ACI 302.1R-15: Guide to Concrete Floor and Slab Construction

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The quality of a concrete oor or slab is highly dependent on

achieving a hard and durable surface that is at, 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 nishing, jointing, and curing—is critical.

Failure to address this issue can contribute to undesirable char-

acteristics 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 oor 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. More-

over, because the drying and cooling rates at the top and bottom

of the slab are not 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. Application of present technology permits only a reduc-

tion in cracking and curling, not elimination. Even with the best

oor designs and proper construction, it is unrealistic to expect

completely crack- and curl-free oors. 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 an occurrence does not necessarily

reect adversely on either the adequacy of the oor’s design or the

quality of its construction (Ytterberg 1987).

This guide describes how to produce high-quality concrete slabs-

on-ground and suspended oors for various classes of service.

It emphasizes such aspects of construction as site preparation,

concrete materials, concrete mixture proportions, concrete work-

manship, joint construction, load transfer across joints, form strip-

ping procedures, nishing methods, and curing. Flatness/levelness

requirements and measurements are outlined. A thorough precon-

struction 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 oor qual-

ities required by the project specications. Adequate supervision

and inspection are required for job operations, particularly those

of nishing.

Keywords: admixture; aggregate; consolidation; contract documents;

curing; curling; deection; durability; form; fracture; joint; mixture propor-

tioning; placing; quality control; slab-on-ground; slabs; slump test.

CONTENTS

CHAPTER 1—INTRODUCTION, p. 2

1.1—Purpose, p. 2

1.2—Scope, p. 2

CHAPTER 2—DEFINITIONS, p. 2

CHAPTER 5—DESIGN CONSIDERATIONS, p. 2

5.1—Scope, p. 2

5.2—Slabs-on-ground, p. 2

5.3—Suspended slabs, p. 7

5.4—Miscellaneous details, p. 9

Joseph F. Neuber Jr., Chair

Patrick J. Harrison, Vice Chair

Russell E. Neudeck, Secretary

ACI 302.1R-15

Guide to Concrete Floor and Slab Construction

Reported by Committee 302

Dennis C. Ahal

Bryan M. Birdwell

Peter A. Craig

Allen Face

C. Rick Felder

Edward B. Finkel

Barry E. Foreman

Greg K. Fricks

Terry J. Fricks

Jerry A. Holland

Philip S. Kopf

Steve R. Lloyd, Sr.

Kevin A. MacDonald

Arthur W. McKinney

Donald M. McPhee

Scott C. Metzger

Jeffrey S. Miller

Scott L. Niemitalo

Nigel K. Parkes

William S. Phelan

Tim H. Robinson

John W. Rohrer

Paul A. Rouis, III

Domenick Thomas Ruttura

Bruce A. Suprenant

Scott M. Tarr

Consulting Members

Carl Bimel*

Michael A. Clark

William C. Panarese

Brian J. Pashina

Boyd C. Ringo*

*Deceased

ACI Committee Reports, Guides, 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 signicance

and limitations of its content and recommendations and who

will accept responsibility for the application of the informa-

tion it contains. ACI disclaims any and all responsibility for

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

loss or damage arising there from.

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 a part of the contract documents,

they shall be restated in mandatory language for incorporation

by the Architect/Engineer.

ACI 302.1R-15 supersedes ACI 302.1R-04 and was adopted and published June 2015.

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 reproduc-

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

writing is obtained from the copyright proprietors.

CHAPTER 1—INTRODUCTION

1.1—Purpose

This guide presents information relative to the construction

of slab-on-ground and suspended-slab oors for industrial,

commercial, and institutional buildings. It is applicable to

the construction of normalweight and structural lightweight

concrete oors and slabs made with conventional portland

and blended cements. This guide identies the various classes

of oors based on use, construction design details, necessary

site preparation, concrete type, and other related materials. In

general, characteristics of the concrete slab surface and joint

performance have a powerful impact on the serviceability

of oors and other slabs. Because the eventual success of a

concrete oor installation depends on the mixture proportions

and oor nishing techniques used, considerable attention is

given to critical aspects of achieving the desired nishes and

the required oor surface tolerances.

1.2—Scope

This guide emphasizes choosing and proportioning of

materials, design details, proper construction methods, and

workmanship. Slabs specically intended for the containment

of liquids are beyond the scope of this guide. Whereas this

guide does provide a reasonable overview of concrete oor

construction, each project is unique and circumstances can

dictate departures from the recommendations given in this

guide. Contractors and suppliers should, therefore, thoroughly

review contract documents before bid preparation (Chapter 3).

