302.1R-04 Guide for Concrete Floor and Slab Construction

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302.1R-1

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Guide for Concrete Floor and Slab Construction

ACI 302.1R-04

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 opera-

tions. The timing of concreting operationsespecially finishing, jointing,

and curing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, poor drainage, and

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, heat, or both, is being

released to the surroundings. Moreover, because 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

and reduced in volume.

This guide contains recommendations for controlling random cracking

and edge curling caused by the concrete’s normal volume change. Applica-

tion of present technology permits only a reduction in cracking and curling,

not elimination. Even with the best floor designs and proper construction,

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

every owner should be advised by both the designer and contractor that it

is normal to expect some amount of cracking and curling on every project,

and that such occurrence does not necessarily reflect adversely on either

the adequacy of the floor’s design or the quality of its construction (Ytter-

berg 1987; Campbell et al. 1976).

Refer to the latest edition of ACI 360R for a detailed discussion of

shrinkage and curling in slabs-on-ground. Refer to the latest edition of ACI

224R 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

aspects of construction such as site preparation, concreting materials,

concrete mixture proportions, concreting workmanship, joint construction,

load transfer across joints, form stripping procedures, finishing methods, and

curing. 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 to achieve the floor qualities

required by the project specifications. Adequate supervision and inspection

are required for job operations, particularly those of finishing.

Keywords: admixture; aggregate; concrete; consolidation; contract docu-

ments; curing; curling; deflection; durability; form; fracture; joint; mixture

proportioning; mortar, paste, placing; quality control; slab-on-ground;

slabs; slump test; specification.

CONTENTS

Chapter 1—Introduction, p. 302.1R-2

1.1—Purpose and scope

1.2—Terminology

1.3—Related work of other committees

Reported by ACI Committee 302

Robert B. Anderson C. Rick Felder John P. Munday

Charles M. Ault Edward B. Finkel Joseph P. Neuber, Jr.

Charles M. Ayers Jerome H. Ford Russell E. Neudeck

Kenneth L. Beaudoin Barry E. Foreman Scott E. Niemitalo

Carl Bimel Terry J. Fricks Mark E. Patton

Michael G. Callas Robert J. Gulyas William S. Phelan

Douglas W. Deno Patrick J. Harrison Dennis W. Phillips

Gregory Dobson Eugene D. Hill, Jr. John W. Rohrer

Alphonse E. Engleman Jerry A. Holland Philip A. Smith

Robert A. Epifano Arthur W. McKinney Bruce A. Suprenant

Samuel A. Face, III Steven N. Metzger R. Gregory Taylor

Eldon Tipping

Chair

Dennis Ahal

Secretary

302.1R-2 ACI COMMITTEE REPORT

Chapter 2—Classes of floors, p. 302.1R-5

2.1—Classification of floors

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-17

4.1—Soil-support system preparation

4.2—Suspended slabs

4.3—Bulkheads

4.4—Setting screed guides

4.5—Installation of auxiliary materials

4.6—Concrete placement conditions

Chapter 5—Materials, p. 302.1R-20

5.1—Introduction

5.2—Concrete

5.3—Portland cement

5.4—Aggregates

5.5—Water

5.6—Curing materials

5.7—Admixtures

5.8—Liquid surface treatments

5.9—Reinforcement

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-27

6.1—Concrete properties

6.2—Recommended concrete mixture

6.3—Concrete mixture analysis

Chapter 7—Batching, mixing, and transporting,

p. 302.1R-34

7.1—Batching

7.2—Mixing

7.3—Transporting

Chapter 8—Placing, consolidating, and finishing,

p. 302.1R-35

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

8.5—Finishing Class 4 and 5 floors

8.6—Finishing Class 6 floors and monolithic-surface

treatments for wear resistance

8.7—Finishing Class 7 floors

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

8.9—Finishing Class 9 floors

8.10—Toppings for precast floors

8.11—Finishing 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-59

9.1—Purpose of curing

9.2—Methods of curing

9.3—Curing at joints

9.4—Curing special concrete

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-61

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-62

11.1—Introduction

11.2—Cracking

11.3—Low wear resistance

11.4—Dusting

11.5—Scaling

11.6—Popouts

11.7—Blisters and delamination

11.8—Spalling

11.9—Discoloration

11.10—Low spots and poor drainage

11.11—Curling

11.12—Analysis of surface imperfections

Chapter 12—References, p. 302.1R-71

12.1—Referenced standards and reports

12.2—Cited references

12.3—Other references

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 normalweight and struc-

tural lightweight concrete floors and slabs made with conven-

tional portland and blended cements. Slabs specifically

intended for the containment of liquids are beyond the scope

of this document.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-3

The design of slabs-on-ground should conform to the

recommendations of ACI 360R. Refer to ACI 223 for

procedures 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. Because the eventual

success of a concrete floor installation depends on 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 propor-

tioning of materials, design details, proper construction

methods, and workmanship.

