224.4r-13 guide to design detailing to mitigate cracking · to offer design and detailing...

ACI 224.4R-13

Guide to Design Detailing to

Mitigate Cracking

Reported by ACI Committee 224

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First PrintingDecember 2013

Guide to Design Detailing to Mitigate Cracking

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Recommendations made in this guide offer performance-based details that can mitigate and control concrete cracking. Structural elements are reviewed individually to identify crack causation and to offer design and detailing recommendations to mitigate crack development. In addition, standard details for various struc-tural members within a building are offered that have been used effectively to mitigate and control crack development in concrete members.

Keywords: cast-in-place; crack mitigation; crack control; cracking; detailing; environmental structure; foundation; prestressed; reinforcement; restraint; shrinkage; slab; tensile stress; thermal effects; wall.

CONTENTS

CHAPTER 1—INTRODUCTION AND SCOPE, p. 21.1—Introduction, p. 21.2—Objective, p. 21.3—Scope, p. 2

CHAPTER 2—NOTATION AND DEFINITIONS, p. 22.1—Notation, p. 22.2—Definitions, p. 3

CHAPTER 3—DESIGN-DETAILING CONSIDERATIONS, p. 3

3.1—Concrete member type and reinforcement, p. 33.2—Overall and local regions, p. 33.3—Framing compatibility, p. 3

CHAPTER 4—DETAILING OF TWO-WAY REINFORCED CONCRETE SLAB SYSTEMS, p. 3

4.1—General, p. 34.2—Causes and types of restraint cracking, p. 34.3—Crack mitigation and control, p. 5

CHAPTER 5—DETAILING OF ONE-WAY REINFORCED CONCRETE SLAB SYSTEMS, p. 9

5.1—General, p. 95.2—Causes and types of restraint cracking, p. 105.3—Crack mitigation and control, p. 11

CHAPTER 6—DETAILING OF COLUMNS, p. 126.1—General, p. 126.2—Short columns, p. 126.3—Columns between walls, p. 136.4—Corner and exterior columns, p. 136.5—Slender columns, p. 136.6—Multistory columns, p. 136.7—Column/slab joints, p. 146.8—Oversized or unique-shaped columns (architectural

columns), p. 14

ACI 224.4R-13


Reported by ACI Committee 224

Jeffrey S. West*, Chair Jacob K. Bice, Secretary

Florian G. Barth*Peter H. Bischoff

David DarwinJohn F. Duntemann

Christopher C. FerraroFouad H. FouadDavid W. FowlerRobert J. Frosch

Grant T. HalvorsenWill Hansen*

Harvey H. Haynes*Mohammad IqbalRalf Leistikow*Malcolm K. LimEdward G. Nawy

Kamran M. Nemati

Keith A. PashinaRandall W. Poston*

Guillermo Alberto RiverosJohn W. RobertsAndrew Scanlon

Andrea J. SchokkerConsulting MembersJulius G. Potyondy

Royce J. RhoadsErnest K. Schrader

_______________*Members of the committee who

prepared this report.Special acknowledgement to Paul

Hedli for his contribution to this report.

1

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 significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom.

Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer.

ACI 224.4R-13 was adopted and published December 2013.Copyright © 2013, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by any

means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduc-tion or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.



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CHAPTER 7—DETAILING OF WALLS, p. 157.1—Gravity load-bearing walls, p. 157.2—Non-load-bearing walls, p. 157.3—Shear walls, p. 157.4—Walls intended to contain liquid, p. 15

CHAPTER 8—DETAILING ON SLABS-ON-GROUND, p. 16

8.1—General, p. 168.2—Contraction joints, p. 168.3—Construction joints, p. 188.4—Expansion and isolation joints, p. 19

CHAPTER 9—REFERENCES, p. 20Cited references, p. 20

CHAPTER 1—INTRODUCTION AND SCOPE

1.1—IntroductionThis guide addresses how to reduce potential cracking in

reinforced concrete buildings in the design process through judicious consideration of building layout, selection of appropriate connections and joint types, and use of good rein-forcement details. Each member within a structure may be subject to different types of cracks. Once the building design is developed, the type of framing system and its geometry selected, and appropriate code-required loads considered, it is then possible for the engineer to understand and identify the possible predominate crack development, crack types, and crack locations for each member within the structure. Predicting possible crack development for members within a building typically allows application of appropriate design details to mitigate and control cracking. Effective detailing of concrete members can improve strength, serviceability (deflection and durability), and aesthetics of a concrete structure.

The terms “crack mitigation” and “crack control” as used in this document have distinct meanings. Crack mitigation involves measures that are intended to prevent cracking from occurring. This includes concepts intended to minimize or eliminate restraint, such as consideration of building layout, the use of connection or element releases, or both. Crack control involves measures that are intended to control where cracks occur, or to limit the width and spacing of cracks. Crack control approaches include the use of joints and rein-forcement detailing. The term “design details” in this docu-ment is a broad term intended to include all design measures intended to mitigate and control cracking.