CHAPTER 2—DEFINITIONS

ACI provides a comprehensive list of denitions through

an online resource, “ACI Concrete Terminology,” http://

www.concrete.org/store/productdetail.aspx?ItemID=CT13.

Denitions provided herein complement that resource.

differential set time—difference in timing from initial

introduction of water to concrete mixture at batch plant to

initial power oating.

dry-shake—dry mixture of hydraulic cement and ne

aggregate (either mineral or metallic) that is distributed

evenly over the surface of concrete atwork and worked into

the surface before time of nal setting and then oated and

troweled to desired nish.

mixture optimization indicator—intersection of the

coarseness factor value and the workability factor on the

coarseness factor chart.

rutting—creation of troughs in the soil support system in

response to applied wheel loads.

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

concrete slab.

soil pumping—vertical displacement and rebound of the

soil support system in response to applied moving loads.

water slump—magnitude of slump, measured in accor-

dance with ASTM C143/C143M, which is directly attributed

to the amount of water in the concrete mixture.

window of nishability—time period available for

nishing operations after the concrete has been placed,

consolidated, and struck-off, and before nal troweling.

workability factor—percentage of combined aggregate

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

CHAPTER 5—DESIGN CONSIDERATIONS

5.1—Scope

Chapter 5 addresses the design of concrete oors as it

relates to their constructibility. Specic design requirements

for concrete oor construction are found in ACI 360R for

slabs-on-ground, ACI 223R for shrinkage-compensating

concrete oors, and ACI 421.1R and 421.2R for suspended

oors.

5.2—Slabs-on-ground

5.2.1 Required design elements—Following are the

minimum items that should be addressed in the construction

documents prepared by the designer (ACI 360R):

a) Slab-on-ground design criteria

b) Base and subbase materials, preparation requirements,

and vapor retarder/barrier, when required

c) Concrete thickness

d) Concrete compressive strength, exural strength, or

both

e) Concrete mixture proportion requirements, ultimate

drying shrinkage strain, or both

f) Joint locations and details

g) Reinforcement (type, size, and location) when required

h) Surface treatment, when required

i) Surface nish

j) Tolerances (base, subbase, slab thickness, and oor at-

ness and levelness)

k) Concrete curing

l) Joint lling material and installation

m) Special embedments

n) Testing requirements

o) Preconstruction meeting, quality assurance, and quality

control

If any of this information is not provided, the contractor

should request it from the slab-on-ground slab designer.

5.2.2 Soil-support system—Because the performance of a

slab-on-ground depends on the integrity of the soil-support

system, specic attention should be given to site preparation

requirements, including proof-rolling (6.1.1). In most cases,

proof-rolling results are much more indicative of the soil-

support system’s ability to withstand loading than from the

results of in-place tests of moisture content or density. A thin

layer of graded, granular, compactible material is normally

used as ne grading material to better control concrete’s thick-

ness and to minimize friction between the base material and

slab. For detailed information on soil-support systems, refer

to ACI 360R.

5.2.3 Moisture protection—Proper moisture protection

is essential for any slab-on-ground where the oor will

be covered by moisture-sensitive ooring materials such

as vinyl; linoleum; wood; carpet; rubber; rubber-backed

carpet tile; impermeable oor coatings; adhesives; or where

moisture-sensitive equipment, products, or environments

exist, such as humidity-controlled or refrigerated rooms.

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2 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)

ACI 302.2R provides recommendations for the design and

construction of concrete slabs that will receive moisture-

sensitive or pH-sensitive ooring materials or coatings for

both slabs-on-ground and suspended slabs.