1.1.1 Prebid meeting—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 herein.

Accordingly, contractors and suppliers are urged to make a

thorough review of contract documents before bid preparation.

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

meeting offers bidders an opportunity to ask questions and

clarify their understanding of contract documents before

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 last-minute

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 slab-

on-ground or suspended floor or slab involves the coordinated

efforts of many subcontractors and material suppliers. It is

strongly recommended that the designer require a precon-

struction meeting to be held to establish and to 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 representa-

tives of organizations and material suppliers directly involved

with either the design or construction of floors.

The preconstruction meeting should confirm and docu-

ment 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 following

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, elec-

trical 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 strength, flexural strength, or both,

and finishability (Section 6.2);

b. Minimum cementitious material content, if appli-

cable (Table 6.2);

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

d. Grading and type of fine aggregate;

e. Combined aggregate grading;

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

g. Slump of concrete (Section 6.2.5);

h. Water-cement ratio (w/c) or water-cementitious

material ratio (w/cm); and

i. 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, length of curing, necessary protec-

tion, and time before opening slabs for traffic (ACI 308R);

17. Testing and inspection requirements; and

18. Acceptance criteria and remedial measures to be used,

if required.

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 and ANSI/ASCE 9 [Section 12.1]);

3. Form stripping procedures, if applicable; and

4. Items listed in Section 3.3 that are appropriate to the

structural system(s) used for the project.

1.1.3 Quality assurance—Adequate provisions 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 main-

tained throughout the entire construction process.

The quality of a completed concrete slab depends on the

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

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

302.1R-4 ACI COMMITTEE REPORT

specifier or owner should consider requiring the use of

prequalified concrete contractors, concrete suppliers,

accredited 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 certify concrete flatwork

finishers and concrete inspectors and testing technicians

throughout the United States, Mexico, and Canada.

1.2—Terminology

adjusted mix optimization indicator (MOI-Adj)—

intersection of the coarseness factor value and the adjusted

workability factor on the coarseness factor chart.

adjusted workability factor (W-Adj)—the workability

factor adjusted for cementitious content. For each 94 lb (43 kg)

of total cementitious material above 564 lb/yd

(335 kg/m

increase the workability factor by 2.5%. For each 94 lb (43 kg)

of total cementitious material below 564 lb/yd

(335 kg/m

decrease the workability factor by 2.5%. (Example for a

workability factor of 33% and 600 lb/yd

[356 kg/m

] of

cementitious material: 600 lb/yd

[356 kg/m

] – 564 lb/yd

[335 kg/m

] = 36 lb/yd

[21 kg/m

]; 36 lb [16 kg]/94 lb [43

kg] = 0.38; 0.38 × 2.5% = 0.95%; W-Adj = 33% workability

factor + 0.95% = 33.95%).

coarseness factor—the percentage of combined aggre-

gate that is larger than the 3/8 in. (9.5 mm) sieve, divided by

the percentage of combined aggregate that is larger than the

No. 8 (2.36 mm) sieve, expressed as a percent. (Example:

33% retained on the 3/8 in. [9.5 mm] sieve/45% retained on

the No. 8 [2.36 mm] sieve = 73.3%).

differential set time—the difference in timing of initial

power floating of sequential truck loads of concrete as they

are delivered to the jobsite.

dry shake—metallic or mineral hardener mixed with

cement and applied dry to the surface of concrete during

finishing operations.

floating—a term used to describe smoothing and subse-

quent compaction and consolidation of the unformed

concrete surface.

mix optimization indicator (MOI)—intersection of the

coarseness factor value and the workability factor on the

coarseness factor chart.

pumping—the vertical displacement and rebound of the

soil support system in response to applied wheel loads.

rutting—the creation of troughs in the soil support system

in response to applied wheel loads.

score—the creation of lines or notches in the surface of a

concrete slab.

water slump—the magnitude of slump, measured in

accordance with ASTM C 143, which is directly attributed to

the amount of water in the concrete mixture.

window of finishability—the time period available for

finishing operations after the concrete has been placed,

consolidated, and struck-off, and before final troweling.

workability factor—the percentage of combined aggre-

gate that passes the No. 8 (2.36 mm) sieve.

1.3—Related work of other committees

1.3.1 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 reference 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.

311—Develops guides and procedures for inspection and

testing.

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.

350—Develops and updates code requirements for

concrete in environmental structures.

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.

435—Provides recommendations for deflection control in

concrete slabs.

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 workers.

1.3.2 The American Society of Civil Engineers—ASCE

publishes documents that can be helpful for floor and slab

construction. Two publications that deal with suspended slab

construction are ASCE Standard for the Structural Design of

Composite Slabs (ANSI/ASCE 3) and ASCE Standard Prac-

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-5

tice for Construction and Inspection of Composite Slabs

(ANSI/ASCE 9).

