stiffen as rapidly as concrete that

does. If the surface crusts over due to drying or to faster setting

caused by solar heat gain, the weight of a power float or

trowel could crack the crusted surface covering a softer layer

of concrete that hasn’t lost water. On-site conditions such as

low humidity, moderate-to-high winds, use of embedded

mineral-aggregate or dry-shake surface hardeners, or a combination

of these can aggravate the problem and increase the

likelihood of cracking.6,8

This Committee recommends that a vapor barrier or vapor

retarder be used only when required by the intended use, and

that installation be in accordance with Section 4.1.5.

3.2.4 Temperature and shrinkage reinforcement—Reinforcement

restrains movement resulting from slab shrinkage

and can actually increase the number of random cracks experienced,

particularly at wider joint spacing (Section

3.2.5.3). Reinforcement in nonstructural slab-on-ground installations

is provided primarily to control the width of

cracks that occur.9,10 This reinforcement is normally furnished

in the form of deformed steel bars, welded wire reinforcing,

steel fibers, or post-tensioning tendons.

Combinations of various forms of reinforcement have

proved successful. The use of each of these types of reinforcement

is discussed in more detail later in this section.

Normally, the amount of reinforcement used in non-structural

slabs is too small to have a significant influence on restraining

movement resulting from volume changes. Refer to

Section 3.2.5 for an expanded discussion of the relationship

between joint spacing and reinforcing quantity.

Temperature and shrinkage cracks in unreinforced slabs

on ground originate at the surface of the slab and are wider

at the surface, narrowing with depth. For maximum effectiveness,

temperature and shrinkage reinforcement in slabs

on ground should be positioned in the upper third of the slab

thickness. The Wire Reinforcement Institute recommends

that welded wire reinforcement be placed 2 in. (50 mm) below

the slab surface or upper one-third of slab thickness,

whichever is closer to the surface.10 Reinforcement should

extend to within 2 in. (50 mm) of the slab edge.

Deformed reinforcing steel or post-tensioning tendons,

when used, should be supported and tied together sufficiently

to prevent displacement during concrete placing and finishing

operations. Chairs with sand plates or precastconcrete

bar supports are generally considered to be the most

effective method of providing the required support. When

precast-concrete bar supports are used, they should be at

least 4 in. (100 mm) square at the base, have a compressive

strength at least equal to the specified compressive strength

of the concrete being placed, and be thick enough to support

reinforcing at the proper elevation while maintaining minimum

coverage of the reinforcing steel

When welded wire reinforcement is used, its flexibility

dictates that the contractor attend closely to establishing and

maintaining adequate support of the reinforcement during

the concrete placing operations. Welded wire reinforcement

should not be laid on the ground and “pulled up” after the

concrete has been placed, nor should the mats be “walked in”

after placing the concrete. Proper support or support-bar

spacing is necessary to maintain welded wire reinforcement

at the proper elevation; supports or support bars should be

close enough that the welded wire reinforcement cannot be

forced out of location by construction foot traffic. Support or

support-bar spacing can be increased when heavier gage

wires or a double mat of small gage wires is used.

Reinforcing bars or welded wire reinforcement should be

discontinued at any joints where the intent of the designer is

to let the joint open and to reduce the possibility of shrinkage

and temperature cracks in an adjacent panel. Where the reinforcement

is carried through the joint, cracks are likely to occur

in adjacent panels because of restraint at the joint.11

When used in sufficient quantity, they will hold out-of-joint

cracks tightly closed. Some engineers prefer partial discontinuation

of the reinforcement at contraction joints in order

to obtain some load transfer capacity without the use of dowel

baskets. See Section 3.2.7.

3.2.4.1 Steel fibers—In some installations, steel fibers specifically

designed for such use can be used with or without

conventional shrinkage and temperature reinforcement in

slab-on-ground floors. As in the case of conventional reinforcement,

steel fibers will not prevent cracking of the concrete.

When used in sufficient quantity, they will hold the

cracks tightly closed.

3.2.4.2 Synthetic fibers—Polypropylene, polyethylene,

nylon, and other synthetic fibers can help reduce segregation

of the concrete mixture and formation of shrinkage cracks

while the concrete is in the plastic state and during the first

few hours of curing. As the modulus of elasticity of concrete

increases, however, most synthetic fibers at typical dosage

302.1R-8 ACI COMMITTEE REPORT

rates recommended by the fiber manufacturers will not provide

sufficient restraint to hold cracks tightly closed.

3.2.4.3 Post-tensioning reinforcement—The use of steel

tendons as reinforcement in lieu of conventional temperature

and shrinkage reinforcement allows the contractor to introduce

a relatively high compressive stress in the concrete by

means of post-tensioning. This compressive stress provides

a balance for the crack-producing tensile stresses that develop

as the concrete shrinks during the curing process. Stage

stressing, or partial tensioning, of the slab on the day following

placement can result in a significant reduction of shrinkage

cracks. Construction loads on the concrete should be

minimized until the slabs are fully stressed.12,13 For guidelines

on installation details, contact a concrete floor specialty

contractor who is thoroughly experienced with this type of

installation.

3.2.4.4 Causes of cracking over reinforcement—Plastic

settlement cracking over reinforcement is caused by inadequate

compaction of concrete, inadequate concrete cover

over reinforcement, use of large-diameter9 bars, high temperature

of bars exposed to direct sunlight, higher-than-required

slump in concrete, revibration of the concrete,

inadequate curing of the concrete, or a combination of these

items.

3.2.5 Joint design—Joints are used in slab-on-ground construction

to limit the frequency and width of random cracks

caused by volume changes. Generally, if limiting the number

of joints or increasing the joint spacing can be accomplished

without increasing the number of random cracks, floor maintenance

will be reduced. The layout of joints and joint details

should be provided by the designer. If the joint layout is not

provided, the contractor should submit a detailed joint layout

and placing sequence for approval of the architect/engineer

prior to proceeding.

As stated in ACI 360R, every effort should be made to

avoid tying the slab to any other element of the structure. Restraint

from any source, whether internal or external, will increase

the potential for random cracking.

Three types of joints are commonly used in concrete slabs

on ground: isolation joints, contraction joints, and construction

joints. Appropriate locations for isolation joints and

contraction joints are shown in Fig. 3.2.5. With the engineer’s

approval, construction joint and contraction joint details

can be interchanged. Refer to ACI 224.3R for an

expanded discussion of joints.

Joints in topping slabs should be located directly over

joints in the base slab.

3.2.5.1 Isolation joints—Isolation joints should be used

wherever complete freedom of vertical and horizontal movement

is required between the floor and adjoining building elements.

Isolation joints should be used at junctions with

walls (not requiring lateral restraint from the slab), columns,

equipment foundations, footings, or other points of restraint

such as drains, manholes, sumps, and stairways.

Isolation joints are formed by inserting preformed joint

filler between the floor and the adjacent element. The joint

material should extend the full depth of the slab and not protrude

above it. Where the joint filler will be objectionably

visible, or where there are wet conditions, hygienic or dustcontrol

requirements, the top of the preformed filler can be

removed and the joint caulked with an elastomeric sealant.

Two methods of producing a relatively uniform depth of

joint sealant are as follow:

1. Score both sides of the preformed filler at the depth to

be removed by using a saw. Insert the scored filler in the

proper location and remove the top section after the concrete

hardens by using a screwdriver or similar tool.

2. Cut a strip of wood equal to the desired depth of the joint

sealant. Nail the wood strip to the preformed filler and install

the assembly in the proper location. Remove the wood strip

after the concrete has hardened.

Alternatively, a premolded joint filler with a removable

top portion can be used. Refer to Figs. 3.2.5.1.a and 3.2.5.1.b

for typical isolation joints around columns. Fig. 3.2.5.1.c

shows an isolation joint at an equipment foundation.

Isolation joints for slabs using shrinkage-compensating

concrete should be treated as recommended in ACI 223.

3.2.5.2 Construction joints—Construction joints are

placed in a slab to define the extent of the individual placements,

generally in conformity with a predetermined joint

layout. If concreting is ever interrupted long enough for the

placed concrete to harden, a construction joint should be

used. If possible, construction joints should be located 5 ft

(1.5 m) or more from any other joint to which they are parallel.

In areas not subjected to traffic, a butt joint is usually adeFig.

3.2.5—Location of joints quate. In areas subjected to hard-wheeled traffic and heavy

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-9

loadings, or both, joints with dowels are recommended (Fig.

3.2.5.2). A keyed joint can be used for low traffic areas

where some load transfer is required. A keyed joint will not

provide the same positive load transfer as a properly constructed

doweled joint because the male and female key

components lose contact when the joint opens due to drying

shrinkage (Section 3.2.7).

3.2.5.3 Contraction joints—Contraction joints are usually

located on column lines, with intermediate joints located at

equal spaces between column lines as shown in Fig. 3.2.5.

The following factors are normally considered when selecting

spacing of contraction joints:

•Method of slab design (refer to ACI 360R)

•Thickness of slab

•Type, amount, and location of reinforcement

•Shrinkage potential of the concrete (cement type and

quantity; aggregate size, quantity, and quality; watercementitious

material ratio; type of admixtures; and

concrete temperature)

•Base friction

•Floor slab restraints

•Layout of foundations, racks, pits, equipment pads,

trenches, and similar floor discontinuities

•Environmental factors such as temperature, wind, and

humidity

Fig. 3.2.5.1.a—Isolation joint at columns

Fig. 3.2.5.1.b—Isolation joints at columns

Fig. 3.2.5.1.c—Isolation joint at equipment pad

302.1R-10 ACI COMMITTEE REPORT

•Methods and quality of concrete curing

As previously indicated, establishing slab joint spacing,

thickness, and reinforcement requirements is the responsibility

of the designer. The specified joint spacing will be a principal

factor dictating both the amount and the character of

random cracking to be experienced, so joint spacing should

always be carefully selected.

For unreinforced, plain concrete slabs, joint spacings of 24

to 36 times the slab thickness up to a maximum spacing of

18 ft (5.5 m) have generally produced acceptable results.

Some random cracking should be expected; a reasonable level

might be random cracks occurring in from 0 percent to 3

percent of the floor slab panels formed by saw-cut or construction

joints or a combination of both.

Joint spacings can be increased somewhat in nominally reinforced

slabs—0.2 percent steel or less placed within 2 in.

(50 mm) of the top of the slab—but the incidence of random

cracking and curling will increase. Reinforcement will not

prevent cracking. However, if the reinforcement is properly

sized and located, crack widths should be held to acceptable

limits.

Transverse contraction joints can be reduced or eliminated

in slabs reinforced with at least 0.5 percent continuous reinforcing

steel placed within 2 in. (50 mm) of the top of the

slab or upper one-third of slab thickness, whichever is closer

to the slab surface. This will typically produce numerous,

closely spaced fine cracks throughout the slab.

Joints in either direction can be reduced or completely

eliminated by post-tensioning to induce a net compressive

force in the slab after all tensioning losses.

The number of joints can also be reduced with the use of

shrinkage-compensating concrete. However, the recommendations

of ACI 223 should be carefully followed.

Contraction joints should be continuous, not staggered or

offset. The aspect ratio of slab panels that are unreinforced,

reinforced only for shrinkage and temperature, or made with

shrinkage-compensating concrete should be a maximum of

1.5 to 1; however, a ratio of 1 to 1 is preferred. L- and Tshaped

panels should be avoided. Fig. 3.2.5.3.a shows various

types of contraction joints. Floors around loading docks

Fig. 3.2.5.2—Doweled construction joint

Fig. 3.2.5.3.a—Types of contraction joints

Fig. 3.2.5.3.b—Joint detail at loading dock

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-11

have a tendency to crack due to their configuration and restraints.

Fig. 3.2.5.3.b shows one method that can be used to

minimize slab cracking at reentrant corners of loading docks.

Plastic or metal inserts are not recommended for creating

a contraction joint in any exposed floor surface that will be

subjected to wheeled traffic.

3.2.5.4 Saw cutting joints—Contraction joints in industrial

and commercial floors are usually formed by sawing a continuous

slot in the slab to form a weakened plane below

which a crack will form (Fig. 3.2.5.3.a). Further details on

saw cutting of joints are given in Section 8.3.12.

3.2.6 Joint filling—Where there are wet conditions, hygienic

and dust-control requirements, or where the floor is

subjected to traffic by small, hard-wheeled vehicles such as

forklifts, contraction and construction joints should be filled

and protected with a semirigid epoxy that gives adequate

support to the joint edges and has sufficient resistance to

wear. Construction joints should be saw cut 1 in. (25 mm)

deep prior to filling. Isolation joints usually are sealed with

an elastomeric sealant. Joints should be as narrow as feasible,

as long as the joint can be properly filled. Refer to Section

5.12 for a discussion of joint materials and Section 9.10

for installation of joint fillers and sealants.

3.2.7 Load transfer mechanisms—Doweled joints (Figs.

3.2.5.2 and 3.2.7.a) are recommended when positive load

transfer is required, unless post-tensioning is provided

across the joint. Dowels force concrete on both sides of a

joint to deflect equally when subjected to a load, and help

prevent damage to an exposed corner when the joint is subjected

to hard-wheeled traffic. Table 3.2.7 provides recommended

dowel sizes and spacing. For dowels to be

effective, they should be smooth, aligned and supported so

they will remain parallel in both the horizontal and the vertical

planes during the placing and finishing operation.

Properly-aligned, smooth dowels allow the joint to open as

concrete shrinks. Dowel baskets (Fig. 3.2.7.b) should be

used to maintain alignment of dowels. Dowels should be

placed no closer than 12 in. (300 mm) from the intersection

of any joints.

As indicated in ACI 223, square dowels cushioned on the

vertical sides by a compressible material to permit movement

parallel and perpendicular to the joint are available.

This type of dowel is useful where the joint must have loadtransfer

capability while allowing some differential movement

in the direction of the joint, such as might be necessary

in post-tensioned slabs on ground.14

In saw-cut contraction joints, aggregate interlock should

not be relied upon for effective load transfer for wheeled

traffic if the expected crack width exceeds 0.035 in. (0.9

mm).15

Deformed reinforcing bars should not be used across contraction

joints or construction joints because they restrain

joints from opening as the slab shrinks during drying. Continuation

of a part of the slab reinforcing through contraction

joints can provide some load transfer capability without using

dowels, but increases the probability of out-of-joint

cracking.

Keyed joints are not recommended in slabs on ground

where heavy traffic is anticipated as they do not provide effective

load transfer. When the concrete shrinks, the keys

and keyways do not retain contact and do not share the load

between panels; this can eventually cause a breakdown of

the concrete edges of the joint.

For long post-tensioned floor strips, care should be taken

to accommodate significant slab movements. In most instances,

post-tensioned slab joints are associated with a jacking

gap. The filling of jacking gaps should be delayed as long

as possible in order to accommodate shrinkage and creep. In

traffic areas, armor plating of the joint edges is recommended.

Fig. 3.2.7.c depicts a doweled joint detail at a jacking gap

in a post-tensioned slab.13,16

3.3—Suspended slabs

3.3.1 Required design elements—In addition to many of

the items listed in Section 1.1.2, the following items specifically

impacting construction of suspended slabs should be

included in the contract documents prepared by the engineer

of record:

•Frame geometry (member size and spacing)

•Reinforcement (type, size, location, and method of support)

•Shear connectors, if required

•Construction joint location

•Metal deck (type, depth, and gage), if required

•Shoring, if required

•Tolerances (forms, structural steel, reinforcement, and

concrete)

3.3.2 Suspended slab types—In general, suspended floor

systems fall into three main categories: (1) slabs with remov-

Table 3.2.7— Dowel size and spacing

Slab depth Dowel diameter Total dowel length* Dowel spacing, center to center

in. in. in. in.

5-6 3/4 16 12

7-8 1 18 12

9-11 11/4 18 12

mm mm mm mm

125-500 19 400 300

175-200 25 450 300

225-275 30 450 300

*Allowance made for joint openings, minor errors in positioning dowels.

Note: Dowels must be carefully aligned and supported during concrete operations. Misaligned dowels cause cracking.

302.1R-12 ACI COMMITTEE REPORT

able forms, (2) slabs on metal decking, and (3) topping slabs

on precast concrete.

Design requirements for cast-in-place concrete suspended

floor systems are covered by ACI 318 and ACI 421.1R. Refer

to these documents to obtain design parameters for various

cast-in-place systems. Slabs on metal decking and

topping slabs on precast concrete are hybrid systems that involve

design requirements established by ANSI, ASCE, the

American Institute of Steel Construction, and the Precast/

Prestressed Concrete Institute, as well as those established

by ACI 117.

Levelness of suspended slabs is dependent on accuracy of

formwork and strikeoff, but is further influenced (especially

in the case of slabs on metal decking) by behavior of the

structural frame during and after completion of construction.

Each type of structural frame behaves somewhat differently;

it is important that the contractor recognize those differences

and plan accordingly.

The presence of camber in some floor members and the

ACI 117 limitation on variation in slab thickness dictate that

concrete be placed to a uniform thickness over the supporting

steel. When placing slabs on metal decking, the contractor

is cautioned that deflections of the structural steel

members can vary from those anticipated by the design engineer.

Achieving a level deflected surface can require increasing

the slab thickness more than 3/8 in. (10 mm) in local

areas. The committee recommends that placement procedures

and the basis for acceptance of the levelness of a completed

floor surface be established and agreed upon by key

parties prior to beginning suspended floor construction.17

3.3.3 Slabs with removable forms—Cast-in-place concrete

construction can be either post-tensioned or conventionally

reinforced. Both of these systems are supported during initial

concrete placement, and they will move when supporting

shores are removed.

Post-tensioned systems are normally used by the designer

when larger spans are necessary or when the structural system

should be shallow for the spans involved. Post-tensioned

systems use high-tensile steel tendons that are stretched beyond

their initial length using a hydraulic jack designed for

that purpose. The tension produced by this stretching operation

has the end result of compressing the concrete. The magnitude

of floor slab deflection after supports are removed is

less than that of comparable floors reinforced with conventional

deformed reinforcing steel. At times, dead load deflection

is entirely eliminated by the use of post-tensioning.

The deformed reinforcing steel in conventionally reinforced

floor systems will start working as the floor deflects.

The magnitude of deflection is dependent on a number of

variables such as span, depth of structure, age at the time

forms are stripped, concrete strength, and amount of reinforcement.

In locations where the anticipated dead load deflection

of a member is deemed excessive by the design

engineer, an initial camber, generally 1/2 in. (13 mm) or

more, can be required. The amount of camber is determined

by the engineer based on an assessment of the impact of the

variables just discussed. Ideally, the cambered parts of the

floor system will deflect down to a level position after removal

of the supporting shores.

3.3.4 Slabs on metal deck—Construction of slabs on metal

deck involves the use of a concrete slab and a supporting

platform consisting of structural steel and metal deck. The

structural steel for this type of construction can be shored or

unshored at the time of concrete placement, and the metal

deck serves as a stay-in-place form for the concrete slab.

This construction can be composite or noncomposite.

The supporting steel platform for slabs on metal deck is

seldom level. Variation in elevations at which steel beams

connect to columns and the presence of camber in some floor

Fig. 3.2.7.a—Doweled contraction joint

Fig. 3.2.7.b—Dowel basket assembly

Fig. 3.2.7.c—Joint detail for post-tensioned slab

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-13

members combine to create variations in the initial elevation

of steel members. Regardless of the initial levelness of the

steel frame, unshored frames will deflect during concrete

placement. These factors make the use of a laser or similar

instrument impractical for the purpose of establishing a uniform

elevation for strikeoff of the concrete surface of a slab

on metal deck. The presence of camber in some floor members

and the ACI 117 limitation on variation in slab thickness

dictate that concrete be placed to a uniform thickness over

the supporting steel.

3.3.4.1 Composite slabs on metal deck—In composite construction,

the composite section (concrete slab and steel

beams) will work together to support any loads placed on the

floor surface after the concrete has hardened. Composite behavior

is normally developed through the use of shear connectors

welded to the structural steel beam. These shear

connectors physically connect the concrete slab to the beam

and engage the concrete slab within a few feet of the steel

beam; the resulting load-carrying element is configured

much like a capital “T.” The steel beam forms the stem of the

“T,” and the floor slab forms the cross-bar. It is important

that construction joints be located far enough from structural

steel beams that they parallel to eliminate their impact on

composite behavior. Questions about the location of construction

joints should be referred to the engineer of record

on the project.

Unshored composite construction is the more common

method used by designers because it is generally less expensive

than shored construction. In unshored construction, the

structural steel beams are sometimes cambered slightly during

the fabrication process. This camber is intended to offset

the anticipated deflection of that member under the weight of

concrete. Ideally, after concrete has been placed and the system

has deflected, the resulting floor surface will be level.

Shored composite concrete slabs on metal deck are similar

to slabs with removable forms, in that both are supported until

the concrete has been placed and reaches the required

strength. Structural steel floor framing members for shored

composite slabs on metal deck are usually lighter and have

less camber than those used for unshored construction with

similar column spacings and floor loadings. One major concern

with shored composite construction is the tendency for

cracks wider than 1/8 in. (3 mm) to form in the concrete slab

when the supporting shores are removed. These cracks do

not normally impair the structural capacity of the floor, but

can become a severe aesthetic problem. The contractor is

cautioned that this issue and any measures taken by the designer

to avoid the formation of cracks should be addressed

to the satisfaction of key parties prior to beginning suspended

floor construction.

3.3.4.2 Noncomposite slabs on metal deck—In noncomposite

construction, the slab and supporting structural steel

work independently to support loads imposed after hardening

of the concrete slab.

3.3.5 Topping slabs on precast concrete—A cast-in-place

concrete topping on precast prestressed concrete units involves

the use of precast elements as a combination form and

load-carrying element for the floor system. The cast-in-place

portion of the system consists of a topping of some specified

thickness placed on top of the precast units. The topping can

be composite or noncomposite. In either case, added deflection

of precast units under the weight of the topping slab is

normally minor, so the finished surface will tend to follow

the surface topography established by the supporting precast

units. The camber in precast members, if they are prestressed,

can change with time as a result of concrete creep.

Depending on the length of time between casting of precast

units and erection, this potential variation in camber of similar

members can create significant challenges for the contractor.

Care should be taken in the scheduling of such

operations to minimize the potential impact of these variations.

Precast members are less flexible and adaptable to

changes or modifications that can be required on the jobsite

than are the previously discussed systems.

3.3.6 Reinforcement—For cast-in-place concrete suspended

slabs, reinforcing steel location will vary as dictated by

the contract documents. Post-tensioning reinforcement,

when used, is enclosed in a plastic or metal sleeve and is

stretched beyond its initial length by means of a hydraulic

jack after the concrete reaches sufficient compressive

strength. Elongation and subsequent anchoring of the ends of

post-tensioning tendons results in transfer of compressive

force to the concrete. See references for installation details.

For slabs on metal deck, reinforcement is normally provided

by deformed reinforcing steel, welded wire reinforcement,

steel fibers, or a combination thereof.

3.3.7 Construction joints—The engineer of record should

provide criteria for location of construction joints in suspended

slabs. Following is a general discussion of criteria

that can influence these decisions.

3.3.7.1 Slabs on removable forms—Construction joints

can introduce weak vertical planes in an otherwise monolithic

concrete member, so they should be located where shear

stresses are low. Under most gravity load conditions, shear

stresses in flexural members are low in the middle of the

span. ACI 318 requires that construction joints in floors be

located within the middle third of spans of slabs, beams and

primary beams. Joints in girders should be offset a minimum

distance of two times the width of any intersecting beams.

3.3.7.2 Composite slabs on metal deck—An important

consideration when locating construction joints in composite

slabs on metal deck is that the joint location can influence

deflection of the floor framing near the joint. A composite

member (steel beam and hardened concrete slab working together)

is stiffer, and deflects less, than a non-composite

member (steel beam acting alone). Most composite slabs on

metal deck are placed on an unshored structural steel floor

frame. Often, structural steel members have fabricated camber

to offset anticipated noncomposite deflection resulting

from concrete placement; during placement of the concrete,

the structural steel deflects a small amount. After hardening

of the concrete, however, the composite member deflects

much less than a comparable noncomposite beam or primary

beam.

Following are general guidelines for locating construction

joints in composite slabs on metal deck.

302.1R-14 ACI COMMITTEE REPORT

1. For slabs that span in one direction between primary

beams, locate construction joints that parallel secondary

beams a sufficient distance from the structural steel member

to allow full flange width to be developed. For slabs that

span between secondary beams, the construction joint should

normally be located near midspan of the slab between

beams.

2. Locate construction joints that parallel primary beams,

and cross secondary beams, near the primary beam. It is important,

however, to allow sufficient distance for development

of the primary beam flange width. Placing the

construction joint a distance of 4 ft (1.2 m) from the primary

beam is usually sufficient for this purpose. This location

allows nearly the full dead load from concrete placement to

be applied to secondary beams on both sides of the primary

beam at one time.

If the primary beam is not cambered, it might be best to

consider including the primary beam in the initial placement.

Dead load deflection will be reduced because a composite

section will be supporting the second concrete

placement at that construction joint. If the primary beam is

cambered, it should be included in the second placement at

the construction joint. This will allow full dead load from

concrete to be present prior to hardening of concrete at the

primary beam.

3. Construction joints that cross primary beams should be

located near a support at one end of the primary beam. This

will allow full dead load from concrete to be present prior to

hardening of concrete at the primary beam.

3.3.7.3 Noncomposite slabs on metal deck—The location

of construction joints in noncomposite slabs on metal deck

should follow the same general guidelines discussed for

slabs on removable forms in Section 3.3.6.1.

3.3.7.4 Topping slabs on precast concrete—Construction

joints in topping slabs on precast concrete should be located

over joints in the supporting precast concrete.

3.3.8 Cracks in slabs on metal deck—Cracks often develop

in slabs on metal deck. These cracks can result from drying

shrinkage and thermal contraction or variations in

flexibility of the supporting structural steel and metal deck.

In a composite floor framing system, primary beams are the

stiffest elements and generally deflect less than secondary

beams. The most flexible part of the floor framing assembly

is the metal deck, which often is designed for strength and

with little thought to its flexibility.

If the metal deck is flexible, vibration as a result of power

floating and power troweling operations can produce cracking

over the structural steel beams during concrete finishing

operations. As the concrete cures and shrinks, these cracks

will open wide if not restrained by reinforcing steel, usually

welded wire reinforcement, located near the top surface of

the slab.

3.4—Miscellaneous details

3.4.1 Heating ducts—Heating ducts embedded in a concrete

slab can be of metal, rigid plastic, or wax-impregnated

cardboard. Ducts with waterproof joints are recommended.

When metal ducts are used, calcium chloride should not be

used in the concrete. Refer to Section 5.6.3 for a discussion

on chlorides in concrete and Section 4.5.2 for installation of

heating ducts.

3.4.2 Edge insulation—Edge insulation for slabs on

ground is desirable in most heated buildings.The insulation

should be in accordance with ASHRAE 90.1. It should not

absorb moisture and should be resistant to fungus, rot, and

insect damage; it should not be easily compressed.

Insulation should preferably be placed vertically on the inside

of the foundation. It can also be placed in an L-shape adjacent

to the inside of the foundation and under the edge of

the slab. In either case, the installation should extend a total

distance of 24 in. (600 mm).

3.4.3 Radiant heating: piped liquids—Slabs can be heated

by circulating heated liquids through embedded piping. Ferrous,

copper, or plastic pipe is generally used, with about 2

in. (50 mm) of concrete [not less than 1 in. (25 mm)] under

the pipe and with 2 to 3 in. (50 to 75 mm) of concrete cover

over the pipe. The slab is usually monolithic, and the concrete

is placed around the piping, which is fixed in place.

Two-course slab construction has also been used, wherein

the pipe is laid, connected, and pressure tested for tightness

on a hardened concrete base course. Too often, however, the

resulting cold joint is a source of later trouble.

Insulating concrete made with vermiculite or perlite aggregate,

or cellular foam concrete can be used as a subfloor.

The piping should not rest directly on this or any other base

material. Supports for piping during concreting should be

inorganic and nonabsorbent; precast concrete bar supports

(Section 3.2.4) are preferred to random lengths of pipe for

use as supports and spacers. Wood, brick, or fragments of

concrete or concrete masonry should not be used.

Sloping of the slab, where possible, can simplify sloping

of the pipe. Reinforcement, such as welded wire reinforcement,

should be used in the concrete over the piping. Where

pipe passes through a contraction joint or construction joint,

provision should be made for possible movement across the

joint. The piping should also be protected from possible corrosion

induced by chemicals entering the joint. The piping

should be pressure-tested before placing concrete and air

pressure (not water pressure) should be maintained in the

pipe during concreting operations. After concreting, the slab

should not be heated until curing of the concrete is complete.

The building owner should be warned to warm the slabs

gradually, using lukewarm liquid in the system, to prevent

cracking of the cold concrete.

3.4.4 Radiant heating: electrical—In some electrical radiant

heating systems, insulated electrical cables are laid

singly in place within the concrete or fastened together on

transverse straps to form a mat. One system employs cable

fastened to galvanized wire sheets or hardware cloth. The

cables are embedded 1 in. to 3 in. (25 mm to 75 mm) below

the concrete surface, depending on their size and operating

temperature. In most systems the wires, cables, or mats are

laid over a bottom course of unhardened concrete, and the

top course is placed immediately over this assemblage with

little lapse of time, thus avoiding the creation of a horizontal

cold joint.18

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-15

Calcium chloride should not be used where copper or aluminum

wiring is embedded in the concrete; damage to insulation

and subsequent contact between the exposed wiring

and reinforcing steel will cause corrosion. If admixtures are

used, their chloride contents should comply with the limits

recommended by ACI 222R.

3.4.5 Snow-melting—Systems for melting snow and ice

can be used in loading platforms or floor areas subjected to

snow and ice. The concrete should be air-entrained for

freeze-thaw resistance. Concrete surfaces should have a

pitch of 1/4 in. per ft (20 mm per m) to prevent puddles from

collecting. Piping systems should contain a suitable liquid

heat-transfer medium that does not freeze at the lowest

temperature anticipated. Calcium chloride should not be

used (Section 5.6.3). Experience has shown these systems

to demand high energy consumption while displaying a

high potential for failure and thermal cracking. The most

successful applications appear to have been at parking garage

entrances.

Some electrical systems are in use. These internally heated

snow-melting systems have not been totally satisfactory.

3.4.6 Pipe and conduit—Water pipe and electrical conduit,

if embedded in the floor, should have at least 11/2 in. (38 mm)

of concrete cover, both on top and bottom.

3.4.7 Slab embedments in harsh environments—Care

should be exercised in using heating, snow-melting, water,

or electrical systems embedded in slabs exposed to harsh environments,

such as parking garages in northern climates,

and marine structures. Embedded systems can accelerate deterioration

by increasing seepage of saltwater through the

slab or by forming electrical corrosion circuits with reinforcing

steel. If concrete deterioration occurs, the continuity and

effective functioning of embedded systems are invariably

disrupted.

CHAPTER 4—SITE PREPARATION AND PLACING

ENVIRONMENT

4.1—Soil support system preparation

The soil support system should be well drained and provide

adequate and uniform load-bearing support.

The ability of a slab to take loads depends on the integrity

of both the slab and full soil support system. As a result, it is

essential that the full soil support system be tested or thoroughly

evaluated before the slab is placed upon it.19

The in-place density of the subgrade, subbase (if used),

and base (Fig. 3.1) should be at least the minimum required

by the specifications, and the base should be free of frost before

concrete placing begins and able to support construction

traffic such as loaded truck mixers.

The base should normally be dry at the time of concreting.

However, if protection from the sun and wind cannot be provided

as mentioned in Section 4.6, or if the concrete is placed

in hot, dry conditions, the base should be lightly dampened

with water in advance of concreting. There should be no free

water standing on the base, nor should there be any muddy

or soft spots, when the concrete is placed (Sections 4.1.1 and

4.1.4).

4.1.1 Proof-rolling—Proof-rolling is one of the most effective

ways to determine if the full soil support system is adequate

to provide a uniformly stable and adequate bearing

support during and after construction. If applicable, this process

should be implemented after completion of the rough

grading and if required can be repeated prior to the placement

of the slab (Fig. 4.1.1).

Proof-rolling, observed and evaluated by the engineer or

the engineer’s representative, should be accomplished by a

loaded tandem axle dump truck, a loaded truck mixer, roller,

or equivalent. In any case, multiple passes should be made

using a preestablished grid pattern.

If rutting or pumping is evident at any time during the

preparation of the subgrade, subbase, or baserolling, corrective

action should be taken.

“Rutting” normally occurs when the surface of the base

or subbase is wet and the underlying soils (subgrade) are

firm. “Pumping” normally occurs when the surface of the

base or subbase is dry and the underlying soils are wet. Any

depression in the surface deeper than 1/2 in. (13 mm) should

be repaired. Repair should include, but not be limited to,

raking smooth or compacting with suitable compaction

equipment.

4.1.2 Subgrade tolerance—The necessary grading of the

subgrade, often referred to as “rough grading,” should conform

to a tolerance of + 0 in./- 11/2 in. (+ 0 mm/- 38 mm).

Compliance should be confirmed prior to removal of excavation

equipment. A rod and level survey should be performed;

measurements should be taken at 20-ft (6-m)

intervals in each direction.

4.1.3 Base tolerance—Base tolerances, often referred to as

“fine grading,” should conform to a tolerance of + 0 in./- 1

in. (+ 0 mm/- 25 mm) for floor Classes 1 through 3 and + 0

in./- 3/4 in. (+ 0 mm/-19 mm) for floor Classes 4 through 9

when measured from bottom of slab elevation. Compliance

with these fine-grade values should be based on the measurements

of individual floor sections or placements. A rod and

level survey should be performed; measurements should be

taken at 20-ft (6-m) intervals in each direction.

Fig. 4.1.1—Proofrolling by loaded ready mix truck

302.1R-16 ACI COMMITTEE REPORT

4.1.4 Base material—Use of the proper materials is essential

in order to achieve the tolerances suggested in Section

4.1.3. The base material should be a compactible, easy-totrim,

granular fill that will remain stable and support construction

traffic. The tire of a loaded concrete truck mixer

should not penetrate the surface more than 1/2 in. (13 mm)

when driven across the base. The use of so-called cushion

sand or clean sand with uniform particle size, such as concrete

sand meeting ASTM C33, will not be adequate. This

type of sand will be difficult, if not impossible, to compact

and maintain until concrete placement is complete.

A clean, fine-graded material with at least 10 percent to 30

percent of particles passing a No. 100 (150 μm) sieve but not

contaminated with clay, silt, or organic material is recommended.

Manufactured sand from a rock-crushing operation

works well; the jagged slivers tend to interlock and stabilize

the material when compacted. It is important that the material

have a uniform distribution of particle sizes ranging from

No. 4 (4.75 mm) through the No. 200 (80 μm) sieve. See

ASTM C33, Table 1, for limitation of deleterious material

finer than No. 200 (80 μm) sieve. Unwashed size No. 10 per

ASTM D 448 works well.