Some of the crack mitigation and control measures described in this document involve reinforcement details and other requirements that may already be required by the Building Code for structural reasons. Other measures recom-mended in this document may require details and reinforce-ment amounts in excess of that required by ACI 315.

1.2—ObjectiveThe ACI Detailing Manual (ACI Committee 315 2004)

provides standard reinforcement details to aid the designer

in addressing concrete cracking. The ACI Detailing Manual shows individual details in isolation, which may be used for a particular member, joint, or cross section. In contrast, the objective of this document is to address the mitigation and control of cracking by considering the overall nature of a structure and how members may experience additional cross-sectional stresses due to the restraint caused by the structural system. The effect of the geometry and layout of the concrete framing system on the cracking of individual members or joints is discussed, and recommendations for more favorable arrangements of structural framing to mini-mize restraint are presented. Additionally, specific framing conditions where the cracking of a particular part of the structure is directly or indirectly affected by the neighboring elements or the overall framing system are discussed, and suggested reinforcement and release details to avoid or mini-mize such cracking are provided.

1.3—ScopeThis document provides recommendations for design

details and structural framing guidelines to mitigate, control, or distribute crack development in concrete members and structural systems. The recommendations and guidelines are presented in terms of concepts and effective practices that have been successfully implemented to mitigate and control cracking. Specific reinforcement details are presented for some situations, but not all, as the document is intended to illustrate concepts and approaches rather than to provide comprehensive details for all situations.

The reinforcement details shown in this document are for Grade 60 deformed steel reinforcing bars. Note that these reinforcement details may not be sufficient when other types of reinforcement are used, and in particular where the elastic properties (for example, fiber-reinforced polymer), bond properties, or strength of the bars are different.

This document specifically excludes the review of concrete member cracking due to unique or special materials used, concrete mixture proportions, or placing and finishing practices. The design details presented herein are limited to building structures and may not apply to special structures or nonbuilding structures. This document is limited to cast-in-place concrete frames only, not to precast concrete and masonry elements.

CHAPTER 2—NOTATION AND DEFINITIONS

2.1—NotationS1, S2 = horizontal spacing of contraction joints in slabs on

ground, in. (mm)SDowel = center-to-center spacing of dowel bars used for

vertical joint load transfer in slabs on ground, in. (mm)

SPlate = center-to-center spacing of steel plates used for vertical joint load transfer in slabs on ground, in. (mm)

r = steel reinforcement ratio

American Concrete Institute Copyrighted Material—www.concrete.org

2 GUIDE TO DESIGN DETAILING TO MITIGATE CRACKING (ACI 224.4R-13)



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2.2—Definitions“2013 ACI Concrete Terminology” provides a compre-

hensive list of definitions that is available online at: http://www.concrete.org/Tools/ConcreteTerminology.aspx.

CHAPTER 3—DESIGN-DETAILING CONSIDERATIONS

3.1—Concrete member type and reinforcementA detailed review of each concrete member type is required

to mitigate or control localized and global cracking. Each member should be reviewed for its predominant cracking behavior. The most common cracks are caused by:

a) Restrained concrete volume changes (creep, shrinkage, elastic shortening, and thermal effects)

b) Concrete member restraint effectsc) Loading effects (flexure, shear, axial tension/compres-

sion, and torsion)This guide addresses horizontal floor framing members

such as one- and two-way slab systems, vertical elements of concrete structures such as walls and columns, and nonstructural slabs-on-ground. This guide may call for minimum reinforcement in excess of that noted in ACI 318 for increased serviceability performance.

3.2—Overall and local regionsDue to the variety of member types and geometries and

the various factors that cause cracks, it is imperative that each concrete member be reviewed individually and as part of the overall framing system during the design-detailing process. Concentrated load application and vulnerable member joint conditions may require a localized review of concrete details. On the other hand, the overall framing layout may cause indirect load transfer due to geometry or member incompatibility that results in concrete cracking based on overall behavior of the framing system. Chapters 4 through 8 describe a localized and overall performance review of members.

3.3—Framing compatibilityFraming compatibility is one of the basic premises of a

properly performing structure. The relative proportioning of member shapes plays an important role in mitigating concrete cracking. An example of framing incompatibility would be found in a structure where concrete member sizes are simply out of scale, such as where oversized architec-tural columns are used to support a thin slab. The restraining effects of oversized members on their surrounding members can be significant. For this reason, it is advisable to main-tain compatible member sizes and connections to mitigate restraint cracking during concrete shortening. If geometry or architectural considerations dictate such incompatibility, special consideration should be given to incompatible member connections. Alternatively, incompatible framing geometry resulting from architectural considerations can be addressed by using built-up nonstructural foam sections or shapes added to the completed concrete frame to obtain

the architectural shapes desired (for example, to increase column size for aesthetic purposes).