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. Intended use requirements

should be considered when selecting concrete properties

(Section 6.2), 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 and impact resistance should also be

considered. Currently, there are no standard criteria for evalu-

ating 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, finishing, curing, and other

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 final

finishing techniques. If abrasion from grit or other materials

is anticipated, a higher-quality floor surface may be required

for satisfactory service (ASTM 1994). Under these conditions,

a higher-class floor, a special mineral or metallic 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 compressive strength than the base

Table 2.1—Classes of floors on the basis of intended use and the suggested final finish technique

Class Anticipated type of traffic Use Special considerations Final finish

1. Single course Exposed surface—foot traffic Offices, churches, commercial,

institutional, multi-unit 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,

multi-unit 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 tolerance, 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

steel-troweled finish

4. Single course Exposed or covered surface—

foot and light vehicular traffic

Institutional or 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 manufac-

turing, 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-troweling

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 well-graded all-

mineral or all-metallic aggregate.

Minimum thickness 3/4 in. (19 mm).

Mineral or metallic 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 steel-troweled finish

8. Two course As in Classes 4, 5, or 6 Unbonded topping—on new or

old floors where construction

sequence or schedule dictates

Bondbreaker on base slab, minimum

thickness 4 in. (100 mm), abrasion

resistance, curing

As in Classes 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 ware-

houses; television studios, ice

rinks, or gymnasiums. Refer to

ACI 360R for design guidance

Varying concrete quality requirements.

Special application procedures and strict

attention to detail are recommended

when shake-on hardeners are used. F

50 to F

125 (“superflat” floor). Curing

Strictly following techniques

as indicated in Section 8.9

302.1R-6 ACI COMMITTEE REPORT

course. Class 8 floors can also make use of an embedded hard

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

metallic hardener for addition to the surface (Section 5.4.5).

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

when it is preferable to not 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 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 commer-

cial or nonindustrial applications, whereas Class 8 floors are

primarily for industrial applications.

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

is 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

should be coordinated with joint spacing in the base slab.

Additional joints should be considered if the topping slab

thickness mandates a closer spacing than the base slab to

limit uncontrolled cracking and slab curl. Curl or warping

will be more probable due to the effects of drying from the

top surface only.

2.3.2 Bonded topping over base slab—Class 3 (bonded

topping) and Class 7 floors use 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 applications subject to heavy

traffic and impact. The base slabs can either be 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.

The topping can be either a same-day installation (before

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 multiple-

pass, 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 (dry-

shake) mineral aggregate or metallic hardener for addition to

the surface (Section 5.4.5). Bonded concrete toppings should

have a minimum thickness of 3/4 in. (19 mm). Proprietary prod-

ucts should be applied per manufacturers’ recommendations.

Joint spacing in the topping should be coordinated with

construction and contraction joint spacing in the base slab.

Saw-cut contraction joints should penetrate into the base slab

a minimum of 1 in. (25 mm).

If the topping is placed on a base slab before the joints are

cut, joints in the topping should extend into the base slab and

depth should be appropriate for the total thickness of the

combined slab. If the topping is installed on a previously

placed slab where joints have activated, additional joints in

the topping are unnecessary as shrinkage relief cannot occur

between the slab joints in the bonded topping. When topping

slabs are placed on shrinkage-compensating base slabs, the

joints in the base slab can only be reflected in the bonded

topping slab if the bonded topping slab is installed shortly

after the maximum expansion occurs. Maximum expansion

usually occurs within seven to 14 days.

2.4—Class 9 floors

Certain materials-handling facilities (for example, high-

bay, 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 appro-

priate sections of Chapter 8.

Floors exposed to mild acids, sulfates, or other chemicals

require 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 the design of concrete floors as it

relates to their constructibility. 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 for structural design of composite

slabs, and ANSI/ASCE 9 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 Required design elements—The following items

should be specified in the contract documents prepared by

the designer:

• Base and subbase materials, preparation requirements,

and vapor retarder, if required;

• Concrete thickness;

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-7

• Concrete compressive strength, flexural strength, or both;

• Concrete mixture proportion requirements;

• Joint locations and details;

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

• Surface treatment, if required;

• Surface finish;

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

• Concrete curing;

• Joint filling material and installation;

• Special embedments; and

• 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; therefore, 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 the results

from in-place tests of moisture content or density are. 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. For detailed information on soil-

support systems, refer to ACI 360R.

3.2.3 Moisture protection—Proper moisture protection is

essential for any slab-on-ground where the floor will be

covered by moisture-sensitive flooring materials such as

vinyl, linoleum, wood, carpet, rubber, rubber-backed carpet

tile, impermeable floor coatings, adhesives, or where moisture-

sensitive equipment, products, or environments exist, such

as humidity-controlled or refrigerated rooms.