4.1.5 Vapor barrier/vapor retarder—If a vapor barrier or

vapor retarder is required due to local conditions, these products

should be placed under a minimum of 4 in. (100 mm) of

trimable, compactible, granular fill (not sand). A so-called

“crusher run” material, usually graded from 11/2 in. to 2 in.

(38 mm to 50 mm) down to rock dust, is suitable. Following

compaction, the surface can be choked off with a fine-grade

material (Section 4.1.4) to reduce friction between the base

material and the slab.

If it is not practical to install a crusher-run material, the vapor

barrier/retarder should be covered with at least 3 in. (75

mm) of fine-graded material, such as crusher fines or manufactured

sand (Section 4.1.4). The granular fill, as well as the

fine-graded material, should have sufficient moisture content

to be compactible, but still be dry enough at the time of

concrete placement to act as a “blotter” (Section 4.1).

If a vapor barrier/retarder is to be placed over a rough

granular fill, a thin layer of approximately 1/2 in. (13 mm) of

fine-graded material should be rolled or compacted over the

fill prior to installation of the vapor barrier/retarder to reduce

the possibility of puncture (Section 4.1.4). Vapor barriers/retarders

should be overlapped 6 in. (150 mm) at the joints and

carefully fitted around service openings. See Section 3.2.3

for more information on vapor barriers/retarders for slabs on

ground.

4.2—Suspended slabs

Prior to concrete placement, bottom-of-slab elevation as

well as the elevation of reinforcing steel and any embedments

should be confirmed. Forms that are too high can often

force reinforcement above the desired elevation for the slab

surface. Screed rails or guides should be set at elevations that

will accommodate initial movement of the forms as they are

loaded. Screed rails may also be set at elevations that will

offset downward deflection of the structure following concrete

placement (Section 3.3).

4.3—Bulkheads

Bulkheads can be wood, metal, or precast concrete20; they

should be placed at the proper elevation with stakes and necessary

support required to keep the bulkheads straight, true,

and firm during the entire placing and finishing procedure.

Keyways are not recommended. However, if specified, small

wood or metal keys should be attached to the inside of the

form.

When it is necessary to set bulkheads on insulation material,

such as in cold storage or freezer rooms, extra attention

should be given to keeping the forms secure during the placing

and finishing process. The insulation material should not

be punctured by stakes or pins. It may be necessary to place

sand bags on top of form supports to ensure stability during

concrete placement.

Circular or square forms can be used to isolate the columns.

Square forms should be rotated 45 deg (Fig. 3.2.5.1.a)

or installed in a pin-wheel configuration as indicated in Fig.

3.2.5.1.b. Walls, footings, and other elements of the structure

should be isolated from the floors. Asphalt-impregnated

sheet or other suitable preformed compressible joint material

(ACI 504R) should be used. These joint materials should

never be used as freestanding forms at construction joints or

column block outs, but should be installed after the original

forms have been removed. After removal of forms around

columns, preformed joint materials should be placed at the

joint to the level of the floor surface, and the intervening area

concreted and finished. These preformed joint materials can

be placed at the proper elevation to serve as screed guides

Table 5.4.1— General guide for preferred grading of fine aggregates for floor concrete

Sieve designations Percent passing

Standard Alternative Normal weight aggregate Light-weight aggregate

Heavy-duty toppings, Class 7

floors

9.5 mm 3/8 in. 100 100 95

4.75 mm No. 4 85-100 85-100 95-100

2.36 mm No. 8 80-90 — 65-80

1.18 mm No. 16 50-75 40-80 45-65

600 m No. 30 30-50 30-65 25-45

300 m No. 50 10-20 10-35 5-15

150 m No. 100 2-5 5-20 0-5

m

m

m

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-17

during the concreting operations. The preformed joint material

should be of the type specified and should conform to

one of the following specifications, depending upon the conditions

of its use: ASTM D 994, D 1751, or D 1752.

4.4—Setting screed guides

The screed guides can be 2-in.-thick (50-mm) lumber,

pieces of pipe, T bars, or rails, the tops of which are set to the

finished concrete grade without changing the design elevation

of the reinforcing steel. Each type should have a tightradius

edge. If the wet-screed approach is used to establish

concrete grade, the finished floor elevation for a slab on

ground may be laid out by driving removable grade stakes

into the subgrade at predetermined intervals that are appropriate

for the width of placement strips being installed. The

tops of these stakes should be set to the required concrete

grade.

4.4.1 Establishing grades for adequate drainage on the

slab surface—When positive drainage is desired, the forms

and screed guides should be set to provide for a minimum

slope of 1/4 in. per ft (20 mm/m) to prevent ponding. Positive

drainage should always be provided for exterior slabs and

can be desirable for some interior slabs.

4.5—Installation of auxiliary materials

4.5.1 Edge insulation—Insulation (Section 3.4.2) should

preferably be placed vertically on the inside of the foundation.

It can also be placed in an inverted L-shape adjacent to

the foundation and under the edge of the slab.

4.5.2 Heating ducts—Metal, rigid plastic, or wax-impregnated

cardboard ducts with watertight joints are recommended;

they can be set on a sand-leveling bed and back-filled

with sand to the underside of the slab. Precautions should be

taken to ensure that the position of the ducts is not disturbed

during concreting, and that they are adequately protected

from corrosion or deterioration.

If the ducts to be used are not waterproof, they should be

completely encased in at least 2 in. (50 mm) of concrete to

prevent the entrance of moisture.

4.6—Concrete placement conditions

When slabs are placed on grade, there should be no more

than 30 F (17 C)—ideally, 20 F (11 C)—difference between

the temperature of the base and concrete at the time of placement.

Floor slab installations should be made in a controlled environment

where possible. Protection from the sun and wind

is crucial to the placing and finishing process. The roof of the

structure should be waterproof, and the walls should be completely

up. The site should provide easy access for concrete

trucks and other necessary materials and suppliers. The site

should be adequately lighted and ventilated. Temperatures

inside the building should be maintained above 50 F (10 C)

while placing, finishing, and curing the concrete. If heaters

are required, they should be vented to the outside.21 Salamanders

or other open flame heaters that might cause carbonation

of the concrete surface should not be used. When

installation procedures are carried out each day under the

same predictable conditions, the resulting floors are significantly

superior to those floors installed under varying or

poor environmental conditions. Also, see Sections 9.5.1 and

9.5.2 for cold and hot weather considerations.

CHAPTER 5—MATERIALS

5.1—Introduction

Concrete ingredients meeting the same ASTM standards

can affect the concrete very differently. These standards offer

a wide window of acceptance.22 It is, therefore, recommended

that the specific characteristics of ingredients be

investigated prior to the preparation of mixture proportions

for floors and slabs.

5.2—Concrete

Since minimizing shrinkage is of prime importance, special

attention should be given to selecting the best possible

concrete mixture proportions. The shrinkage characteristics

of a concrete mixture can be determined by ASTM C 157.

Should it be necessary to determine if a proposed concrete

mixture has other than normal shrinkage, approximately .05

percent, the proposed concrete mixture should be compared

to the specified or a reference concrete mixture using ASTM

C 157. It is essential that the concrete used in these tests be

made with the same materials that will be used in the actual

construction.

Approval of a concrete mixture for use in a floor should

not be solely based on its meeting the specified compressive

strength related to standard laboratory cured cylinders. The

portland cement content, and the content of other cementitious

products if used, should be sufficient to allow satisfactory

finishability. The setting characteristics of the concrete

should be relatively predictable. It should be verified that the

concrete will not experience excessive retardation, differential

set time, or surface crusting difficulties under the conditions

of temperature and humidity expected on the project.

Some admixture-cement combinations can cause these difficulties,

particularly when multiple admixtures are used.

Since there is not a generally-recognized procedure for establishing

these performance characteristics, the committee

recommends placement of a sample floor slab as indicated in

Section 6.2.4. Floor concrete requirements differ from those

of other concrete used in the structure. Project requirements

should be reviewed thoroughly prior to mixture proportioning.

If possible, the concrete contractor should have the opportunity

to review the proposed mixture proportions, and to

prepare a sample placement to verify the workability, finishability,

and setting time for the proposed usage.

5.3—Portland cement

5.3.1—Concrete floors can incorporate a variety of portland

cements that meet ASTM Specifications C 150, C 595,

C 845, and C 1157.

Of the four cements used in floors and slabs described in

ASTM C 150, Type I is the most common, and it is used

when the special properties of another type are not required.

Type II is also for general use, especially when moderate sul302.1R-

18 ACI COMMITTEE REPORT

fate resistance or moderate heat of hydration is desired. Type

III is used when high early strength is desired. Type V is used

where high sulfate resistance is required.

If air-entrained concrete is required, air-entrainment

should be obtained with an admixture, rather than by using

an air-entraining cement; this allows for better control of air

content.

5.3.2 Blended hydraulic cements—Blended hydraulic cements

are produced by intimately and uniformly blending

two or more types of fine materials, such as portland cement,

ground granulated blast furnace slag, fly ash and other pozzolans,

hydrated lime, and preblended cement combinations

of these materials.

There are six recognized classes of blended cements that

conform with ASTM C 595: Type IS portland blast-furnace

slag cement; Type IP and P portland-pozzolan cements;

Type I (PM) pozzolan-modified portland cement; Type S

slag cement; and Type I (SM) slag-modified portland cement.

However, Types P and S are normally not available for

use in general concrete construction. It is strongly recommended

that the manufacturers of these cements be contacted

for information regarding the specific product and the

impact its use will have on setting time, strength, water demand,

and shrinkage of concrete proposed for the project under

anticipated field conditions. Conformance to the

requirements of ASTM C 150 does not impose sufficient restrictions

on the cement to be used; if the 28-day design

strength is achieved, but shrinkage is excessive and retardation

is significant, the cement may not be suitable for the

project.

ASTM C 1157 is a performance specification that establishes

physical requirements for six types of blended cements

mirroring the attributes of ASTM C 150 cement types.

For information on pozzolans used as cement replacements

or cementitious additions, see Section 5.6.5.

5.3.3 Expansive cements—Types K, M, and S are expansive

cements meeting ASTM C 845 specifications that are

used in shrinkage-compensating concrete floors. See ACI

223 for specific details on shrinkage-compensating concrete

floors. Shrinkage-compensating concrete can also be made

by adding an expansive admixture as discussed in Section

5.6.4.

5.4—Aggregates

Aggregates should conform to ASTM C 33 or to ASTM C

330. These specifications are satisfactory for most Class 1, 2,

3, 4, 5, and 6 floors. Additional limitations on grading and

quality can be required for the surface courses of heavy-duty

Class 7 and 8 floors.

Although these ASTM standards set guidelines for source

materials, they do not establish combined gradation requirements

for the aggregate used in concrete floors A uniform

gradation is necessary to produce a desirable matrix while

reducing water demand of the concrete mixture and reducing

the amount of cement paste required to coat the aggregate.23

5.4.1 Fine aggregate grading—Although ASTM C 33 and

C 330 are acceptable specifications, Table 5.4.1 contains

preferred grading specifications for the toppings for Class 7

floors. The amount of material passing the No. 50 and 100

sieves (300 and 150 μm) should be limited as indicated for

heavy-duty floor toppings for Class 7. However, when fine

aggregates contain minimum percentages of material passing

the No. 50 and 100 sieves (300 and 150 μm), the likelihood

of excessive bleeding is increased and limitations on

water content of the mixture become increasingly important.

Natural sand is preferred to manufactured sand; the gradation

indicated in Table 5.4.1 will minimize water demand.

5.4.2 Coarse aggregate grading—The maximum size of

coarse aggregate should not exceed three-fourths the minimum

clear spacing of the reinforcing bars in structural

floors, nor one-third the thickness of nonreinforced slabs. In

general, natural aggregate larger than 11/2 in. (38 mm) or

lightweight aggregate larger than 1 in. (25 mm), is not used.

Although the use of large aggregate is generally desired for

lower water demand and shrinkage reduction, it is important

to recognize the overall gradation of all the aggregate (Section

5.1). When aggregate sizes larger than 1 in. (25 mm) are

used, the coarse aggregate can be batched as two sizes to prevent

segregation. Drying shrinkage can be minimized by the

use of the largest practical size coarse aggregate. However,

if flexural strength is of primary concern, the use of smaller

size coarse aggregate can help achieve better uniformity in

strength.

5.4.3 Combined aggregate grading—Gradations requiring

between 8 percent and 18 percent for large top size aggregates

(such as 11/2 in.) or 8 percent and 22 percent for smaller

top size aggregates (such as 1 in. or 3/4 in.) retained on each

sieve below the top size and above the No. 100 sieve have

proven to be satisfactory in reducing water demand while

providing good workability. The ideal range for No. 30 and

No. 50 sieves is 8 percent to15 percent retained on each. Often,

a third aggregate is required to achieve this gradation.23

Typically, 0 percent to 4 percent retained on the top size

sieve and 1.5 percent to 5.0 percent on the No. 100 sieve will

be a well graded mix. This particle size distribution is appropriate

for round or cubically-shaped particles in the No. 4

through the No. 16 sieve sizes. If the available aggregates for

these sizes are slivered, sharp, or elongated, 4 percent to 8

percent retained on any single sieve is a reasonable compromise.

Mixture proportions should be adjusted whenever individual

aggregate grading varies during the course of the

work.

5.4.4 Aggregate quality—Compliance with ASTM C 33

and C 330 generally ensures aggregate of adequate quality,

except where one of the following conditions will be severe:

chemical attack, or abrasion in Class 7 and 8 floors. See ACI

201.2R for a more complete discussion of precautions under

these conditions. Sections 5.4.6 and 5.4.8 discuss special

abrasion-resistant and nonslip aggregates respectively. The

guidelines of ACI 201.R and ASTM C 33 and its appendix

should be followed where there is concern about the possibility

of alkali-aggregate reaction.

5.4.5 Special-purpose aggregates—Decorative and nondecorative

mineral aggregate and metallic hardeners are

used to improve the properties of the slab surface. These materials

applied as dry shakes on top of the concrete are floated

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-19

and troweled into the floor surface to improve the abrasion

resistance, impact resistance, achieve nonslip surfaces, or to

obtain a decorative finish. In this document, the term “dryshake”

is applied to premixed materials, which may be mineral

aggregate, metallic, or colored. The term “embedded” is

a more generic term used where the material can be furnished

in either premixed or bulk form. Trap rock and emery

are two examples of materials that can be furnished in bulk

form.

5.4.6 Wear-resistant aggregates—Hard, abrasion-resistant

aggregates, such as quartz, emery, and traprock, as well

as malleable metallic hardeners, are frequently used as surface

treatments.3 They are applied as dry shakes and finished

into the surface of the floor to improve its abrasion and wear

resistance.

Nonmetallic surface hardeners should be used on floors

subjected to heavy frequent forklift or hard-wheeled traffic

(Table 2.1). Metallic hardeners in sufficient quantity should

be considered for use when heavy steel wheel or intense

point impact loading is anticipated. Chloride-bearing admixtures

should not be used in conjunction with a metallic floor

hardener.

Mineral aggregate and metallic surface hardeners are factory

premixed with specially selected portland cement and

plasticizers. Some mineral aggregates can be supplied in

bulk and mixed with cement onsite. These aggregates, in

properly graded sizes, can also be used in topping mixes.

5.4.7 Surface treatment for electrically conductive

floors—Concrete floors can be made electrically conductive

by using specially prepared metallic hardeners (dry shakes).

Electrically conductive floors are also required to be sparkresistant

under abrasion or impact. For protection against

abrasion sparks, care should be taken in the choice of aggregates.

Since construction techniques for these floors are rather

specialized, specific recommendations of the product

manufacturer and engineer should be followed.24

The electrical resistance of such floors can be determined

by reference to the appropriate specification of the Naval Facilities

Engineering Command.25 A typical test for spark resistance

under abrasion or impact is given in the above

specification, as well as the National Fire Protection Association,

NFPA 99 specification. A factory premixed metallic

surface hardener containing a conductive binder is commonly

used for these floors. This hardener is floated and troweled

into the surface of freshly placed concrete (Section 8.6).

Special conductive curing compounds should be used to

cure these floors. Conductive floors should not be used in areas

expected to be continuously moist.

5.4.8 Slip-resistant aggregates—Slip-resistant aggregates

should be hard and nonpolishing. Fine aggregates are usually

emery or a manufactured abrasive. The slip resistance of

some aggregates can be improved by replacing the fines with

those of a more slip-resistant aggregate. To improve slip resistance,

extremely soft aggregates like vermiculite can be

troweled into the surface of freshly placed concrete, and then

removed later by scrubbing after the concrete has hardened.

5.4.9 Decorative aggregates—Decorative aggregates can

be of many minerals and colors. They should be sound,

clean, nonreactive, and of good quality. The most common

are quartz, marble, granite, and some ceramics. Rocks,

shells, brass turnings or other brass pieces, and ball bearings

have also been used. Shapes resembling spheres and cubes

are preferable to flat or highly irregularly-shaped pieces,

which can become dislodged easily. It is usually preferable

to have aggregate of only one sieve size.

5.5—Water

Mixing water should be potable. Nonpotable water can be

used if 7- and 28-day strengths of 2-in. (50-mm) mortar

cubes made with it are equal to at least 90 percent of the

strengths of cubes made from similar mixtures using distilled

water and tested in accordance with ASTM C 109. ACI

301 discusses mixing water, as do Steinour26 and others.27

Also see AASHTO T 26.

5.6—Admixtures

Admixtures should be used when they will effect a specific

desired change in the properties of the freshly mixed or

hardened concrete. They should be used in accordance with

the instruction and principles given in ACI 212.1R and

212.2R and the guidelines for chloride limits given in Section

5.6.3. If more than one type of admixture is used in the

same concrete, each should be batched separately. A second

admixture can significantly affect the required dosage of

both admixtures; therefore, preliminary tests are recommended

to assure compatibility. Sample slabs made under

the anticipated job conditions of temperature and humidity

can also be used to help evaluate admixture performance,

and to allow necessary adjustments affecting workability,

finishability, and setting time prior to the start of the slab installation.

Some admixtures are not compatible with shrinkage-

compensating concrete because they adversely affect

expansion, bond to steel, and shrinkage (ACI 223).

5.6.1 Air-entraining admixtures—Concrete for use in areas

that will be exposed to freezing temperatures while moist

should contain entrained air (Section 6.2.7). Entrained air is

not recommended for concrete to be given a smooth, dense,

hard troweled finish since blistering and delamination may

occur. Smaller percentages of entrained air may reduce

bleeding and segregation, and may be used for floors and

slabs using other finishes when they improve finishability of

concretes not exposed to freezing. Air-entraining admixtures,

when used in the concrete as recommended in Chapter

6, should meet the requirements of ASTM C 260. Consistent

control of air entrainment is necessary.

In most cases, concrete for trowel-finished interior concrete

floors made with normal weight aggregates should not

include an air-entraining admixture; the maximum air content

for these concretes should normally be 3 percent. Higher

air contents make the surface difficult to finish, and can lead

to surface blistering and peeling during finishing.

5.6.2 Chemical admixtures—Chemical admixtures should

meet the requirements of ASTM C 494 for whichever of the

following types are to be used:

•Type A water-reducing

•Type B retarding

302.1R-20 ACI COMMITTEE REPORT

•Type C accelerating

•Type D water-reducing and retarding

•Type E water-reducing and accelerating

•Type F high-range water-reducing (superplasticizer)

•Type G high-range water-reducing (superplasticizer)

and retarding.

The superplasticizers should also meet the requirements of

ASTM C 1017. Water-reducing and combination admixtures

should provide the additional advantage of increased compressive

and flexural strength at ages less than 6 months. The

retarding admixtures can be useful in delaying initial set and

extending time available for final finishing in hot weather;

however, excessive retardation can cause surface crusting or

plastic shrinkage cracking. Accelerating admixtures increase

the rate of strength gain at early ages and can be useful in

cold weather.

High-range water-reducing admixtures (superplasticizers)

can be used to greatly reduce the water content in concrete

while maintaining a given consistency. The resultant shrinkage

reduction, if any, may not parallel the water reduction.

They also can be used to increase slump significantly without

the need to increase the water content of the original mixtures.

High slumps, however, can cause consolidation and

finishing problems; if high slumps are used, consolidation

and finishing methods should be modified to avoid segregation

of the concrete and finishing before the concrete is sufficiently

stiff.

Admixtures conforming to ASTM C494 will not necessarily

reduce shrinkage nor improve the finishing characteristics

of the concrete.28 Shrinkage tests as indicated in Section

5.2 can be performed.

The Committee recommends that a representative test slab

be cast at the jobsite so that the workability, finishability, and

setting time of the proposed mixture can be evaluated by the

project team (ACI 212.3R and ACI 212.4R).

5.6.3 Chlorides—Studies have shown that chlorides are

significant contributors to corrosion of steel in concrete. The

problem is particularly severe when dissimilar metals are

embedded in concrete, or when reinforced concrete is placed

over galvanized decking. Corrosion products can cause expansion,

cracking, and spalling.

Limits on chloride in fresh concrete mixtures are based on

the recommendations of ACI 222R. The following concrete

should not include any intentionally added calcium chloride:

Prestressed concrete; floors over prestressed concrete or

galvanized deck; floors containing two kinds of embedded

metals; conventionally reinforced concrete in a moist environment

and exposed to deicing salts or saltwater mist; parking

garage floors in northern climates; structures near bodies

of saltwater; floors or slabs containing snow-melting electrical

radiant heating systems; and floors finished with metallic

dry shakes.

Noncorrosive, nonchloride accelerators are available for

use in cold weather. The admixture manufacturer should be

able to provide long-term data (of at least a year’s duration)

demonstrating noncorrosivity using an acceptable accelerated

corrosion test method such as one using electrical potential

measurements. Data from an independent laboratory are

preferable.

If accelerated set or high early strength is desired, either a

noncorrosive nonchloride accelerator or high-early strength

(Type III) cement can be used; alternatively, 100 to 150 lb

per cu yd (59 to 89 kg/m3) of additional Type I or Type II cement

can be used in the mixture. A significant decrease in

setting time may not be realized with the increased cement

content. The increased cement and water demand can increase

shrinkage and curling.

Heated concrete may be required for cold weather construction

(ACI 306R). The use of additional Type I or Type

II cement is recommended in lieu of using chloride-based accelerators.

When used, calcium chloride should be added as a water solution

in amounts of not more than 1 percent to 2 percent by

weight of cement. It will accelerate the rate of strength development

and decrease setting time. Calcium chloride, in dosages

as high as 1 percent to 2 percent, does not significantly

lower the temperature at which the concrete will freeze. It accelerates

the rate of strength development and thereby decreases

the length of time during which protection against

freezing must be provided. Setting time is decreased, thereby

reducing finishing time.

Calcium chloride tends to darken the color of concrete and

can cause variations in color of the hardened concrete. The

difference in color is most noticeable when slabs with calcium

chloride are adjacent to those without (Fig. 11.9). If concrete

containing calcium chloride is not adequately cured,

the surface can show light and dark spots. Calcium chloride

should not be dispensed dry from bags. Dry-flake material

frequently absorbs moisture and becomes lumpy. Pellet-type

calcium chloride must be completely dissolved prior to addition

to concrete or pop-outs will result from any undissolved

pellets.

5.6.4 Expansive cementitious admixtures—Specifically

formulated dry-powder admixtures can be blended with

portland cement at the batch plant to produce shrinkagecompensating

concrete. Concrete incorporating the same

materials that will be used for the anticipated project should

be tested for expansion by ASTM C 878 (see ACI 223 for

full details). It is also recommended that the compatibility of

the expansive cementitious admixture and portland cement

be checked by the use of ASTM C 806.

5.6.5 Pozzolans—A number of natural materials, such as

diatomaceous earth, opaline cherts, clays, shales, volcanic

tuffs, and pumicites are used as pozzolans. Pozzolans also

include fly ash and silica fume.29 Information on the use of

slag can be found in ACI 226.1R and on fly ash in ACI

226.3R. For information on silica fume, see Reference 29.

When these materials are used in concrete, except silica

fume, the time of set is frequently extended and the color of

concrete can be different from that produced when portland

cement is the only cementitious component.

ASTM C 618 fly ash, Class F or Class C, is frequently incorporated

in concrete. Fly ash can affect the setting time,

and it is often helpful in hot weather by delaying set time, or

as an aid in pumping concrete (Refer to ACI 226.3R). In

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-21

floors and slabs, fly ash is often substituted for portland cement

in quantities up to about 20 percent fly ash by mass of

cementitious materials.

In cool weather, fly ash will usually delay the setting and

finishing of the concrete unless measures—increasing the

concrete temperature or using an accelerator—are taken to

compensate for the low temperatures.

Silica fume is used as a portland cement replacement or

cementitious addition in an amount typically between 5 percent

and 10 percent by mass of the total cementitious material.

The use of silica fume can increase both the

impermeability and the strength of the concrete. Special attention

should be given to avoiding plastic shrinkage cracking

during placing and finishing, by using evaporationretardant

chemicals sprayed onto the plastic concrete surface

or by using fog sprays in the air above the concrete. Early

and thorough curing of the slab is also very important to minimize

cracking.

5.6.6 Coloring admixtures—Pigments for colored floors

should be either natural or synthetic mineral oxides or colloidal

carbon. Synthetic mineral oxides can offer more intensity

in color, but they are normally more expensive. Pigments can

be purchased alone or interground with a water-reducing admixture

for mixing into the batched concrete to produce integrally-

colored concrete. Colored aggregate-type surface

hardeners containing pigments can also be used. These pigmented

mineral aggregates or metallic hardeners contain mineral

oxide pigment, portland cement, a well-graded mineral

aggregate or metallic hardener, and plasticizers. Pigments for

integrally-colored concrete should conform to ASTM C 979

and have uniform color. It should be recognized that carbonblack

pigments especially manufactured for this purpose will

appear lighter in color at an early age. The prepared mixtures

should not contain pigments that are not mineral oxides. Jobproportioning

or job-mixing of material for monolithic colored

surfaces is not recommended. The use of these materials

is described in Section 8.6. Coloring admixtures should be

lime-proof and contain no calcium chloride. Curing compounds

for these slabs should be the same as those used on the

approved sample panels (Chapter 8).

5.7—Liquid surface treatments

Some floor slabs, improperly constructed, can have relatively

pervious and soft surfaces that wear or dust rapidly.

Though the life of such surfaces can be short, it can be extended

by using surface treatments containing certain chemicals,

including sodium silicate and the fluosilicates of

magnesium and zinc. When these compounds penetrate the

floor surface, they react chemically with calcium hydroxide

(a product of cement hydration) to form a hard, glassy substance

within the pores of the concrete, thereby reducing

dusting of the floor and creating a denser, harder surface.

Liquid surface treatments should be considered only as

emergency measures for treatment of deficiencies.30 They

are not intended to provide additional wear resistance in

new, well-designed, well-constructed and cured floors, nor

to permit the use of lower quality concrete. The most effective

use of liquid surface treatments is on existing floors.

New floors should be of sufficiently good quality that such

treatments are not required.

If for any reason these surface treatments are to be applied

to new concrete floors, the floor should be moist cured. Liquid

membrane-forming curing compounds should not be

used because they prevent penetration of the liquid treatment.

These surface treatments should be applied only to

concrete floors that are at least 28 days old, and that have

been thoroughly moist cured and allowed to air dry.

5.8—Reinforcement

5.8.1 Reinforcing steel, mats, or welded wire reinforcement—

Deformed bars, bar mats, or welded wire reinforcement

usually are required in suspended structural floors as

part of the structural design. They can also be called for in

the specifications for slabs on ground as discussed in Section

3.2.4. Deformed bars should conform to the requirements of

ASTM A 615, A 616, or A 617. Bar mats conforming to

ASTM A 184 can also be used. Welded wire reinforcing

should conform to ASTM A 185 or A 497.

5.8.2 Post-tensioning—Post-tensioning can be used in

slabs on ground and suspended slabs to address specific design

requirements. Prestressing steel for use in floors and

slabs should conform to the requirements of ASTM A 416.

The post-tensioning tendons can be bonded or unbonded.

Unbonded tendons should meet or exceed specifications

published by the Post-Tensioning Institute.16

5.8.3 Synthetic fibers—Synthetic fibers for use in concrete

floors increase the cohesiveness of concrete and should meet

the requirements outlined in ASTM C 1116. The most widely

used synthetic fibers are polypropylene and nylon, although

other types are available. Polypropylene fibers are

available in both fibrillated and monofilament form; nylon

fibers are only available in monofilament form.

Synthetic fibers are added to the concrete mixer in quantities

generally less than 0.2 percent by volume of the concrete.

They are generally used in floors and slabs in

quantities of from 0.75 to 1.5 lb per cu yd (0.44 to 0.89

kg/m3). Synthetic fibers are used in floors to minimize plastic

shrinkage cracking of concrete. These fibers should not

be used to replace temperature and shrinkage reinforcement

because they have little impact on the behavior of concrete

after it hardens.

5.8.4 Steel fibers—Steel fibers for use in floors and slabs

should conform to the requirements of ASTM A 820. Steel fibers

made from wire, slit sheet, milled steel, and melt extract

are available and are normally deformed or hooked to improve

bond to the hardened matrix. Steel fibers are added to the concrete

mixer in quantities ranging from 0.0625 percent to 1 percent

by volume of the concrete (8 to 132 lb per cu yd; 4.7 to

78 kg/m3). Quantities of from 0.25 percent to 0.50 percent by

volume of the concrete (34 to 68 lb per cu yd; 20 to 40 kg/m3)

are typical.

Steel fibers are used in floors to minimize visible cracking,

increase shear strength, increase the flexural fatigue endurance

and impact resistance, and increase flexural toughness.

The increases in mechanical properties achieved depend primarily

on the type and amount of fiber used, and can result

302.1R-22 ACI COMMITTEE REPORT

in reduced floor thickness and increased contraction joint

spacing.31

5.8.5 Fiber characteristics—Crack reduction, material

properties, and mixture proportions are thoroughly discussed

by Balaguru.32 Additional information is available in ACI

544.1R, 544.2R, 544.3R, and 544.4R.

5.8.6 Dowels and load transfer devices—Dowels required

for load transfer can be round or square. Square dowels are

available with expansion material on the vertical sides to allow

for some horizontal movement. Round dowels for slabon-

ground installation should meet ASTM A36 or ASTM

A615, Grade 40 minimum. Square dowels should meet

ASTM A36. The diameter or cross sectional area, length,

and specific location of dowels as well as the method of support

should be specified by the architect/engineer. See Section

3.2.7 for more information on load transfer mechanisms

for slabs on ground.

5.9—Curing materials

ACI 308 lists many coverings and membrane-forming liquids

that are acceptable for curing concrete floors. Since curing

is so vital to good flatwork, the characteristics of curing

materials suitable for flatwork are set forth here in great detail.

Also see Chapter 9 for the purpose, methods, and length

of curing.

5.9.1 Wet burlap—If kept continually moist, burlap is an

effective material for curing concrete surfaces. Old burlap

from which the sizing has disappeared (or has been removed)

is easier to wet than new burlap.

Care should be taken that the burlap used does not stain the

concrete or come from sacks that once contained sugar; sugar

retards the hardening of concrete and its presence could

result in a soft surface. The requirements for burlap are described

in AASHTO M182. White, polyethylene-coated burlap

is available; the polyethylene is helpful in keeping the

burlap moist longer, but it makes rewetting more difficult.

Refer to ASTM C171.

5.9.2 Plastic film, waterproof paper, or combination

polyethylene/burlap sheets—Plastic film, waterproof paper

or polyethylene/burlap sheets for curing should allow a

moisture loss of no more than 0.055 g/cm3 in 72 hrs when

tested according to ASTM C 156. Polyethylene plastic film

with the same thickness and permeance used for vapor retarders

below slabs on ground (Section 3.2.3) should be satisfactory.

Waterproof paper should meet the requirements

of ASTM C 171.

5.9.3 Spray-applied membranes—Liquid membraneforming

curing compounds should meet the provisions of

ASTM C 309, which describes the requirements for both

clear and pigmented types. White or gray compounds are

used for their good light-reflectance. Colored curing compounds

are available for colored concrete. Dissipating resinbased

materials can be used on slabs receiving applied finishes

or subsequent liquid surface treatments. ASTM C 309

allows moisture loss of 0.55 kg/m2 in 72 hours at a curing

compound coverage of 200 sq ft per gal (4.91 m2/L) when

applied in compliance with ASTM C 156. Special conductive

curing compounds should be used to cure electrically

conductive and spark-resistant floors. It is always important

Table 6.2.1— Recommended strength and maximum slump at point of placement for each class of

concrete floor

Floor class*

28-day compressive strength Maximum slump

psi MPa in. mm

1, 2, and 3 3000 21 5 125

4 and 5 4000 28 5 125

6 4500 31 5 125

7 base 3500 24 5 125

7 bonded topping† 5000-8000 35-55 3 75

8 unbonded topping†§ 4000-8000 28-55 3 75

9 superflat† 4000 or higher 278 or higher 5 125

*Refer to Table 2.1 for floor class definitions.

†The strength required will depend on the severity of usage.

§Maximum aggregate size not greater than one-quarter the thickness of unbonded topping.

Table 6.2.4— Minimum cementitious materials requirements for floors*

Nominal maximum size aggregate Cementitious material content

in. mm lb/yd3 kg/m3

11/2 38 470 279

1 25 520 308

3/4 19 540 320

1/2 13 590 350

3/8 10 610 362

*See Section 5.6.5 for minimum portland cement requirements.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-23

to determine if a dissipating or nondissipating product

should be used. The use of a nondissipating compound can

be incompatible with the installation or application of future

floor coverings.

For floors designed for high wear resistance, optimum top

surface strength development, and minimal cracking, it is

desirable to use curing compounds that offer high water retention.

When a mineral aggregate or metallic surface hardener

is used, the curing procedure and specific product used

for curing should be approved by the manufacturer of the

hardener. A high-solids-type curing compound can limit

maximum moisture loss to 0.030 g/cm2 at a coverage of 300

sq ft per gal (7.36 m2/L)—less than 50 percent of that allowed

by ASTM C 309 (ACI 308).

Compounds should also be tested in accordance with

ASTM C1151; results less than or equal to 3.7 x 10-6 cm2/s for

the difference between the top and bottom Ka (absorptivity)

values represent an acceptably cured sample. More stringent

criteria can be appropriate for some projects. Manufacturer’s

written instructions should be followed for both the number of

coats and the coverage rate needed to meet the appropriate

ASTM or project requirements. Periodic field testing to evaluate

actual performance is recommended. One practical test

for concrete surfaces to receive a moisture-sensitive covering

is to apply a 5x5-ft (1.5x1.5-m) black polyethylene sheet,

sealed to the slab with tape at the edges. No significant amount

of moisture should be present when the sheet is removed after

24 hours. Tests should be conducted at intervals of approximately

15,000 sq ft (1400 m2).

It is always important to determine if a dissipating or nondissipating

product should be used. When a mineral aggregate

or metallic surface hardener is used, it is important that

the curing method be compatible with recommendations of

the hardener manufacturer.