5.2.3.1 Vapor retarder permeance—A vapor retarder/

barrier is a material that is intended to minimize the trans-

mission of water vapor upward through the slab from sources

below. The performance requirements for plastic vapor

retarder/barrier materials in contact with soil or granular ll

under concrete slabs are listed in ASTM E1745. According

to ASTM E1745 a vapor retarder/barrier material is to have a

permeance level, also known as the water vapor transmission

rate, not exceeding 0.1 perms as determined by ASTM E96/

E96M or ASTM F1249. However, most ooring installations

will benet by using a material with a permeance level well

below 0.1 perms (0.0659 metric perms = 5.72 ng/s

–1

–2

–1

The selection of a vapor retarder/barrier material and

its level of permeance should be made on the basis of the

protective requirements of the material being applied to the

oor surface or the environment being protected. Although

conventional 6, 8, and 10 mil (0.15, 0.20, and 0.25 mm)

polyethylene has been used in the past, this class of mate-

rial does not fully conform to the requirements of ASTM

E1745 and should not be considered for use as below-slab

moisture protection. Any plastic vapor retarder/barrier

material to be used below slabs should be in full compli-

ance with the minimum requirements of ASTM E1745 and

the thickness and permeance of the material be selected

on the basis of protective needs and durability during and

after installation.

However, for a material to be considered a true barrier it

would need to have a permeance level of 0.0 perms when

tested in accordance with ASTM E96/E96M or ASTM

F1249. The industry has not established a permeance level

that serves as the dividing point between materials classed

as vapor barriers or vapor retarders. It is most likely that

when a dividing point between barrier and retarder is estab-

lished it will be at 0.01 perms or less. The laps or seams for

a vapor retarder/barrier should be overlapped 6 in. (150 mm)

(ASTM E1643) or as instructed by the manufacturer. Joints

and penetrations should be sealed with the manufacturer’s

recommended adhesive, pressure-sensitive tape, or both.

5.2.3.2 Vapor retarder/barrier location—The decision

to locate the vapor retarder/barrier in direct contact with

the slab’s underside had long been debated. Experience

has shown, however, that the greatest level of protection

for oor coverings, coatings, or building environments is

provided when the vapor retarder/barrier is placed in direct

contact with the slab. Placing concrete in direct contact with

the vapor retarder/barrier eliminates the potential for water

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

compaction to become trapped within the ll course. Wet

or saturated ll above the vapor retarder/barrier can signi-

cantly lengthen the time required for a slab to dry to a level

acceptable to the manufacturers of oor coverings, adhe-

sives, and coatings. A ll layer sandwiched between the vapor

retarder/barrier and the concrete also serves as an avenue for

moisture to enter and travel freely beneath the slab, which

can lead to an increase in moisture within the slab once it

is covered. Moisture can enter the ll layer through voids,

tears, or punctures in the vapor retarder/barrier.

Placing concrete in direct contact with the vapor retarder/

barrier requires additional design and construction consider-

ations 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/

barrier shows more settlement and exhibits signicantly

larger length change in the rst hour after casting, during

drying shrinkage, and when subject to environmental

change (Suprenant 1997). Joints that open wider than what

is normally anticipated are called dominant joints (Walker

and Holland 2007). Dominant joint behavior can be made

worse when the slab is placed in direct contact with a vapor

retarder/barrier that reduces friction from the base. Where

reinforcing steel is present, settlement cracking over the

steel is more likely because of increased settlement resulting

from a longer bleeding period. There is also increased poten-

tial for a greater measure of slab curl.

Concrete that does not lose excess water to the base does not

stiffen as rapidly as concrete that does. If rapid surface drying

conditions are present, the surface of concrete placed directly

on a vapor retarder/barrier has a tendency to dry and crust over

whereas the concrete below the top fraction of an inch (milli-

meter) 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

sufciently set. Under such conditions, a reduction in surface

atness and some blistering or delamination can occur as air,

water, or both, become trapped below the nish surface.

Each proposed installation should be independently evalu-

ated for moisture sensitivity of anticipated subsequent oor

nishes and the level of protection and material strength

they might need. When placing concrete in direct contact

with the vapor retarder/barrier, the potential effects of slab

curling, crusting, and cracking should be considered. Design

and construction measures should be implemented to offset

or to minimize these effects. The anticipated benets and

risks associated with the specied location of the vapor

retarder/barrier should be reviewed (Fig. 5.2.3.2) with all

parties before construction (ACI 302.2R).

5.2.4 Supporting reinforcement—Deformed reinforcing

steel or post-tensioning tendons should be supported and tied

together sufciently to minimize movement during concrete

placing and nishing 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 compres-

sive strength at least equal to the specied compressive

strength of the concrete being placed, and be thick enough

to support reinforcing steel or post-tensioning tendons at the

proper elevation while maintaining minimum concrete cover

requirements.