A vapor retarder is a material that is intended to minimize

the transmission of moisture upward through the slab from

sources below. The performance requirements for plastic

vapor retarder materials in contact with soil or granular fill

under concrete slabs are listed in ASTM E 1745. It is gener-

ally recognized that a vapor retarder should have a perma-

nence (water vapor transmission rate) of less than 0.3 perms,

as determined by ASTM E 96.

The selection of a vapor retarder or barrier material should

be made on the basis of protective requirements and the

moisture-related sensitivity of the materials to be applied to

the floor surface. Although conventional polyethylene film

with a thickness of as little as 6 mils (0.15 mm) has been

used, the committee strongly recommends that the material

be in compliance with ASTM E 1745 and that the thickness

be no less than 10 mils (0.25 mm). The increased thickness

offers increased resistance to moisture transmission while

providing greater durability during and after installation.

A number of vapor retarder materials have been incorrectly

referred to and used by designers as vapor barriers. True vapor

barriers are products that have a permanence (water-vapor

transmission rating) of 0.00 perms when tested in accordance

with ASTM E 96. The laps or seams in either a vapor retarder

or barrier should be overlapped 6 in. (150 mm) (ASTM E 1643)

or as instructed by the manufacturer. The joints and penetra-

tions should be sealed with the manufacturer’s recommended

adhesive, pressure-sensitive tape, or both.

The decision whether to locate the vapor 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 (Suprenant and

Malisch 1998b). For moisture-sensitive flooring materials and

environments, placing concrete in direct contact with the vapor

retarder or barrier eliminates the potential for water from

sources such as rain, saw-cutting, curing, cleaning, or

compaction to become trapped within the fill course. Wet or

saturated fill above the vapor retarder can significantly increase

the time required for a slab to dry to levels required by the

manufacturers of floor coverings, adhesives, and coatings.

Placing concrete in direct contact with the vapor retarder

or barrier, however, requires additional consideration if

potential slab-related problems are to be avoided. When

compared with identical concrete cast on a draining base,

concrete placed in direct contact with a vapor retarder or

barrier has been shown to exhibit significantly larger length

change in the first hour after casting, during drying

shrinkage, and when subject to environmental change; there

is also more settlement (Suprenant 1997). Care should be

taken in design detailing to minimize restraint to such move-

ment (Anderson and Roper 1977). Where reinforcing steel is

present, settlement cracking over the steel is more likely

because of the increased settlement resulting from a longer

bleeding period. The potential for a greater measure of slab

curl is also increased.

Concrete that does not lose water to the base does not

stiffen as rapidly as concrete that does lose part of its excess

water to the base. If rapid, surface drying conditions are

present, the surface of concrete placed directly on a vapor

retarder will have a tendency to dry and crust over while the

concrete below the top fraction of an inch remains relatively

less stiff or unhardened. When this occurs, it may be necessary

to begin machine operations on the concrete surface before

the concrete below the top surface is sufficiently set. Under

such conditions, a reduction in surface flatness and some

blistering or delamination can occur as air, water, or both

become trapped below the finish surface.

The committee recommends that each proposed installation

be independently evaluated as to the moisture sensitivity of

subsequent floor finishes, anticipated project conditions, and

the potential effects of slab curling, crusting, and cracking.

The anticipated benefits and risks associated with the speci-

fied location of the vapor retarder should be reviewed with

all appropriate parties before construction. Figure 3.1 can be

used to assist this evaluation process.

3.2.4 Reinforcement for crack-width control—Reinforce-

ment restrains movement resulting from slab shrinkage and

can actually increase the number of random cracks experi-

enced, 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 (Dakhil, Cady, and Carrier 1975; CRSI 1990). This

reinforcement is normally furnished in the form of deformed

steel bars, welded wire reinforcing, steel fibers, or post-

302.1R-8 ACI COMMITTEE REPORT

tensioning tendons. Combinations of various forms of

reinforcement have proved successful.

Normally, the amount of reinforcement used in nonstruc-

tural slabs is too small to have a significant influence on

restraining movement resulting from volume changes. Refer

to Section 3.2.5 for a detailed discussion of the relationship

between joint spacing and amount of reinforcement.

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

Fig. 3.1—Decision flow chart to determine if a vapor retarder/barrier is required and where it is to be placed.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-9

surface or within the upper third of slab thickness, whichever

is closer to the surface (CRSI 2001; Snell 1997). Rein-

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

side edge.

Deformed reinforcing steel or post-tensioning tendons

should be supported and tied together sufficiently to minimize

movement during concrete placing and finishing operations.

Chairs with sand plates or precast-concrete 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 steel or

post-tensioning tendons at the proper elevation while main-

taining minimum concrete cover requirements.

When welded wire reinforcement is used, its larger flexi-

bility dictates that the contractor pay close attention to estab-

lishing and maintaining adequate support of the

reinforcement during the concrete placing operations.

Welded wire reinforcement should not be placed on the

ground and pulled up after placement of the concrete, nor

should the mats be walked in after placing the concrete.