5.10—Evaporation reducers

Evaporation reducing chemicals can be sprayed on the

plastic concrete one or more times during the finishing operation

to minimize plastic shrinkage cracking when the evaporation

rate is high. These products should be used in strict

accordance with the manufacturer’s directions; they should

never be used during the final troweling operations because

they discolor the concrete surface.

5.11—Gloss-imparting waxes

Concrete waxes to impart gloss to concrete surfaces are

available from various manufacturers. Some also are curing

compounds; for such use, they should meet or exceed the

water-retention requirements of ASTM C 309.

5.12—Joint materials

Certain two-component semirigid epoxy resins, polysulfides,

and urethanes can be used to fill joints where the joint

edges need support to withstand the action of small, hardwheeled

traffic. These are the only materials known to the

Committee that can provide sufficient shoulder support to the

edges of the concrete and prevent joint breakdown. Two-component

epoxy resins are desirable because their curing is independent

of job site conditions. Such joint materials should be

100 percent solids and have a minimum Shore A hardness of

80 when measured in accordance with ASTM D 2240. See

Section 9.10 for more details on joint filling and sealing.

Preformed elastomeric sealants are useful for some applications.

They should not be used where subjected to the traffic

of small, hard wheels. They can be quickly installed, they

require no curing, and if properly chosen, they can maintain

a tight seal in joints that are subject to opening and closing.

See ACI 504R for more information on preformed elastomeric

sealants.

Preformed asphalt impregnated or plain fiber materials or

compressible foam are used in expansion and isolation joints,

depending on the anticipated movement. These materials and

their appropriate use are described in detail in ACI 504R.

5.13—Volatile organic compounds (VOC)

Many users and some states require materials to meet

VOC limits. Liquid materials are of greatest concern since

they are often solvent-based. Certification of compliance

with the applicable VOC limits should be required before the

products are used.

Many curing compounds that comply with limits on VOC

are water-based. They should not be permitted to freeze. In

many cases, they cannot be reconstituted after freezing.

CHAPTER 6—CONCRETE PROPERTIES AND

CONSISTENCY

6.1—Concrete properties

A concrete mixture should incorporate the most economical

combination of available materials that will consistently

produce concrete with the required workability, abrasion resistance,

durability, strength, and shrinkage characteristics

(ACI 211.1 and 211.2).

In most flatwork, the placeability of the concrete and finishability

of the surface are at least as important as the abrasion

resistance, durability, and strength. The former qualities

will have a significant effect on the quality of the top 1/16 or

1/8 in. (1.5 or 3 mm) of the concrete surface. If the slab is

floated while there is still free water on the surface, the finished

surface will be of poorer quality than if it were properly

floated (Section 8.3.3). Unfortunately, placeability and

finishability are not easily measured. There is a tendency for

specifiers to emphasize more easily determined properties

such as slump and compressive strength.

Other parameters being equal, a given concrete’s strength

and shrinkage properties will improve as its water content is

reduced. Therefore, the use of the minimum amount of water

necessary to produce the required slump and workability is

highly important. However, the particular cementitious materials,

aggregates, and admixtures used can significantly affect

the strength, setting characteristics, workability, and

shrinkage of the concrete at a given water-cementitious material

ratio.33,34 Furthermore, the amount of water required

to produce a given slump depends on the maximum size of

coarse aggregate, aggregate gradation, particle shape and

surface texture of both fine and coarse aggregates, air con302.1R-

24 ACI COMMITTEE REPORT

tent, and the admixtures used, as well as the temperature and

humidity at the time of placement. Using larger maximumsize

aggregate or improving the over-all aggregate gradation

reduces the mixing-water requirement.

Air-entraining admixtures produce a system of small air

bubbles that reduce the mixing water requirement. Concretes

containing entrained air are generally proportioned to have

the same amount of coarse aggregate as similar non-air-entrained

concretes. They are made with less mixing water and

less fine aggregate; however, in richer mixes this may not

offset the strength reduction that can result from intentional

entrainment of air. It is preferable not to use air-entraining

admixtures in floors that are to have a dense, smooth, hardtrowelled

surface.

The optimum quality and content of fine aggregate in concrete

for floors should be related to the slump of the concrete

and the abrasive exposure to which the floor will be subjected.

Concretes should be sufficiently plastic and cohesive to

avoid segregation and bleeding.34 Less fine aggregate

should be used in concrete with low slump—less than 1 in.

(25 mm)—since this concrete does not normally bleed or

segregate. Decreased fine aggregate contents can improve

resistance to abrasion if the concrete exhibits little bleeding

and segregation.

Laboratory trial batches should be used to establish optimum

proportions of ingredients. If concrete mixtures have

been used successfully under similar conditions in other

jobs, the laboratory trial batches can be omitted. Records of

gradations of fine and coarse aggregates from concrete mixtures

should be retained.

Trial batch proportions should generally be in accordance

with ACI 211.1 or 211.2. However, adjustments of fine aggregate

content may be necessary to obtain the best workability.

35

6.2—Recommended concrete mixture

6.2.1 Required compressive strength and slump—Two approaches

for selecting mixture proportions are discussed in

Section 6.2.4. Regardless of the approach, the design

strengths shown in Table 6.2.1 should be used for the various

classes of concrete floors.

The architect/engineer should be consulted as to the

strength to be achieved by concrete prior to subjecting the

slab to early construction loads. To obtain this strength

quickly, it may be necessary to use more cementitious materials

than the minimum amount shown in Table 6.2.4, or to

proportion the concrete for a 28-day strength higher than that

shown in Table 6.2.1. Compressive strengths should be used

for jobsite control.

The slump indicated for each floor class shown in Table

6.2.1 is the recommended maximum at the point of placement

to prevent segregation, and yet provide adequate workability

of the concrete. A one-time jobsite slump adjustment

should be permitted as outlined in the “Tempering and control

of mixing water” provisions of ACI 301, or the “Mixing

and delivery” provisions of ASTM C 94 (Section 7.3.2).

6.2.2 Required finishability—Concrete for floors should

have other desirable characteristics in addition to strength.

There should be sufficient paste to allow the finisher to completely

“close” the surface and to achieve the required surface

tolerances, hardness, and durability.35

6.2.3 Required durability—The procedures for producing

durable concrete outlined in ACI 201.2R apply to floors and

slabs. Concrete floors exposed to freezing and thawing while

moist should have a water-cementitious material ratio not

greater than the values given in the following paragraph.

These w/cm ratio requirements can be lower than those required

for strength alone. Additionally, these concretes

should have adequate entrained air.

Requirements based only on durability may yield concrete

compressive strengths much higher than normally required

for structural concerns. Concrete floors and slabs subjected

to moderate and severe exposures to freezing and thawing, as

defined in ACI 201.2R, should have a w/cm ratio no greater

than 0.50. Concrete subjected to deicing chemicals should

have a w/cm ratio no greater than 0.45. Reinforced concrete

exposed to brackish water, seawater, deicing chemicals, or

other aggressive materials should have a w/cm ratio no

greater than 0.40. The Committee recognizes that there is no

direct correlation between compressive strengths and w/cm

ratios and suggests that the two not be combined in a specification.

When durability is a concern, w/cm ratios should be

specified. For informational purposes, various w/cm ratios

are likely to produce the following relative compressive

strengths or higher: .50 [4000 psi (28 MPa)]; 0.45 [4500 psi

(31 MPa)]; 0.40 [5000 psi (34 MPa)].

Entrained air is necessary in concrete subjected to freezing

and thawing when moist, or subjected to deicing chemicals.

Recommended air contents for hardened concrete for various

exposure conditions, aggregate types, and maximum

size aggregates are given in ACI 201.2R. Properly air-entrained

concrete should achieve a compressive strength of

4000 psi (28 MPa) prior to being subjected to freezing and

thawing in a moist condition. Prior to the application of any

deicing chemicals, floors should receive some drying and

should reach a strength level of 4000 psi (28 MPa).

Air contents within the limits recommended will cause

significant strength reductions in rich concretes, but the effect

will be less important in lean concretes. Air contents in

excess of the recommended quantities will reduce strength in

rich mixtures approximately 3 percent to 5 percent per 1 percent

increase in air content, and will reduce abrasion resistance

correspondingly.

6.2.4 Concrete mixture—In addition to meeting structural

requirements, concrete for floors should provide adequate

workability necessary to obtain the required finish and floor

surface profile. Floors that are required to be impermeable,

resistant to freezing and thawing and deicing chemicals, or

to meet the requirements of ACI 211.2, 223, or 318, should

conform to more stringent criteria. In general, w/cm ratios in

the range of 0.47 to 0.53 are applicable for most interior

floors of Class 4 and higher.

Total water content can have a major impact on the bleeding

characteristics of the concrete, as well as the potential for

shrinkage, so use of the lowest practical quantity of water in

the concrete mixture is recommended.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-25

The Committee recommends that the concrete mixture be

accepted on the basis of (1) a minimum cementitious material

content as indicated in Table 6.2.4, or (2) a demonstration

to the architect/engineer that a proposed concrete

mixture will be capable of producing a floor of acceptable

finish and appearance while meeting the strength requirements

of Table 6.2.1 and the project.

If a history of finishing properties is not available for a

concrete mixture, a trial slab of concrete should be placed

under job conditions to evaluate the workability, finishability,

setting time, slump loss, hardness, and appearance of the

concrete proposed for use. Materials, equipment, and personnel

proposed for the project should be used. A test panel

measuring at least 8x8 ft (2.5x2.5 m) and of the specified

thickness can provide confirmation of some characteristics,

but a test placement of a non-critical floor section is more

likely to provide useful information about the finishability

and setting time of concrete proposed for the work. It is recommended

that the contractor who will actually finish the

concrete be supplied with the information available on the

proposed concrete mixture.

6.2.5 Consistency and placeability—The maximum slump

recommended for each class of floor is given in Table 6.2.1.

These slumps are intended to produce concrete of sufficient

workability to be properly consolidated in the work without

excessive bleeding or segregation during placing and finishing.

Excessive bleeding and segregation can contribute

greatly to poor performance in concrete floors. If the finished

floor is to be uniform in appearance and grade, it is important

that successive batches placed in the floor have very nearly

the same slump and setting characteristics. See Sections 6.1,

6.2.1 and 7.3.2 regarding jobsite slump adjustment. Workability

of a concrete mixture is not directly proportional to

the slump. Properly proportioned concrete with slumps less

than shown in Table 6.2.1 can respond very well to vibration

and other consolidation procedures. Increased slump alone

does not assure satisfactory workability characteristics; a

discussion of the practical aspects of slump is given in Reference

35.

Slump limits in Table 6.2.1 are for concrete made with

both normal weight and structural lightweight aggregate and

assume the use of a normal water reducer, if required.

Slumps in excess of those shown in the table may be acceptable

when mid-range or high-range water reducers are used.

If structural lightweight-aggregate concrete is placed at

slumps higher than shown in Table 6.2.1, the coarse lightweight-

aggregate particles can rise to the surface and the

concrete can bleed excessively, particularly if the concrete

does not contain an adequate amount of entrained air.

6.2.6 Maximum size of coarse aggregate—The maximum

aggregate sizes in Table 6.2.4 apply to normal weight aggregates.

The largest practical size aggregate should be used if

economically available, and if it will satisfy the requirements

that maximum size not exceed three-quarters of the minimum

clear spacing of reinforcing bars nor one-third of the

depth of the section. Structural lightweight aggregates are

not generally furnished in sizes larger than 3/4 or 1 in. (19 or

25 mm); however, some lightweight aggregates provide

maximum strength with relatively fine gradings.

6.2.7 Air content—Moderate amounts of entrained air for

purposes other than durability as described in Section 6.2.3

can be used to improve workability, particularly with lean

and harsh concrete mixtures, or with poorly graded aggregates.

The Committee recommends that concretes made with

structural lightweight aggregates contain some entrained air.

Specific recommendations for air content should be secured

from the concrete supplier, the manufacturer of the lightweight

aggregate, or both, but the air content should not be

lower than 4 percent.

It is recommended that an air entraining agent not be specified

or used for concrete to be given a smooth, dense, hardtroweled

finish since blistering or delamination may occur.

These troublesome finishing problems can develop any time

the total air content is in excess of 3 percent. This is particularly

true when monolithic surface treatments are applied.

Some variation in the air content of air-entrained concrete

is common, and this can make it difficult to time the finishing

operations. Exposure conditions that dictate the need for

air-entrainment should be discussed with the architect/engineer

before proceeding.

CHAPTER 7—BATCHING, MIXING, AND

TRANSPORTING

Detailed provisions relating to batching, mixing, and

transporting concrete are available in ASTM C 94, ASTM C

1116, and ASTM C 685.

7.1—Batching

Whether the concrete is mixed onsite or in a ready-mixed

concrete operation, the materials should be batched within

the following limits:

Cement +1 percent

Added water +1 percent

Fine and coarse aggregate +2 percent

Admixtures and pigments +3 percent

Except for site mixing on small jobs, cement should be

weighed on a scale separate from that used for weighing aggregates.

If batching is by the bag, no fractional bags should

be used.

Aggregate should be batched by weight. Batching by volume

should not be permitted, except with volumetric batching

and continuous-mixing equipment (Section 7.2.1). Batch

weights should be adjusted to compensate for absorbed and

surface moisture. When the mixture contains special aggregates,

particular care should be exercised to prevent segregation

or contamination.

Water can be batched by weight or volume. The measuring

device used should have readily adjustable positive cutoff

and provisions for calibration.

Accurate batching of admixtures and colored pigments is

critical, since they are used in relatively small quantities. Admixtures

should be accurately batched at the batch plant. Admixtures

that are designed to be added to the concrete at the

jobsite should be incorporated in accordance with the manufacturer’s

recommendations. When more than one admixture

302.1R-26 ACI COMMITTEE REPORT

is batched, each should be batched separately and in such a

way that the concentrated admixtures do not come into contact

with each other. Care should be taken to avoid the freezing

of admixtures in cold weather, as this can damage some

of them. It is preferable to purchase pigments or colored admixtures

prepackaged in batch-sized quantities. Powdered

admixtures should be batched by weight, and paste or liquid

admixtures by weight or volume. The volume of admixture

batched should not be controlled by timing devices. Liquid

admixtures are preferred but can require agitation to prevent

the settling of solids.

7.2—Mixing

7.2.1 Ready-mixed concrete—Mixing should be in accordance

with ASTM C 94 or ASTM C 1116 and should produce

the required slump and air content without exceeding

the authorized or approved water-cementitious material ratio.

Close attention should be given to the moisture content

of the aggregate. In critical jobs, or when specifically required,

truck mixers should be in compliance with requirements

of the project specification. In order to assure

consistent slump at the point of placement, it is recommended

that a small quantity of “trim water” be held out at the

batch plant. The amount of withheld water should be indicated

on the ticket; the truck should then leave the plant with a

full water tank.

7.2.2 Site mixing—Mixers that produce a volume of concrete

requiring less than one bag of cement should not be

used. For small quantities of concrete, packaged products

meeting ASTM C 387 are more convenient, and can be more

accurately proportioned.

Mixing time should be sufficient to produce uniform concrete

with the required slump and air content. Site mixers

less than 1 cu yd (0.76 m3) in capacity should mix for not less

than 3 minutes; ordinarily 15 seconds should be added for

each additional cubic yard (0.76 m3) of capacity or fraction

thereof, unless a turbine mixer is used. A longer mixing time

is required for concrete with a slump of less than 3 in. (75

mm).

Equipment for volumetric batching and continuous mixing

at the jobsite is available. Concrete produced in this manner

should comply with ASTM C 685.

7.2.3 Architectural concrete—When special architectural

concretes are produced using special aggregates, white cement,

special cements or pigments, mixer drums and equipment

should be kept clean, and any wash water should be

disposed of before a new batch is introduced. Identical ingredients

and quantities of materials should be used, and not less

than 1/3 of the capacity of the mixing drum, a minimum of

three yards in a nine yard drum, and should always be in full

yard increments. See ACI 303 for additional details.

7.2.4 Shrinkage-compensating concrete—When expansive

cement or an expansive-component type admixture specifically

designed for producing shrinkage-compensating

concrete is required, refer to ACI 223 for details.

7.3—Transporting

7.3.1 Discharge time—Concrete mixed or delivered in a

truck mixer should be completely discharged while the concrete

still has sufficient workability to respond properly during

the placing and finishing operations. The period after

arrival at the jobsite during which the concrete can be properly

worked will generally vary from less than 45 minutes to

more than 2 hours, depending on the weather and the concrete

proportions. Prolonged mixing accelerates the rate of

stiffening and can greatly complicate placing and timing of

finishing operations.

7.3.2 Jobsite slump control—When concrete arrives at the

point of delivery with a slump below that which will result in

the specified slump at the point of placement and is unsuitable

for placing at that slump, the slump may be adjusted to

the required value by adding water up to the amount allowed

in the accepted mixture proportions unless otherwise permitted

by the architect/engineer. Addition of water should be in

accordance with ASTM C 94. The specified water-cementitious

material ratio or slump should not be exceeded. After

plasticizing or high-range water-reducing admixtures are

added to the concrete at the site to achieve flowable concrete,

do not add water to the concrete. Water should not be added

to concrete delivered in equipment not acceptable for mixing.

Testing samples should be taken after any necessary adjustment.

See ACI 301 for further details.

7.3.3 Delivery to point of discharge—Concrete for floor

and slab placement can be delivered to the forms directly

from a truck mixer chute, or by pump, belt conveyor, buggy,

crane and bucket, or a combination of these methods. It is

important that delivery of concrete be at a consistent rate appropriate

to the size of the placement, and that the concrete

be deposited as close as possible to its final location. Concrete

should not be moved horizontally by vibration, as this

contributes to segregation. See ACI 304R for recommended

procedures.

CHAPTER 8—PLACING, CONSOLIDATING, AND

FINISHING

Most of this chapter applies to both normal weight and

lightweight-aggregate concrete. The proper procedures for

finishing structural-lightweight concrete floors differ somewhat,

however, from finishing normal weight concrete; they

are discussed separately in Section 8.11.

Various finishing procedures should be executed sequentially

and within the proper time period, neither too early nor

too late in the concrete-hardening process. This time period

is called the “window of finishability.” It refers to the time

available for operations taking place after the concrete has

been placed, consolidated, and struck off. Surface finish, surface

treatment, and flatness/levelness requirements dictate

the type and number of finishing operations. All should take

place within the proper time period. If the floor slab is placed

during a time period of rapid hardening, this window becomes

so narrow that it can present considerable difficulties

to the floor contractor. The preconstruction meeting should

include discussion of the measures necessary to assure a satisfactory

“window of finishability.”

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-27

8.1—Placing operations

8.1.1 Caution—All concrete handling operations should

minimize segregation, since it is difficult to remix concrete

after it has been placed.

8.1.1.1 Placing sequence—In many cases, the most efficient

way to place concrete in large areas is in long strips as

illustrated in Fig. 8.1.1.1. Strip placements allow superior

access to the sections being placed. Intermediate contraction

joints are installed at specified intervals transverse to the

length of the strips. Wide strip placements can require installation

of longitudinal contraction joints.

Large block placements with interior contraction joints are

an acceptable alternative to strip placements if the contraction

joints are installed at specified intervals in a timely manner.

The use of shrinkage-compensating concrete or some types of

laser screeds are compatible with large block placements.

A checkerboard sequence of placement with side dimensions

of 50 ft (15.2 m) or less as shown in Fig. 8.1.1.1 (right)

has been used in the past in an effort to permit earlier placements

to shrink and to obtain minimum joint width. Experience

has shown that shrinkage of the earlier placements

occurs too slowly for this method to be effective. Access is

more difficult and expensive, and joints may not be as

smooth. The Committee recommends that the checkerboard

sequence of placement not be used.

8.1.1.2 Placing sequence for shrinkage-compensating

concrete—Neither the strip method nor the checkerboard

method described in Section 8.1.1.1 should be used with

shrinkage-compensating concrete. Refer to ACI 223 for specific

recommendations concerning placement configuration

and sequence.

8.1.2 Discharge of concrete—The rate of discharge of

concrete from a truck mixer can be controlled by varying the

drum speed.

8.1.3 Jobsite transfer—Chutes should have rounded bottoms

and be constructed of metal or be metal-lined. The

chute slope should be constant and steep enough to permit

concrete of the slump required to flow continuously down

the chute without segregation. Long flat chutes should be

avoided because they encourage the use of high-slump

concrete. A baffle at the end of the chute helps to prevent

segregation. The discharge end of the chute should be near

the surface of previously deposited concrete. When concrete

is being discharged directly onto the base, the chute

should be moved at a rate sufficient to prevent accumulation

of large piles of concrete. Allowing an excessively

steep slope on chutes can result in high concrete velocity

and segregation.

Regardless of the method of transportation and discharge,

the concrete should be deposited as near as possible

to its final position, and toward previously placed concrete.

Advance planning should include access to and around the

site, suitable runways, and the use of other devices to avoid

the use of concrete with a high water-cementitious material

ratio or excessive delays.

8.1.4 Placing on base—Mixing and placing should be

carefully coordinated with finishing operations. Concrete

should not be placed on the base at a faster rate than it can be

spread, bull floated or darbied, and restraightened, since

these latter operations should be performed before bleeding

water has an opportunity to collect on the surface.

Proper sizing of finishing crews, with due regard for the

effects of concrete temperature and atmospheric conditions

on the rate of hardening of the concrete, will assist the contractor

in obtaining good surfaces and avoiding cold joints.

If construction joints become necessary, they should be produced

using suitably placed bulkheads, with provisions

made to provide load transfer between current and future

work (Section 3.2.5.2 and 3.2.7).

Fig. 8.1.1.1—Placing sequence: long-strip construction (left) is recommended; checkerboard construction

(right) is not recommended

302.1R-28 ACI COMMITTEE REPORT

8.2—Tools for spreading, consolidating, and

finishing

The sequence of steps commonly used in finishing unformed

concrete floor surfaces is illustrated in Figure 8.3.

Production of high-quality work requires that proper tools be

available for the placing and finishing operations. Following

is a list and description of typical tools that are commonly

available. Refer to Section 8.3 for suggestions and cautions

concerning uses of these tools. Definitions for many of these

tools can be found in ACI 116R.

8.2.1 Tools for spreading—Spreading is the act of extending

or distributing concrete or embedding hardeners—often

referred to as “shake-on” or “dry-shake”—or other special

purpose aggregate over a desired area.

8.2.1.1 Spreading concrete—The goal of spreading operations

for concrete is to avoid segregation.

8.2.1.1.1 Hand spreading—Short-handled, square-ended

shovels, or come-alongs—hoe-like tools with blades about 4

in. (100 mm) high, 20 in. (500 mm) wide, and curved from

top to bottom—should be used for the purpose of spreading

concrete after it has been discharged.

8.2.1.2 Spreading dry-shake hardeners, colored dry-shake

hardeners, or other special-purpose material—The goal of

spreading operations for these materials is to provide an even

distribution of product over the desired area. Generally, hand

application should be used for distribution of these materials

only in areas where a mechanical spreader cannot be used.

8.2.1.2.1 Mechanical spreaders—Mechanical spreaders

are the best method of uniformly applying dry-shake hardeners,

colored dry-shake hardeners, or other special purpose

materials to concrete during the finishing process. These devices

generally consist of (1) a bin or hopper to hold the material,

(2) a vibrator or motorized auger to assist in

distribution of the material, and (3) a supporting framework

that allows the hopper to move smoothly over the concrete

surface while distributing the material. (Fig. 8.2.1.2.1)

8.2.2 Tools for consolidating—Consolidation is the process

of removing entrapped air from freshly placed concrete,

usually by vibration. Internal vibration and surface vibration

are the most common methods of consolidating concrete in

supported slabs and slabs on ground. Refer to ACI 309R for

additional discussion of topics related to the consolidation of

concrete.

8.2.2.1 Internal vibration—This method employs one or

more vibrating elements that can be inserted into the fresh

concrete at selected locations. Internal vibration is generally

most applicable to supported cast-in-place construction.

8.2.2.2 Surface vibration—This process employs a portable

horizontal platform on which a vibrating element is

mounted. Surface vibration is commonly used in slab-onground,

strip-type placements with edge forms. Refer to

8.2.3.2 for additional discussion.

8.2.3 Tools for screeding—Screeding is the act of striking

off concrete lying above the desired plane or shape to a predetermined

grade. Screeding can be accomplished by hand,

using a straightedge consisting of a rigid, straight piece of

wood or metal, or by using a mechanical screed.

8.2.3.1 Hand screeding—Hollow magnesium or solid

wood straightedges are commonly used for hand-screeding

of concrete. The length of these straightedges generally varies

up to approximately 20 ft (6 m). Straightedge cross-sectional

dimensions are generally 1 to 2 in. (25 to 50 mm) wide

by 4 to 6 in. (100 to 150 mm) deep. Tools specifically made

for screeding, such as hollow magnesium straightedges,

should be used in lieu of randomly selected lumber.

8.2.3.2 Mechanical screeding—Various types of surface

vibrators, including vibrating screeds, vibratory tampers,

and vibratory roller screeds are used mainly for screeding

slab-on-ground construction. They consolidate concrete

from the top down while performing the screeding function.

Refer to ACI 309 for a detailed discussion of equipment and

parameters for proper usage.

Vibrating screeds generally consist of either hand-drawn

or power-drawn single-beam, double-beam, or truss assemblies.

They are best suited for horizontal or nearly horizontal

surfaces. Vibrating screeds should be of the low-frequency—

3000 to 6000 vibrations per min (50 to 100 Hz)—highamplitude

type, to minimize wear on the machine and provide

adequate depth of consolidation without creating an objectionable

layer of fines at the surface. Frequency and

amplitude should be coordinated with the concrete mixture

designs being used (Refer to ACI 309).

Fig. 8.2.1.2.1—Mechanical spreader Fig. 8.2.3.2—Laser controlled screed

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-29

Laser-controlled variations of this equipment can be used

to produce finished slabs on ground with improved levelness

over that which might otherwise be achieved. Laser-controlled

screeds can ride on supporting forms, or they can operate

from a vehicle using a telescopic boom (Fig. 8.2.3.2).

Plate-tamper screeds are vibratory screeds that are adjusted

to a lower frequency and amplitude. Tamper screeds work

best on very stiff concrete. These screeds are generally used to

embed metallic or mineral aggregate hardeners. The contractor

is cautioned that improper use of this screed could embed

the hardener too deeply and negate the intended benefit.

Vibratory-roller screeds knock down, strike off, and provide

mild vibration. They can rotate at varying rates up to

several hundred revolutions per minute, as required by the

consistency of the concrete mixture. The direction of rotation

of the rollers on the screed is opposite to the screed’s direction

of movement. These screeds are most suitable for

more plastic concrete mixtures.

8.2.4 Tools for floating—Floating is the act of consolidating

and compacting the unformed concrete surface in preparation

for subsequent finishing operations. Initial floating of

a concrete floor surface takes place after screeding and before

bleed water comes to the surface and imparts a relatively

even but still open texture to the fresh concrete surface. After

evaporation of bleed water, additional floating operations

prepare the surface for troweling.

8.2.4.1 Bull floats (long-handled)—Bull floats are used to

consolidate and compact unformed surfaces of freshly

placed concrete immediately after screeding operations,

while imparting an open texture to the surface. They are usually

composed of a large, flat, rectangular piece of wood or

magnesium and a handle. The float part of the tool is usually

4 to 8 in. (100 to 200 mm) wide and 3.5 to 10 ft (1.1 to 3 m)

long. The handle is usually 4 to 20 ft (1.2 to 6.1 m) long. The

handle is attached to the float by means of an adjustable head

that allows the angle between the two pieces to change during

operation.

8.2.4.2 Darby—A darby is a hand-manipulated float, usually

31/2 in. (90 mm) wide and 3 to 8 ft (1 to 2.4 m) long. It

is used in early-stage-floating operations near the edge of

concrete placements.

8.2.4.3 Hand floats—Hand tools for basic floating operations

are available in wood, magnesium, and composition

materials. Hand float surfaces are generally about 31/2 in. (90

mm) wide and vary from 12 to 20 in. (300 to 500 mm) in

length.

8.2.4.4 Power floats—Also known as rotary floats, power

floats are engine-driven tools used to smooth and to compact

the surface of concrete floors after evaporation of the bleed

water. Two common types are heavy, revolving, single-diskcompactor

types that often incorporate some vibration, and

troweling machines equipped with float shoes. Most troweling

machines have four blades mounted to the base and a ring

diameter that can vary from 36 to 46 in. (1 to 1.2 m); weight

generally varies from about 150 to 250 lbs (68 to 113 kg).

Two types of blades can be used for the floating operation.

Float shoes are designed to slip over trowel blades; they are

generally 10 in. (250 mm) wide and 14 to 18 in. (350 to 450

mm) long. Both the leading edge and the trailing edge of

float shoes are turned up slightly. Combination blades are

usually 8 in. (200 mm) wide and vary in length from 14 to 18

in. (350 to 450 mm). The leading edges of combination

blades are turned up slightly. The use of float shoes is recommended

(Section 8.3.10).

Another attachment that is available to assist in power

float operations is a pan with small brackets that slide over

the trowel blades. These pans are normally used on doubleor

triple-platform ride-on machines and are very effective on

concrete surfaces requiring an embedded hardener or coloring

agent. The use of mechanical pan floating (Fig. 8.2.4.4)

can also materially improve flatness of the finished floor.

8.2.5 Tools for restraightening—Straightedges are used to

create and to maintain a flat surface during the finishing process.

Straightedges vary in length from 8 to 12 ft (2.4 to 3.7

m) and are generally rectangular in cross section (though designs

differ among manufacturers). When attached to a handle

with an adjustable head (that is, a bull-float handle and

head), these tools are frequently referred to as “modified

highway” straightedges (Fig. 8.2.5).

Fig. 8.2.4.4—Double-riding trowel with clip-on pans Fig. 8.2.5—“Modified highway” straightedge

302.1R-30 ACI COMMITTEE REPORT

8.2.6 Tools for edging—Edgers are finishing tools used on

the edges of fresh concrete to provide a rounded edge. They

are usually made of stainless steel and should be thin-lipped.

Edgers for floors should have a lip radius of 1/8 in. (3 mm).

8.2.7 Tools for troweling—Trowels are used in the final

stages of finishing operations to impart a relatively hard and

dense surface to concrete floors and other unformed concrete

surfaces.

8.2.7.1 Hand trowels—Hand trowels generally vary from

3 to 5 in. (75 to 125 mm) in width and from 10 to 20 in. (250

to 500 mm) in length. Larger sizes are used for the first troweling

in order to spread the troweling force over a large area.

After the surface has become harder, subsequent trowelings

use smaller trowels to increase the pressure transmitted to

the surface of the concrete.

8.2.7.2 Fresno trowels—A fresno is a long-handled trowel

that is used in the same manner as a hand trowel. Fresnos are

useful for troweling slabs that do not require a hard-troweled

surface. These tools are generally 5 in. (125 mm) wide and

vary in length from 24 to 48 in. (0.6 to 1.2 m).

8.2.7.3 Power trowels—Power trowels are gasoline engine-

driven tools used to smooth and compact the surface of

concrete floors after completion of the floating operation.

Ring diameters on these machines generally vary from 36 to

46 in. (0.9 to 1.2 m); their weight generally varies from about

150 to 250 lbs (68 to 113 kg). Trowel blades are usually 6 in.

(150 mm) wide and vary in length from 14 to 18 inches (350

Fig. 8.3—Typical finishing procedures (subject to numerous

conditions and variables)

to 450 mm). Neither the leading nor the trailing edge of trowel

blades is turned up. Power trowels can be walk-behind machines

with one set of three or four blades or ride-on

machines with two or three sets of four blades.

8.2.8 Tools for jointing—These tools are used for the purpose

of creating contraction joints in slabs. Contraction

joints can be created by using groovers, also called jointers,

or by saw-cutting.

8.2.8.1 Groovers—Groovers can be of the hand-held or

walk-behind type. Stainless steel is the most common material.

Hand-held groovers are generally from 2 to 43/4 in. (50

to 120 mm) wide and from 6 to 71/2 in. (150 to 190 mm) long.

Groove depth varies from 3/16 to 11/2 in. (5 to 38 mm). Walkbehind

groovers usually have a base with dimensions that

vary from 31/2 to 8 in. (90 to 200 mm) in width and from 6 to

10 in. (150 to 250 mm) in length. Groove depth for these

tools varies from 1/2 to 1 in. (13 to 25 mm).

8.2.8.2 Saw-cutting—The following three families of tools

can be used for saw-cutting joints: conventional wet-cut (water-

injection) saws; conventional dry-cut saws; and early-entry

dry-cut saws. Timing of the sawing operations will vary

with manufacturer and equipment. The goal of saw-cutting is

to create a weakened plane as soon as the joint can be cut,

preferably without creating spalling at the joint.

Both types of dry-cut tools can use either electrical or gasoline

power. They provide the benefit of being generally

lighter than wet-cut equipment. Early-entry dry-cut saws do

not provide as deep a cut—generally 11/4 in. (32 mm) maximum—

as can be achieved by conventional wet-cut and drycut

saws.

Early-entry dry-cut saws use diamond-impregnated blades

and a skid plate that helps prevent spalling.Timely changing

of skid plates is necessary to effectively control spalling. It is

best to change skid plates in accordance with manufacturer’s

recommendations.

Conventional wet-cut saws are gasoline powered and,

with the proper blades, are capable of cutting joints with

depths of up to 12 in. (300 mm) or more.

8.3—Spreading, consolidating, and finishing

operations

This section describes the manner in which various placing

and finishing operations can be completed successfully.

The finishing sequence to be used after completion of the initial

screeding operation depends on a number of variables related

to project requirements or to the concrete finishing

environment.

Project variables are generally controlled by requirements

of the owner and are specified by the designer. Some examples

are the choice of additives used in concrete, the requirement

for an embedded hardener, and the final finish desired.

Variables subject to the environment include such items as

setting time of the concrete, ambient temperature, timeliness

of concrete delivery, consistency of concrete at the point of deposit,

and site accessibility. Figure 8.3 is a flowchart that illustrates

the normal sequence of steps in the finishing process.

8.3.1 Spreading and compacting—Concrete, whether

from a truck mixer chute, wheelbarrow, buggy, bucket, belt

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-31

conveyor, pump, or a combination of these methods, should

be delivered without segregation of the concrete components

(Section 8.1). Spreading, the first operation in producing a

plane surface (not necessarily a level surface, since in many

cases it can be sloped for surface drainage) should be performed

with a come-along or a short-handled, square-ended

shovel (Section 8.2.1.1.1).

Long-handled shovels, round-ended shovels, or gardentype

rakes with widely-spaced tines should not be used to

spread concrete. Proper leverage, of prime importance for

manipulating normal weight concrete, is lost with a longhandled

shovel. Round-ended shovels do not permit proper

leveling of the concrete. The tines of garden-type rakes can

promote segregation and should not be used in any concrete.