When welded wire reinforcement is used, its larger

exibility dictates that the contractor pay close atten-

tion to establishing and maintaining adequate support of

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GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 3

the reinforcement during the concrete placing operations

(Neuber 2006). 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 such that welded wire reinforcement cannot

be forced out of location by construction foot trafc. Support

spacing can be increased when heavier gauge wires or a

Fig. 5.2.3.2—Flow chart to determine when and where a vapor retarder/barrier should be used.

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4 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)

double mat of small-gauge wires is used. Installation should

be in accordance with the Wire Reinforcing Institute’s

(2008) recommendations.

5.2.5 Steel bers—Steel bers are used to provide rein-

forced concrete slabs-on-ground with increased strain

strength, impact resistance, exural toughness, fatigue

endurance, crack-width control, and tensile strength (ACI

544.4R). Finishing oors with steel bers is similar to other

oors, except for the potential for steel bers to become

exposed on the surface of the slab. Typically, however, occa-

sional exposed bers do not cause a problem. Section 7.2.4

discusses types of steel bers and dosages in detail.

5.2.6 Synthetic bers—Synthetic bers are used to rein-

force concrete against plastic shrinkage and drying shrinkage

stresses. Section 7.2.3 discusses types of synthetic bers and

dosages in detail.

5.2.7 Post-tensioning reinforcement—The use of high-

strength steel tendons as reinforcement instead of conven-

tional mild steel temperature and shrinkage reinforcement

introduces 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 drying

process. Stage stressing or partial tensioning of the slab on

the day following placement can result in a signicant reduc-

tion of shrinkage cracks. Construction loads on the concrete

should be minimized until the slabs are fully stressed (Post-

Tensioning Institute 1990, 1996). For guidelines on instal-

lation details, contact a concrete oor specialty contractor

who is thoroughly experienced with this type of installation.

5.2.8 Causes of cracking over reinforcement—Bar shad-

owing and subsidence cracking directly over reinforce-

ment is caused by inadequate consolidation of concrete,

inadequate concrete cover over the reinforcement, use of

large-diameter bars (Babaei and Fouladgar 1997; Dakhil et

al. 1975), higher temperature of reinforcing bars exposed

to direct sunlight, higher-than-required slump in concrete,

revibration of the concrete, inadequate control of evapora-

tion rate before concrete curing begins, or a combination of

these items.

5.2.9 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 magni-

tude of slab curling. Slab designs with an increased number

of joints can result in decreases in curling and individual

joint opening width, resulting in less overall maintenance.

The joint details and layout of joints should be provided by

the designer. When the joint layout and joint details are not

provided before project bid, the designer should provide a

detailed joint layout along with the joint details before the

slab preconstruction meeting or commencing construction.

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

isolate the slab from restraint that might be provided by

another element of the structure. Restraint from any source,

whether internal or external, will increase the potential for

random cracking.

Isolation, contraction, and construction joints are commonly

used in concrete slabs-on-ground. Appropriate locations for

isolation joints and contraction joints are shown in Fig. 5.2.9.

With the designer’s approval, construction joint and contrac-

tion joint details can be interchanged. Refer to ACI 360R for

a detailed discussion of joints. Joints in topping slabs should

be located directly over joints in the base slab.

5.2.9.1 Isolation joints—Isolation joints should be used

wherever complete freedom of vertical and horizontal

movement is required between the oor and adjoining

building elements. Isolation joints should be used at junc-

tions with walls that do not require lateral restraint from the

slab, columns, equipment foundations, footings, or other

points of restraint such as drains, manholes, sumps, and

stairways. An isolation joint may be composed of sheet

material or a preformed joint material separating two adja-

cent concrete elements; one example is where a slab abuts

a wall. Where the isolation joint will restrain shrinkage,

exible closed-cell foam plank should be used with a thick-

ness that accommodates the anticipated shrinkage move-

ment. The joint material should extend the full depth of

the slab or slightly below its bottom to ensure complete

separation. Where the joint ller will be objectionably

visible, or where there are wet conditions or hygienic or

dust-control requirements, the top of the preformed ller

can be removed and the joint caulked with an elastomeric

sealant. Three methods for producing a relatively uniform

depth of joint sealant depth are:

1. Use a saw to score both sides of the preformed ller at

the depth to be removed. Insert the scored ller in the proper

location. After the concrete hardens, use a screwdriver or

similar tool and remove the top section.

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

joint sealant. Nail the wood strip to the preformed ller

Fig. 5.2.9—Appropriate locations for joints.