Proper support spacing is necessary to maintain welded wire

reinforcement at the proper elevation; supports should be

close enough that the welded wire reinforcement cannot be

forced out of location by construction foot traffic. Support

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 reduce the possibility of shrinkage and

temperature cracks in an adjacent panel. Where the reinforce-

ment is continued through the joint, cracks are likely to occur

in adjacent panels because of restraint at the joint (WRI/

CRSI 1991). When used in sufficient quantity, reinforce-

ment will hold out-of-joint cracks tightly closed. Some

designers prefer partial discontinuance of the reinforcement

at contraction joints to obtain some load-transfer capacity

without the use of dowel baskets. Refer to 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 mild steel 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. Use of steel fibers through the

contraction joints reduces the width of joint openings and

that increases the likelihood of cracking occurring between

joints. The crack width, however, should remain narrow and,

in most cases, there are nondetectible microcracks providing

sufficient quantities of fibers used for the given slab joint

spacing and thickness, and subgrade conditions and concrete

material shrinkage properties are taken into consideration.

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 with hardening of concrete, however, most

synthetic fibers at typical dosage rates recommended by the

fiber manufacturers will not provide sufficient restraint to

inhibit cracking.

3.2.4.3 Post-tensioning reinforcement The use of

high-strength steel tendons as reinforcement instead of

conventional mild steel temperature and shrinkage reinforce-

ment 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 place-

ment can result in a significant reduction of shrinkage

cracks. Construction loads on the concrete should be mini-

mized until the slabs are fully stressed (PTI 1990; PTI 1996).

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 inade-

quate consolidation of concrete, inadequate concrete cover

over the reinforcement, use of large diameter bars (Dakhil,

Cady, and Carrier 1975), higher temperature of reinforcing

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 and to reduce the magnitude of slab

curling. Generally, if limiting the number of joints by

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 designer before proceeding.

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

isolate the slab from restraint that might be provided by 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. With the designer’s approval,

construction joint and contraction joint details can be inter-

changed. Refer to ACI 360R for a detailed 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 members. Isolation joints should be used at junc-

tions 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

302.1R-10 ACI COMMITTEE REPORT

between the floor and the adjacent member. The joint mate-

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

above it. The joint filler will be objectionably visible where

there are wet conditions, or hygienic or dust-control require-

ments. Two methods of producing a relatively uniform depth

of joint sealant are as follows:

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 Fig. 3.3 and 3.4 for typical

isolation joints around columns. Figure 3.5 shows an isolation

joint at an equipment foundation.

Isolation joints for slabs using shrinkage-compensating

concrete should be dealt with 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 concrete

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

adequate. In areas subjected to hard-wheeled traffic, heavy

loadings, or both, joints with dowels are recommended

(Fig. 3.6). Refer to Section 3.2.7 for a detailed discussion on

dowel joints. Keyed joints are not recommended where load

transfer is required because the two sides of the keyway 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. The following factors are normally considered

when selecting spacing of contraction joints:

• Method of slab design (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; w/cm;

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; and

Fig. 3.2—Appropriate locations for joints.

Fig. 3.3—Typical isolation joints at tube columns.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-11

• Methods and quality of concrete curing.

As previously indicated, establishing slab joint spacing,

thickness, and reinforcement requirements is the responsi-

bility 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.

Curling of the floor surface at joints is a normal consequence

of volume change resulting from differential moisture loss

from concrete slab to the surrounding environment. This

distortion can result in conflict with respect to installation of

some floor coverings in the months after concrete placement.

Current national standards for ceramic tile and wood

flooring, such as gymnasium floors, are two instances that

require the concrete slab surface to comply with stringent

surface tolerances that cannot be met under typical slab

curling behavior. The designer should consider the require-

ments for successful installation of floor coverings contained

in Division 9 of the project specifications when performing

the concrete slab design (ACI 360R).

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 produced acceptable results. Some

random cracking should be expected; a reasonable level

might be random visible cracks to occur in 0 to 3% of the

surface area floor slab panels formed by saw-cutting,

construction joints, or a combination of both. If slab curl is

of greater concern than usual, joint spacing, mixture propor-

tion, and joint details should be carefully analyzed.

Joint spacing in nominally reinforced slabs (approximately

0.2% steel placed within 2 in. [50 mm] of the top of the slab)

can be increased somewhat beyond that recommended for

unreinforced, plain concrete slabs, but the incidence of random

cracking and curling will increase. Reinforcement will not

prevent cracking. If the reinforcement is properly sized and

located, cracks that do occur should remain tightly closed.

Contraction joints can be reduced or eliminated in slabs

reinforced with at least 0.5% 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

top surface. This will typically produce a larger number of

closely spaced fine cracks throughout the slab.