Initial compacting of concrete in floors, with the exception

of heavily reinforced slabs, is usually accomplished in the

first operations of spreading, vibrating, screeding, darbying

or bull floating, and restraightening. The use of grate

tampers or mesh rollers is usually neither desirable nor necessary

if cement paste splatters when they are used. If grate

tampers are used on lightweight-concrete floors, only one

pass over the surface with a very light impact should be permitted.

Spreading by vibration should be minimized. See

ACI 309R for detailed discussion.

8.3.1.1 Structural floors—Both suspended and on-ground

structural floors can be reinforced with relatively heavy deformed

reinforcing bars or with post-tensioning tendons, and

typically contain other embedded items such as piping and

conduit. Proper consolidation around reinforcing steel, posttensioning

anchorages, and embedded elements requires internal

vibration, but care should be taken not to use the vibrator

for spreading the concrete, especially in deeper sections

where over-vibration can easily cause segregation.

The vibrator head should be completely immersed during

vibration. Where slab thickness permits, it is proper to insert

the vibrator vertically. On thinner slabs, the use of short 5 in.

(125 mm) vibrators permits vertical insertion. Where the

slab is too thin to allow vertical insertion, the vibrator should

be inserted at an angle or horizontally. The vibrator should

not be permitted to contact the base since this might contaminate

the concrete with foreign materials.

8.3.2 Screeding—Screeding is the act of striking off the

surface of the concrete to a predetermined grade, usually set

by the edge forms. This should be done immediately after

placement. See Section 8.2.3 for tools used for screeding.

Of all the floor-placing and finishing operations, form setting

and screeding have the greatest effect on achieving the

specified grade. Accuracy of the screeding operation is directly

impacted by the stability of the edge forms or screed

guides selected by the contractor. Consequently, care should

be taken to match the forming system and the screeding

method to the levelness tolerance specified.

Edge forms for slab-on-ground and suspended-slab placements

are normally constructed of wood or metal. Some edge

forms are constructed of concrete. The spacing between edge

forms, and the support provided for them, will influence the

accuracy of the screeding operation. Where edge-form spacing

exceeds the width of the screed strip, intermediate screed

guides can improve the accuracy of the screeding operation.

The width of these screed strips will generally vary between

10 ft (3 m) and 16 ft (5 m) and will be influenced by column

spacings. Generally, screed strips should be equal in width,

and should have edges that fall on column lines.

In general, slab-on-ground placements are either block

placements or strip placements. Block placements generally

have edge dimensions that exceed 50 feet (15 m). Strip

placements are generally 50 feet (15 m) or less in width and

vary in length up to several hundred feet. Suspended-slab

placements are usually block placements. Where wood is

used for edge forms, the use of dressed lumber is recommended.

The base should be carefully fine-graded to ensure

proper slab thickness.

Selection of the type of screed guide to be used for screeding

operations is somewhat dependent on placement configuration.

The maximum practical strip width for hand

screeding is about 20 feet (6 m). Where strict elevation tolerances

apply, it is wise to limit strip width for hand screeding

to about 16 feet (5 m). Screeding of strip placements for

slabs on ground is generally completed using some type of a

vibrating screed supported by edge forms. Screeding of

block placements for slabs on ground is usually accomplished

using wet-screed guides, dry-screed guides, a combination

of these two, or some type of laser-guided screed. For

slabs on ground, an elevation change no greater than 3/8 in.

(10 mm) in 10 ft (3 m), approximately FL35, can be achieved

routinely through use of laser-guided screeds. Screeding of

block placements for suspended slabs is usually accomplished

using either wet-screed guides, dry-screed guides, or

a combination of the two.

Wet-screed guides, when used between points or grade

stakes, are established immediately after placement and

spreading; see Section 4.4 for setting of dry-screed guides.

At the time of floor placement, before any excess moisture

or bleed water is present on the surface, a narrow strip of

concrete not less than 2 ft (600 mm) wide should be placed

from one stake or other fixed marker to another, and straightedged

to the top of the stakes or markers; then another parallel

strip of concrete should be placed between the stakes or

markers on the opposite side of the placement strip. These

two strips of concrete, called “wet-screed guides,” are used

in establishing grade for the concrete located between the

guides. Immediately after wet-screed guides have been established,

concrete should be placed in the area between,

then spread and straightedged to conform to the surface of

the wet-screed guides. It is important that the contractor confirm

that proper grade has been achieved following strikeoff.

High spots and low spots should be identified and immediately

corrected. Low spots left behind should be filled by

placing additional concrete in them with a shovel, carefully

avoiding segregation. Nonconforming areas should then be

rescreeded. Difficulty in maintaining the correct grade of the

floor while working to wet-screed guides is an indication

that the concrete mixture is too wet or that vibration is causing

the guides to move.

Elevation stakes placed at regular intervals are one method

of establishing grade for wet-screed guides in slab-on302.1R-

32 ACI COMMITTEE REPORT

ground construction. As screeding progresses the stakes can

be driven down flush with the base if expendable, or pulled

out one at a time to avoid walking back into the screeded

concrete. This early removal of stakes is one of the big advantages

in the use of wet-screeds; in addition, grade stakes

are much easier and faster to set than dry-screeds. Screeding

should be completed before any excess moisture or bleed

water is present on the surface.

Benefits of using wet-screed guides include economical

and rapid placement of the concrete. However, successful

use of wet-screed guides requires careful workmanship by

craftspeople who strike off the concrete because vibration

can change the elevation of the wet-screed. Wet-screed

guides are difficult to employ when varying surface slopes

are required and can produce inconsistent results when variations

in slab thickness are required to compensate for deflection

of a suspended slab. Special care is necessary to

avoid poor consolidation or cold joints adjacent to wetscreed

guides.

Wet-screed guides should not be used in suspended-slab

construction unless the finished floor surface is level and

formwork is shored at the time of strikeoff. During construction

activity, vibration of reinforcing steel and the supporting

platform may result in an incorrect finished grade when wetscreed

guides are used. It is imperative, therefore, that grade

be confirmed after strikeoff and that errors be corrected at

that time by restriking the area.

Wet-screed guides should be used only for surfaces where

floor levelness is not critical. For slabs on grade where floor

levelness requirements are important, it is recommended that

dry-screed guides be used instead of wet-screed guides. In

general, surfaces produced using wet-screed guides will exhibit

maximum elevation changes of at least 5/8 in. (16 mm)

in 10-ft (3-m). This corresponds to an FL20 floor.

Elevation variation of surfaces produced using dry-screed

guides is dependent on placement-strip width and the accuracy

with which the guides are installed. Generally, the maximum

elevation changes that can be anticipated will be

reduced as the dry-screed guides are moved closer together.

For suspended-slab construction, the desirability of utilizing

dry-screed guides on both sides of each placement strip

is diminished by the damage done when the contractor retrieves

the guide system. For this reason, it is recommended

that a combination of dry-screed guide and wet-screed guide

techniques be employed on suspended slabs.

The first placement strip should always start against a

bulkhead or edge of the building. Strikeoff on the interior

side of the strip should be controlled through use of moveable

dry-screed guides, which will provide positive control

over the surface elevation along that line. The concrete edge

along the moveable guide should be kept near vertical and

straight. As concrete is placed and struck off, these guides

are removed. When the next strip is placed, preferably in the

same direction as the initial strip, the prior strip will normally

have been in place for 30 or more minutes. The contractor

can extend the straightedge 2 ft (600 mm) or more over the

previous partially-set placement to control grade of strikeoff

on that side of the strip and use moveable dry-screed guides

to control grade on the side of the strip not adjacent to previously

placed concrete.

For suspended-slab construction, the procedure described in

the previous paragraph has several advantages over unmodified

“wet-screed” techniques or those techniques that employ

dry-screed guides on both sides of each placement strip.

1. Where previously placed concrete is used as a guide for

strikeoff, it provides a relatively stable guide, because it will

have been in place for some time before it is used.

2. Retrieval of the dry-screed guide from areas surrounded

by previously placed concrete is unnecessary, because dryrigid

guides are not used in these locations.

Moveable dry-screed guides should be used to establish

grade on any suspended slabs that are not level and shored at

the time of strikeoff, and for any suspended slab where increases

in local slab thickness might be used to compensate

for anticipated or identified differential deflection of the

structure. When an increase in local slab thickness is used to

compensate for differential floor deflection, it is likely that

the resulting slab will be more than 3/8 in. (10 mm) thicker

than design thickness. The contractor should secure permission

to exceed the plus tolerance for slab thickness prior to

beginning construction. Refer to Section 3.3 for a discussion

of suspended slab deflection and suggested construction

techniques.

For construction of slabs on ground, the use of vibrating

screeds—where edge forms or screed-guide rails can be

used—will facilitate strike-off operations. By using a vibrating

screed, crews can place concrete at a lower slump than

might be practical if screeding were done by hand. Suspended

slabs are seldom both level and supported at the time of

construction. Vibrating screeds and roller screeds similar to

those used for slab-on-ground strip placements are generally

not appropriate for use in suspended-slab construction because

of the probability that their use will result in slabs that

are too thin in localized areas. It is essential that minimum

slab thickness be maintained at all locations on suspended

slabs because of fire separation requirements.

Slumps up to 5 in. (125 mm) are often recommended for

concrete consolidated by vibrating screeds. If slumps in excess

of 4 in. (100 mm) are used, the amplitude of vibration

should be decreased in accordance with the consistency of

the concrete so that the concrete does not have an accumulation

of excess mortar on the finished surface after vibration.

Vibrating screeds strike off and straightedge the concrete

in addition to providing consolidation. To perform significant

consolidation, the leading edge of the shoe should be at

an angle to the surface, and the proper surcharge (height of

uncompacted concrete required to produce a finished surface

at the proper elevation) should be carried in front of the leading

edge.

Vibrating screeds should be moved forward as rapidly as

proper consolidation allows. If not used in this manner, too

much mortar will be brought to the surface in normal weight

concrete; conversely, too much coarse aggregate will be

brought to the surface in structural-lightweight-aggregate

concrete.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-33

8.3.3 Floating—The term “floating” is used to describe

compaction and consolidation of the unformed concrete surface.

Floating operations take place at two separate times

during the concrete finishing process.

The first floating, generally called “bull floating,” is by

hand and takes place immediately after screeding. Initial

floating should be completed before any excess moisture or

bleeding water is present on the surface. Any finishing operation

performed while there is excess moisture or bleed water

on the surface will cause dusting or scaling. This basic

rule of concrete finishing cannot be over-emphasized. The

first floating operation is performed using a bull float, darby,

or modified highway straightedge. The second floating operation

takes place after evaporation of most of the bleed water

and is usually performed using a power trowel with float

shoes or a pan attached. The second floating operation is described

in Section 8.3.10.

8.3.3.1 Bull floating—One of the bull float’s purposes is to

eliminate ridges and to fill in voids left by screeding operations.

Bull floating should embed the coarse aggregate only

slightly. This process prepares the surface for subsequent

edging, jointing, floating, and troweling.

When the specified finished floor flatness using the Fnumber

system restricts the difference between successive 1-

ft (300-mm) slopes to a maximum of 1/4 in. (6 mm), approximately

FF20 (Section 8.15), a traditional-width bull float of

4 to 5 ft (1.2 to 1.5 m) can be used to smooth and to consolidate

the concrete surface after screeding. The use of this

width bull float, however, can adversely affect floor flatness

and make achievement of higher flatness extremely difficult.

When the magnitude of difference between successive 1-ft

(300-mm) slopes is limited to less than 1/4 in. (6 mm)—floor

flatness greater than FF20 (Section 8.15)—an 8- to 10-ftwide

(2.4- to 3-m) bull float can be very useful in removing

surface irregularities early in the finishing process. This is

particularly true for suspended-slab construction, where local

irregularities caused by form- or metal-deck deflection

and concrete leakage can be significant.

Many contractors use an 8- to 10-ft-wide (2.4- to 3-m) bull

float or modified highway straightedge after initial strikeoff

to restraighten any local irregularities that can be present.

Use of a traditional 4- to 5-ft-wide (1.2- to 1.5-m) bull float

will provide little assistance to the finisher in correcting

these irregularities. Using the wider bull float or modified

highway straightedge allows the finisher to recognize and to

correct irregularities at a time when significant amounts of

material can be moved with relatively little effort. This simple

substitution of tools can routinely produce up to a 50 percent

increase in floor flatness.

In block placements for slabs on ground, and for suspended-

slab placements, a wide bull float or modified highway

straightedge can also be used to advantage. Applied at an angle

of approximately 45 to the axis of the placement strip and

extending across the joint between the current strip and the

strip just previously placed, these tools can remove many irregularities

that would otherwise remain if they were used

only in a direction perpendicular to the axis of the placement

strip.

A magnesium bull float can be used for lightweight concrete

and sticky mixes, or where it is desirable to partially

close the surface until it is time to float. The magnesium face

of the bull float slides along the fines at the surface and thus

requires less effort, and is much less likely to tear the surface.

When an embedded hardener or other special purpose aggregate

is required and rapid stiffening is expected, the use

of a bull float, preferably wooden, can be helpful in initially

smoothing the surface after the aggregate is applied and before

the modified highway straightedge is used in the initial

cutting and filling operation. Inevitable variations in the uniformity

of coverage when an embedded hardener or other

special purpose aggregate is applied will create slight irregularities

in the slab surface. Restraightening operations necessary

to remove these irregularities will remove embedded

material in some locations while adding to the thickness of

embedded material in other locations. Experience has shown

that some variation in the uniformity of embedded material

coverage does not adversely impact the floor’s function.

Wooden bull floats are preferable for use on normal

weight concrete that receives an embedded hardener. The

wood’s texture moves a mortar mixture of cement and fine

aggregate on the surface, permits normal bleeding, and

leaves the surface open. If a magnesium bull float is used for

normal weight concrete, the embedded hardener should first

be forced into the concrete using a wooden float. This brings

moisture to the surface and ensures proper bond of the hardener

to the base slab. This is particularly important where dry

shakes will be applied for color or increased wear resistance.

8.3.3.2 Darbying—Darbying serves the same purpose as

bull floating, and the same rules apply. Since bull floating

and darbying have the same effect on the surface of fresh

concrete, the two operations should never be performed on

the same surface. Because of its long handle, the bull float is

easy to use on a large scale, but the great length of the handle

detracts from the attainable leverage, so high tolerances are

more difficult to achieve. A darby is advantageous on narrow

slabs and in restricted spaces. Long-handled darbies should

be used for better leverage and control of level. Metal darbies

are usually unsatisfactory for producing surfaces meeting

high-tolerance requirements. The same principles regarding

the use of wooden or magnesium bull floats (Section 8.3.3.1)

apply to darbies, since both darbies and bull floats are used

for the same purpose following screeding.

8.3.3.3 Hand floating—Wooden hand floats encourage

proper workmanship and timing. If used too early on any

type of concrete, they stick, dig in, or can tear the surface.

Used too late, they roll the coarser particles of fine aggregate

out of the surface, at which time use of a magnesium float

held in a flat position would be preferable. Wooden floats

more easily fill in low spots with mortar; they should also be

used in areas where embedded hardeners or other special

purpose aggregates will be applied, floated, and finished by

hand only. The use of wooden hand floats has declined largely

due to the need for periodic replacement because of wear

or breakage, and the greater effort and care in timing required

in using them. Used at the proper time, their floating

action is unequaled by other hand tools.

302.1R-34 ACI COMMITTEE REPORT

Magnesium hand floats require less effort. Like magnesium

bull floats, they slide along largely on fines. They can

be used on concrete from the time of placement to beyond

the point of stiffening when a wooden float cannot be used.

Magnesium floats are best used in the initial smoothing of

the surface near screeds, walls, columns, or other projections,

and during placing, screeding, and bull floating, when

a wooden float would dig in or tear the surface. Magnesium

floats can also be used on air-entrained concrete that is not to

receive a troweled finish, or following wooden or power

floating to produce a more uniform swirl finish not quite as

roughly textured. Well-worn magnesium floats develop an

edge almost as sharp as a steel trowel’s, so care should be exercised

to use them flat to avoid closing the surface too early

or causing blisters.

Composition hand floats using resin-impregnated canvas

surfaces are smoother than wooden floats and only slightly

rougher than magnesium floats. They are similar to magnesium

hand floats and should be used in the same manner.

8.3.4 Highway-type straightedging—The use of a modified

highway straightedge for restraightening of the surface

varies with the type of slab being installed. Experienced finishers

can use this tool early in the finishing process instead

of an 8- to 10-ft-wide (2.4- to 3-m) bull float. Care is needed,

however, because the straightedge tends to dig into the concrete

if it is used improperly. Initial restraightening with the

modified highway straightedge, should immediately follow

screeding. Restraightening should be completed before any

excess moisture or bleed water is present on the surface.

When specified differences between successive 1-ft (300-

mm) slopes are 3/16 in. (5 mm) or less—flatness higher than

FF20 (Section 8.15)—a modified highway straightedge is

recommended to smooth and to restraighten the surface after

power floating or any floating operation that generates significant

amounts of mortar. A weighted modified highway

straightedge can also be used after power-trowel operations

to scrape the surface, reducing local high spots. Filling of

low spots is generally not appropriate after scraping with a

weighted modified highway straightedge.

The flatness exhibited by any concrete floor will be determined

almost exclusively by the effectiveness of corrective

straightedging employed after each successive strikeoff,

floating, and troweling step. Without restraightening, each

step performed in a conventional concrete floor installation

tends to make the surface less flat. Straightedges are capable

of restraightening, or reflattening the plastic concrete, since

they alone contain a reference line against which the resulting

floor profile can be compared. Restraightening operations

are most effective when new passes with the modified

highway straightedge overlap previous passes by about 50

percent of the straightedge width. In contrast, traditional 4-

to 5-ft-wide (1.2- to 1.5-m) bull floats, power floats, and

power trowels are by nature wave-inducing devices. To the

extent that further restraightedgings can only reduce floorwave

amplitudes and enlarge floor-wave lengths, floor surface

flatness can be further improved until Class 9 floor surface

quality is obtained.

The modified highway straightedge is used in a cutting

and filling operation to achieve surface flatness. When using

this, or any, restraightening tool, it is desirable to overlap

previous work with the tool by at least 50 percent of the tool

width. It is also desirable to use the tool in at least two directions,

preferably in perpendicular directions to each other.

For strip placements, this can be accomplished by using the

straightedge at a 45 angle to the axis of the strip, and toward

the end of the strip, followed by use of the straightedge at a

45 angle toward the beginning of the strip. The cutting and

filling operation taking place in these two directions from the

edge of a placement strip will enable the straightedge passes

to cross at right angles, and to produce a flatter, smoother

floor. Straightedging in a direction parallel to the strip-cast

operation and to the construction joints is possible, but less

desirable because this would require the finisher to stand in

the plastic concrete or on a bridge spanning the strip. This

“cut and fill” process can also be performed after powerfloating

operations (Section 8.3.10) to further improve the

floor’s flatness.

For slabs on ground with an embedded metallic or mineral

hardener, coloring agents, or other special-purpose material,

the use of a modified highway straightedge plays an important

part in reestablishing surface flatness after application of

the material. These products are generally applied after initial

screeding or strikeoff, and even the best of applications

will create minor irregularities in the surface. After the hardener

or special-purpose material has been worked into the

surface of the concrete using a wooden bull float, a followup

pass using the modified highway straightedge is desirable

to restraighten the surface after the embedded metallic, mineral,

and special-purpose material or its coating has absorbed

sufficient moisture.

Some embedded metallic dry-shake hardeners and colored

dry-shake hardeners are applied immediately after the initial

power float pass. When these materials are relatively fine, it

is necessary to wait until this point in the finishing operation

to begin their application. When applied too early in the finishing

process, they tend to be forced below the surface by

finishing operations. The use of a modified highway

straightedge to embed these materials and to restraighten the

surface after their application is a critical component of the

finishing process.

The committee recommends mechanical spreaders for use

in the application of metallic or mineral hardeners, colored

dry-shake hardeners, or other special-purpose materials.

Hand spreading sometimes results in an inadequate and uneven

application of the material.

8.3.5 Waiting—After initial floating and restraightening

have been completed, a slight stiffening of concrete is necessary

before proceeding with the finishing process. Depending

on job conditions, it is usually necessary to wait for this

stiffening to occur. Waiting time can be reduced or eliminated

by the use of dewatering techniques. No subsequent operation

should be done until the concrete will sustain foot

pressure with only about 1/4 in. (6 mm) indentation.

8.3.6 Dewatering techniques—For slabs on ground, the

use of dewatering techniques as an alternative to waiting

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-35

should be thoroughly discussed by key parties before implementation

by the contractor. Either vacuum mats, or a blotter

of cement on top of damp burlap, applied to the surface of

freshly placed concrete, can be used to remove significant

amounts of water. While this process quickly prepares the

surface for final floating and troweling, it should only be undertaken

by those with successful experience in the use of

these techniques. The application of dry cement directly to

the surface of freshly placed concrete should be avoided; this

practice promotes dusting of the floor surface and can result

in reduced abrasion resistance.

Vacuum mats, or a blotter of cement on top of damp burlap,

are applied after the concrete has been placed, compacted,

and floated. If vacuum mats are used, vacuum is applied

for about 3 to 5 min. per 1 in. (25 mm) thickness of

slab.36,37,38

Vacuum dewatering has been used extensively in Europe.

More detailed information is presented in References 36

through 38.

8.3.7 Edging—Edging is not required or recommended on

most floors. Edgers should be used only when specifically

required by the project documents. Where edging is required,

utilization of walk-behind edgers is discouraged because

their use can yield inconsistent results. If the floor is to

be covered with tile, an edger should not be used. If required,

a 1/8 in. (3 mm) or smaller radius edge should be used for

construction joints subjected to regular vehicular traffic, although

saw-cutting is the preferred method for this type of

surface.

The edger is used to form a radius at the edge of the slab

(Section 8.2.6). Edging or stoning will also allow construction

joints to be readily visible for accurate location of sawing,

when used. The second placement at a construction joint

will often bond to the first placement. Sawing this joint encourages

development of a clean, straight crack at the construction

joint. Edging is most commonly used on sidewalks,

driveways, and steps; it produces a neater looking edge that

is less vulnerable to chipping. Edging should not commence

until most bleed water and excess moisture have left or been

removed from the surface. Instead of being edged, construction

joints of most floor work can be lightly stoned after the

bulkheads, or edge forms, are stripped and before the adjacent

slab is placed.

8.3.8 Hand-tooled joints—Slabs on ground are jointed immediately

following edging, or at the same time, unless the

floor is to be covered with tile. If the floor is to be covered

with tile, jointing is unnecessary because random cracks are

preferable to tooled joints under tile. For floors to be covered

with quarry tile, ceramic tile, terrazzo pavers, or cast-inplace

terrazzo, the joints in slabs on ground should be

aligned with joints in the rigid coverings.

The cutting edge, or bit, of the jointing tool creates

grooves in the slab, called contraction joints (Section

3.2.5.3). For contraction joints, the jointing tool should have

a bit deep enough to cut grooves that are one-quarter of the

thickness of the slab. This forms a plane of weakness along

which the slab will crack when it contracts. Jointers with

worn-out or shallow bits should not be used except for forming

decorative, nonfunctional groves in the concrete surface.

The jointer should have a 1/8-in. (3-mm) radius for floors.

Because of limitations on bit length, hand-tooled joints

aren’t practical for slabs greater than 5 in. (125 mm) thick

where the groove depth is 1/4 of the slab thickness.

It is good practice to use a straight 1x8- or 1x10-in. board

(25x200- or 25x250-mm) as a guide when making the joint,

or groove, in a concrete slab. If the board is not straight it

should be planed true. The same care should be taken in running

joints as in edging, because a hand-tooled joint can either

add to or detract from the appearance of the finished

slab.

8.3.9 Preformed joints—Preformed plastic and metal

strips are also available as an alternative to the use of jointers

or saw cuts for making contraction joints. If used, they are inserted

in the fresh concrete at the time hand-tooled jointing

would take place. Proper performance of these strips is extremely

sensitive to installation. Plastic or metal inserts are

not recommended in any floor surface subjected to wheeled

traffic (Section 3.2.5.3).

8.3.10 Power floating—After edging and hand-jointing

operations (if used), slab finishing operations should continue

with use of either the hand float or the power float. Power

floating is the normal method selected. The purposes of power

floating are threefold: (1) to embed the large aggregate

just beneath the surface of a mortar composed of cement and

fine aggregate from the concrete; (2) to remove slight imperfections,

humps, and voids; and (3) to compact the concrete

and consolidate mortar at the surface in preparation for other

finishing operations. In the event that multiple floating passes

are required, each floating operation should be made perpendicular

to the direction of the immediately previous pass.

Nonvibratory, 24- to 36-in. diameter (0.6- to 1-m) steel

disk-type floats are usually employed to float low-slump or

zero-slump concrete or toppings. They can also be used for

additional compacting or floating following normal floating

operations when the surface has stiffened to a point where it

can support the weight of the machine without disturbing the

flatness of the concrete.

Troweling machines equipped with float-shoe blades can

be used for floating. Later float passes can use pans clipped

to trowel blades.Troweling machines with combination

blades could be used, but are not recommended. Floating

with a troweling machine equipped with normal trowel

blades should not be permitted. Contract documents should

also prohibit the use of any floating or troweling machine

that has a water attachment for wetting the concrete surface

during finishing of a floor. Application of water by brush or

machine during finishing promotes dusting of the floor surface.

Many variables—concrete temperature, air temperature,

relative humidity, and wind—make it difficult to set a definite

time to begin floating. The concrete is generally

ready for hand floating when the water sheen has disappeared

or has been removed, and the concrete will support

a finisher on kneeboards without more than approximately

a 1/8 in. (3 mm) indentation. The slab surface is ready for

machine floating with the lightest machine available when

302.1R-36 ACI COMMITTEE REPORT

the concrete will support a finisher on foot without more

than approximately a 1/4 in. (6 mm) indentation, and the

machine will neither dig in nor disrupt the levelness of the

surface.

Normally, concrete will be ready for power floating in the

same order in which it was placed. On a given placement,

however, certain areas can become ready for power floating

before others. The areas that should be floated first generally

include surfaces adjacent to screed guides, edge forms,

blockouts, walls, and columns. Areas exposed to sun tend to

set more quickly than those protected by shade; surfaces exposed

to wind also require attention before those protected

from the wind. Generally, one or more finishers should be

assigned to look after those areas that will set faster than the

overall placement.

As a general rule, and under slow-setting conditions when

flatness tolerances are not high, power floating should be

started as late as possible; this is indicated by minimum machine

indentation or when a footprint is barely perceptible.

Under fast-setting conditions or when high-flatness tolerances

are required, and with the understanding that abrasion resistance

of the slab can be reduced, floating should be started

as soon as possible; the maximum practical indentation is

about ¼ in. (6 mm). When higher-flatness quality is required,

the floating operation should generate sufficient mortar

to assist in restraightening operations with the modified

highway straightedge. Flatness/levelness tolerances can require

restraightening of the surface before and after the floating

operation.

The marks left by the edger and jointer should be removed

by floating, unless such marks are desired for decoration, in

which case the edger or jointer should be rerun after the

floating operation.

Generally, when the floating operation produces sufficient

mortar, restraightening after the floating operation is very

beneficial. After the initial power-float pass, and while the

surface mortar is still fresh, the modified highway straightedge

can be used to restraighten the slab surface by removing

the troughs and ridges generated by the power float. This is

accomplished by cutting down the ridges and using that mortar

to fill the troughs. These operations should be completed

during the “window of finishability” described in the second

paragraph of this chapter.

The use of the power float, as previously described, tends

to create troughs under the center of the machine in the direction

of travel, with ridges of mortar occurring just outside

the perimeter of the blades. Around projections such as columns

and sleeves, the power float tends to push mortar up

against the projection. If this mortar buildup is not removed

by the hand finisher, it will remain when the concrete hardens

and the surface will be at a higher elevation than desired.

One method that allows proper grade to be maintained at

these locations is to place a bench mark a specified distance

above design grade on the projection for subsequent use by

the finisher. While completing hand work around the column

or sleeve, the finisher can use a template to confirm that

proper grade has been maintained. Excess material can then

be removed as required.

8.3.11 Troweling—The purpose of troweling is to produce

a dense, smooth, hard surface. Troweling is done immediately

following floating; no troweling should ever be done on a

surface that has not been floated by power or by hand. Use

of a bull float or darby without following by hand or machine

floating is not sufficient.

If troweling is done by hand, it is customary for the concrete

finisher to float and then steel trowel an area before

moving kneeboards. If necessary, tooled joints and edges

should be rerun before and after troweling to maintain uniformity

and true lines.

Hand trowels that are short, narrow, or of inferior construction

should not be used for first troweling. Mechanical

troweling machines can be used. The mechanical trowel can

be fitted with either combination blades or with those intended

specifically for the troweling operation.

For the first troweling, whether by power or by hand, the

trowel blade should be kept as flat against the surface as possible;

in the case of power troweling, use a slow speed. If the

trowel blade is tilted or pitched at too great an angle, an objectionable

“washboard” or “chatter” surface will result. A

trowel that has been properly “broken in” can be worked

quite flat without the edges digging into the concrete. Each

subsequent troweling should be made perpendicular to the

previous pass. Smoothness of the surface can be improved

by restraightening operations with the modified highway

straightedge and by timely additional trowelings. There

should be a time lapse between successive trowelings to permit

concrete to become harder. As the surface stiffens, each

successive troweling should be made with smaller trowel

blades or with blades tipped at a progressively higher angle

to enable the concrete finisher to apply sufficient pressure

for proper finishing. Additional troweling increases the compaction

of fines at the surface and decreases the water-cementitious

material ratio of concrete near the slab surface

where the trowel blades agitate surface paste and hasten the

evaporation rate of water within the paste; this process results

in increased surface density and improved wear resistance.

Extensive steel-troweling of surfaces receiving a

colored dry-shake hardener can have a negative impact on

the uniformity of color. Refer to Section 8.6.2 for a detailed

discussion.

The formation of blisters in the surface of the concrete during

troweling can be the result of entrained air or excessive

fines in the concrete mixture, of early troweling, or of an excessive

angle of the trowel blades. Purposefully-added (entrained)

air is not recommended on surfaces to receive a

hard-troweled finish (Section 6.2.7). If the air content is acceptable,

then blister formation is an immediate indication

that the angle of the trowel blade is too great for the surface

in that area at that particular time for the concrete and job

conditions involved.

Extensive steel-troweling leaves the concrete surface with

a very high sheen. Such surfaces become quite slippery when

wet, and should be slightly roughened to produce a nonslip

surface if they are to be exposed to the weather. A smoothtextured

swirl finish can be produced by using a steel trowel

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-37

in a swirling motion (also known as a sweat finish) or by

brooming the freshly troweled surface.

A fine-broomed surface is created by drawing a soft-bristled

broom over a freshly troweled surface. When coarser

textures are desired, a stiffer bristled broom can be used after

the floating operation. A coarse-textured swirl finish can be

created after completion of the power float pass and subsequent

restraightening using a modified highway straightedge.

A coarse-swirl pattern is normally created using a

hand-held wood or magnesium float (Section 8.13.4).

During periods of hot, dry, and windy weather, troweling

should be kept to the minimum necessary to obtain the desired

finish. When ambient conditions create high water loss

due to slab evaporation, fog spraying above the concrete or

use of an evaporation retardant is necessary. After finishing,

any delay in protecting the slab with curing compounds or

other water-retaining materials can result in an increase in

plastic-shrinkage cracking, crazing, low surface strength,

dusting, and early deterioration.

8.3.12 Saw-cut joints—On large flat concrete surfaces,

rather than hand-tooling joints, it can be more convenient to

cut joints with an electric or gasoline-driven power saw fitted

with an abrasive or diamond blade, and using one of the

following three types of saws: conventional wet-cut; conventional

dry-cut; or early-entry dry-cut.

The early-entry dry-cut process is normally used when

early sawing is desired. Early-entry dry-cut joints are

formed using diamond-impregnated blades. The saw cuts

resulting from this process are not as deep as those produced

using the conventional wet-cut process—11/4 in. (32

mm) maximum. The timing of the early-entry process,

however, allows joints to be in place prior to development

of significant tensile stresses in the concrete; this increases

the probability of cracks forming at the joint when sufficient

stresses are developed in the concrete. Care should be

taken to make sure the early-entry saw does not ride up

over hard or large coarse aggregate. The highest coarse aggregate

should be notched by the saw to ensure the proper

function of the contraction joint. State-of-the-art early-entry

saws have an indicator that shows the operator if the

saw cut becomes too shallow.

Typically, joints produced using conventional processes

are made within 4 to 12 hours after the slab has been finished

in an area—4 hours in hot weather to 12 hours in cold weather.

For early-entry dry-cut saws, the waiting period will typically

vary from 1 hour in hot weather to 4 hours in cold

weather after completing the finishing of the slab in that joint

location. Longer waiting periods can be necessary for all

three types of sawing for floors with steel-fiber reinforcement

or embedded-mineral-aggregate hardeners with longslivered

particles, such as traprock.

The depth of saw cut using a conventional saw should be at

least 1/4 of the slab depth or a minimum of 1 in. (25 mm),

whichever is greater. The depth of saw cut using an early-entry

dry-cut saw should be 1 in. (25 mm) minimum, unless experience

has shown that a shallower saw cut is sufficient. This recommendation

assumes that the early-entry dry-cut saw is used

within the time constraints noted above. For steel-fiber-reinforced

slabs, the saw cut using the conventional saw should be

1/3 of the slab depth. Typically, experience has shown that

when timely cutting is done with an early-entry saw, the depth

can be the same as for plain concrete.

Regardless of the process chosen, saw cutting should be

performed (1) before concrete starts to cool, (2) as soon as

the concrete surface is firm enough not to be torn or damaged

by the blade, and (3) before random-drying-shrinkage cracks

can form in the concrete slab. Shrinkage stresses start building

up in the concrete as it sets and cools. If sawing is unduly

delayed, the concrete can crack randomly before it is sawed.

Additionally, delay can generate cracks that run off from the

saw blade toward the edge of the slab at an obtuse or skewed

angle to the saw cut.

8.4—Finishing Class 1, 2, and 3 floors (tilecovered,

offices, churches, schools, hospitals,

ornamental, and garages)

The placing and finishing operations described under Section

8.3 should be followed. Multiple restraightening operations

and two hand or machine trowelings are recommended,

particularly if a floor is to be covered with thin-set flooring

or resilient tile; this will give closer surface tolerances and a

better surface for application of the floor covering.

The use of silica fume concrete for parking garage construction

lends itself to a “one pass” finishing approach. After

initial strikeoff and bull floating have been completed, the

concrete placement strips can be textured using a broom.

Normally, a light broom with widely-spaced, stiff bristles

will be satisfactory for this purpose.

Since silica-fume concrete exhibits virtually no bleeding,

it is necessary to keep the surface moist during concrete

finishing operations to prevent plastic shrinkage cracking.

This normally requires use of a pressure fogger with a reach

capable of covering the entire surface. Fogging should be

performed continuously between finishing operations until

the surface has been textured. The goal of the fogging operation

should be to keep the concrete surface moist, but

not wet. It is critical that curing operations commence as

quickly as possible after texturing has been completed.29

If decorative or nonslip finishes are desired, refer to procedures

described in Section 8.13.