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GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 5

and install the assembly in the proper location. Remove the

wood strip after the concrete has hardened.

3. Use a premolded joint ller with a removable top portion.

Refer to ACI 223R for guidance on isolation joints for

slabs using shrinkage-compensating concrete.

5.2.9.2 Construction joints—Construction joints are

placed in a slab to dene the extent of the individual concrete

placements, generally in conformity with a predetermined

joint layout. If concrete placement 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 trafc, a butt joint is usually

adequate. In areas subjected to hard-wheeled trafc, heavy

loadings, or both, joints with dowels are recommended.

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.

5.2.9.3 Contraction joints—Contraction joints are usually

located on column lines with intermediate joints located at

equal spaces between column lines, as shown in Fig. 5.2.9.

Factors considered when selecting spacing of contraction

joints are:

a) Slab design method (ACI 360R)

b) Slab thickness

c) Type, amount, and location of reinforcement

d) Shrinkage potential of the concrete, including cement

type and quantity; aggregate type, size, gradation, quantity, and

quality; w/cm; type of admixtures; and concrete temperature

e) Base friction

f) Floor slab restraints

g) Layout of foundations, racks, pits, equipment pads,

trenches, and similar oor discontinuities

h) Environmental factors such as temperature, wind, and

humidity

As previously indicated, establishing slab joint spacing,

thickness, and reinforcement requirements is the responsi-

bility of the designer. Because the specied joint spacing

will be a principal factor dictating both the amount and

the character of random cracking to be experienced, joint

spacing should always be carefully selected.

Floor surface curling at joints is a normal consequence

of volume change resulting from differential moisture loss

from concrete slab to the surrounding environment. This

distortion can lead to conicts with respect to installation of

some oor coverings in the months after concrete placement.

Current national standards for ceramic tile and wood ooring,

such as gymnasium oors, are two instances that require the

concrete slab surface to comply with stringent surface toler-

ances that cannot be met under typical slab curling behavior.

The designer should correlate the slab design with the require-

ments imposed by the oor covering specication in the

design documents. The potential for the joints to telegraph

through the ooring should be addressed by the design.

Some random cracking should always be expected, even

with sufciently close joint spacing. It is reasonable to expect

random visible cracks to occur in 0 to 3 percent of the surface

area oor 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 proportion, and

joint details should be carefully analyzed. Reinforcement will

not prevent cracking. If the reinforcement is properly sized

and located, cracks should remain tightly closed.

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 recommendations of ACI 223R

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. Plastic or metal inserts

are not recommended for constructing or forming a contrac-

tion joint in any exposed oor surface that will be subjected

to wheeled trafc.

5.2.10 Saw-cutting joints—Contraction joints in indus-

trial and commercial oors are usually formed by sawing a

continuous slot in the slab to create a weakened plane, below

which a crack will form. Further details on saw cutting of

joints are given in 10.3.12.

5.2.11 Joint lling—Contraction and construction joints in

oor areas subject to the hard wheels of material-handling

vehicle trafc should be lled with a semi-rigid ller to

minimize wear and damage to joint edges. Construction

joints should be saw-cut 1 in. (25 mm) deep before lling.

Joints should be as narrow as possible to minimize damage

due to wheels loads while still being wide enough to be

properly lled.

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 indus-

trial vehicular trafc in these areas, consideration should be

given to armoring the edge of the joint through alternative

means. Refer to 7.8 for a discussion of joint materials and

11.10 for installation of joint llers.

5.2.12 Load-transfer mechanisms—Use of load-transfer

devices at construction and contraction joints is recom-

mended when positive load transfer is required, unless

a sufcient post-tensioning force is provided across the

joint to transfer the shear. Load-transfer devices force the

concrete sections on both sides of a joint to undergo approx-

imately equal vertical displacements when subjected to a

load and help prevent damage to an exposed edge when the

joint is subjected to vehicles with hard wheels such as lift

trucks. Dowel baskets should be used to maintain alignment

of dowels when used in contraction joints, and alignment

devices should be used in construction joints.

Deformed reinforcing bars should not be continued across

contraction joints or construction joints because they restrain

joints from opening as the slab shrinks during drying, unless

the reinforcement is properly designed as an enhanced

aggregate interlock load-transfer mechanism (ACI 360R).