Joints in either direction can be reduced or eliminated by

post-tensioning that introduces 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 recommen-

dations 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 T-

shaped panels should be avoided. Figure 3.7 shows various

types of contraction joints. Floors around loading docks have

a tendency to crack due to their configuration and restraints.

Fig. 3.4—Typical isolation joint at wide flange column.

Fig. 3.5—Typical isolation joint around an equipment

foundation.

Fig. 3.6—Typical doweled construction joint.

302.1R-12 ACI COMMITTEE REPORT

Figure 3.8 shows two methods that can be used to minimize

slab cracking at reentrant corners of loading docks.

Plastic or metal inserts are not recommended for

constructing or forming 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 result in a weakened plane,

below which a crack will form (Fig. 3.7). Further details on

saw cutting of joints are given in Section 8.3.12

3.2.6 Joint filling—Contraction and construction joints in

floor areas subject to the hard wheels of material handling

vehicle traffic should be filled with a semirigid filler to

minimize wear and damage to joint edges. Construction joints

should be saw-cut 1 in. (25 mm) deep before filling. Joints

should be as narrow as possible to minimize damage due to

wheels loads while still being wide enough to be properly filled.

Where wet conditions or hygienic requirements exist, joints

should be sealed with an elastomeric liquid sealant or a

preformed elastomeric device. If there is also industrial vehic-

ular traffic in these areas, consideration should be given to

strengthening the edge of the joint through alternative means.

Refer to Section 5.12 for a discussion of joint materials,

Section 9.10 for installation of joint fillers, and ACI 504R for

joint sealants.

3.2.7 Load-transfer mechanisms—Doweled construction

and contraction joints (Fig. 3.6 and 3.9) are recommended

when load transfer is required, unless a sufficient

post-tensioning force is provided across the joint to transfer

the shear. Dowels force the concrete sections on both sides of

a joint to undergo approximately equal vertical displacements

subjected to a load and help prevent damage to an exposed

edge when the joint is subjected to vehicles with hard-wheels

such as forklifts. Table 3.1 provides recommended dowel

sizes and spacing for round, square, and rectangular dowels.

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 opera-

tion. All dowels should be sawn and not sheared. Properly

aligned, smooth dowels allow the joint to open as concrete

shrinks. Dowel baskets (Fig. 3.9 to 3.11) should be used to

maintain alignment of dowels in contraction joints, and

alignment devices similar to the one shown in Fig. 3.6 should

be used when detailing the doweled construction joints.

Dowels should be placed no closer than 12 in. (300 mm)

from the intersection of any joints.

Diamond-shaped load plates (a square plate turned so that

two corners line up with the joint, Fig. 3.12) can be used to

replace dowels in construction joints (Walker and Holland

1998). The diamond shape allows the slab to move horizontally

without restraint when the slab shrinkage opens the joint

(Fig. 3.13). Table 3.2 provides the recommended size and

Fig. 3.7—Saw-cut contraction joint.

Fig. 3.8—Joint details at loading dock.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-13

spacing of diamond-shaped load plates. This type of load-

transfer device can be placed within 6 in. (150 mm) of an

intersection (Fig. 3.13). Square and rectangular dowels

cushioned on the vertical sides by a compressible material

also permit movement parallel and perpendicular to the joint

(Fig. 3.14). These types of load-transfer devices are useful in

other slab types where the joint should have load-transfer

capability while allowing some differential movement in the

direction of the joint, such as might be necessary in

post-tensioned and shrinkage-compensating concrete slabs

or in slabs with two-directional doweling (Schrader 1987).

In saw-cut contraction joints, aggregate interlock should not

be relied upon for effective load transfer for wheeled traffic

if the expected joint width exceeds 0.035 in. (0.9 mm)

(Colley and Humphrey 1967).

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 significantly increases the probability

of out-of-joint random cracking.

Round, square, and rectangular dowels for slab-on-ground

installation should meet ASTM A 36. The diameter or cross-

sectional area, length, shape, and specific location of dowels

as well as the method of support should be specified by the

designer. Refer to Table 3.1 and Fig. 3.9 to 3.14.

Fig. 3.9—Typical doweled contraction joint.

Fig. 3.10—Dowel basket assembly.

Table 3.1—Dowel size and spacing for round, square, and rectangular

dowels (ACI Committee 325 1956)

Slab depth, in.

(mm)

Dowel dimensions

, in. (mm)

Dowel spacing

center-to-center, in. (mm)

Round Square Rectangular

†

Round Square Rectangular

5 to 6

(125 to 150)

3/4 x 14

(19 x 350)

3/4 x 14

(19 x 350)

3/8 x 2 x 12

(10 x 50 x 300)

12 (300) 14 (350) 19 (475)

7 to 8

(175 to 200)

1 x 16

(25 x 400)

1 x 16

(25 x 400)

1/2 x 2-1/2 x 12

(12 x 60 x 300)

12 (300) 14 (350) 18 (450)

9 to 11

(225 to 275)

1-1/4 x 18

(30 x 450)

1-1/4 x 18

(30 x 450)

3/4 x 2-1/2 x 12

(19 x 60 x 300)

12 (300) 12 (300) 18 (450)

Total dowel length includes allowance made for joint opening and minor errors in positioning dowels.