8.5—Finishing Class 4 and 5 floors (light-duty

industrial and commercial)

The placing and finishing operations described in Section

8.3 should be followed. Three machine trowelings can be

specified for increased wear resistance.

8.6—Finishing Class 6 floors (industrial) and

monolithic-surface treatments for wear resistance

Floors using embedded mineral or metallic hardeners are

usually intended for moderate or heavy traffic and, in some

cases, to resist impact. It is essential that these hardeners be

properly embedded near the top surface of the slab in order

to provide the required surface hardness, toughness, and impact

resistance.

Air content of the concrete should not be more than 3 percent

if the concrete is not subject to freezing and thawing cy302.1R-

38 ACI COMMITTEE REPORT

cles under service conditions. As with any commercial or

industrial floor subjected to wheeled traffic, special care

should be exercised to obtain flat and level surfaces and

joints. Metallic hardeners should not be placed over concrete

with chloride ion contents greater than 0.1 percent by weight

of cementitious material. The proposed mixture proportions

should be used in the installation of any test panel or test

placement. If adjustments to the concrete mixture are required,

they can be made at that time.

8.6.1 Embedded mineral-aggregate hardener—The application

and finishing of embedded mineral-aggregate hardeners

should follow the basic procedures outlined below.

Concrete installations are subject to numerous conditions

and variables. Experience is necessary to determine proper

timing for the required procedures. It is recommended that

these procedures be discussed and agreed upon at the preconstruction

meeting:

1. Place, consolidate, and strike off concrete to the proper

grade.

2. Compact and consolidate the concrete surface using a

bull float.

3. Restraighten the surface using a modified highway

straightedge. Occasionally, compacting, consolidating, and

restraightening are accomplished in one step by using a wide

bull float or a modified highway straightedge with the

straightedge rotated so its wide dimension is in contact with

the surface.

4. Evenly distribute approximately two-thirds of the specified

amount of mineral-aggregate hardener immediately following

the restraightening operation, and prior to the

appearance of bleed water on the slab surface. The first application

generally consists of a larger, coarser material than

will be used in the final application. Distribution of the hardener

by mechanical spreader is the preferred method. The

concrete mixture should have proportions such that excessive

bleed water does not appear on the surface after application

of the hardener.

5. As soon as the hardener darkens slightly from absorbed

moisture, a modified highway straightedge should be used to

embed the hardener as well as to remove any irregularities in

the surface.

6. Wait until the concrete sets up sufficiently to support the

weight of a power trowel with float shoes or a pan attached.

Combination blades should not be used. The float breaks the

surface and agitates concrete paste at the surface of the slab.

The first power-float passes should be across the placement

strip in the short direction. This will ensure that irregularities

resulting from the power floating can be easily identified and

corrected in subsequent operations.

7. Apply the remaining one-third of the specified mineral

aggregate, preferably at right angles to the first application.

This material generally consists of finer-size aggregate and

is most often broadcast over the surface of the slab by hand.

8. Restraighten the surface using a modified highway

straightedge. Remove irregularities and move excess material

to low spots.

9. Embed the mineral-aggregate fines using a power trowel

with float shoes or a pan attached.

10. Restraighten the surface following the power-floating

operation using a weighted modified highway straightedge if

its use is seen to be effective or necessary to achieve required

surface tolerances. One method of increasing the weight of a

modified highway straightedge is to wedge a #11 bar inside

the rectangular section of the straightedge.

11. Continue finishing with multiple power trowelings as

required to produce a smooth, dense, wear-resistant surface

(Section 8.3.11). Provide a burnished (hard) troweled surface

where required by specification.

12. Cure immediately after finishing by following the curing

material manufacturer’s recommendations. Curing

methods should be in accordance with those used and approved

in construction of any test panel.

8.6.2 Metallic dry-shake hardeners and colored dry-shake

hardeners—Metallic dry-shake hardeners and colored dryshake

hardeners can be finer in texture than uncolored-mineral-

aggregate-dry-shake hardeners. This difference, along

with the fact that the metallic dry-shake hardener has a higher

specific gravity, dictates that the material normally be embedded

in the concrete later in the setting process than is

common for uncolored-mineral-aggregate-dry-shake hardeners.

Some metallic dry-shake hardeners are designed by

their manufacturers to allow application of all the hardener

at one time. When such procedures are used, however, caution

should be exercised to ensure that manufacturer’s recommendations

are followed, and that the material is

thoroughly wetted-out, since a one-time application significantly

increases the possibility of surface delamination or related

finishing problems. Typical installation techniques for

metallic dry-shake hardeners and colored dry-shake hardeners

are similar to those described in Section 8.6.1, but the following

sequence is recommended: Refer to Section 8.13.1

for additional discussion.

1. Place, consolidate, and strike off concrete to the proper

grade.

2. Compact and consolidate the concrete surface using a

bull float.

3. Restraighten the surface using a modified highway

straightedge. A wide bull float or a modified highway

straightedge can be used to accomplish both steps in one operation.

4. Open the surface to promote movement of bleed water

to the top of the slab by using a wooden bull float. Steps 3

and 4 can be accomplished in one operation if the wide bull

float or modified highway straightedge is made of wood.

5. Wait until the concrete sets up sufficiently to support the

weight of a power trowel.

6. Break the surface using a power trowel with float shoes

or a pan attached.

7. Evenly distribute approximately two-thirds of the specified

amount of metallic dry-shake hardener or colored dryshake

hardener. Application of the material by mechanical

spreader is the preferred method.

8. Restraighten the surface after application of the metallic

dry-shake hardener or colored dry-shake hardener to remove

irregularities. Some contractors find that embedding the maCONCRETE

FLOOR AND SLAB CONSTRUCTION 302.1R-39

terials and restraightening can be accomplished in one step

using a modified highway straightedge.

9. Complete initial embedment and prepare the surface for

additional material by using a power trowel with float shoes

or a pan attached.

10. Apply the remaining one-third of the specified amount

of metallic dry-shake hardener or colored dry-shake hardener,

preferably at right angles to the first application.

11. Embed metallic dry-shake hardener or colored dryshake

hardener using a power trowel with float shoes or a

pan attached. Thorough embedment and integration of the

metallic dry-shake hardener or colored dry-shake hardener

with the concrete by floating is very important. Failure to accomplish

this goal can result in blistering or delamination of

the slab.

12. Restraighten the surface following the power-floating

operation using a weighted modified highway straightedge,

if effective.

13. Continue finishing with multiple power trowelings as

required to produce a smooth, dense, wear-resistant surface

(Section 8.3.11). Proper and uniform troweling is essential.

Colored surfaces should not be burnished—hard-troweled;

the result would be uneven color and a darkening of the surface.

14. Cure immediately after finishing by following the curing

material manufacturer’s recommendations. Curing

methods should be in accordance with those used and approved

in construction of any test panel. Colored floors

should not be cured with plastic sheeting, curing paper,

damp sand, or wet burlap. These materials promote uneven

color, staining, or efflorescence.

8.7—Finishing Class 7 floors (heavy-duty

industrial)

The topping course of heavy-duty industrial floors should

have a minimum thickness of 3/4 in. (19 mm). The concrete

topping used should have a maximum slump of 3 in. (75

mm) unless a water-reducing admixture or high-range waterreducing

admixture is used to increase the slump, or unless

dewatering techniques are used. Because of the relatively

small amount of concrete in the topping course and the low

slump required, concrete for the topping could be job-mixed.

Embedded metallic dry-shake hardeners, mineral-aggregate

dry shakes, and colored dry-shakes can be applied to

produce the desired combination of increased wear resistance

or color as described in Sections 8.6.1 and 8.6.2, respectively.

The base course should be screeded and bull floated; close

maintenance of the elevation tolerance for the base course

surface is important. Class 7 floors can be constructed in two

ways: (1) the topping installation can be bonded monolithically

to the base slab before the base slab has completely set,

or (2) the topping can be deferred for several days.

For suspended slabs, it is recommended that the deferred

bonded approach be used. This will allow the structure to deflect

under its own weight prior to application of the topping.

The additional weight of the topping will have little impact

on subsequent deflection of the slab.

8.7.1 Bonded monolithic two-course floors—For these

floors, the topping course is placed before the base course

has completely set. Any excess moisture or laitance should

be removed from the surface of the base course, and the surface

floated before the top course is placed. When the topping

is being placed, the concrete in the base slab should be

sufficiently hard that footprints are barely perceptible. The

use of a disk-type power float can be necessary to bring sufficient

past to the surface to allow restraightening to take

place. The power-floating operation should be followed by a

minimum of two power trowelings. This method of topping

application is generally not appropriate for a suspended slab.

8.7.2 Deferred bonded two-course floors—Caution: Bonding

of two-course floors is a highly critical operation requiring

the most meticulous attention to the procedure

described. Even with such care, such bonding has not always

been successful. As a result, it is recommended that contractors

using this type of construction for heavy-duty industrial

applications be experienced and familiar with the challenges

presented.

Locations of joints in the base course should be marked so that

joints in the topping course can be placed directly over them.

After the base course has partially set, the surface should

be brushed with a coarse-wire broom. This removes laitance

and scores the surface to improve bond of the topping course.

It is recommended that concrete base courses be wet-cured

a minimum of 3 days (Sections 9.2.1 and 9.2.2). Shrinkagecompensating

concrete base courses should be wet-cured a

minimum of 7 to 10 days, and preferably until the topping is

applied. Refer to ACI 223 for additional information.

If the topping is to be applied immediately after the minimum

3-day curing time has elapsed, the curing cover or water

should be removed from the slab and any collected dirt

and debris washed or hosed off. After most free water has

evaporated or has been removed from the surface, a bonding

grout should be scrubbed in. The bonding grout should be

composed of one part cement, 1.5 parts fine sand passing the

No. 8 sieve (2.36 mm), and sufficient water to achieve the

consistency of thick paint. The grout should be applied to the

floor in segments, keeping only a short distance ahead of the

concrete topping placing operations that follow it.

While the bonding grout is still tacky, the topping course

should be spread and screeded. The use of a disk-type power

float is suggested, followed by a minimum of two power

trowelings.

If 3 to 7 days are to elapse between placing the base and

the topping course, the surface of the base course should be

protected from dirt, grease, plaster, paint, or other substances

that would interfere with the bond. Immediately before placing

the topping, the base course should be thoroughly

cleaned by scrubbing with a brush and clean water. Most excess

water should be removed and a thin coat of grout, as described

previously, scrubbed in. While this grout is still

tacky, the topping course should be spread and screeded, as

described previously.

If the floor is to be subjected to construction activities after

curing and before application of the topping, more thorough

cleaning can be necessary. One method of cleaning the base

302.1R-40 ACI COMMITTEE REPORT

slab is to scrub the surface with water containing detergent.

If oil or grease has been spilled on the floor, a mixture of sodium

metasilicate and resin soap is useful. If this method is

used, the floor should then be rinsed thoroughly with water.

Shot-blasting, sand-blasting, or mechanical scarification by

scabbling can also be employed in lieu of cleaning with detergent

to achieve a bondable surface.

In some circumstances, it can be convenient or desirable to

bond the topping with an epoxy adhesive appropriate for the

particular application. Methods are described in ACI 503R,

and a standard specification is given in ACI 503.2.

Joints in the topping above the joints in the base slab

should be saw-cut to a depth equal to twice the thickness of

the topping, and should match the location of joints in the

base slab, where applicable.

8.8—Finishing Class 8 floors (two-course

unbonded)

The unbonded topping for Class 8 floors should be a minimum

of 4 in. (100 mm) thick. An unbonded topping thickness

of 3 in. (75 mm) has been used with some success for

Class 3 floors, but thickness for strength and control of curling

is less important for a Class 3 slab because of its duty,

loading, and because it may also be covered. A Class 8 floor

is intended for industrial applications where strength and

control of curling is more important. The base course,

whether old or new, should be covered with plastic sheet,

felt, a sand cushion, or other approved bond-breaker, spread

as wrinkle-free as possible.

The topping slab should contain sufficient steel reinforcement

to limit the width of shrinkage cracks in the topping and

the displacement of the topping concrete on either side of

any cracks that might form. Although reinforcing steel is

normally discontinued at joints, engineering considerations

can make it desirable to carry reinforcement through construction

joints in specified locations in a topping. Reinforcement

that is continuous through contraction and

construction joints will cause restraint against movement

that will inevitably result in cracks in the concrete.

Concrete for the top course should comply with the requirements

of Table 6.2.1.

Power floats and power trowels are recommended and

usually required. The practice of completing troweling by

hand is counterproductive because hand troweling is less effective

than power troweling in compacting the surface.

Embedded mineral-aggregate hardeners for increased

wear resistance can be applied as described in Section 8.6.1.

Embedded metallic dry-shake hardeners and colored dryshake

hardeners can be applied as described in Section 8.6.2.

8.9—Finishing Class 9 floors (superflat or critical

surface tolerance required)

Floor surfaces of this quality can be subdivided by function

into two separate groups. Refer to Section 8.9.1 for special

considerations dealing with construction of Class 9 floor

surfaces.

The more common group of these floor surfaces should support

vehicular traffic along paths that are defined prior to construction

and that do not change during the life of the floor

surface (that is, defined traffic). A typical example of a defined-

traffic floor would be a distribution center that uses

very-narrow aisles and high-bay racking systems. In this type

of facility, tolerances across aisles and the joints that parallel

them are less critical than those along the axis of the aisle. This

type of floor surface is often referred to as “superflat.”

Floor surfaces in the second group are less common, but

should support traffic in all directions (that is, random traffic).

A typical example of a random-traffic floor would be a

television or movie studio. The random nature of traffic in

these facilities requires that tolerances across placement

strips and their joints should match those achieved parallel to

the axis of the strip.

Finishing procedures required to produce Class 9 floors

represent the most rigorous and demanding floor installation

technology now being performed. If discipline and preplanning

are a part of the overall process, however, installation of

Class 9 floors is neither complex nor especially difficult.

Proper timing and execution of various procedures will usually

ensure that the floor produced is of a predictable quality.

Class 9 floor construction requires that (1) the slabs be

constructed in long strips less than 20 ft (6 m) in width; (2)

the concrete slump be adjusted onsite to within ± 1/2 in. (± 13

mm) of the target slump; (3) the slump at point of deposit be

sufficient to permit use of the modified highway straightedge

to close the floor surface without difficulty after the initial

strikeoff; (4) the “window of finishability” be sufficient for

the concrete contractor to perform the necessary finishing

operations; and (5) the concrete supplier use enough trucks

to ensure an uninterrupted concrete supply. In addition, since

environmental factors can significantly alter the setting rate

of concrete, an effort is usually made to construct Class 9

floors out of the weather.

On Class 9 defined-traffic floors, construction joints between

placement strips are located out of the traffic pattern

where racks abut each other. These surfaces are evaluated by

taking measurements only in locations matching the wheelpaths

of the vehicles that will eventually use the floor. The

part of the floor surface falling under racks is not tested.

While the same construction techniques are required to

produce Class 9 random-traffic floors—television studios or

similar surfaces—the entire floor surface should be evaluated

because the entire surface will be subjected to traffic. The

contractor is cautioned that grinding of the entire length of

the joints will be necessary to produce Class 9 quality across

the width of concrete placement strips.

On most projects with Class 9 defined-traffic floors, surfaces

are measured for flatness and levelness immediately

following the final troweling of each placement; placements

are frequently scheduled for consecutive days. Where Class

9 random-traffic quality should be achieved across multiple

strips, initial testing should take place as each strip is placed,

but final testing should be deferred until the installation is

complete.

Nonetheless, it is imperative that surface-profile testing

and defect identification be accomplished on each new slab

as soon as possible. To maintain satisfactory results, the conCONCRETE

FLOOR AND SLAB CONSTRUCTION 302.1R-41

tractor requires continuous feedback to gage the effectiveness

of construction techniques against ever-changing job

conditions (Section 8.9.1).

Achieving Class-9-quality levels on suspended slabs is

impractical in a one-course placement. Deflection of the surface

between supports occurs after removal of supporting

shores. If the surface were to meet Class 9 requirements in a

shored condition, it is very likely that the deflected surface

after shores are removed would be less level than is required

to meet Class 9 requirements. Two-course placements utilizing

methods similar to those discussed for Class 7 and Class

8 floors provide the best opportunity for achieving Class-9-

quality levels on suspended floors.

8.9.1 Special considerations for construction of Class 9

floor surfaces—Certain specialized operations—narrowaisle

warehouses, ice rinks, television studios, and air-pallet

systems—require extraordinarily flat and level floors for

proper equipment performance. Such superflat floors generally

exhibit FF numbers and FL numbers above 50 in the direction

of travel for the particular application. Refer to

Section 8.15 for additional discussion.

The floor-finish tolerance employed in the contract specification

should meet the equipment supplier’s published requirements

unless there is reason to doubt the validity of

such requirements. In any case, written approval of the contract

floor tolerance should be obtained from the appropriate

equipment supplier prior to finalizing the bid package. In this

way, equipment warranties will not be jeopardized, and the

special superflat nature of the project will be identified to

key parties from the outset.

Superflat floors have very specific design requirements.

Chief among these is the limit imposed on placement width.

In general, superflat floors cannot be produced if construction

joint spacing exceeds 20 ft (6 m). Since hand-finishing

procedures and curling effects are known to make floors in

the vicinity of construction joints less flat than in the “middle”

of the slab, joints should be located out of the main traffic

areas, or provision should be made for their correction.

Contraction joints oriented transverse to the longitudinal

axis of a Class-9-placement strip can curl and reduce surface

flatness along aisles. Limited placement width, consequent

increased forming requirements, and reduced daily-floor

production are primary factors that increase the cost of Class

9 floors.

The prebid meeting is an essential component of any superflat

project. Since floor flatness/levelness is one of the

primary construction requirements, a thorough prebid review

of the design, specification, and method of compliance

testing is required. This will enable the prospective contractor

to price the project realistically (thereby avoiding costly

misunderstandings and change orders), and will greatly increase

the chances of obtaining the desired results at the lowest

possible cost.

To further reduce the risk of significant problems, the installation

of test slabs has become a standard part of superflat-

floor construction. Generally, if the contractor is

inexperienced with superflat construction or with the concrete

to be used, at least two test slabs should be installed and

approved before the contractor is permitted to proceed with

the balance of the superflat-floor construction.

Superflat-floor tolerances should be inspected within 24

hours after slab installation. This eliminates the possibility of

large areas being placed before any tolerance problem is discovered.

In narrow-aisle warehouses, tolerances are measured

using a continuous recording floor profileograph or

other device. In these facilities, floor tolerances are generally

written around the lift-truck wheel dimensions, and compliance

measurements and corrections are required only in the

future wheel tracks.

In television studios and other similar random-traffic installations,

the use of FF and FL to specify the floor-surface

tolerances is appropriate. Measurements for compliance

should be made in accordance with ASTM E1155 (Section

8.15).

8.10—Toppings for precast floors

Many types of precast floors require toppings. These include

double-tees, hollow-core slabs, and other kinds of precast

floor elements. When these floors are to be covered with

bonded or unbonded toppings, the procedures in Section

8.7.2 or 8.8 should be followed as appropriate. High-strength

concrete is often used for precast floor elements; roughening

of the surface of such members can be difficult if delayed too

long.

8.11—Finishing structural lightweight concrete

This section concerns finishing structural lightweight concrete

floors. Finishing very-lightweight insulating-type concretes—

having fresh weights of 60 lb per cu ft (960 kg/m3)

or less—that are sometimes used below slabs generally involves

little more than screeding.

Structural lightweight concrete for floors usually contains

expanded shale, clay, slate, or slag coarse aggregate; expanded

shale is most common. The fine aggregate can consist

of manufactured lightweight sand, natural sand, or a

combination of the two, but natural sand is most common.

The finishing procedures differ somewhat from those used

for a normal weight concrete; in lightweight concrete, the

density of the coarse aggregate is generally less than that of

the sand and cement. Working the concrete has a tendency to

bring coarse aggregate rather than mortar to the surface. This

should be taken into account in the finishing operations.39

Observing the following simple rules will control this tendency

so that structural lightweight concrete can be finished

as easily as normal weight concrete, provided the mixture

has been properly proportioned:

1. The mixture should not be over-sanded in an effort to

bring more mortar to the surface for finishing. This usually

will aggravate rather than eliminate finishing difficulties.

2. The mixture should not be under-sanded in an attempt

to meet the unit weight requirements. Neither mixing to the

recommended slump nor entrainment of air will effectively

control segregation in such a mixture.

3. The lightweight-concrete mixture should be proportioned

to provide proper workability, pumpability, finishing

characteristics, and required setting time, to minimize segre302.1R-

42 ACI COMMITTEE REPORT

gation or the tendency for coarse-aggregate particles to rise

above the heavier mortar.

4. Some lightweight aggregates can require further control

of segregation or bleeding, or both. For this purpose, use not

less than 4 percent entrained air in accordance with ACI

211.2.

5. Presaturate lightweight aggregates for use in concrete

that will be pumped, in accordance with the manufacturer’s

recommendations.

6. Overworking or over-vibrating lightweight concrete

should be avoided. A well-proportioned mixture can generally

be placed, screeded, and bull floated with approximately

half the effort considered good practice for normal weight

concrete. Excess darbying or bull floating are often principal

causes of finishing problems, since they only serve to drive

down the heavier mortar that is required for finishing, and to

bring an excess of the coarse aggregate to the surface.

7. A magnesium darby or bull float should be used in preference

to wood. Metal will slide over coarse aggregate and

embed it rather than tear or dislodge it.

8. The surface should be floated and flat troweled as soon

as surface moisture has disappeared and while the concrete

is still plastic. If floating is being done by hand, use a magnesium

float. If evaporation is not taking place soon enough

(while concrete is still plastic), other measures should be taken.

Water and excess moisture should be removed from the

surface with as little disturbance as possible. A simple but reliable

method is to drag a loop of heavy-rubber garden hose

over the surface.

8.12—Nonslip floors

Nonslip surfaces are produced by using the following finishing

procedures: swirl or broom finish (Section 8.13.4), or

nonslip special-purpose aggregate (Section 8.13.2). The

nonslip special-purpose aggregate is recommended for

heavy-foot traffic.

References 40 and 41 describe methods of measuring and

evaluating the relative skid resistance of floors.

8.13—Decorative and nonslip treatments

8.13.1 Colored dry-shake-hardener surface treatment—

The installation of a colored surface treatment is particularly

sensitive to the finishing and curing techniques employed by

the contractor. It is recommended that sample panels be constructed

prior to beginning actual placement on the project in

order to confirm that the proposed procedures are adequate,

and that the uniformity of color is acceptable. Any sample

panel should be larger than 100 sq ft (9 m2) and should be

prepared using the concrete mixture and finishing and curing

techniques planned for the project.42

Coloring agents are normally included with an embedded

hardener when a hardener is applied and color is desired.

Finishing procedures should follow the steps described in

Section 8.6.2.

8.13.2 Nonslip monolithic surface treatment—Before being

applied to the surface, the slip-resistant material (Section

5.4.8) should be mixed with dry portland cement if not already

so formulated. Proportions usually range from 1:1 to

1:2; but the manufacturer’s directions should be followed.

The nonslip monolithic surface treatment procedure is the

same as that outlined for the colored treatment (Section

8.6.2). A swirl finish produced using natural or colored embedded

mineral or metallic hardeners provides increased

wear resistance and also produces a long-lasting, nonslip finish

(Section 8.13.1).

8.13.3 Exposed aggregate surface treatment—Exposed

aggregate surfaces are commonly used to create decorative

effects. Both the selection of the aggregates and the techniques

employed for exposing them are important to the effect

obtained; test panels should be made before the job is

started. Colorful, uniform-sized gravel or crushed aggregate

is recommended.

Aggregates should not be reactive with cement (ACI

201.2R). Aggregates can be tested by using ASTM C 227, or

by petrographic examination (ASTM C 295). If information

or a service record is lacking, the aggregates and the cement

aggregate combinations should be evaluated using the guideline

in the appendix to ASTM C 33.

Flat particles, sliver-shaped particles, and particles smaller

than 1/4 in. (6 mm), do not bond well. As a result, they can

easily become dislodged during the operation of exposing

the aggregate. The use of aggregate ordinarily used in concrete

is not satisfactory unless the aggregate is sufficiently

uniform in size, bright in color, and can be closely packed,

and uniformly distributed.

Immediately after the slab has been screeded, and darbied

or bull floated, the selected aggregate should be broadcast

and evenly distributed so that the entire surface is completely

covered with one layer of the select aggregate. Initial embedding

of the aggregate is usually done by patting with a darby

or the broad side of a short piece of 2- x 4-in. (50- x 100-mm)

lumber. After the aggregate has been thoroughly embedded

and as soon as the concrete will support the weight of a finisher

on kneeboards, the surface should be floated using a

magnesium hand float, darby, or bull float until aggregate is

entirely embedded and slightly covered with mortar. This

operation should leave no holes in the surface.

Shortly after floating, a reliable surface set retarder can be

sprayed over the surface in accordance with the manufacturer’s

recommendations. Retarders may not be necessary on

small jobs, but they are generally used on large jobs to ensure

better control of the exposing operations. Use of a surface set

retarder ordinarily permits several hours to elapse before

brushing and hosing the surface with water exposes the aggregate.

It is important to note that the proper timing for exposing

the aggregate is critical whether or not a retarder has

been used, and this timing is very dependent upon the temperature

and other weather conditions. Recommendations of

the retarder manufacturer should be followed closely.

Operations to expose the aggregate should begin as soon

as the surface can be brushed and washed without overexposing

or dislodging the aggregate. If it becomes necessary

for finishers to move about on the newly exposed surface,

kneeboards should be used, gently brought into contact with

the surface, and neither slid nor twisted on it. If possible,

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-43

however, finishers should stay off the surface entirely because

of the risk of breaking the aggregate bond.

If a smooth surface is desired, as might be the case in an interior

area, no retarder should be used. The aggregate is not exposed

until the surface has hardened. Exposure is accomplished

after hardening entirely by grinding. If grinding is followed by

polishing, a terrazzo-like surface can be produced.

Alternative methods of placement are available. A top

course, 1 in. (25 mm) or more thick, that contains the select

aggregate can be applied, or the monolithic method can be

used. The monolithic method does not use aggregate seeding;

the select aggregate to be exposed is mixed throughout

the concrete during batching.

Tooled joints are not practical in exposed-aggregate concrete

because the aggregate completely covers the surface.

Decorative or working joints are best produced by wet-cut

sawing (Section 8.3.12). Another method of providing joints

is to install permanent strips of wood (redwood, cypress, or

cedar) before placing concrete (Fig 3.2.5.3.a).

Exposed-aggregate slabs should be cured thoroughly.

Care should be taken that the method of curing does not stain

the surface. Straw, earth, and any type of sheet membrane,

such as polyethylene or building paper, can cause discoloration

(Section 9.2.2).

8.13.4 Geometric designs, patterns, and textures—Concrete

surfaces are frequently scored or tooled with a jointer

to produce various decorative patterns. For random geometric

designs, the concrete should be scored after it has been

screeded, bull floated or darbied, and excess moisture has

left the surface. Scoring can be done using a jointer, a

groover, or a piece of pipe bent to resemble an S-shaped

jointer tool. The tool is made of 1/2- or 3/4-in. (15- or 19-mm)

pipe, about 18 in. (450 mm) long. Cobblestone, brick, tile,

and many other patterns can be impressed deeply into partially-

set concrete slabs with special imprinting tools (Reference

42). See PCA’s Finishing Concrete Slabs with Color

and Texture (PA 124M) for patterned concrete.

A swirl-float finish or swirl design can be produced using

a magnesium or wooden hand float, or a steel finishing trowel.

After the concrete surface has received the first powerfloat

pass and subsequent restraightening using a modified

highway straightedge, a float should be worked flat on the

surface in a semicircular or fanlike motion using pressure. A

finer-textured swirl design can be obtained with the same

motion by using a steel finishing trowel held flat. An alternative

method is to draw a soft-bristled broom across the slab

in a wavy motion.

After the concrete has set sufficiently that these surface

textures or patterns will not be marred, the slab should be

moist-cured. Plastic membranes or waterproof curing paper

should not be used on colored concrete (Sections 9.2.1 and

9.2.2).

8.14—Grinding as a repair procedure

Grinding can be used to repair certain surface defects.

Grinding has been used successfully to repair the following

kinds of problems:

1. Unacceptable flatness and levelness

2. Curled joints

3. Surface irregularities that might show through thin

floor coverings, such as resilient tile

4. Poor resistance to wear, when this is due to a weak

floor surface with sound concrete underneath

5. Rain damage

8.14.1 Cautions—Grinding does not always produce the

desired effect, and it sometimes makes the floor look worse.

It can also adversely affect the floor’s resistance to wear, particularly

in industrial applications where the surface is subject

to heavy traffic and abuse. For these reasons, it is usually

wise to make a small trial section before starting full-scale

repairs. Only wet grinding should be used, primarily to minimize

dust, and also because diamond-disk grinders are more

effective when used with water.

8.14.2 Types of grinders—Many types of grinders are

available. The two types most often used on floor slabs are

diamond-disk grinders and stone grinders.

8.14.2.1 Diamond-disk grinder—This grinder uses one or

more diamond-impregnated steel disks. Each disk is mounted

horizontally and is driven by a vertical shaft. The most

common type of diamond-disk grinder has a single 10-in.

(250-mm) grinding disk powered by a gasoline engine or

electric motor of 5 to 10 hp (3.7 to 7.4 kW). Bigger, more

powerful machines are available for floors that need extensive

grinding. Diamond-disk grinders are much faster than

stone grinders, and are usually the better choice to correct

Problems 1 and 2 above.

8.14.2.2 Stone grinder—This grinder uses multiple abrasive

blocks, called stones, mounted on one or more steel

disks. The abrasive material is usually silicon carbide. The

most widely used type of stone grinder has two disks with

three stones on each disk. One-disk and four-disk machines

are also available. Stone grinders can be effective on Problems

3 through 5 above, particularly where the floor surface

is soft, or where the amount of material to be removed is

small.

8.15—Floor flatness and levelness

8.15.1 Floor flatness/levelness tolerances—Tolerances for

various floor uses should conform to the requirements set

forth in ACI 117. A discussion of floor flatness/levelness is

given in the commentary to ACI 117.

ACI 117 specifies that overall conformance to design

grade shall be within 3/4 in. (19 mm) of design elevation. For

suspended cast-in-place concrete slabs, this tolerance is to be

achieved before removal of any supporting shores. For suspended

slabs on metal deck, this tolerance for overall conformance

to design grade does not apply, since tolerances for

erected steel frames are not consistent with those for formwork

in cast-in-place concrete frames.

8.15.1.1 F-number system—It is recommended that both

flatness and levelness requirements be described by Face

Floor Profile Numbers.43 Two separate F-numbers are required

to define the required flatness and levelness of the

constructed floor surface. Refer to the Commentary on ACI

117 for additional discussion of this method.

302.1R-44 ACI COMMITTEE REPORT

SLABS ON GRADE

Composite

flatness (FF)

Composite

levelness (FL) Typical use Typical class

20 15

Noncritical: mechanical rooms, nonpublic

areas, surfaces to have raised computer

flooring, surfaces to have thick-set

tile, and parking structure slabs

1 or 2

25 20

Carpeted areas of commercial office

buildings or lightly-trafficked office/industrial

buildings

2

35 25 Thin-set flooring or warehouse floor with

moderate or heavy traffic

2, 3, 4, 5,

6, 7, or 8

45 35

Warehouse with air-pallet use, ice, or

roller rinks 9

>50 >50 Movie or television studios 3 or 9

SUSPENDED SLABS

Composite flatness

(FF)

Composite

levelness (FL) Typical use Typical class

20 152 or N/A

Noncritical: mechanical rooms, nonpublic

areas, surfaces to have raises computer

flooring, surfaces to have thick-set

tile, and parking structure slabs

1 or 2

25 201 or N/A

Carpeted areas of commercial office

buildings 2

30 202 or N/A Surfaces to receive thin-set flooring 2, 3, or 4

>50 >501,3 Movie or television studios 3 and 9

NOTES:

1. Multidirectional quality of this level requires grinding of joints.

2. Levelness F-number only applies to level slabs shored at time of testing.

3. This levelness quality on a suspended slab requires a two-course placement.

Fig. 8.15.1.1

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-45

The Flatness F-number (FF) controls local surface bumpiness

by limiting the magnitude of successive 1-ft (300-mm)

slope changes when measured along sample measurement

lines in accordance with ASTM E 1155.

The Levelness F-number (FL) controls local conformance

to design grade by limiting differences in departures from

design grade over distances of 10 ft (3 m) when measured

along sample measurement lines in accordance with ASTM

E 1155.

The F-number pair is always written in the order FF/FL. In

theory, the range of flatness and levelness F-numbers extends

from zero to infinity. In practice, FF and FL values generally

fall between 12 and 45. The scale is linear, so the

relative flatness/levelness of two different floors will be in

proportion to the ratio of their F-numbers. For example, an

FF30/FL24 floor is twice as flat and twice as level as an

FF15/FL12 floor.

On random-traffic floors—those with varied and unpredictable

traffic patterns—two tiers of specified FF/FL values

should be indicated: one for the composite values to be

achieved (Specified Overall Value), and one for the minimum

quality level that will be accepted without repair (Minimum

Local Value).

Compliance with the Specified Overall Value is based on

the composite of all measured values. For any given floor,

the composite FF/FL values are derived in accordance with

ASTM E 1155, Section 9.10.

Minimum Local Values represent the minimum acceptable

flatness and levelness to be exhibited by any individual floor

section. Minimum Local Values are generally set at 67 percent

of the Specified Overall Values, and are not normally set lower

than 50 percent of the specified overall FF/FL requirements.

Minimum Local Values should never be less than FF13/FL10,

since these values represent the worst local results to be expected

from any concrete floor construction method.

Remedial measures can be required:

If the composite value of the entire floor installation

(when completed) measures less than either of the specified

overall F-numbers, or

If any individual section measures less than either of the

specified Minimum Local FF/FL numbers. Sectional

boundaries are usually set at the column and half-column

lines on suspended slabs, or at the construction

and contraction joints for slabs on ground. They should

be no closer together than one-half bay.

Remedial measures for slabs on ground might include

grinding, planing, surface repair, retopping, or removal and

replacement. For suspended slabs, remedial measures are

generally limited to grinding or use of an underlayment or

topping material. Contract documents should clearly identify

the acceptable corrective method(s) to be used.

The selection of proper FF/FL tolerances for a project is

best made by measurement of a similar satisfactory floor.

This measurement is then used as the basis for the FF/FL tolerance

specification for the new project. If this method is

used, it is important to note that slab-on-ground floor surfaces

change after construction as a result of shrinkage and curling,

and that the surfaces of suspended slabs change as a

result of deflection. Because of these post-construction

changes, it is likely that measurements of an existing project

will yield results of a lower quality than can be achieved by

the contractor because all of the post-construction changes

just described create slightly diminished FF/FL measurement

results. When measurement of a similar satisfactory floor is

not possible or practical, the flatness/levelness quality levels

provided in Figure 8.15.1.1 have been found to be reasonable

for the stated applications.