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6 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)

Keyed joints are not recommended for load transfer in

slabs-on-ground where heavy-wheeled trafc 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 oor strips and oors using

shrinkage-compensating concrete with long joint spacing,

care should be taken to accommodate signicant slab move-

ments. In most instances, post-tensioned slab joints are asso-

ciated with a jacking gap. The lling of jacking gaps should

be delayed as long as possible to accommodate shrinkage

and creep (Post-Tensioning Institute 1990, 2000). Where

signicant slab movement is expected, steel plating of the

joint edges is recommended for strengthening the edges.

5.3—Suspended slabs

5.3.1 Required design elements—The following items

specically impact the construction of suspended slabs and

should be included in the construction documents prepared

by the designer:

a) Frame geometry (member size and spacing)

b) Concrete reinforcement (type, size, location, and

method of support)

c) Shear connectors, if required

d) Construction joint location and details

e) Steel deck (type, depth, gauge, and installation require-

ments), if required

f) Shoring, if required

g) Tolerances (forms, structural steel, reinforcement, and

concrete)

5.3.2 Suspended slab types—In general, suspended oor

systems fall into four main categories:

a) Cast-in-place suspended oors

b) Slabs with removable forms

c) Slabs-on-composite and noncomposite steel decking

d) Topping slabs on precast concrete

Design requirements for cast-in-place concrete suspended

oor 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-steel decking and topping

slabs-on-precast-concrete are hybrid systems that involve

design requirements established by The Steel Deck Insti-

tute, 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 strike-off, but is further inuenced, espe-

cially in the case of slabs-on-steel decking, by the behavior

of the structural frame during and after completion of

construction. The contractor should recognize that each type

of structural frame behaves differently and plan accordingly.

The slab designer should discuss with the owner if the

slab surface is to be placed as near level as possible with

a varying slab thickness, or if it is more important to have

a slab with a uniform thickness, and then design the slab

and framing system accordingly. When placing slabs-on-

steel decking, the contractor is cautioned that deections

of the structural steel members can vary from those antici-

pated by the designer. Achieving a level deected surface

can require increasing the slab thickness more than 3/8 in.

(9.5 mm) in local areas. Concrete placement procedures,

increasing slab thickness to place level surfaces, and

the basis for acceptance of the levelness of a completed

concrete oor surface should be established and agreed

upon by key parties before beginning suspended oor

construction (Tipping 1992). In many cases, the deection

will not allow a level slab to be placed.

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

construction can be either post-tensioned or convention-

ally reinforced. Both of these systems are supported during

initial concrete placement and will deect when supporting

shores are removed.

Post-tensioned systems are normally used when larger

spans are necessary or when the structural system is rela-

tively shallow for the spans considered. Post-tensioned

systems use high-strength steel tendons that are tensioned

after the concrete hardens using a hydraulic jack designed

for that purpose. The magnitude of oor slab deection after

supports are removed is less than that of comparable oors

reinforced with conventional deformed reinforcing steel. At

times, dead load deection is entirely eliminated by the use

of post-tensioning.

The magnitude of deection in a conventionally reinforced

oor system depends 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 deection of a member is deemed

excessive by the designer, an initial 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 oor

system will deect down to a level position after removal of

the supporting shores.

5.3.4 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 signicant

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, resulting in a loss of part or all

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

This failure can occur immediately or up to 30 or 45 minutes

after strike-off. 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

veried, before concrete placement.

American Concrete Institute – Copyrighted © Material – www.concrete.org

GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 7

5.3.5 Slabs-on-steel-deck—Construction of slabs-on-steel-

deck involves the use of a concrete slab and a supporting

platform consisting of structural steel and steel deck. The

structural steel can be shored or unshored at the time of

concrete placement. The steel 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-steel-deck is

seldom level. Variation in elevations at which steel beams

connect to columns and the presence of camber in some oor

members combine to create variations in the initial eleva-

tion of steel members. Regardless of the initial levelness of

the steel frame, unshored frames will deect during concrete

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

instrument impractical for the purpose of establishing a

uniform elevation for strike-off of the concrete surface of

a slab-on-steel-deck, unless the frame is preloaded to allow

deection to take place before strike-off and slab thickness

is allowed to vary.

5.3.6 Composite slabs-on-steel-deck—In composite

construction, the composite section, which consists of the

concrete slab and steel beams, will work together to support

any loads placed on the oor surface after the concrete

has hardened. Composite behavior is normally developed

through the use of shear connectors welded to the struc-

tural 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 with a resulting load-

carrying element that is congured much like a capital T.