†

Rectangular plates are typically used in contraction joints.

Notes: Table values based on a maximum joint opening of 0.20 in. (5 mm). Dowels must be carefully aligned and supported during

concrete operations. Misaligned dowels cause cracking.

Table 3.2—Dowel size and spacing for diamond-

shaped load plates

Slab depth, in. (mm)

Diamond load plate

dimensions, in. (mm)

Diamond load plate spacing

center-to-center, in. (mm)

5 to 6 (125 to 150)

1/4 x 4-1/2 x 4-1/2

(6 x 115 x 115)

18 (450)

7 to 8 (175 to 200)

3/8 x 4-1/2 x 4-1/2

(10 x 115 x 115)

18 (450)

9 to 11 (225 to 275)

3/4 x 4-1/2 x 4-1/2

(19 x 115 x 115)

20 (500)

Notes: Table values based on a maximum joint opening of 0.20 in. (5 mm). The

construction tolerances required make it impractical to use diamond-shaped load

plates in contraction joints.

Fig. 3.11—Rectangular load plate basket assembly.

Fig. 3.12—Diamond-shaped load plate at construction joint.

302.1R-14 ACI COMMITTEE REPORT

Keyed joints are not recommended for load transfer in

slabs-on-ground where heavy-wheeled traffic load is antici-

pated, because 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 joint

edges. For long post-tensioned floor strips and floors using

shrinkage-compensating concrete with long joint spacing, 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 to accommodate shrinkage and

creep (PTI 1990; PTI 2000). Where significant slab movement

is expected, steel plating of the joint edges is recommended;

for strengthening the edges (Fig. 3.15 and 3.16).

A doweled joint detail at a jacking gap in a post-tensioned

slab (ASTM 1994; Spears and Panarese 1992) is shown in

Fig. 3.16.

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 specifi-

cally impact the construction of suspended slabs and should

be included in the contract documents prepared by the

designer:

• 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; and

• Tolerances (forms, structural steel, reinforcement, and

concrete).

3.3.2 Suspended slab types—In general, suspended floor

systems fall into four main categories:

1. Cast-in-place suspended floors;

2. Slabs with removable forms;

3. Slabs on metal decking; and

4. 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, Precast/

Prestressed Concrete Institute, and tolerances of ACI 117.

The levelness of suspended slabs depends on the accuracy

of formwork and strikeoff but is further influenced (espe-

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

of the structural frame during and after completion of

construction. Each type of structural frame behaves some-

what differently; it is important for the contractor to recog-

nize these differences and plan accordingly.

Fig. 3.14—Doweled joint detail for movement parallel and

perpendicular to the joint.

Fig. 3.13—Diamond-shaped load plates at slab corner.

Fig. 3.15—Typical armored construction joint detail.

Fig. 3.16—Typical doweled joint detail for post-tensioned slab.

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-15

The presence of camber in some floor members and the

ACI 117 limitation on tolerances in slab thickness dictate

that concrete be placed at 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 designer.

Achieving a level deflected surface can require increasing

the slab thickness more than 3/8 in. (9.5 mm) in local areas.

The committee recommends that concrete placement

procedures and the basis for acceptance of the levelness of a

completed concrete floor surface be established and agreed

upon by key parties before beginning suspended floor

construction (Tipping 1992).

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 deflect when supporting

shores are removed.

Post-tensioned systems are normally used when larger

spans are necessary or when the structural system is relatively

shallow for the spans considered. Post-tensioned systems use

high-strength steel tendons that are tensioned using a

hydraulic jack designed for that purpose. 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 magnitude of deflection in a conventionally mild steel

reinforced floor system 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 designer, an initial

required camber, generally 1/2 in. (13 mm) or more, can be

required. The amount of camber is determined by the designer

based on an assessment of the loading conditions discussed.

Ideally, the cambered floor system will deflect down to a level

position after removal of the supporting shores.

3.3.4 Slabs on carton forms—Slabs on carton forms are a

special application of slabs with removable forms (Tipping

and North 1998). These slabs are necessary when slabs at

ground level should remain independent of soil movement.

Slabs on carton forms are most commonly used when soils at

the building site are expansive clays subject to significant

movement as a result of moisture variation. They provide a

more economical construction solution than conventional

framing systems, which require a crawl space to remove

forms. The cardboard carton forms deteriorate in the months

following construction, eventually leaving the desired void

space below the slab and forcing the slab to span between

supporting foundation elements.

Experience has shown that certain types of wet cardboard

carton forms can fail locally under the weight of concrete

and construction activities, with a resultant loss of part or all

of the desired void space in the vicinity of the form failure.