8.15.1.2 The 10-ft (3-m) straightedge method—The older

method of using a 10-ft (3-m) straightedge can also be used

to measure floor flatness, but it is much less satisfactory than

the F-number system. There is no nationally accepted method

for taking measurements or for establishing compliance

of a test surface using this tolerance approach. This lack of

an accepted standard test procedure often leads to conflict

and litigation. The straightedge-tolerance method also has a

number of other serious deficiencies. Refer to the Commentary

on ACI 117 for additional discussion.

When straightedge tolerances are specified, the Committee

emphasizes that 100 percent compliance with 10-ft (3-m)

straightedge tolerances is unrealistic. Compliance with four

of five consecutive measurements is more realistic, with a

provision that obvious faults be corrected.

8.15.1.3 Other measurement methods—Measurement

methods are not limited to the F-number (ASTM E 1155) or

the 10-ft (3-m) straightedge systems. Alternative tolerancing

systems that adequately control critical floor surface characteristics

can be used.

8.15.2 Precautions—Floor tolerance specification and

measurement procedures are currently undergoing technological

change. Much remains to be learned about which tolerances

can be reasonably expected from a given

construction method. On those projects where floor flatness/

levelness constitutes a potential issue, the following

precautions are suggested:

•The exact meaning of the flatness/levelness requirement,

and the exact method and time of measurement to

determine compliance, should be established prior to

beginning construction.

•The contractor should confirm an ability to satisfy the

floor tolerance requirement by profiling previous installations.

•Where feasible, test slabs should be installed to verify

the effectiveness of proposed installation procedures

under actual job conditions. If necessary, methods and

procedures should then be modified for the actual job

installation based on these results. The acceptance of

the test slab by the owner as to tolerances and surface

finish should clarify requirements for the project slab.

•The exact remedy to be applied to every possible floor

tolerance deficiency should be confirmed.

8.15.3 Factors influencing floor flatness and levelness—

The flatness and levelness exhibited by a newly installed

concrete slab on ground will depend upon the effectiveness

of the specific placement and finishing procedures employed

during its construction. In general, the forming, placement,

and initial strikeoff phases of the installation will establish

302.1R-46 ACI COMMITTEE REPORT

Table 8.15.3.a— Slabs on grade: flatness

Typical specification

requirements Typical finishing requirements

Specified overall value - 20

Minimum local value - 15

1. Smooth surface using 4 to 5 ft

wide bull float.

2. Wait until bleed water sheen has

disappeared.

3. Float surface with one or more

passes using a power float (float-shoe

blades or pans).

4. Make multiple passes with a power

trowel (trowel blades).

Specified overall value - 25

Minimum local value - 17

1. Smooth and restraighten surface

using 8 to 10 ft wide bull float.

2. Wait until bleed water sheen has

disappeared.

3. Float surface with one or more

passes using a power float (float-shoe

blades or pans).

4. Restraighten surface following

paste-generating float passes using

10 ft wide highway straightedge.

5. Make multiple passes with a power

trowel (trowel blades).

Specified overall value - 35

Minimum local value - 24

1. Smooth and restraighten surface

using 8 to 10 ft wide bull float. Apply

in two directions at 45 deg angle to

strip.

2. Wait until bleed water sheen has

disappeared.

3. Float surface with one or more

passes using a power float (float-shoe

blades or pans).

4. Restraighten surface following

paste-generating float passes using

10 ft wide highway straightedge. Use

in two directions at 45 deg angle to

strip. Use supplementary material to

fill low spots.

5. Make multiple passes with a power

trowel (trowel blades are preferable).

6. Restraighten surface after trowel

passes using multiple passes with

weighted highway straightedge to

scrape the high spots. No filling of

low spots is done at this stage.

Specified overall value - 50

Minimum local value - 35

1. Smooth and restraighten surface

using 8 to 10 ft wide bull float or

highway straightedge. Apply in two

directions at 45 deg angle to strip.

2. Wait until bleed water sheen has

disappeared.

3. Float surface with one or more

passes using a power float (float-shoe

blades or pans). First float pass

should be across width of strip.

4. Restraighten surface following

paste-generating float passes using

10 ft wide highway straightedge. Use

in two directions at 45 deg angle to

strip. Use supplementary material to

fill low spots.

Table 8.15.3.a— Slabs on grade: levelness

Typical specification

requirements

Typical forming and strikeoff

requirements

Specified overall value - 15

Minimum local value - 10

1. Set perimeter forms (optical or

laser instruments).

2. Use block placements of varying

dimensions. Use wet screed strikeoff

techniques to establish initial

grade.

Specified overall value - 20

Minimum local value - 15

1. Set perimeter forms (optical or

laser instruments).

2. Use block placements of varying

dimensions. Use wet screed strikeoff

techniques to establish initial grade.

3. Check grade after strikeoff. Repeat

strikeoff as necessary.

Specified overall value - 25

Minimum local value - 17

1. Set edge forms using optical or

laser instruments. Optical instruments

provide more accurate elevation

control.

2. Use strip placements with maximum

widths of 50 ft. Utilize edge

forms to establish initial grade.

3. Use vibratory screed for initial

strikeoff.

Specified overall value - 30

Minimum local value - 20

1. Set edge forms using optical or

laser instruments. Optical instruments

provide more accurate elevation

control.

2. Use strip placements with maximum

widths of 30 ft. Utilize edge

forms to establish initial grade.

3. Use vibratory screed for initial

strikeoff.

4. Check grade after strikeoff. Repeat

strikeoff as necessary.

5. Use a laser screed in lieu of rigid

strikeoff guides and vibratory screed

to produce this same quality.

Specified overall value - 50

Minimum local value - 35

1. Set edge forms using optical

instrument to ± 1/16 in accuracy.

Use straightedge to identify form

high spots; place top surface to fit

inside 1/16 in envelope.

2. Use strip placements with maximum

widths of 20 ft. Utilize edge

forms to establish initial grade.

3. Use vibratory screed for initial

strikeoff.

4. Check grade after strikeoff. Repeat

strikeoff as necessary.

5. Follow vibratory screed pass with

two or three hand straightedge passes

along the axis of the strip.

6. Use a laser screed in lieu of rigid

strikeoff guides and vibratory screed

to produce this same quality.

Flatness/Levelness (FF/FL) Construction Guide

Notes:

1. These descriptions illustrate typical tolerance levels and construction procedures for floor surfaces in which direction and location of traffic may vary (random-traffic

pattern). Most surfaces must accommodate random-traffic patterns.

2. The use of F-numbers to specify tolerances allows the specifier and contractor independent control of surface waviness and levelness. The Flatness F-number (FF)

controls waviness; the Levelness F-number (FL) controls local levelness.

Flatness quality is primarily a function of finishing technique (tools and procedures used after initial strikeoff). Levelness quality is mainly dependent on accuracy of

formwork and initial strikeoff.

3. The tolerance examples illustrate average to high floor tolerances; specified quality levels should be dictated by facility use.

4. Descriptions of placing and finishing methods are intended to assist the contractor in evaluation and “fine-tuning” of relative costs associated with producing the various

levels of quality in flatness and levelness.

5. Finishing sequences described in this table require a slight modification when a metallic hardener, mineral-aggregate hardener, pigmented hardener, or pigment is to

be applied. Refer to Section 8.6 for detailed discussion of suggested techniques. Proposed techniques for application of hardener and finishing concrete should be

confirmed with a successful panel installation.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-47

Table 8.15.3.b— Suspended slab: flatness

Typical specification

requirements Typical finishing requirements

Level and shored until after

testing:

Specified overall value - 20

Minimum local value -15

Unshored:

Specified overall value - 20

Minimum local value - 15

1. Smooth surface using 4 to 5 ft

wide bull float.

2. Wait until bleed water sheen has

disappeared.

3. Float surface with one or more

passes using a power float (float-shoe

blades or pans).

4. Make multiple passes with a power

trowel (trowel blades).

Level and shored until after

testing:

Specified overall value - 25

Minimum local value -17

Unshored:

Specified overall value - 25

Minimum local value - 17

1. Smooth and restraighten surface

using 8 to 10 ft wide bull float.

2. Wait until bleed water sheen has

disappeared.

3. Float surface with one or more

passes using a power float (float-shoe

blades or pans).

4. Restraighten surface following

paste-generating float passes using

10 ft wide highway straightedge.

5. Make multiple passes with a power

trowel (trowel blades).

Level and shored until after

testing:

Specified overall value - 30

Minimum local value -24

Unshored:

Specified overall value - 30

Minimum local value - 24

1. Smooth and restraighten surface

using 8 to 10 ft wide bull float. Apply

in two directions at 45 deg angle to

strip.

2. Wait until bleed water sheen has

disappeared.

3. Float surface with one or more

passes using a power float (float-shoe

blades or pans).

4. Restraighten surface following

paste-generating float passes using

10 ft wide highway straightedge. Use

in two directions at 45 deg angle to

strip. Use supplementary material to

fill low spots.

5. Make multiple passes with a power

trowel (trowel blades are preferable).

Level and shored until after

testing:

Specified overall value - 50

Minimum local value - 35

Unshored:

Specified overall value - 50

Minimum local value - 35

1. Smooth and restraighten surface

using 8 to 10 ft wide bull float or

highway straightedge. Apply in two

directions at 45 deg angle to strip.

2. Wait until bleed water sheen has

disappeared.

3. Float surface with one or more

(float-shoe blades or pans). First float

pass should be across width of strip.

4. Restraighten surface following

paste-generating float passes using

10 ft wide highway straightedge. Use

in two directions at 45 deg angle to

strip. Use supplementary material to

fill low spots.

5. Make multiple passes with a power

trowel (trowel blades are preferable).

6. Restraighten surface after trowel

passes using multiple passes with

weighted highway straightedge to

scrape the high spots. No filling of

low spots is done at this stage.

Table 8.15.3.b— Suspended slab: levelness

Typical specification

requirements

Typical forming and strikeoff

requirements

Level and shored until after

testing:

Specified overall value - 15

Minimum local value -10

Unshored:

Specified overall value - N/A

Minimum local value - N/A

1. Set perimeter forms (optical or

laser instruments).

2. Use block placements of varying

dimensions. Use wet screed strikeoff

techniques to establish initial grade.

Level and shored until after

testing:

Specified overall value - 20

Minimum local value -15

Unshored:

Specified overall value - N/A

Minimum local value - N/A

1. Set perimeter forms (optical or

laser instruments).

2. Use block placements of varying

dimensions. Use wet screed strikeoff

techniques to establish initial grade.

3. Check grade after strikeoff. Repeat

strikeoff as necessary.

Level and shored until after

testing:

Specified overall value - N/A

Minimum local value -N/A

Unshored:

Specified overall value - 50

Minimum local value - 30

1. Use a two-course placement to

achieve this levelness quality. Topping

slab must be placed using slab

on grade techniques after shoring has

been removed.

2. Set edge forms using optical

instrument to ±1/16 in accuracy. Use

straightedge to identify form high

spots; place top surface to fit inside

1/16 in envelope.

3. Use strip placements with maximum

widths of 20 ft. Utilize edge

forms to establish initial grade.

4. Use vibratory screed for initial

strikeoff.

5. Check grade after strikeoff. Repeat

strikeoff as necessary.

6. Follow vibratory screed pass with

two or three hand straightedge

passes along the axis of the strip.

Metric Equivalents

1/16 in = 1.5 mm

4 ft = 1.2 m

5 ft = 1.5 m

8 ft = 2.4 m

10 ft = 3 m

20 ft = 6.1 m

30 ft = 9.1 m

50 ft = 15.2 m

302.1R-48 ACI COMMITTEE REPORT

the floor’s relative levelness, while subsequent finishing operations

(floating, restraightening, and troweling) will determine

the floor’s relative flatness. Any factor that

complicates placing or finishing operations will have an adverse

effect upon the flatness/levelness produced.

The flatness and levelness F-numbers normally obtained

using a given floor construction procedure are summarized

in Tables 8.15.3.a and 8.15.3.b.

These are the floor-finish tolerances expected to be

achieved by competent, knowledgeable finishers under

“standard” job conditions. Difficult job environments could

result in significantly lower values. Both specifiers and contractors

should approach each new concrete floor project using

the guidelines set forth in Section 8.15.2.

8.15.3.1 Flatness—On those projects where flatness is an

important consideration, precautions should be taken to provide

an adequate construction environment. Of particular

concern for both slabs on ground and suspended slabs are:

•Workability, finishability, and setting times of concrete

to be used

•The “window of finishability,” which should be sufficient

for the contractor to perform the required finishing

operations

•Sun, wind, rain, temperature, other exposure conditions,

and their effects on personnel and concrete

•Light levels

•Timeliness of concrete delivery

•Consistency of delivered slump

•Consistency of final setting time

•Site accessibility

8.15.3.2 Levelness—For slabs on ground, accuracy of

formwork and initial strikeoff establish the overall levelness

of the surface. Form spacing, therefore, is an important consideration

when developing a construction program intended

to produce a certain quality. The use of block-placement

techniques with wet-screed strikeoff provides the least accurate

control of grade. Block placements with moveable rigidscreed

guides provides an improvement in the levelness

quality that can be achieved. Further improvement in levelness

generally requires the use of either strip placements and

vibrating screeds, or self-propelled laser-guided strike-off

equipment. Strip widths up to 50 ft (15.2 m) have provided

levelness quality comparable to that which can be achieved

using moveable dry-screeds in a block placement. Reducing

the width of strips improves the ability of the contractor to

produce level surfaces because there is less tendency for the

vibrating screed to oscillate or deflect, and the controlling

edge form elevations are closer together. The highest quality

of levelness can be achieved using strip widths between 10

and 20 ft (3 and 6 m). This width allows the contractor to follow

the vibrating screed with hand straightedging operations

to remove any imperfections in the surface left by the vibrating

screed.

Levelness of suspended slabs is dependent on accuracy of

formwork and strikeoff, but is further influenced by behavior

of the structural frame during and after completion of construction.

Each type of structural frame behaves somewhat

differently; it is important that the contractor recognize those

differences and plan accordingly. Refer to Chapter 3 for

more detailed discussion of behavior of different types of

structural systems.

The FL levelness tolerance should only be applied to slabs

on ground that are level and to suspended slabs that are both

level and shored at the time data are taken. The FL levelness

tolerance should not apply to slabs placed on unsupported

form surfaces. It should not be applied to cambered or inclined

slab surfaces. Concrete slabs placed over unshored

structural steel and metal deck surfaces can exhibit significant

deflection in the hardened state. The resulting slab surfaces

have occasionally required extensive repair to achieve

a product satisfactory for applied finishes or partitions.

8.15.4 Timeliness of tolerance measurement—To establish

the flooring contractor’s compliance with specified floor tolerances,

the contract documents should stipulate that floor

tolerance compliance tests be performed and defective areas

identified. This should be completed by the owner’s agent as

soon as possible, preferably within 24 hours after placement,

and be reported to key parties as soon as possible, but not later

than 72 hours after installation. For suspended cast-inplace

slabs, tests for acceptance should be conducted before

forms and shoring have been removed. In this way, the effects

of deflection and shrinkage on the tolerance data can be

minimized.

As a practical matter, measurements for suspended-slab

construction should usually be made within a few hours of

slab placement. In vertical construction, the only available

surface for staging materials is often the slab that has just

been placed and finished. Failure to take advantage of this

very short window of availability following completion of

finishing operations will hamper, if not preclude, the tolerance

data collection.

Early measurement also relates directly to the contractor’s

performance. If methods and procedures require modification,

changes can be made early on, minimizing the amount

of unsatisfactory floor surface and repair required. At times,

later measurements will be needed to see whether other influences

have impacted flatness or levelness. For example,

slabs on ground are subject to edge curling in the weeks following

construction; cast-in-place suspended slabs deflect

from their supported position when shores are removed.

These possible later changes are affected by various design

choices as well as the implementation of these choices by the

contractor. For slabs on ground, such design choices include

slab thickness, joint spacing, use of reinforcing steel, and vapor

retarders. Inadequate curing can also accelerate curling

of slabs on ground. For cast-in-place suspended slabs, deflection

can be influenced by a number of variables, including

depth of the structure, quantity of reinforcing steel, formstripping

procedures, and concrete strength when shoring is

removed.

Since curling of slabs on ground will adversely affect flatness/

levelness in service, methods to limit curling (Section

11.11) should be identified in the contract documents. Concrete

with the lowest practical water content and low-shrinkage

characteristics should be required. In addition, base

conditions should not be such that the concrete underside reCONCRETE

FLOOR AND SLAB CONSTRUCTION 302.1R-49

mains wet while the top dries out. Joint spacings, load transfer

device, and reinforcement should be designated to

minimize curling. Proper curing measures are essential, and

should be started as soon as possible after final finishing.

These requirements should be clearly defined in the contract

documents and adhered to during the concreting operations.

8.16—Treatment when bleeding is a problem

Prolonged bleeding can occur with poorly-proportioned

mixtures, poorly-graded aggregates, excessive slump, or under

conditions of low temperature, high humidity, or no air

circulation. Bleed water may not evaporate, and the surface

may not be sufficiently dry for floating and troweling.

One method to remedy the problem is to use fans or blower

heaters of adequate size and in sufficient numbers to evaporate

the excess moisture while the concrete is still plastic.

Avoid using nonvented heaters, particularly those impinging

on the surface of new concrete. They will cause carbonation

of the surface, which can create a soft, dusty, chalky surface

(Section 11.4).

If the concrete is firm enough for floating, but the surface

is still wet, the following methods can be used to obtain a drier

surface:

1. Drag a rubber hose slowly over the entire surface; the

concrete should be stiff enough so that only water is removed.

In limited small areas that are difficult to reach with

a hose, a single pass of a trowel tipped on edge can be used

to remove water; however, slowly dragging a hose is much

less likely to damage the surface, and this method should be

used for the problem whenever possible.

2. Where required after removal of bleed water, apply additional

concrete to fill low spots. This can be accomplished

by discharging a small amount of concrete in a container during

placing operations. The material in the container should

have setting characteristics similar to those of the in-place

concrete.

In general, the bleeding tendencies of concrete can be reduced

significantly by the following actions. Every reasonable

effort should be made to take such measures when

bleeding is a problem:

1. Correct any aggregate gradation deficiency problem

where materials of the required size gradations are economically

available. The use of gap-graded aggregates results in

increased bleeding. Ideally, combined gradation of all aggregates

should yield a percent retained on each sieve below the

largest and above the No. 100 (150 μm) of somewhere between

8 percent and 18 percent. The most common deficiency

is in the 3/8 in., No. 4, No. 8, or No. 16 (9.5 mm, 4.75 mm,

2.36 mm, or 1.18 mm) sieve sizes.

2. Use more cement if paste content is low.

3. Use pozzolan to replace part of cement or as an addition

to the cement. (Note: Pozzolan should be finer than cement,

and if the pozzolan is fly ash, it should conform to the requirements

of ASTM C 618.)

4. Use the maximum allowable amount of entrained air.

The use of air-entrained concrete containing in excess of 3

percent air for hard-troweled surfaces can promote development

of blisters, delamination, and surface peeling.

5. Increase the amount of fine aggregate passing the No.

50, 100, and 200 (300, 150, and 75 μm) sieves to near the

maximum allowable amount. More water (and possibly

more cement) can be needed due to more paste being required;

more shrinkage could result.

6. Use the lowest practicable water content.

7. Avoid admixtures that augment bleeding.

8. Use an accelerating admixture (see Section 5.6.3 for potentially

deleterious effects).

9. Use concrete approaching the highest as-placed temperature

permitted by the contract documents. (Note: Except for

bleeding, there are benefits to be derived from placing concrete

at the lowest permissible temperature.)

10. Use dewatering techniques (Section 8.3.6).

8.17—Delays in cold-weather finishing

Since concrete sets more slowly in cold weather and can be

damaged by freezing, measures should be taken to keep the

concrete temperature above 50 F (10 C). Appropriate curing

procedures (Section 9.5.1) should be provided to prevent

moisture loss and to keep every portion of the slab (including

the edges) above freezing temperature. Any of the concrete’s

tendencies toward bleeding will be considerably aggravated

by the slower setting, and more work will be required to take

care of it properly (Section 8.16). Many extra hours of finishers’

time will be required unless acceptable means can be

found to shorten the setting time. Often, some extra expense

to speed up the operation is justified by whichever of the following

methods are most appropriate and least costly for a

particular situation. Before adopting any method, tests should

be made with job materials at job temperature conditions to

confirm that acceptable results will be obtained.

1. Where it can be used without violating the precautions

of Section 5.6.3, a 1 percent to 2 percent addition of calcium

chloride by weight of cement will accelerate setting significantly.

When used, it should be added as a water solution.

2. Where the use of calcium chloride is prohibited:

a. A change to high-early-strength cement (Type III), or

use of a larger amount of Type I or II cement than usually

required, can provide sufficient acceleration. Use

of these modifications can modestly increase shrinkage

and curling. Increases in cement content above approximately

600 to 625 lb per cu yd (355 to 370 kg/m3) can

cause additional drying shrinkage and cracking in the

hardened concrete.

b. Noncorrosive, nonchloride accelerating admixtures are

available. The dosage rate can be varied to provide the

optimum acceleration.

c. An increase in concrete temperate to 70 F (21 C) will

noticeably reduce the setting time (although a low asplaced

concrete temperature has many benefits). ACI

306R.

d. Early access for floating can be achieved by the application

of dewatering techniques after first strikeoff and

bull floating (Section 8.3.6).

3. The water-cementitious material ratio should be reduced

and the minimum slump selected that can be easily

handled and placed.

302.1R-50 ACI COMMITTEE REPORT

4. Overworking the concrete should be avoided during the

strikeoff and bull-floating operations.

CHAPTER 9—CURING, PROTECTION, AND JOINT

FILLING

9.1—Purpose of curing

After proper placement and finishing of suitable quality

concrete, curing is the single most important factor in

achieving a high quality slab. The primary purpose of curing

is to slow the loss of moisture from the slab. A longer period

of moisture retention permits more complete hydration of the

cement, resulting in greater strength. See ACI 308 for details

as to recommended curing time and minimum recommended

temperatures.

9.2—Methods of curing

Moisture retention can be enhanced by several methods including

moisture addition, moisture retaining covers, and

liquid membrane-forming curing compound. The characteristics

of curing materials are set forth in detail in Section 5.9.

9.2.1 Water curing—Water curing by ponding, sprinkling,

or soaking should only be employed on slab areas without

joints or where the water is positively confined by dams to

prevent flooding the base course or saturating the subbase/

subgrade. This is necessary to limit potential slab curling

due to moisture gradients and to preserve compaction of

the soil support system. Water used for curing should closely

match the concrete temperature at time of application. Fogging

can provide the most satisfactory approach. Care should

be taken to maintain continuous wetting and to prevent isolated

dry spots. Water curing should be used for shrinkagecompensating

concrete slabs.

9.2.2 Wet covering—When properly applied and maintained,

burlap and other wet coverings provide a continuous

supply of moisture uniformly distributed on the slab surface.

Burlap has been the most commonly used wet covering; wet

burlap tends to reduce the temperature of the hydrating concrete

slabs. Moist hay, straw, earth, or sand have been used,

but their use is usually too labor-intensive for large projects

and can discolor the surface. If sand or earth is used, it should

be applied at least 1 in. (25 mm) deep and kept continuously

wet during the curing period. Wet coverings should be laid

over the concrete as soon as finishing operations are complete

and surface marring can be avoided. Exposed concrete

edges should be carefully covered. The coverings should be

kept wet so that a film of moisture remains continuously in

contact with the concrete throughout the curing period. Burlaps

are available that resist rot and fire or that reflect light—

reducing heat absorption from sunlight—or a combination

thereof. Coverings with burlap on one side and polyethylene

on the other are also available; the polyethylene is helpful in

keeping the burlap moist longer, but it makes rewetting more

difficult.

9.2.3 Moisture retaining coverings—Although not usually

as effective as water curing and wet coverings, moisture retaining

coverings are widely used due to their convenience.

9.2.3.1 Polyethylene (plastic) film—Polyethylene film and

other plastic sheet materials are available in clear, white, or

black and are easily handled; the white is especially good for

covering fresh concrete subject to sunlight. These films

avoid leaving a residue that can prevent the bond of new concrete

to hardened concrete or the bond of resilient floor coverings

to concrete. Plastic films are particularly effective for

curing the base slab of two-course floors. However, they can

leave blotchy spots on the slab and should not be used for

colored concrete or where appearance of the slab surface is

important. The sheets should be spread as soon as possible

after finishing operations without marring the surface finish.

Edges of sheets should be lapped a sufficient distance to prevent

moisture loss and sealed with tape, mastic, or glue, or

held in place with wood planks or sand. Construction traffic

should be restricted because the film can be extremely slippery.

9.2.3.2 Waterproof paper—Waterproof paper has the

same advantages and disadvantages as plastic film, except

that discoloration is less likely. It should be light in color; the

edges should be lapped and sealed, and left in place for the

duration of the curing period. Tears caused by construction

traffic should be repaired to maintain proper moisture retention.

9.2.4 Liquid membrane-forming curing compounds—Application

of liquid membrane-forming curing compounds is

the most widely used method for curing concrete. Advantages

are relatively low in-place cost, early access to the floor,

and the elimination of subsequent monitoring of the curing

process. Disadvantages include the potential for insufficient

and uneven coverage, conflict with regulations on the release

of volatile organic compounds, interference with bond of

surfacing materials, and variability of quality and solids content.

Liquid membrane-forming curing compounds should

be applied as soon as finishing operations are complete,

while the surface is still damp, but without free water. Machine

spraying is preferable, but manual spraying is acceptable

if accomplished with sufficient care to ensure uniform

and complete coverage. White-pigmented or fugitive-dye

compounds help assure even coverage and can be considered

to reflect light and heat for floors exposed to sunlight. Generally,

the curing compound should meet or exceed the minimum

moisture retention requirements of ASTM C 309

(Section 5.9.3).

Curing compounds leave a film that can interfere with the

adhesion of other materials to the treated surface; they

should not be used on the base slab of a bonded two-course

floor. Their use should also be avoided on surfaces that will

later be covered with resilient floor coverings, protective

coatings, sealers, or other special treatments. Where applicable,

a letter of compatibility should be issued prior to the use

of a curing compound on a floor receiving a subsequent finish.

Curing compounds can also aggravate tire marking

problems from forklift traffic; special nonmarking tires can

be effective in minimizing these problems.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-51

9.3—Curing at joints

It is important that edges of joints be cured to ensure maximum

concrete strength and further reduce the potential for

curling. Joints are cured adequately when wet coverings or

moisture-retaining coverings are used. If a liquid membraneforming

curing compound is used, it may have to be removed

later if a joint filler is installed. Alternatively, joints can be

temporarily filled with wet sand or compressed backer rod

during the curing period. If sand is used, it should be rewetted

periodically.

9.4—Curing of special concretes

Colored concretes and metallic-hardened floors require

special curing techniques. Refer to Section 8.6 and recommendations

of the materials manufacturer.

9.5—Length of curing

Regardless of the method employed, the curing process

should begin as soon as finishing operations are complete.

If concrete begins to dry excessively prior to completion of

finishing operations, the surface should be protected by

fogging or use of a monomolecular film. The duration of

curing will vary with the method, ambient temperature, humidity,

and type of cement. With any type of cement, in

temperatures above 40 F (5 C) 7 days of uninterrupted curing

is normally recommended for water curing or moistureretaining-

cover curing. This time period can be reduced to

3 days when high-early-strength concrete is used and temperatures

are 73 F (23 C) or higher.

9.5.1 Cold-weather considerations—Slabs should not be

placed on a frozen base. Cold-weather protective measures

should maintain a concrete temperature above 50 F (10 C),

and appropriate curing procedures should be provided to

minimize moisture loss. Insulating blankets placed over the

top of a curing membrane will retain heat, provided there is

no danger of serious loss of heat from below the slab—for

example, an upper floor of an open, unheated building. Particular

care should be given to the corners and edges of the

slab, which are more vulnerable to rapid heat loss. The

amount of insulation required can be calculated from tables

furnished in ACI 306R. When there is a danger of freezing—

particularly when mean daily temperatures are lower

than 40 F (4  C)—insulation is frequently not sufficient to

protect thin slabs of concrete used for floors, and auxiliary

heat is required. The area should be enclosed with tarpaulins

or plastic sheeting and heated with live steam or vented

heaters. The use of salamanders, or other equipment that

exhaust carbon dioxide gases into the area above the concrete

floor, should be avoided because of the danger of carbonation

of the fresh concrete; carbonation will result in a

soft, dusty surface.21

Where freezing is anticipated during or within a few days

following the curing period, consideration should be given

to protection of the concrete. Concrete saturated with water

is vulnerable to freezing damage; the use of thermal blankets

or other protective measures can be necessary. The

curing method and procedure should cure the concrete satisfactorily

and allow appropriate drying of the concrete before

freezing. See Section 4.6 for other concrete placement

conditions and refer to ACI 306R for more information on

cold-weather concreting procedures.

9.5.2 Hot-weather considerations—In hot weather, curing

procedures should begin immediately following or even before

the final finishing operations to prevent surface drying.

Continuous moist-curing methods—water curing and wet

coverings—are the most effective because they provide adequate

moisture and tend to prevent excessive heat build-up.

Moisture-retaining coverings limit evaporation; conditions

creating temperature gradients in the slab should be avoided.

Curing compounds used for exterior work should be whitepigmented.

See Section 4.6 for other concrete placement

conditions, and refer to ACI 305R for more information on

hot-weather concreting procedures.

9.6—Preventing plastic shrinkage cracking

Plastic shrinkage cracking occurs in newly floated or troweled

slabs when the rate of drying at the surface is more rapid

than the upward movement of bleed water. Plastic shrinkage

cracking occurs in the presence of such factors as moderate

to high winds, low relative humidity, and high concrete and

air temperatures. The use of latex-modified concrete, highrange

water reducers, and silica fume tends to increase plastic

shrinkage cracking potential since these materials usually

reduce the bleed rate of concrete.

Vapor retarders/barriers immediately under the concrete

may aggravate plastic and drying-shrinkage cracking and

slab curling because the bottom of the slab loses little or no

moisture while the top dries and shrinks at a faster rate.6,7,8

If the rate of water evaporation from the concrete exceeds

0.2 lb per sq ft per hr (1.0 kg/m2/hr), precautions

should be taken to reduce evaporation (Fig. 2.1.5 of ACI

305R). Measures helpful in preventing or reducing plastic

shrinkage cracking are given in Section 11.2.1; additional

information is presented in ACI 305R.

9.7—Curing after grinding

If grinding is required, it should be initiated as soon as the

floor is hard enough to avoid dislodging aggregate particles.

Curing should be maintained both before and after early

grinding.

9.8—Protection of slab during construction

Protection should be provided against:

a. Heavy construction traffic

b. Hard-wheeled traffic

c. Impact and abrasion

d. Imposed loads (cranes, concrete trucks)

e. Stains (grease, oil, chemicals, paints, plaster, clay soil)

f. Rubber tire marks

g. Deicers

h. Freezing

9.9—Temperature drawdown in cold storage and

freezer rooms

The temperature reduction in freezer and cold storage

rooms should be gradual to control cracking caused by dif302.1R-

52 ACI COMMITTEE REPORT

differential thermal contraction and to allow drying to remove

excess moisture from the slab after curing. A typical

drawdown schedule might be as follows:

Temperature Time

1. Ambient to 35 F (1.7 C) 10 F (5.6 C) Per day (24 hours)

2. Hold at 35 F (1.7 C) —— 2 to 5 days

3. 35 F (1.7 C) to final 10 F (5.6 C) Per day

9.10—Joint filling and sealing

Materials for joint fillers and sealants are discussed in Section

5.12. Construction and contraction joints are normally

formed and sawn, or sawn only. Saw cuts should not be made

at the construction joints until a crack is perceptible where

the adjacent placements abut each other. Isolation joints can

be formed with preformed fiberboard, polyethylene foam, or

similar materials before concrete placement begins. This is

described in Section 3.2.5.1 and detailed in ACI 504R. Backer

rods should not be used in joints that will be exposed to

heavy traffic. Isolation joints are sometimes sealed with

an elastomeric sealant to prevent moisture, dirt, or debris

accumulation.

9.10.1 Time of filling and sealing—Concrete slabs on

ground continue to shrink for years; most shrinkage takes

place within the first year. It is advisable to defer joint filling

and sealing as long as possible to minimize the effects of

shrinkage-related joint opening on the filler or sealant. This

is especially important where semirigid epoxy fillers are

used in traffic-bearing joints; such epoxies have minimal extensibility.

If the joint should be filled before most of the

shrinkage has occurred, separation should be expected between

the joint edge and the joint filler, or within the joint

filler itself. These slight openings can subsequently be filled

with a low-viscosity epoxy adhesive. If construction traffic

dictates that joints be filled early, provisions should be made

to require that the contractor return at a preestablished date

to complete the necessary work using the same manufacturer’s

product. Earlier filling will result in greater separation,

and will lead to the need for more substantial correction; this

separation does not indicate a failure of the filler. For cold

storage and freezer room floors, the joint filler should be installed

only after the room has been held at its planned operating

temperature for 48 hours.

9.10.2 Installation—Elastomeric sealants should be installed

over a backer rod or other bondbreaker as described

in ACI 504R. The use of elastomeric sealants over backer

rods is not recommended in joints exposed to solid-wheel

traffic. Semirigid epoxy and polyurea fillers should be installed

full-depth in saw-cut joints. Joints should be suitably

cleaned to provide optimum contact between the filler or

sealant and bare concrete. Vacuuming is recommended rather

than blowing the joint out with compressed air. Dirt, debris,

saw cuttings, curing compounds, and sealers should be

removed. Cured epoxy fillers should be installed flush with

the floor surface to protect the joint edges and re-create an interruption-

free floor surface.

CHAPTER 10—QUALITY CONTROL CHECKLIST

10.1—Introduction

The Committee recommends that details on a Quality Control

Program be included in the contract documents. To ensure

that the program will be fully complied with for the duration

of the project, procedures should be presented to the involved

parties in the prebid meeting, and reviewed in detail at the preconstruction

meeting. Since the eventual success of any

project is the result of a team effort, there should be a complete

understanding and agreement regarding the provisions of the

Program before any concrete construction is started.

Many items involved with quality control will be covered

in the preconstruction meeting, but some questions or concerns

will invariably come up on site that are not covered in

the bid documents or at the meeting. Therefore, it is essential

to have a person on site who has the experience and background

necessary to use the best possible judgment. Personnel

with ACI certification can contribute greatly toward

resolving these concerns, as well as ensuring quality construction

in the field.

10.2—Partial list of important items to be observed

Additional background information regarding important

items such as concrete reinforcement, surface hardeners, and

joint sealants can be applicable during the actual construction

phase.