The steel beam forms the stem of the T, and the oor slab

forms the crossbar. 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 project designer (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 struc-

tural steel beams are sometimes cambered slightly during

the fabrication process. This camber is intended to offset

the anticipated deection of that member under the weight

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

system has deected, the resulting oor surface will be level

(Tipping 2002).

Shored composite concrete slabs-on-steel-deck are similar

to slabs with removable forms; both are supported until the

concrete has been placed and reaches the required strength.

Structural steel oor framing members for shored composite

slabs-on-steel deck are usually lighter and have less camber

than those used for unshored construction with similar

column spacings and oor 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 oor but can

become an 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

oor construction.

5.3.7 Noncomposite slabs-on-steel-deck—In noncom-

posite construction, the slab and supporting structural steel

work independently to support loads imposed after hard-

ening of the concrete slab.

5.3.8 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 oor system. The cast-in-

place portion of the system consists of a topping of some

specied thickness placed on top of the precast units. The

topping can be composite or noncomposite. In either case,

added deection of precast units under the weight of the

topping slab is normally minor, so the nished 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 signicant chal-

lenges for the contractor. Care should be taken scheduling

such operations to minimize the potential impact of these

variations. The potential for signicant variations in the

preplacement camber of supporting precast members and

the relative lack of options associated with eld changes

or modications create a unique challenge to the contractor

when the desire is to produce a surface that is uniformly

level or uniformly sloped. It is imperative that preplace-

ment discussions be held concerning anticipated deec-

tion, minimum required topping thickness, and allowable

increase in topping thickness beyond that required by the

drawings that may be necessary to achieve the desired

product. A preplacement survey of the erected precast is a

critical component of those discussions.

5.3.9 Reinforcement—For cast-in-place concrete

suspended slabs, reinforcing steel location varies as

dictated by the construction documents. The reinforcement

amount as a minimum should meet the ACI 318 building

code requirements. However, for slabs that will have

surfaces exposed to view, the slab designer should discuss

with the owner any additional reinforcement that may be

required to control the crack widths to meet the owner’s

crack width expectations. It should be noted that, for

exposed slabs, the minimum amount of steel required by

the building code may not be sufcient to meet the owner’s

expectations for crack widths (ACI 224R). For suspended

slabs supporting vehicular trafc such as cars, trucks, and

lift trucks, reinforcement amounts should be carefully

considered to maintain the crack width sufciently tight to

minimize crack spalling due to this fatigue-loading condi-

tion (ACI 215R). Post-tensioning reinforcement, when

used, is enclosed in a plastic or metal sleeve, unbonded,

and tensioned by a hydraulic jack after the concrete reaches

sufcient compressive strength. Elongation and subsequent

anchoring of the ends of post-tensioning tendons result in

the transfer of compressive force to the concrete (Post-

Tensioning Institute 1990).

8 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)

5.3.10 For slabs-on-composite-steel-deck, the composite

steel deck provides the positive moment reinforcement for

static gravity loads and, if needed, negative moment rein-

forcement can be used. Composite steel deck is not recom-

mended as the only reinforcement for use in applications

where the oor is subjected to repeated vehicular trafc

such as lift trucks or similar heavy wheeled trafc (ANSI/

SDI C-2011). Temperature and shrinkage reinforcement for

composite steel deck slabs can be provided by deformed

reinforcing steel, welded wire reinforcement, steel bers,

macrosynthetic bers, or a combination thereof (ANSI/SDI

C-2011). For noncomposite slabs, the reinforcement is the

same as for cast-in-place suspended slabs with the steel deck

acting as a stay-in-place form (ANSI/SDI NC-2010).

5.3.11 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 inuence these decisions.

5.3.11.1 Slabs on removable forms—Construction joints

can introduce weak vertical planes in an otherwise mono-

lithic concrete member, so they should be located where

shear stresses are low. Under most gravity load conditions,

shear stresses in exural members are low in the middle

of the span. ACI 318 requires that construction joints in

oors 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.