This failure can be instantaneous but can also occur 30 or

45 min after strikeoff. The latter type of failure, in addition

to reducing desired void space, can result in a loss of local

slab levelness. Forms that have been damaged by rain should

be replaced or allowed to dry thoroughly, with their capacity

verified, before placement of concrete.

3.3.5 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 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

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, unless the frame is preloaded to allow

deflection to take place before strikeoff, and slab thickness is

allowed to vary outside norms dictated by ACI 117. The

presence of camber in some floor members and the ACI 117

limitation on variation in slab thickness generally dictates

that concrete be placed to a uniform thickness over the

supporting steel.

3.3.5.1 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. Construction joints that are

parallel to structural steel beams should be located far enough

away to eliminate their impact on composite behavior.

Questions about the location of construction joints should be

referred to the designer on the project (Ryan 1997).

Unshored composite construction is the more common

method used by designers because it is 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

(Tipping 2002).

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

302.1R-16 ACI COMMITTEE REPORT

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 this type of crack should

be addressed to the satisfaction of key parties before beginning

suspended floor construction.

3.3.5.2 Noncomposite slabs on metal deck—In noncom-

posite construction, the slab and supporting structural steel

work independently to support loads imposed after hard-

ening of the concrete slab.

3.3.6 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.

3.3.7 Reinforcement—For cast-in-place concrete suspended

slabs, reinforcing steel location varies as dictated by the

contract documents. Post-tensioning reinforcement, when

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

by a hydraulic jack after the concrete reaches sufficient

compressive strength. Elongation and subsequent anchoring

of the ends of post-tensioning tendons results in the transfer

of compressive force to the concrete (PTI 1990).

For slabs on metal deck, reinforcement is normally

provided by deformed reinforcing steel, welded wire reinforce-

ment, or a combination thereof.

3.3.8 Construction joints—The designer should provide

criteria for location of construction joints in suspended slabs.

The following is a general discussion of criteria that can

influence these decisions.

3.3.8.1 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.

3.3.8.2 Composite slabs on metal deck—An important

consideration when deciding on the location of 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, therefore, deflects less

than a noncomposite 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 initial camber to offset anticipated noncom-

posite deflection resulting from concrete placement. 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 deciding on the location

of construction joints in composite slabs on metal deck:

1. Construction joints that parallel secondary structural

steel beams should normally be placed near the midspan of

the slab between beams;

2. Construction joints that parallel primary structural steel

beams and cross secondary structural steel beams should be

placed near the primary beam. The primary structural steel

beam should not be included in the initial placement. It is

important to place the construction joint far enough away from

the primary beam 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 construction joint

location allows nearly the full dead load from concrete place-

ment to be applied to secondary beams that are included in the

initial concrete placement. The primary beam should gener-

ally be included in the second placement at the construction

joint. This will allow the primary beam to deflect completely

before concrete at the primary beam hardens; and

3. Construction joints that cross primary structural steel

beams should be placed near a support at one end of the

primary beam. This will allow the beam to deflect

completely before concrete at the beam hardens.

3.3.8.3 Noncomposite slabs on metal deck—The place-

ment of construction joints in noncomposite slabs on metal

deck should follow the same general guidelines discussed for

slabs on removable forms in Section 3.3.7.1.

3.3.8.4 Topping slabs on precast concrete—Construc-

tion joints in topping slabs on precast concrete should be

placed over joints in the supporting precast concrete.

3.3.9 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 is often designed for strength rather

than for flexibility consideration.

Vibration as a result of power floating and power troweling

operations can produce cracking over the structural steel

beams during concrete finishing operations if the metal deck

is flexible. As the concrete cures and shrinks, these cracks

will open wide if not restrained by reinforcing steel, usually

CONCRETE FLOOR AND SLAB CONSTRUCTION 302.1R-17

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-impreg-

nated cardboard. Ducts with waterproof joints are recom-

mended. When metal ducts are used, calcium chloride

should not be used in the concrete. Refer to Section 5.7.3 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

configuration adjacent to the inside of the foundation and

under the edge of the slab. If the L-shape configuration is

used, the installation should extend horizontally under the

slab 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

approximately 2 in. (50 mm) of concrete cover (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 distress during

the service life.

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 or

construction joint, a 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 pres-

sure-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 is complete. The building owner

should be warned to warm the slabs gradually using luke-

warm 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 to 3 in. (25 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 (ASHVE 1955).

Calcium chloride should not be used where copper 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

freezing-and-thawing resistance. Concrete surfaces should

have a slope not less than 1/4 in./ft (20 mm/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 5.7.3). Experience has shown that these

snow-melting piping systems demand high energy consump-

tion 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 1-1/2 in. (38 mm)

of concrete cover on both the 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. If not properly embedded, systems

can accelerate deterioration by increasing seepage of salt-

water 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.