10.2.1 Slump control and testing—The addition of water to

the concrete at the jobsite (Section 7.3.2) can be required to

ensure consistent placeability, workability, and finishability;

it is essential that no more water be added than is necessary

to meet the overall project requirements. The Committee

recommends that a specific amount of “trim water”—part of

the design mix water—be withheld at the plant to permit this

onsite adjustment. Two procedures that help ensure adequate

control of slump at the jobsite are: (1) be sure truck mixers

come to the site with full water tanks, and (2) designate one

specific person to authorize adding water at the site.

Testing, including provisions for handling and storing cylinders

or cores, should be completed in accordance with ASTM

procedures. This is particularly significant when air-entrained

concrete is used; the actual air content is subject to change and

requires repeated testing. When entrained air is prohibited, or

less than 3 percent total air is desired, the air content should be

checked on the first truck, and occasionally thereafter.

10.2.2 Avoid delays—Anything that would result in slump

loss should be avoided—delays in delivery of concrete, delays

in placing or finishing operations, and interruptions by

other trades. Although the mixture proportions may have

been approved, some minor adjustments could be required

due to locally available materials or jobsite conditions.

10.2.3 Forms, reinforcement, dowels, and joints—Forms,

reinforcement, and dowels should be secured and remain

straight and true during the entire placing and finishing operation.

Unless otherwise stated in the contract documents,

reinforcement should be discontinued at joints. If the contract

documents indicate that reinforcement should continue

through joints, the engineer should be told that some random

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-53

cracking could result. The alignment of reinforcement along

joints should permit a straight sawcut to be effective and allow

joints to open. Smooth dowels should be used in joints

where load transfer is required. Dowels in contraction joints

should be positively supported and aligned. Any conditions

that create restraint to the normal shrinkage process should

be noted—for example, the condition of the base on which

the concrete is placed. Although the practice of cutting every

other bar or wire has been used with some success, there is

always the possibility of some cracks forming in the intermediate

panels due to partial restraint at the joint (Section

3.2.4).

10.2.4 Finishing—The finishing process should be discussed

with the finishing foreman since no specification can

be sufficiently accurate as to the actual timing of most finishing

operations. Slab edges should be given special attention,

beginning with the initial floating step and continuing

through the entire finishing process.

If an aggregate- or metallic-surface hardener is used, the

hardener should be completely “wetted out” so no dry material

will be floated into the surface prior to machine floating.

10.2.5 Curing, saw cutting, joint filling, and tolerances—

The proposed method of curing, the necessary timing for

sawing joints, the protection of joint edges until the joints are

filled, the timing of joint filling, and the protection required

of the completed floor should be reviewed in detail. There

should be a complete understanding regarding the order in

which the curing, sawing, and floor tolerance testing are to

be performed.

10.2.6 On-site meeting—After initial placement, it is possible

that additional onsite meetings may be necessary to review

actual results and discuss any required adjustments in

the overall plan. Also, backup procedures for equipment

breakdowns should be discussed with the concrete superintendent—

for example, pumps, troweling machines, spreaders,

and saws.

CHAPTER 11—CAUSES OF FLOOR AND SLAB

SURFACE IMPERFECTIONS

11.1—Introduction

Concrete is a forgiving material. But, concrete quality can

be adversely affected by conditions over which the engineer

or the contractor has little control. This chapter lists the conditions

and circumstances that can cause imperfections in

concrete floor and slab surfaces. Concrete is capable of providing

a highly durable, serviceable, and attractive surface.

When it does not do so, there are always reasons. By keeping

the causes of certain imperfections in mind, it is possible to

reduce the likelihood of unsatisfactory results; these causes

will be described briefly in this chapter. When the corrective

action to eliminate a particular cause is not obvious, the most

promising suitable procedure described in preceding chapters

will be referenced.

In reviewing the causes of floor and slab surface imperfections,

the reader should keep in mind the inherent characteristics

of portland cement concrete, such as drying shrinkage

cracking. Some curling and cracking can be expected on every

project. Also, it will be evident that the most common

imperfections stem from failure to obey the basic rules of

concrete finishing given in Section 8.3.3, such as “Any finishing

operation performed while there is excess moisture or

bleed water on the surface will cause dusting or scaling” [and

also cause crazing and reduced resistance to wear]; and as is

stated in Section 8.3.5, “No subsequent operation [after bullfloating

and restraightening] should be done until the concrete

will sustain foot pressure with only about 1/4-in. (6-

mm) indentation” [that is, no premature finishing].

Another common cause of floor and slab surface imperfections

is the lack of prompt curing. The keyword is “prompt,”

and the degree to which this can be accomplished, especially

in dry or windy weather, will improve the quality of floor

and slab surfaces tremendously. Moist curing is best, provided

the slab is kept continuously moist (Section 9.2).

Rarely will there be a single cause for a given imperfection;

usually some combination will be responsible. The influence

of any cause will vary with the degree of its

departure from best practice, with the properties of the materials

used, and with the ambient temperature and other

weather conditions present during the work. Satisfactory results

are more likely to be obtained if the causes mentioned

for the various kinds of imperfections are carefully avoided.

11.2—Cracking

Cracking of concrete (Fig. 11.2) is a frequent complaint.

Cracking is caused by restraint (internal or external) of volume

change, commonly brought about by a combination of

factors such as drying shrinkage, thermal contraction, curling,

settlement of the soil support system, and applied loads.

Cracking can be significantly reduced when the causes are

understood and preventive steps are taken. For example,

joints that are properly designed, detailed, and installed at

the proper spacing and time during construction will cause

cracks to occur in the joints, where they remain inconspicuous,

instead of random locations.

Contractors are not necessarily responsible for all cracks.

Many floor or slab design features and concrete mixture proportions

are responsible for, or contribute to, cracking of

concrete construction. If a contractor believes there are prob-

Fig. 11.2—Drying shrinkage cracks such as these are a frequent

cause of complaint

302.1R-54 ACI COMMITTEE REPORT

lems with slab design, mixture proportions, or other problems,

they should be pointed out prior to installation; the

prebid and preconstruction meetings should be used for this

purpose. Designers should pay careful attention to the causes

of cracking, and contractors need to understand floor and

slab design and concrete mixture proportioning in order to

avoid problems. Designers also should understand slab construction

in order to avoid “building in” problems for the

contractor. For more information on control, causes, evaluation,

and repair of cracks in concrete structures, see ACI

224R and 224.1R.

11.2.1 Restraint—Since cracking is caused by restraint of

volume changes, normal volume changes would be of little

consequence if concrete were free of any restraint. But, since

concrete in service is usually restrained by foundations, subgrade,

reinforcement, or connecting members, significant

stresses can develop—particularly tensile stresses. The

amount of drying shrinkage will be reduced somewhat by taking

practical measures to place the concrete with the lowest

possible water content. Water reduction through use of admixtures—

water-reducing admixtures meeting ASTM C 494,

Types A and D, and air-entraining admixtures—has little effect

on drying shrinkage.1,28,44 Thus, drying shrinkage of concrete

containing water reducers can still result in undesirable

cracking unless the following poor practices are prevented:

1. Contraction joints spaced too far apart (Section 3.2.5.3).

2. Contraction joints not deep enough.

3. Contraction joints not sawn early enough.

4. Slabs strongly restrained at their perimeters by bond of

floor or slab concrete to foundation walls or other construction,

or by tying-in reinforcement to foundations, docks, and

tilt-up walls (Section 3.2.5).

5. Isolation joints not provided around columns (Figs.

3.2.5.1.a and 3.2.5.1.b).

6. Omission of joint or omission of extra reinforcing steel

placed diagonally to reentrant corners.

7. Concrete mixtures of low strength with too little cement,

too much water, or both. Also, mixtures that include

any ingredient, such as aggregates or admixtures, with highshrinkage

characteristics.

8. Deficient curing, or no curing.

9. Slabs restrained by a rutted or uneven base, and changes

in slab thickness.

10. Reinforcement continuous through joints, thus preventing

joints from opening.

11. Slabs cast upon a base which has a high coefficient of

friction, such as an open-graded crushed stone. Such bases

can be “choked-off” with a 1/2-in.-thick (13-mm) layer of

sand or a suitable fine-graded crushed-stone material. This

will provide a smoother surface on which the slab can slide

(Section 4.1.4).

11.2.2 Early cracking—Some cracking can occur before

the concrete has hardened. This can complicate the finishing

operations considerably. Some examples are:

1. Plastic-shrinkage cracking (Sections 9.6 and 11.2.2.1).

Plastic-shrinkage cracks in the still-unhardened concrete can

sometimes be closed by tamping and beating the surface

with a hand float. While this should be done, the more effective

protective measures listed in Section 11.2.2.1 also

should be undertaken immediately to remove the causes of

plastic-shrinkage cracking in the remaining work.

2. Cracking from settlement of concrete around reinforcing

bars or other embedments (Sections 5.8 and 6.2.5).

3. Cracking along edges where forms are not rigid.

4. Early thermal cracking.

5. Damage from form removal.

11.2.2.1 Plastic-shrinkage cracking—Plastic-shrinkage

cracks (Fig. 11.2.2.1) are relatively short, shallow, random

(but sometimes parallel) cracks that can occur before final

finishing on days when wind, low humidity, and high concrete

and ambient temperatures occur. Surface moisture

evaporates faster than it can be replaced by rising bleed water,

causing the surface to shrink more than the interior concrete.

As the interior concrete restrains shrinkage of the

surface concrete, stresses that exceed the concrete’s tensile

strength develop, resulting in surface cracks. These cracks

range from a few inches to a few feet (about 100 mm to 1 m

or more) in length. They can be roughly parallel to one another

and spaced from a few inches to two feet (about 100 to

600 mm) apart, but usually occur in a random, irregular pattern.

Crack formation begins at the surface and continues

downward for some distance, rapidly becoming narrower

Fig. 11.2.2.1—Plastic shrinkage cracks are caused by

rapid loss of mix water from the surface while the concrete

is still plastic

Fig. 11.2.2.2—Crazing is a network of very fine superficial

surface cracks

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-55

with depth. Though usually only a few inches (approximately

25 to 75 mm) deep, they can go completely through the

slab. Following are some helpful measures to prevent or reduce

plastic-shrinkage cracking:

1. Dampen the base when no vapor retarder is used.

2. Erect windbreaks.

3. Erect sunshades.

4. Cool aggregates and mixing water before mixing.

5. Prevent rapid drying by one of the following:

a. Protect concrete with moisture-retaining coverings

(Section 9.2.3) during any delay between placing and

finishing.

b. Cover with damp burlap or with white-polyethylene

sheeting (Section 9.2.2) immediately after screeding

and bull-floating. Keep burlap moist until the concrete

is ready for finishing. Uncover only a small area at one

time, just ahead of the finishers. Begin curing as soon

as possible.

c. Use monomolecular films to reduce evaporation

between the various placing and finishing operations.

d. Use a fog spray located upwind of the freshly-placed

concrete. The spray device should use metered heads

and discharge spray into the air above the concrete.

6. Postpone each step of finishing (and its inherent reworking

of the surface) as long as possible without endangering

results.

7. Avoid the use of a vapor retarder where not needed.

11.2.2.2 Crazing—Crazing, a pattern of fine cracks that do

not penetrate much below the surface, is caused by minor

surface shrinkage (Fig. 11.2.2.2). Crazing cracks are very

fine and barely visible, except when the concrete is drying

after the surface has been wet. They are similar to mud

cracking in shape and in generation. The cracks encompass

small concrete areas less than about 2 in. (50 mm) in dimension,

forming a chicken-wire-like pattern. The term “map

cracking” is often used to refer to cracks that are similar to

crazing cracks only more visible and involving larger areas

of concrete. Although crazing cracks can be unsightly and

can collect dirt, crazing is not structurally serious and does

not necessarily indicate the start of future deterioration in interior

slabs.

When concrete is just beginning to gain strength, climatic

conditions, particularly the relative humidity during the drying

period in a wetting and drying cycle, are an important cause of

crazing. Low humidity, high air and concrete temperatures,

hot sun, or drying wind, either separately or in any combination,

can cause rapid surface drying that encourages crazing.

The conditions that contribute to dusting, as described in Section

11.4, also will increase the tendency to craze.

To prevent crazing, curing procedures should begin immediately—

within minutes after final finishing, particularly after

hard troweling. This is especially important when

weather conditions are adverse. When the temperature is

high, and the sun is shining with high winds and low humidity,

some method of moist curing should be used to stop rapid

drying. The concrete should be protected against rapid

changes in temperature and moisture wherever feasible. Other

conditions to be avoided that can cause craze cracking are:

1. Curing with water that is more than 20 F (11 C) cooler

than the concrete.

2. Alternate wetting and drying of the concrete surface at

early ages.

3. Overuse of jitterbugs, vibrating screeds, and bull floats

(Section 8.3.2).

4. Overworking and overtroweling, especially when the

surface is too wet (Sections 8.3.10 and 8.3.11).

5. Premature floating and troweling (Section 8.3.3).

6. Dusting dry cement onto a surface to hasten drying before

finishing.

7. Too much clay and dirt in aggregates.

8. Sprinkling water onto the surface of a slab during finishing.

11.2.3 Other causes—Cracking can result from causes

other than shrinkage. Prominent causes are:

1. Uneven support by a poorly prepared subgrade, subbase

or base, poor drainage, or uneven support due to curling of

slab edges (Section 11.11).

2. Expansive clay in the subgrade.

3. Sulfates in subgrade soil or groundwater.

4. Placing concrete over preformed joint filler (when placing

adjacent concrete).

5. Improper jointing and sealing (Sections 3.2.5, 5.12,

9.10, and ACI 504R).

6. Structural overloading, especially following the floor

construction phase of a building project.

7. Impact loads.

8. Disruption from expansive alkali-silica reaction.

9. Disruption from corrosion of reinforcing steel.

10. Disruption from freezing and thawing along edges and

at corners.

11. Earth movements from contiguous construction—for

example, blasting or pile driving.

12. Thermal contraction, such as a sharp drop in ambient

temperature shortly after casting a floor or slab.

13. Early or excessive construction traffic.

14. Improper design (for example, selection of an inadequate

safety factor), resulting in a slab of inadequate thickness

for service conditions.

11.3—Low resistance to wear

Low wear resistance is due primarily to low-strength concrete,

particularly at the surface. Such low strengths result

from:

1. Too much mixing water.

2. Use of concrete with too high a water-cementitious material

ratio.

3. Excessive slump, which promotes bleeding and carries

softer, lighter-weight material (laitance) to the surface. After

considering the unavoidable causes for slump loss, use the

lowest practical water content and slump (Table 6.2.1).

4. Overworking overwet concrete. This should not be construed

to mean that it is acceptable to use overwet concrete

under any conditions. Whatever is placed must be worked,

but if concrete is overwet, the ready-mix plant should be

called to make sure no more wet batches are delivered. In addition

to producing a surface with low resistance to wear,

302.1R-56 ACI COMMITTEE REPORT

overworking of overwet concrete also will cause segregation;

fluid mortar will flow into low areas, settle, and leave

low spots.

5. Premature floating and troweling, which works bleed water

into the surface (see Section 8.16 if bleeding is a problem).

6. Excessive use of water by finishers (Section 8.3.3).

7. Excessive entrained air in the surface mortar (although

occurrence of this is not common).

8. Deficient curing (Chapter 9).

9. Surface carbonation from unvented heaters used for

cold-weather protection (ACI 306R).

10. Impairment of surface strength potential by early-age

freezing (ACI 306R).

11. Opening slab to abrasive traffic before sufficient

strength has developed.

12. Poor finishing techniques and improper timing during

and between finishing operations (Section 8.3).

11.4—Dusting

Dusting (Fig. 11.4) is another aspect of weak concrete at

the surface of a floor or slab. Dusting (the development of a

fine, powdery material that easily rubs off the surface of

hardened concrete) can occur either indoors or outdoors, but

is more likely to be a problem when it occurs indoors. Dusting

is the result of a thin, weak surface layer, called laitance,

which is composed of water, cement, and fine particles.

Fresh concrete is a fairly cohesive mass, with the aggregates,

cement, and water uniformly distributed throughout.

A certain amount of time must elapse before the cement and

water react sufficiently to stiffen and develop hardened concrete.

During this period, the cement and aggregate particles

are partly suspended in the water. Because the cement and

aggregates are heavier than water, they tend to sink. As they

move downward, the displaced water and fines move upward

and appear at the surface, resulting in more water and

fines near and at the surface than in the lower portion of the

concrete. This laitance—the weakest, most permeable, and

least wear-resistant material—is at the top surface, exactly

where the strongest, most impermeable, and most wear-resistant

concrete is needed. Floating and troweling concrete

with bleed water on the surface mixes the excess water back

into the surface, further reducing the strength and wear resistance

at the surface, and giving rise to dusting (Section

8.3.3). Dusting can also be caused by:

1. Overly wet mixes with poor finishing characteristics.

2. Insufficient cement (Table 6.2.4).

3. Excessive clay, dirt, and organic materials in the aggregate.

4. Use of dry cement as a blotter to speed up finishing.

5. Water applied to the surface to facilitate finishing.

6. Carbonation of the surface during winter concreting,

caused by unvented heaters (ACI 306R).

7. Inadequate curing, allowing rapid drying of the surface,

especially in hot, dry, and windy weather.

8. Freezing of the surface (ACI 306R).

11.5—Scaling

Scaling is the loss of surface mortar and mortar surrounding

the coarse aggregate particles (Fig. 11.5). The aggregate

is usually clearly exposed and often stands out from the concrete.

Scaling is primarily a physical action caused by hydraulic

pressure from water freezing within the concrete; it is

not usually caused by chemical corrosive action. When pressure

exceeds the tensile strength of concrete, scaling can result

if entrained-air voids are not present in the surface

concrete to act as internal pressure relief valves. The presence

of a deicing solution in water-soaked concrete during

freezing causes an additional buildup of internal pressure.

However, properly designed and placed air-entrained concrete

will withstand deicers for many years.

Deicers such as sodium chloride, urea, and weak solutions

of calcium chloride do not chemically attack concrete; however,

deicers containing ammonium sulfate or ammonium nitrate

will rapidly disintegrate concrete and should not be

used. Several deicers, particularly those containing chloride

ions, can accelerate corrosion of embedded steel. Prominent

among the causes of scaling are:

1. Permeable and poor-quality concrete due to:

a. High water-cementitious material ratio (over 0.50).

Fig. 11.4—Dusting is evident when a fine powdery material

can be easily rubbed off the surface of a slab

Fig. 11.5—Scaling is the loss of surface mortar, usually

exposing the coarse aggregate

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-57

b. Excessive slump for prevailing job conditions (Table

6.2.1).

c. Overworking of wet concrete (Section 8.3.10).

d. Premature finishing operations (Section 8.3.3).

e. Inadequate curing (Chapter 9).

f. Low compressive strength at the surface (Section

6.2.3).

2. In concrete to be exposed to freezing and thawing in service,

little or no entrained air due to

a. Failure to use an air-entraining agent.

b. Air worked out by overworking overwet concrete in

premature finishing operations (Sections 8.3.10 and

8.3.3).

c. Air content too low due to: mixing too long, concrete

temperature too high for a given dosage of air-entraining

agent, or improper dispensing of air-entraining

agent.

3. Air content too low to resist the effect of chemicals used

for snow and ice removal (Section 6.2.3).

4. Inadequate thermal protection, allowing freezing of the

surface at an early age.

5. Exposure of new concrete to freezing and thawing before

it has been adequately cured, achieved a compressive

strength of 4000 psi (28 MPa), and allowed to air dry. Application

of deicing chemicals at this early age greatly increases

the likelihood of scaling.

6. Blistering (Section 11.7), which increases vulnerability

to scaling.

7. Inadequate slope to properly drain water away from the

slab; saturated concrete is more susceptible to damage from

freezing and thawing than drier concrete.

11.5.1 Mortar flaking—Mortar flaking over coarse aggregate

particles (Fig. 11.5.1) is another form of scaling. Aggregate

particles with flat surfaces are more susceptible to this

type of imperfection than round particles. Mortar flaking occasionally

precedes more widespread surface scaling, but its

presence is not necessarily an indication of an onslaught of

more extensive scaling.

Mortar flaking over coarse aggregate particles is caused

essentially by the same actions that cause regular scaling and

often results from placing concrete on hot, windy days. Excessive

and early drying out of the surface mortar can alone

aggravate scaling; however, the moisture loss is accentuated

over aggregate particles near the surface because bleed water

beneath the aggregates cannot readily migrate to the surface

to replenish the evaporated water. This combination of

bleed-water blockage, high rate of evaporation, and lack of

moisture necessary for cement hydration results in a drymortar

layer of low strength, poor durability, high shrinkage,

and poor bond with the aggregate. Upon freezing in a saturated

condition, this thin, weakened mortar layer breaks

away from the aggregate. Poor finishing practices can also

aggravate mortar flaking.

11.6—Popouts

Popouts are roughly cone-shaped pits left in the surface of

flatwork after a small piece of concrete has broken away by

internal pressure (Fig. 11.6). This pressure is generated by

the expansion of a piece of chert, soft fine-grained limestone,

shale, hard-burned lime, hard-burned dolomite, pyrite, or

coal. The first two are natural constituents of some aggregates;

the others sometimes find their way into aggregates as

impurities. In some materials, the expansion is caused by

freezing or absorption of moisture; in others it is caused by a

chemical change. For example, popouts can occur from the

chemical reaction between alkalies in concrete and reactive

siliceous aggregates. Popouts range in size from about 1/4 to

2 in. (6 to 50 mm) or more in diameter.

\Because popouts usually do not significantly diminish the

integrity of concrete flatwork, they are sometimes tolerated.

Nevertheless, they are usually unsightly and interfere with

the performance of any slab required to be smooth. On floors

with hard-wheeled traffic, popouts can degenerate into larger

imperfections. Early repair should minimize further problems

in high-traffic areas.

The occurrence of impurities in the concrete can be beyond

the control of the floor constructor, since it usually occurs

inadvertently in the production and handling of readymixed

concrete or its constituents. The presence of naturally

occurring chert or soft fine-grained limestone, however, can

Fig. 11.5.1—Mortar flaking over coarse aggregate particles

is another form of scaling that resembles a surface with

popouts

Fig. 11.6—A popout is a small fragment of concrete broken

away from the surface of a slab due to internal pressure,

leaving a shallow, typically conical, depression

302.1R-58 ACI COMMITTEE REPORT

be a continuing problem in some locales. Measures that can

be taken to alleviate the problem are:

1. Switching to a nonoffending source of aggregate for

floors and slabs, if possible.

2. Using two-course construction with selected or imported

aggregate without popout potential for the topping course.

3. Using aggregates from which the offending particles

have been removed by heavy-media separation, if available

and economically feasible.

4. Using wet-curing methods such as continuous fogging,

or covering with wet burlap immediately after final finishing.

Wet-cure for a minimum of 7 days, since wet curing can

greatly reduce or eliminate popouts caused by alkali-aggregate

reactivity.44 Avoid plastic film, curing paper, and especially

curing compounds, since they allow an accumulation

of alkalies at the surface. Impervious floor coverings or

membranes, such as wax, epoxy, or other coatings, should be

avoided because they can aggravate popout development.

5. Using the lowest practical slump possible to prevent potential

popout-causing particles from floating to the surface.

In some areas and situations, these measures may not be practical.

Specific local practices have been developed that have

been helpful in minimizing popouts. For example, in some regions

ready mix producers can supply popout-free concrete.

11.7—Blisters

The appearance of blisters (Fig. 11.7) on the surface of a

concrete slab during finishing operations is annoying and an

imperfection not easily repaired once the concrete hardens.

These “bumps” can range in size from 1/4 to 4 in. (6 to 100

mm) in diameter with a depth of about 1/8 in. (3 mm). They

appear when bubbles of entrapped air or water rise through

the plastic concrete and are trapped under an already sealed,

airtight surface. This early closing of the surface frequently

happens when the top of a slab stiffens, dries, or sets faster

than the underlying concrete. Experienced finishers attribute

blistering to the following:

1. An excessive amount of entrapped or entrained air held

within the concrete by excessive fines—material passing the

Nos. 30, 50, and 100 sieves (600, 300, and 150 μm)—resulting

in a sticky mixture that can become more easily sealed during

the closing and troweling operations. Sticky mixtures have a

tendency to crust under drying winds, while the remainder of

Fig. 11.7—Blisters

the concrete stays plastic. Usually, what is needed to relieve

this condition is to reduce the amount of sand in the mix by

100 to 200 lb per cu yd (60 to 120 kg/m3), and to replace the

removed sand with a like amount of the smallest-size coarse

aggregate available. The resulting slightly harsher mix should

release most of the entrapped air using normal vibration. On

days when surface crusting occurs, slightly modified finishing

techniques may be needed, such as the use of wooden floats to

keep the surface open, and flat troweling to avoid enfolding air

into the surface under the blade action.

2. Insufficient vibration during compaction that does not

adequately release entrapped air, or overuse of vibration that

leaves the surface with excessive fines, inviting crusting and

early finishing.

3. Finishing when the concrete is still spongy. Any tool

used to compact or finish the surface will tend to force the

entrapped air toward the surface. Blisters may not appear after

the first finishing pass, but later, as the work progresses

to the second or third troweling. At this stage in finishing, the

trowel blade is tilted to increase surface density; air and water

just under the surface are forced ahead of the blade until

enough is concentrated (usually near a piece of large aggregate)

to form a blister. Blisters, which can be full of air

and/or water when punctured, also can appear at any time

during finishing operations and without apparent cause.

Floating the concrete a second time helps to reduce blistering.

Delayed troweling will depress the blisters even though

it may not reestablish complete bond.

To avoid blisters, the following should be considered:

1. Avoid the use of concrete with excessively high slump,

water content, air content, or fines.

2. Use appropriate cement contents (Table 6.2.4).

3. Warm the base before placing concrete during cool

weather. During hot, dry, windy weather, reduce evaporation

over the slab by using an evaporation retardant (monomolecular

film), a fog spray, or a slab cover (polyethylene film or

wet burlap).

4. Avoid placing a slab directly on polyethylene film or any

other vapor retarder. Use a minimum 4-in.-thick (100-mm)

layer of trimmable, compactible granular fill (not sand) to separate

the vapor retarder from the concrete (Section 4.1.5).

5. Avoid overworking the concrete, especially with vibrating

screeds, jitterbugs, or bull floats. Overworking causes

coarse aggregate to settle, and bleed water and excess fines

to rise to the surface. Properly vibrate concrete to release entrapped

air.

6. Do not use pans for initial machine floating.

7. Do not attempt to seal (finish) the surface too soon.

Hand floating should be started when a worker standing on a

slab makes a 1/4-in. (6-mm) footprint; for machine floating,

the footprint should be only about 1/8 in. (3 mm) deep. If

moisture is deficient, a magnesium float should be used.

8. Use a wooden bull float on non-air-entrained concrete

to avoid early sealing. Magnesium or aluminum tools should

be used on air-entrained concrete. Slabs that incorporate a

surface hardener are more prone to blister if not properly finished

(Sections 8.6.1 and 8.6.2).

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-59

9. Use proper finishing techniques and proper timing during

and between finishing operations (Section 8.3). The formation

of blisters is an immediate indication that the angle of

the trowel is too great for the surface in that area at that particular

time with the concrete and job conditions involved.

The position of the trowel should be flattened, and the blistered

area re-troweled immediately to eliminate and rebond

the blisters. If frequent blistering occurs despite reasonable

care in the timing and technique employed in the finished

troweling, attention should be directed to the job and climatic

conditions, and to the concrete mixture.

Most skilled finishers know when a concrete surface is

ready for the raised and final troweling and closing of the

surface, and how to accomplish this operation. However, circumstances

are often beyond their control. For instance, if

there are too few finishers for the climatic conditions, finishers

may have to close some portions of a floor too early in

order to get it troweled before it has set too much. Similarly,

if supervisors insist that a floor be finished by a certain time,

whether it is ready or not, blisters, trowel marks, and poor

surfaces can result.

11.8—Spalling

Unlike scaling and blistering, spalling is a deeper surface

imperfection, often extending to the top layers of reinforcing

steel or to the horizontal joint between the base and topping

in two-course construction. Spalls can be 6 in. (150 mm) or

more in diameter and 1 in. (25 mm) or more in depth, although

smaller spalls also occur. Spalls are caused by pressure

or expansion within the concrete, bond failure in twocourse

construction, impact loads, fire, or weathering. Joint

spalls are often caused by improperly constructed joints.

Spalls can occur over corroding reinforcing steel because the

corrosion products (rust) occupy more volume than the original

steel, and the resultant pressure spalls the concrete.

In addition to its poor appearance, spalling can seriously

impair the strength or serviceability of a floor or slab. Indoor

spalling is more likely to result from improper joint design

or installation or bond failure in two-course floor construction,

but obviously this can happen outdoors as well. Causes

for the various kinds of spalling include:

1. Insufficient depth of cover over reinforcement.

2. Inferior concrete in the cover over reinforcing steel.

Such concrete can fail to protect the steel from disruptive

corrosion because of its high permeability due to:

a. Overworking overwet concrete during finishing (Sections

8.3.10 and 8.3.3).

b. Serious loss of entrained air during such wet-finishing

operations.

c. Problems with excessive bleeding during finishing,

especially in cold weather (Sections 8.16 and 8.17).

d. Inadequate or delayed curing.

e. Severe cracking that permits water and salts to attack

the steel.

f. Loss of bond between concrete and reinforcing steel

bars, caused by placement of concrete on top of excessively

hot steel during hot-weather concreting.

3. Joint edge spalls caused by small-hard-wheeled vehicles

traveling across improperly installed or filled joints

(Sections 3.2.6, 5.12, and 9.10), and spalls on the upper

flange of the female side of keyed-construction joints.

4. Poor bonding of topping to base course in two-course

floors (Sections 8.7.1 and 8.7.2) due to:

a. Inferior quality of surface concrete in the base course.

b. Unremoved contamination in, or poor preparation of,

the surface of the base course.

c. Differences in shrinkage between topping and base

courses.

d. Drying of the bonding grout before the topping concrete

is placed.

e. Excessive pressure developed at joints, where preformed

joint material was topped by continuous concrete.

f. Restraint of movement of deck slabs on supporting

walls and piers due to inadequate provision for such

movement.

11.9—Discoloration

Surface discoloration of concrete flatwork can appear as

gross color changes in large areas of concrete, as spotted or

mottled light or dark blotches on the surface, or as early light

patches of efflorescence. Laboratory studies to determine the

effects of various concrete materials and concreting procedures

show that no single factor is responsible for discoloration.

46 Factors found to influence discoloration are calcium

chloride admixtures, concrete alkalies, hard-troweled surfaces,

inadequate or inappropriate curing, variations in watercementitious

material ratio at the surface, and changes in the

concrete mixture. Like many other surface imperfections,

discoloration is generally a cosmetic nuisance, rather than a

structural or serviceability problem.

Dark areas do not necessarily denote inferior serviceability

unless there is evidence that dry cement has been troweled

into the surface to absorb excess bleed water (Section 8.16).

The following are causes of dark areas:

1. The use of calcium chloride in concrete can discolor the

surface (Fig. 11.9). Calcium chloride accelerates the overall

hydration process, but has a retarding effect on the hydration

of the ferrite compounds in portland cement. The ferrite

Fig. 11.9—Concrete slab discoloration due to the use of calcium

chloride admixture. Concrete in the upper part of the

photo did not contain the admixture

302.1R-60 ACI COMMITTEE REPORT

phases normally become lighter with hydration; however, in

the presence of calcium chloride the retarded, unhydrated

ferrite phases remain dark in color.

2. Low spots where water stands longer before evaporating.

3. Curing with waterproof paper and plastic sheets can

cause a lighter color where the sheet is in contact with the

surface, and a darker color where the sheet is not in contact

with the surface. This type of discoloration is aggravated

when concrete contains calcium chloride.

4. Changes in the water-cementitious material ratio of concrete

mixtures can significantly affect color. Such a change

can result from localized changes in construction practices,

from a batch-to-batch variation in the concrete’s water or cementitious

material content, or from steel troweling. A high

water-cementitious material ratio will usually produce a

light-colored concrete, a low ratio a darker color. Repeated

hard-steel troweling in areas of advanced setting reduces the

water-cementitious material ratio at the surface, darkening

its color.

5. Changes in source or type of cement. Individual brands

and types of cement can differ in color; therefore, changing

brand or type of cement in the middle of a job can noticeably

change the color of concrete.

6. Uneven application of dry-shake materials, such as mineral-

aggregate or metallic hardeners.

7. Changes in the amount, source, and chemistry of a mineral

admixture. The extent of the discoloration will depend

upon the color and the amount of admixture used. Some mineral

admixtures resemble portland cement and have no effects

on concrete color. Silica fume can give concrete a darkgray

tint. Dark-gray fly ashes can also give concrete a darker

color, whereas tan- or beige-colored fly ashes, if used in

large quantities, can produce a tan color in concrete.

Light-colored areas can simply be the result of contrast to

adjacent dark areas; these would not normally impair serviceability.

However, if light-colored areas are caused by local

overworking of excessively wet concrete, the surface will

be weaker and serviceability can be impaired. This can be

caused by high concrete water content or finishing while

there is excess moisture or bleed water on the surface.

Light-colored areas also can be caused by efflorescence (a

crystalline deposit—usually white in color—that occasionally

develops on the surface of concrete slabs after construction

is completed). Moisture present in hardened concrete

dissolves soluble salts. These salts in solution migrate to the

surface by evaporation or hydraulic pressure where the water

evaporates and leaves a deposit of salt at the surface. If the

water, the evaporation, or the salts are not present, efflorescence

will not occur.

11.10—Low spots and poor drainage

Puddles or “bird baths” on an outdoor concrete slab after a

rain, or on a floor after hosing, characterize poor slab or floor

surface drainage or serviceability. Among the primary causes:

1. Inadequate slope. Positive drainage requires a slope of

1/4 in. per ft (20 mm/m) for an exterior slab; for an interior

floor slab, 1/16 in. per ft (5 mm/m) minimum is adequate for

drainage, but 1/8 in. per ft (10 mm/m) is preferred.

2. Inaccuracy in setting grades for forms and screeds.

3. Damage to grade settings of forms and screeds during

construction.

4. Strikeoff operation in which low spots are filled in with

extra-wet concrete. The wetter concrete settles more than the

surrounding areas during the interval between strikeoff and

floating operations.

5. Fresh concrete that is too wet or variably wet. A little

working of such concrete results in areas with excessive

mortar at the surface, which settles more than the surrounding

areas.

6. Failure to frequently check grades, levels, and slopes

with long straightedges (Sections 8.2.5 and 8.3.4), and to

properly build up low spots in areas thus detected.

7. Tooling joint grooves without removing the small

amount of mortar displaced. The ridge of mortar formed in

this way can act as a dam.

8. Failure to check the finished grade following strikeoff

when wet-screeds are used (Section 8.3.2).

9. Poor lighting during placing and finishing.

10.Deflection of suspended slabs between supports after

removal of supporting shores.

11.11—Curling

Curling is the distortion (rising up) of a slab’s corners and

edges due to differences in moisture content or temperature

between the top and bottom of a slab. The top dries out or

cools, and contracts more than the wetter or warmer bottom.