5.3.11.2 Composite slabs-on-steel-deck—An important

consideration when deciding on the location of construc-

tion joints in composite slabs-on-steel-deck is that the joint

location can inuence deection of the oor framing near

the joint. A composite member, which is the steel beam

and hardened concrete slab working together, is stiffer and,

therefore, deects less than a noncomposite member, which

is the steel beam acting alone. Most composite slabs-on-

steel-deck are placed on an unshored structural steel oor

frame. Often, structural steel members have initial camber

to offset anticipated noncomposite deection resulting

from concrete placement. After hardening of the concrete,

however, the composite member deects much less than a

comparable noncomposite beam or primary beam.

The following are general guidelines for deciding

on the location of construction joints in composite

slabs-on-steel-deck:

a) Construction joints that parallel secondary structural

steel beams should be placed near the middle third of the

slab between beams.

b) 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 be excluded from the initial placement. Place

the construction joint far enough away from the primary

beam to allow sufcient distance for development of the

primary beam ange width. Placing the construction joint

a distance of 4 ft (1.2 m) from the primary beam is usually

sufcient for this purpose. This construction joint location

allows nearly the full dead load from concrete placement

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 deect completely

before concrete at the primary beam hardens; 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 deect completely before concrete at

the beam hardens.

5.3.11.3 Noncomposite slabs-on-steel-deck—The place-

ment of construction joints in noncomposite slabs-on-steel-

deck should follow the same general guidelines discussed

for slabs-on-removable-forms in 5.3.11.1.

5.3.11.4 Topping slabs-on-precast-concrete—Construc-

tion joints in topping slabs-on-precast-concrete should be

placed over joints in the supporting precast concrete.

5.3.12 Cracks in slabs-on-steel-deck—Cracks often

develop in slabs-on-steel-deck. These cracks can result from

loadings, drying shrinkage, thermal contraction, or variations

in exibility of the supporting structural steel and steel deck.

In a composite oor framing system, primary beams are the

stiffest elements and generally deect less than secondary

beams. The most exible part of the oor framing assembly

is the steel deck, which is often designed for strength rather

than for exibility consideration.

Vibration as a result of power oating and power trow-

eling operations can produce cracking over the structural

steel beams during concrete nishing operations if the steel

deck is exible. As the concrete cures and shrinks, these

cracks will open wide if not restrained by reinforcing steel—

usually deformed or welded wire reinforcement—located

near the top surface of the slab. Steel and macrosynthetic

bers have also been effective in controlling crack widths.

5.4—Miscellaneous details

5.4.1 Heating ducts—Heating ducts embedded in a

concrete slab can be 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.

5.4.2 Edge insulation—Edge insulation for slabs-on-

ground is desirable in most heated buildings. The insulation

should be in accordance with ANSI/ASHRAE/IES 90.1. It

should not absorb moisture; be resistant to fungus, rot, and

insect damage; and 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

conguration adjacent to the inside of the foundation and

under the edge of the slab. If the L-shape conguration is

used, the installation should extend horizontally under the

slab a total distance of 24 in. (600 mm).

5.4.3 Radiant heating: piped liquids—Slabs can be heated

by circulating heated liquids through embedded piping.

Ferrous, copper, or plastic pipe is generally used with not

less than 1 in. (25 mm) of concrete under the pipe and 2 to

3 in. (50 to 75 mm) of concrete cover over the pipe. The

slab is usually monolithic and the concrete placed around the

piping, which is xed in place. Two-course slab construc-

GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 9

tion 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 suboor.

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

material. Supports for piping during concrete placement

should be inorganic and nonabsorbent; precast concrete bar

supports 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 provi-

sion 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 by air pressure,

but not water pressure, and should be maintained in the pipe

during concrete placement operations. After concreting, the

slab should not be heated until concrete curing 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.

5.4.4 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 to avoid 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 rein-

forcing steel will cause corrosion. If admixtures are used,

their chloride contents should comply with the limits recom-

mended by ACI 222R.

5.4.5 Snow melting—Systems for melting snow and ice

can be used in loading platforms or oor 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 admix-

tures should not be used with snow-melting systems. Expe-

rience has shown that these snow-melting piping systems

demand high energy consumption while displaying a high

potential for failure and thermal cracking.

5.4.6 Pipe and conduit—Water pipe and electrical conduit,

if embedded in the oor, should have at least 1-1/2 in.

(38 mm) of concrete cover on both the top and bottom.

5.4.7 Slab embedments in harsh environments—Care

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

electrical systems embedded in slabs exposed to harsh envi-

ronments such as parking garages in northern climates and

marine structures. If not properly 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.

10 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)