If the curled section of a slab is loaded beyond the flexural

strength of the concrete, cracks will develop parallel to the

joints at which curling occurs.

Slabs also can be dished in the center because the centers

were finished lower than the screeds. This is readily apparent

from straightedging after finishing. There are a number of

ways to reduce slab curling:

1. Equalize moisture content and temperature between the

top and bottom of a slab.

2. Use a concrete mixture with low-shrinkage characteristics,

that is, a stony concrete mixture with large maximum-size

coarse aggregate at the highest quantity consistent with the required

workability. Such mixtures minimize water content.

3. Use a permeable (porous) dry—or almost dry—base.

4. Use shrinkage-compensating concrete.

5. Place a generous amount of reinforcement in the top

third of the slab. One percent reinforcement could be justified

in the direction perpendicular to the slab edge or construction

joint, and for about 10 ft (3 m) in from the slab edge

or construction joint.

6. Use post-tensioning.

Some of the measures that can reduce moisture differentials

between the top and the bottom of a slab are:

1. Cure the slab well, particularly during early ages. Use of

a continuous moist cure or a high-solids curing compound

(Sections 5.9.3 and 9.2.4)—especially during the first few

days—can greatly reduce the rate of water lost from the concrete

and help reduce moisture differentials.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-61

2. After proper curing, further reduce moisture loss from

the top of slabs by using coatings, sealers, and waxes. These

also reduce carbonation, which adds to surface shrinkage.

3. If a vapor retarder is necessary, use a minimum 4-in.-

thick (100-mm) layer of trimable, compactible granular fill

(not sand) between the vapor retarder and concrete slab (Section

4.1.5). Material conforming to ASTM D 448, No. 10,

with plenty of rock fines, has been used successfully. If the

fill is dry—or almost dry—this will permit some moisture

loss from the slab bottom. The fill should be designed so that

it does not retain water.

Measures to reduce the shrinkage potential of a concrete

mixture include:

1. Reduce total water content of concrete by:

a. Maintaining the proper slump (Table 6.2.1).

b. Reducing the as-mixed temperature of the concrete.

c. Avoiding delays in placement that require large quantities

of retempering water.

d. Selecting hard aggregates that are well graded for

good workability at minimum water contents, and contain

a minimum of fines. Aggregates should be generally

rounded or cubical in shape, with a minimum of

flat or elongated particles.

e. Increasing the maximum size of coarse aggregate and

using coarser sand.

f. Reducing the sand content to the lowest level consistent

with adequate workability and mixing water

requirements.

g. Using a high-range water-reducing admixture (superplasticizer)

with good shrinkage-reduction history and

tests.

2. Avoid aggregates known to have high-shrinkage potential,

such as sandstone, slate, hornblende, and some types of

basalt. Hard, rigid aggregates that are difficult to compress

provide more restraint to shrinkage of cement paste in concrete

than softer aggregates. Quartz, granite, feldspar, limestone,

dolomite, and some basalt aggregates generally

produce concretes with low drying shrinkage (ACI 224R).

3. Minimize aggregate gap-grading.

4. Avoid admixtures or concrete constituents that increase

drying shrinkage.33,44 Use of a water-reducing admixture—

or other admixture conforming to ASTM C 494 and intended

for reducing the water demand of concrete—will not necessarily

decrease the drying shrinkage of concrete. Unless concretes

contain very low levels of calcium chloride or

triethanolamine, drying shrinkage generally will be increased.

Chlorides can get into concrete from admixtures,

water, aggregates, or cement.

5. Dewatering techniques (Section 8.3.6) of fresh concrete

slab surfaces can significantly reduce water content, and thus

help reduce slab curling. However, since vacuum mats do

not extend fully to the edges of the forms and screeds, it is

possible for the joints at the forms and screeds to end up

slightly higher than the overall slab surface after vacuum dewatering

is completed. Where wheeled traffic—especially

automated guided vehicles—will be involved, this should be

taken into consideration during screeding, leveling, and bull

floating to the forms and screeds.

Placing concrete at lower temperatures can reduce thermal

contraction from cooling. Curling magnitude can diminish

with age as moisture and temperature equalize throughout

the slab thickness. In addition, creep probably reduces curling

over a period of months.

Concrete strength should be only as high as necessary for

the floor or slab to fulfill its function (Tables 2.1 and 6.2.1).

Excessively high strengths reduce creep and this can accentuate

curling. However, high strength quality concrete slabs

have less cracking due to higher early flexural and tensile

strengths.

11.12—Analysis of surface imperfections

The cause of most surface imperfections can be determined

by petrographic (microscopic) analysis on samples of

the concrete. A petrographic analysis of concrete is performed

in accordance with ASTM C 856.

Samples for the analysis are usually 4-in.-diameter (100-

mm) drilled cores or saw-cut sections. Broken sections can

be used, but cores or saw-cut sections are preferred because

they are less apt to be disturbed. Samples should represent

concrete from both the problem and the nonproblem areas.

The petrographer should be provided with a description and

photographs of the problem, in addition to information on

the concrete mixture proportions, construction practices

used, and environmental conditions. A field review by a petrographer,

engineer, or concrete technologist is also helpful

in analyzing the imperfection.

The petrographic report often includes the probable cause

of the problem, extent of distress, the general quality of the

concrete, and expected durability and performance of the

concrete. Corrective action, if necessary, would be based to

a great extent on the petrographic report.

CHAPTER 12—SELECTED REFERENCES

12.1—Specifications and recommended

references

The documents of the various standards-producing organizations

referred to in this Guide are listed below with their

serial designation:

American Association of State Highway and Transportation

Officials (AASHTO)

M 182 Standard Specification for Burlap Cloth Made from

Jute or Kenaf

T 26 Standard Method of Test for Quality of Water to Be

Used in Concrete

American Concrete Institute (ACI)

116R Cement and Concrete Terminology

117 Standard Specifications for Tolerances for Concrete

Construction and Materials

201.2R Guide to Durable Concrete

211.1 Standard Practice for Selecting Proportions for

Normal, Heavyweight and Mass Concrete

211.2 Standard Practice for Selecting Proportions for

Structural Lightweight Concrete

302.1R-62 ACI COMMITTEE REPORT

211.3 Standard Practice for Selecting Proportions for No-

Slump Concrete

212.3R Chemical Admixtures for Concrete

212.4R Guide for the Use of High-Range Water-Reducing

Admixtures (Superplasticizers) in Concrete

222R Corrosion of Metals in Concrete

223 Standard Practice for the Use of Shrinkage-Compensating

Concrete

224R Control of Cracking in Concrete Structures

224.1R Causes, Evaluation, and Repair of Cracks in Concrete

Structures

224.3R Joints in Concrete Construction

226.1R Ground Granulated Blast-Furnace Slag as a Cementitious

Constituent in Concrete

226.3R Use of Fly Ash in Concrete

301 Specifications for Structural Concrete for Buildings

304R Guide for Measuring, Mixing, Transporting, and

Placing Concrete

305R Hot Weather Concreting

306R Cold Weather Concreting

308 Standard Practice for Curing Concrete

309R Guide for Consolidation of Concrete

311.1R ACI Manual of Concrete Inspection (SP-2)

311.4R Guide for Concrete Inspection

318 Building Code Requirements for Reinforced Concrete

with Commentary

325.9R Recommendations for Construction of Concrete

Pavements and Concrete Bases

330R Guide for Design and Construction of Concrete

Parking Lots

332R Guide to Residential Cast-in-Place Concrete Construction

347R Guide to Formwork for Concrete

360R Design of Slabs on Grade

421.1R Shear Reinforcement for Slab

423.3R Recommendations for Concrete Members Prestressed

with Unbonded Tendons

503R Use of Epoxy Compounds with Concrete

503.2 Standard Specifications for Bonding Plastic Concrete

to Hardened Concrete with a Multi-Component

Epoxy Adhesive

504R Guide to Sealing Joints in Concrete Structures

515.1R A Guide to the Use of Waterproofing, Dampproofing,

Protective and Decorative Barrier Systems for

Concrete

544.1R State-of-the-Art Report on Fiber Reinforced Concrete

544.2R Measurement of Properties of Fiber Reinforced

Concrete

544.3R Guide for Specifying, Proportioning, Mixing, Placing,

and Finishing Steel Fiber Reinforced Concrete

544.4R Design Considerations for Steel Fiber Reinforced

Concrete

C-640 Craftsman Certification

American National Standards Institute (ANSI)

A122.1 Specifications for Vermiculite Concrete Roofs and

Slabs on Grade

American Society of Civil Engineers (ASCE)

ANSI/ASCE 3 Standard for the Structural Design of

Composite Slabs

ANSI/ASCE 9 Standard Practice for Construction and Inspection

of Composite Slabs

American Society of Heating, Refrigerating, and Air-Conditioning

Engineers (ASHRAE)

90.1 Energy Conservation in New Building Design

(Sections 1 through 9)

American Society for Testing and Materials (ASTM)

A 36 Specification for Structural Steel

A 184 Specification for Fabricated Deformed Steel Bar

Mats for Concrete Reinforcement

A 185 Specification for Steel Welded Wire Fabric, Plain,

for Concrete Reinforcement

A 416 Specification for Steel Strand, Uncoated Seven-

Wire for Prestressed Concrete

A 497 Specification for Steel Welded Wire Fabric, Deformed,

for Concrete Reinforcement

A 615 Specification for Deformed and Plain Billet-Steel

Bars for Concrete Reinforcement

A 616 Specification for Rail-Steel Deformed and Plain

Bars for Concrete Reinforcement

A 617 Specification for Axle-Steel Deformed and Plain

Bars for Concrete Reinforcement

A 820 Specification for Steel Fibers for Use in Fiber Reinforced

Concrete

C 33 Specification for Concrete Aggregates

C 94 Specification for Ready-Mixed Concrete

C 109 Test Method for Compressive Strength of Hydraulic

Cement Mortars (Using 2-in. or 50-mm Cube

Specimens)

C 150 Specification for Portland Cement

C 156 Test Method for Water Retention by Concrete Curing

Materials

C 157 Test Method for Length Change of Hardened Hydraulic-

Cement Mortar and Concrete

C 171 Specification for Sheet Materials for Curing Concrete

C 227 Test Method for Potential Alkali Reactivity of Cement-

Aggregate Combinations (Mortar-Bar Method)

C 260 Specification for Air-Entraining Admixtures for

Concrete

C 295 Practice for Petrographic Examination of Aggregates

for Concrete

C 309 Specification for Liquid Membrane-Forming Compounds

for Curing Concrete

C 330 Specification for Lightweight Aggregates for

Structural Concrete

C 387 Specification for Packaged, Dry, Combined Materials

for Mortar and Concrete

C 494 Specification for Chemical Admixtures for Concrete

C 595 Specification for Blended Hydraulic Cements

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-63

C 618 Specification for Fly Ash and Raw or Calcined Natural

Pozzolan for Use as a Mineral Admixture in

Portland Cement Concrete

C 685 Specification for Concrete Made by Volumetric

Batching and Continuous Mixing

C 806 Test Method for Restrained Expansion of Expansive

Cement Mortar

C 845 Specification for Expansive Hydraulic Cement

C 878 Test Method for Restrained Expansion of Shrinkage-

Compensating Concrete

C 979 Specification for Pigments for Integrally Colored

Concrete

C 989 Specification for Ground Granulated Blast-Furnace

Slag for Use in Concrete and Mortars

C 1017 Specification for Chemical Admixtures for Use in

Producing Flowing Concrete

C 1116 Specification for Fiber-Reinforced Concrete and

Shotcret

C 1151 Test Method for Evaluating the Effectiveness of

Materials for Curing Concrete

C 1157 Performance Specification for Blended Hydraulic

Cement

D 448 Classification for Sizes of Aggregate for Road and

Bridge Construction

D 994 Specification for Preformed Expansion Joint Filler

for Concrete (Bituminous Type)

D 1751 Specification for Preformed Expansion Joint Filler

for Concrete Paving and Structural Construction

(Nonextruding and Resilient Bituminous Types)

D 1752 Specification for Preformed Sponge Rubber and

Cork Expansion Joint Fillers for Concrete Paving

and Structural Construction

D 2240 Test Method for Rubber Property—Durometer

Hardness

E 96 Test Method for Water Vapor Transmission of Materials

E 1155 Test Method for Determining Floor Flatness and

Levelness Using the F-Number System

The above publications may be obtained from the following

organizations:

American Association of State Highway and Transportation

Officials

333 West Capitol Street, NW, Suite 225

Washington, D.C 20001

American Concrete Institute

P.O. Box 9094

Farmington Hills, Mich. 48333-9094

American National Standards Institute

1430 Broadway

New York, N.Y. 10018

American Society of Civil Engineers

345 East 47th Street

New York, N.Y. 10017-2398

American Society of Heating, Refrigerating, and Air-

Conditioning Engineers

1791 Tullie Circle, NE

Atlanta, Ga. 30329

American Society for Testing and Materials

100 Barr Harbor Dr.

West Conshohocken, Pa. 19428-2959

12.2—Cited references

1. Ytterberg, R. F., “Shrinkage and Curling of Slabs on Grade,” Concrete

International: Design and Construction, American Concrete Institute, Detroit,

Apr. 1987, pp. 22-31; May 1987, pp. 54-61; and June 1987, pp. 72-81.

2. Campbell, Richard H., et al., “Job Conditions Affect Cracking and

Strength of Concrete In Place,” ACI JOURNAL, Proceedings V. 73, No. 1,

Jan. 1976, pp. 10-13.

3. Significance of Tests and Properties of Concrete and Concrete-Making

Materials , STP 169-C, American Society for Testing and Materials,

Philadelphia, 1994, (Note especially Chapter 19, “Abrasion Resistance,”

pp. 182-191.)

4. Spears, Ralph, and Panarese, William C., Concrete Floors on Ground ,

EB075D, Portland Cement Association, Skokie, 2nd Edition, 1983, revised

1992.

5. Gustaferro, Armand H., “Are Thickness Tolerances for Concrete

Floors on Grade Realistic?,” Concrete Construction, Apr. 1989, pp. 389-

391.

6. Anderson, T., and Roper, H., “Influence of an Impervious Membrane

Beneath Concrete Slabs on Grade,” Symposium, Concrete for Engineering ,

Institute of Engineers, Brisbane, Australia, Aug. 1977, pp. 51-56.

7. Nicholson, Leo P., “How to Minimize Cracking and Increase Strength

of Slabs on Grade,” Concrete Construction, Sept. 1981, pp. 739-742.

8. Turenne, R. G., “The Use of Vapor Barriers under Concrete Slabs on

Ground,” Building Practice Note No. 8, Division of Building Research,

National Research Council of Canada, August 1978, 3 pp.

9. Dakhil, Fadh H.; Cady, Philip D.; and Carrier, Roger E., “Cracking of

Fresh Concrete as Related to Reinforcement,” ACI JOURNAL, Proceedings

V. 72, No. 8, Aug. 1975, pp. 421-428.

10. Manual of Standard Practice , MSP-1-90, 25th Edition, Concrete

Reinforcing Steel Institute, Schaumburg, 1990.

11. “Reinforcing Steel in Slabs-on-Grade,” WRI/CRSI Engineering

Data Report #37, Concrete Reinforcing Steel Institute, Schaumburg, 1991.

12. Design and Construction of Post-Tensioned Slabs on Ground, Post-

Tensioning Institute, Phoenix, 1980.

13. Post-Tensioning Manual, 3rd Edition, Post-Tensioning Institute,

Phoenix, 1981.

14. Schrader, Ernest K., “A Proposed Solution to Cracking by Dowels,”

Concrete Construction, Dec. 1987, pp. 1051-1053.

15. Colley, B. E., and Humphrey, H. A., Aggregate Interlock at Joints in

Concrete Pavements, Development Department Bulletin DX124, Portland

Cement Association, Skokie, 1967.

16. “Specifications for Unbonded Single Strand Tendons,” PCI Journal,

V. 30, No. 2, Post-Tensioning Institute, Phoenix, Mar./Apr. 1993.

17. Tipping, Eldon, “Building Superior Quality Elevated Floors,” Concrete

Construction, Apr. 1992, pp. 285-288.

18. “Panel Heating,” Heating, Ventilating, Air-Conditioning Guide, V.

36, American Society of Heating and Ventilating Engineers, Atlanta, 1955,

pp. 605-644.

19. Ringo, Boyd, “Basics of Subgrade Preparation for Industrial Floors,”

Concrete Construction, Feb. 1958, pp. 137-140.

20. “Precast Leave-in-Place Screed Rails for Floor Construction,” Concrete

Construction, May 1984, pp. 483-486.

21. Kauer, J. A., and Freeman, R. L., “Effects of Carbon Dioxide on

Fresh Concrete,” ACI JOURNAL, Proceedings V. 52, No. 4, Dec. 1955, pp.

447-454.

22. Bimel, Carl, “ASTM Specifications are a Start, But...,” Concrete

International, Dec. 1993, pp. 55.

23. Shilstone, James M., Sr., “Concrete Mixture Optimization,” Concrete

International, June 1990, pp. 33-39.

24. Boone, T. H., et al., “Conductive Flooring for Hospital Operating

Rooms,” Journal of Research , V. 630, No. 2, U.S. Department of Com302.1R-

64 ACI COMMITTEE REPORT

merce, National Bureau of Standards, Washington, D.C., Oct.-Dec. 1958,

pp. 125-140.

25. Metallic Type Conductive and Spark-Resistant Concrete Floor Finish,

Guide Specification No. NFGS-09785, Naval Facilities Engineering

Command, Apr. 1984, 14 pp.

26. Steinour, Harold H., “Concrete Mix Water—How Impure Can It

Be?,” Research Department Bulletin RX119, Portland Cement Association,

Skokie, Sept. 1960.

27. Kosmatka, Steven H., and Panarese, William C., Chapter 3, “Mixing

Water for Concrete,” Design and Control of Concrete Mixtures, EB001T,

13th Edition, Portland Cement Association, Skokie, 1988, Revised 1994,

pp. 26-29.

28. Martin, Ross, and Phelan, William S., “How Do Admixtures Influence

Shrinkage?,” Concrete Construction, July 1995, pp. 611-617.

29. ACI Committee 226, “Silica Fume in Concrete,” ACI Materials

Journal, American Concrete Institute, Detroit, Mar.-Apr. 1987.

30. “The Effect of Various Surface Treatments Using Magnesium and Zinc

Fluosilicate Crystals on Abrasion Resistance of Concrete Surfaces,” Concrete

Laboratory Report No. C-819, U.S. Bureau of Reclamation, Denver.

31. Tatnall, P. C., and Kuitenbrouwer, L., “Steel Fiber Reinforced Concrete

in Industrial Floors,” Concrete International, Dec. 1992, pp. 43-47.

32. Balaguru, P. N., and Shah, S. P., Fiber Reinforced Cement Composites,

McGraw-Hill, New York, 1992, 530 pp.

33. Tremper, Bailey, and Spellman, D. C., “Shrinkage of Concrete—Comparison

of Laboratory and Field Performance,” Highway Research Record,

No. 3, Highway Research Board, Washington, D.C., 1963, pp. 30-61.

34. Kosmatka, Steven H., and Panarese, William C, Chapter 7, “Proportioning

Normal Concrete Mixtures,” Design and Control of Concrete Mixtures,

EB001T, 13th Edition, Portland Cement Association, Skokie, 1988,

Revised 1994, pp. 77-93.

35. Martin, Ross, “Discussion, Proposed Revisions to Specifications for

Structural Concrete for Buildings (ACI 301-72) (Rev. 1981),” ACI JOURNAL,

Proceedings V. 80, No. 6, Nov.-Dec. 1983, p. 548.

36. Wenander, Harold, “Vacuum Dewatering Is Back,” Concrete Construction,

V. 20, No. 2, Feb.1975, pp. 40-42.

37. Wenander, Harold; Danielsson, Jan. L.; and Sendker, F. T., “Floor

Construction by Vacuum Dewatering,” Concrete Construction, V. 20, No.

2, Feb. 1975, pp. 43-46.

38. Malinowski, Roman, and Wenander, Harold, “Factors Determining

Characteristics and Composition of Vacuum-Dewatered Concrete,” ACI

JOURNAL, Proceedings V. 72, No. 3, Mar. 1975, pp. 98-101.

39. “Floor Finishing,” Lightweight Concrete Information Sheet No. 7,

Expanded Shale, Clay and Slate Institute, Bethesda, 1958, 4 pp.

40. Brungraber, Robert J., “New Portable Tester for the Evaluation of the

Slip-Resistance of Walkway Surfaces,” NBS Technical Note No. 953, U.S.

Department of Commerce, National Bureau of Standards, Washington,

D.C., July 1977, 43 pp.

41. Brungraber, Robert J., “Overview of Floor Slip-Resistance Research

with Annotated Bibliography,” NBS Technical Note No. 895, U.S. Department

of Commerce, National Bureau of Standards, Washington, D.C., Jan.

1976, 108 pp.

42. Kosmatka, Steven H., Finishing Concrete Slabs With Color and Texture,

PA124H, Portland Cement Association, Skokie, 1991, 40 pp.

43. Face, Allen, “Floor Flatness and Levelness—The F-Number System,”

Construction Specifier, V. 40, No. 4, Apr. 1987, pp. 24-32.

44. Whiting, D., and Dziedzic, W., “Effects of Conventional and High-

Range Water Reducers on Concrete Properties,” Research and Development

Bulletin RD107T, Portland Cement Association, Skokie, 1992.

45. “Popouts: Causes, Prevention, Repair,” PCA’s Concrete Technology

Today, June 1995.

46. Greening, N. R., and Landgren, R., “Surface Discoloration of Concrete

Flatwork,” Research Department Bulletin RX203, Portland Cement

Association, Skokie, 1966.

12.3—Additional references

Chapter 1

“Concrete Specifications: Read and Write Them Carefully,” Concrete

Technology Today, PL941B, Portland Cement Association, Skokie, Mar.

1994.

Hoff, Philip L., “Industrial Floors—Before You Build,” Concrete Technology

Today, PL863B, Portland Cement Association, Skokie, Sept. 1986.

Chapter 2

Gray, J. E., Report on Skid Resistance of Portland Cement Mortar Surfaces,

Projects 61-34-36-36, National Crushed Stone Association, Washington,

D.C., Mar. 1962, 22 pp.

Chapter 3

“Concrete Myths: Vapor Barriers are Always Required under Slab-on-

Grade Floors,” Concrete Technology Today, PL823B, Portland Cement

Association, Skokie, Sept. 1982.

Garber, George, “Post-Tensioning for Crack-Free Superflat Floors,”

Concrete Construction, May 1983, pp. 396-400.

“Load Transfer Across Joints in Floors,” Concrete Technology Today,

PL814B, Portland Cement Association, Skokie, Dec. 1981.

Nussbaum, Peter J., “Reflections on Reinforcing Steel in Slabs on

Grade,” Concrete Technology Today, PL922B, Portland Cement Association,

Skokie, July 1992.

Packard, Robert G., “Slab Thickness Design for Industrial Concrete

Floors on Grade,” Concrete Information, IS195D, Portland Cement Association,

Skokie, 1976, 16 pp.

“Reinforcing Steel in Slabs on Grade,” Concrete Technology Today,

PL921B, Portland Cement Association, Skokie, Mar. 1992.

Ringo, Boyd, “Effect of Design Variables on Floor Thickness Requirements,”

Concrete Construction, Jan. 1992, pp. 13-14.

Ringo, Boyd C., and Anderson, Robert B., “Choosing Design Methods for

Industrial Floor Slabs,” Concrete Construction, Apr. 1994, pp. 346-352.

Suprenant, Bruce, “Construction of Elevated Concrete Slabs—Understanding

the Effect of Structural Systems,” Concrete Construction, V. 35,

No. 11, Nov. 1990, pp. 910-917.

Tipping, Eldon, and Suprenant, Bruce, “Construction of Elevated Concrete

Slabs—Practice and Procedures,” Concrete Construction , V. 36, No.

1, Jan. 1991, pp. 32-42.

Tipping, Eldon, and Suprenant, Bruce, “Construction of Elevated Concrete

Slabs—Measuring and Evaluating Quality,” Concrete Construction,

V. 36, No. 3, Mar. 1991, pp. 260-268.

Tipping, Eldon, “Tolerance Conflicts and Omissions in Suspended Slab

Construction,” Concrete International: Design & Construction, V. 14, No.

8, Aug. 1992, pp. 33-37.

Tipping, Eldon, “Controlling the Quality of Suspended Slab Construction,”

Concrete International: Design & Construction , V. 14, No. 8, Aug.

1992, pp. 38-40.

Tobin, Robert E., “How to Double the Value of Your Concrete Dollar,”

Concrete Technology Today, PL852B, Portland Cement Association,

Skokie, June 1985.

Chapter 4

Anderson, Robert B., “Soil Information Needed for Slab Design,” Concrete

Construction, Apr. 1992, pp. 289-290.

ACI Committee 332, “Guide for Construction of Concrete Floors on

Grade,” ACI JOURNAL, Proceedings V. 59, No. 10, Oct. 1962, pp. 1377-

1390.

“Concrete Screed Rails Used for Concrete Placed on Metal Decks,”

Concrete Construction, Apr. 1991, pp. 341-342.

Fitzpatrick, Robert, “Designing Durable Industrial Floor Slabs,” Concrete

International, Jan. 1996, pp. 38-39.

Schmidt, Norbert O., and Riggs, Charles O., “Methods for Achieving

and Measuring Soil Compaction,” Concrete Construction, Aug. 1985, pp.

681-689.

“Subgrades and Subbases for Concrete Pavements,” Concrete Information

, IS029P, Portland Cement Association, Skokie, 1991, 24 pp.

Chapter 5

Bimel, Carl, “Trap Rock Aggregates for Floor Construction,” Concrete

Construction, Oct. 1988, pp. 946 and 948.

“Concrete Sealers for Protection of Bridge Structures,” NCHRP Report

No. 244, Transportation Research Board, Washington, D.C., 1981, 138 pp.

“A Contractor’s Guide to Air-Entraining and Chemical Admixtures,”

Concrete Construction, Mar. 1990, pp. 279-286.

“Fly Ash in Concrete,” Concrete Construction, May 1982, pp. 417-427.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-65

“Fly Ash—Its Effect on Concrete Performance,” Concrete Technology

Today, PL833B, Portland Cement Association, Skokie, Sept. 1983.

Goeb, Eugene O., “Do Plastic Fibers Replace Wire Mesh in a Slab on

Grade?,” Concrete Technology Today, PL891B, Portland Cement Association,

Skokie, Apr. 1989.

Gulyas, Robert J., “Dry Shake for Floors,” Concrete Construction, Mar.

1984, pp. 285-289.

Kosmatka, Steven H., “Floor Covering Materials and Moisture in Concrete,”

Concrete Technology Today, PL853B, Portland Cement Association,

Skokie, Sept. 1985.

Moens, J., and Nemegeer, D., “Designing Fiber Reinforced Concrete

Based on Toughness Characteristics,” Concrete International , Nov. 1991,

pp. 38-43.

“Polymeric Fiber Reinforced Concrete,” Concrete Technology Today,

PL893B, Portland Cement Association, Skokie, Nov. 1989.

Robinson, Chuck; Colasanti, Angelo; and Boyd, Gary, “Steel Fibers

Reinforce Auto Assembly Plant Floor,” Concrete International, Apr. 1991,

pp. 30-35.

Rocole, Larry, “Silica-Fume Concrete Proves to Be an Economical

Alternative,” Concrete Construction, June 1993, pp. 441-442.

Symposium on Use of Pozzolanic Materials in Mortars and Concretes,

STP-99, American Society for Testing and Materials, Philadelphia, Aug.

1950, 203 pp.

“Working with Steel Fiber Reinforced Concrete,” Concrete Construction,

Jan. 1985, pp. 5-11.

Zollo, Ronald F., and Hays, Carol D., “Fibers vs. WWF as Non-Structural

Slab Reinforcement,” Concrete International, Nov. 1991, pp. 50-55.

Chapter 6

Kosmatka, Steven H., “ACI 318-89 and Deicer Exposure,” Concrete

Technology Today, PL903B, Portland Cement Association, Skokie, Sept.

1990.

Rose, Jerry G., “Yield of Concrete,” Concrete Construction, Mar. 1986,

pp. 313-316.

Shilstone, James M., “Concrete Strength Loss and Slump Loss in Summer,”

Concrete Construction, May 1982, pp. 429-432.

Chapter 7

“Control of Quality of Ready-Mixed Concrete,” Publication No. 44, 5th

Edition, National Ready Mixed Concrete Association, Silver Spring, Oct.

1962, 51 pp.

Hover, Ken, “Investigating Effects of Concrete Handling on Air Content,”

Concrete Construction, Sept. 1995, pp. 745-750.

Suprenant, Bruce A., “Adjusting Sump in the Field,” Concrete Construction,

Jan. 1994, pp. 38-44.

Chapter 8

Concrete Manual, 8th Edition, U.S. Bureau of Reclamation, Denver,

1975, pp. 457-467.

“Early Sawing to Control Slab Cracking,” Concrete Technology Today,

PL 953B, Portland Cement Association, Skokie, Nov. 1995.

“Finishing $$Ahead with Surface Vibration,” Concrete Technology

Today, PL822B, Portland Cement Association, Skokie, June 1982.

Fling, Russell S., “A Screeding Machine That’s More Than a Strike-

Off,” Concrete Construction, Apr. 1987, pp. 351-353.

Fricks, Terry J., “Misunderstandings and Abuses in Flatwork Specifications,”

Concrete Construction, June 1994, pp. 492-497.

Hester, Weston T., “Superplasticizers in Ready Mixed Concrete (A Practical

Treatment of Everyday Operations),” NRMC Publication No. 158,

National Ready Mixed Concrete Association, Silver Spring, Jan. 1979.

Kosmatka, Steven H., “Repair with Thin-Bonded Overlay,” Concrete

Technology Today, PL 851B, Portland Cement Association, Skokie, Mar.

1985.

Lien, Rob, “Pan Floats Help Make Nestlè’s Floors Sweet,” Concrete

Construction, May 1995, pp. 439-444.

Panarese, William C., Cement Mason’s Guide, PA 122H, 6th Edition,

Portland Cement Association, Skokie, 1995, 20 pp.

Phelan, William C., “Floors that Pass the Test,” Concrete Construction ,

Jan. 1989, pp. 5-11.

Reed, Ray, and Schmidt, Greg, “Long-Strip Concrete Placement,” Concrete

Construction, Jan. 1994, pp. 46-50.

Suprenant, Bruce A., “Finishing Non-Bleeding Concrete,” Concrete

Construction, May 1992, pp. 386-389.

Tipping, Eldon, “Bidding and Building to F-Number Floor Specs,” Concrete

Construction, May 1992, pp. 18-19.

Tipping, Eldon, “Using the F-Number System to Manage Floor Installations

(Part 1 of a 2-Part Series),” Concrete Construction, V. 41, No. 1, Jan.

1996, pp. 28-34.

Ytterberg, C. Fred, “Good Industrial Floors: What It Takes to Get Them

and Why. Part 1: Monolithic Floors; Part 2: Concrete Toppings,” Civil

Engineering, V. 31, No. 2, Feb. 1961, pp. 55-58, and No. 4, Apr. 1961, pp.

60-63.

Chapter 9

Kosmatka, Steven H., and Panarese, William C., Chapter 10, “Curing,”

Chapter 11, “Hot-Weather Concreting,” and Chapter 12, “Cold-Weather

Concreting,” Design and Control of Concrete Mixtures, EB001T, 13th Edition,

Portland Cement Association, Skokie, 1988, Revised 1994, pp. 123-

129, 130-136, and 137-150.

Metzger, Steven N., “Better Industrial Floors through Better Joints,”

Concrete Construction, Aug. 1988, pp. 749-754.

Metzger, Steven N., “Repairing Joints in Industrial Floors,” Concrete

Construction, June 1989, pp. 548-551

“Proper Curing—Preventive Medicine for Concrete,” Concrete Technology

Today, PL 823B, Portland Cement Association, Skokie, Sept. 1982.

Chapter 10

Hays, Raymond C., “Achieving Quality in Concrete Construction,” Concrete

International, Nov. 1995, pp. S-2 - S-3.

Shilstone, James M., “Quality Assurance and Quality Control,” Concrete

Construction, Nov. 1983, pp. 813-816.

Chapter 11

Bimel, Carl, “Concrete Contractors Do Not Create All Cracks,” Concrete

International, Jan. 1993.

Concrete Slab Surface Defects: Causes, Prevention, Repair, IS 177T,

Portland Cement Association, Skokie, 1987.

Concrete Slabs on Grade: Design, Specification, Construction, and

Problem Solving, SCM-25(92), American Concrete Institute, Detroit, 1992,

429 pp.

Kosmatka, Steven, H., “Petrographic Analysis of Concrete,” Concrete

Technology Today, PL 862B, Portland Cement Association, Skokie, July

1986.

“Slab Curling Is Not a Game Played on Ice,” Concrete Technology

Today, PL822B, Portland Cement Association, Skokie, June 1982.

302.1R-66 ACI COMMITTEE REPORT

The report of ACI Committee 302, “Guide for Concrete

Floor and Slab Construction (ACI 302.1R-96)” states in

section 4.1.5 that “if a vapor barrier or retarder is required

due to local conditions, these products should be placed

under a minimum of 4 in. (100 mm) of trimable, compactible,

granular fill (not sand).” ACI Committee 302 on Construction

of Concrete Floors, and Committee 360 on Design of Slabs on

Ground have found examples where this approach may have

contributed to floor covering problems.

Based on the review of the details of problem installations,

it became clear that the fill course above the vapor retarder

can take on water from rain, wet-curing, wet-grinding or cutting,

and cleaning. Unable to drain, the wet or saturated fill

provides an additional source of water that contributes to

moisture-vapor emission rates from the slab well in excess of

the 3 to 5 lb/1000 ft2/24 h (1.46 to 2.44 kg/100 m2/24 h)

recommendation of the floor covering manufacturers.

As a result of these experiences, and the difficulty in adequately

protecting the fill course from water during the construction

process, caution is advised on the use of the

granular fill layer when moisture-sensitive finishes are to be

applied to the slab surface.

The committees believe that when the use of a vapor retarder

or barrier is required, the decision whether to locate the

retarder or barrier in direct contact with the slab or beneath a

layer of granular fill should be made on a case-by-case basis.

Each proposed installation should be independently evaluated

by considering the moisture sensitivity of subsequent

floor finishes, anticipated project conditions and the potential

effects of slab curling and cracking.

The following chart can be used to assist in deciding where to

place the vapor retarder. The anticipated benefits and risks associated

with the specified location of the vapor retarder should be

reviewed with all appropriate parties before construction.

ADDENDUM

GUIDE FOR CONCRETE FLOOR AND SLAB CONSTRUCTION

(302.1R-96)

Vapor Retarder Location

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-67

ADDENDUM

GUIDE FOR CONCRETE FLOOR AND SLAB CONSTRUCTION

(302.1R-96)

Flow Chart for Location of Vapor Retarder/Barrier