CHAPTER 4—DETAILING OF TWO-WAY REINFORCED CONCRETE SLAB SYSTEMS

4.1—GeneralACI 318-11 provides minimum requirements related to

shrinkage and temperature reinforcement (7.12) and outlines the need for restraint crack control (7.12.1.2, 7.12.3, 10.6, 11.6.7, 18.2.4, and 18.10.2). However, specific requirements for structural and restraint crack control in two-way rein-forced concrete slab systems are not provided. As a result, some designers use the provisions in 10.6 of ACI 318-11 for flexural crack control in beams and one-way slabs in both principal directions for two-way slab systems. The ACI 318 provisions for one-way action may not be adequate for two-way slab systems, where the effects of restrained volume changes from shrinkage and thermal strains are significant. Crack-control equations for beams and one-way slabs gener-ally underestimate the crack widths developed in two-way slabs. ACI 224R and Nawy (2005) present specific proce-dures for determining conventional flexural crack control in two-way slab systems.

In contrast to cracking that may be induced by the action of structural loading, unintended cracking due to restraint of slab shortening by structural elements can be quite pronounced. This type of cracking tends to be through-slab cracking because it results from direct in-plane tension (ACI 224.2R). This chapter covers restraint cracking and mitiga-tion strategies for two-way slab systems. Much of the infor-mation presented has been adapted from the work by Aalami and Barth (1989) on post-tensioned building structures.

4.2—Causes and types of restraint crackingSeveral factors can combine to lead to restraint cracks

in two-way reinforced concrete slabs. Structurally stiff elements such as walls, elevator and stairwell cores, and columns restrain the slab from shortening. If the tensile stress resulting from the restrained shortening exceeds the tensile strength of the concrete, a restraint crack occurs (ACI 224.2R). Depending on the stiffness of the restraining elements and the length of the slab spans, multiple restraint cracks may form.

The specific factors that cause shortening of concrete slabs include:

a) Shrinkage of concreteb) Creep of concrete due to sustained loadsc) Temperature decreased) Elastic shortening (post-tensioned slabs only)Figure 4.2a illustrates typical restraint cracking that may

occur in two-way slab systems. The figure illustrates that for reinforced concrete, the bonded reinforcement in the column strips tends to control restrained shrinkage cracking and more cracks of smaller size tend to form because of the bonded reinforcement. In contrast, in unbonded post-tensioned construction, restrained shrinkage cracking tends


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to be less distributed because there is less bonded mild rein-forcement used.

To illustrate the potential total restraint that can develop in an 8 in. (200 mm) slab with a standard 4000 psi (28 MPa) normalweight concrete mixture, consider a 100 x 200 ft (30 x 60 m) concrete slab subjected to 70 percent ambient humidity. Table 4.2 summarizes the anticipated total contri-butions of various factors to shortening of the slab based on Martin and Perry (2004). As noted in the table, a signifi-cant portion of the slab shortening after 1 year results from concrete shrinkage.

For illustration purposes, the total long-term unrestrained concrete shrinkage for the slab is assumed to be approxi-mately 400 microstrain (accounting for slab characteris-tics and ambient humidity; refer to ACI 209.2R for more information on estimation of shrinkage). If the slab is fully restrained, the elastic stress in the slab is calculated as the product of the unrestrained strain times the modulus of elas-ticity. Based on an estimated modulus of elasticity of 3.6 × 106 psi (25,000 MPa), then the induced elastic stress, when the slab is fully restrained, is approximately 1400 psi (9.6 MPa). Because this calculated tensile stress will greatly exceed the tensile strength of concrete, restraint (tension) cracks will occur. Of course, no slab is fully restrained, and a countervailing effect of long-term creep is a reduction in effective modulus, which reduces the theoretical computed tensile stress by at least half (Kim and Lee 1998; Altoubat and Lange 2001). It is clear, however, that the interaction of restraining structural elements is the critical factor in crack formation.

A secondary factor that can predispose a restraint crack to form at a specific location is the termination (cutoff) of reinforcement and location of combined maximum flexural and axial shortening tensile stress, as illustrated in Fig. 4.2b. The termination of all or most slab reinforcement at a single

location tends to produce a stress discontinuity that can define the location of cracks. Section 13.3.8 of ACI 318-11 prescribes requirements for termination of top (negative moment) reinforcement in the column and middle strips of two-way slabs. Potential crack formation can be minimized by extending alternating bar termination locations beyond the minimum requirements. In structures where restraint is significant, it is recommended to make some portion of the top reinforcement continuous over the slab length to control restrained shrinkage and temperature cracks.

4.2.1 Overall slab cracks—The primary cause of overall restraint cracking in two-way slab systems is the layout of gravity- and lateral-load-carrying structural elements, the manner in which the slab is tied into those elements, and irregularities in the slab geometry. Figure 4.2.1 illustrates the probable locations of restraint cracks in a two-way slab system relative to the location of structural elements and discontinuities. The two-way concrete slab will shorten toward the center of rigidity or point of no movement. The wall surrounding the slab limits possible shortening move-ment, resulting in cracking along the length of the slab wall connection. Note that restraint may result in top, bottom, or through cracking of the slab.

4.2.2 Localized slab cracks—Figure 4.2.2 presents exam-ples of localized cracks in slabs. This type of cracking is associated with slab openings and reentrant corners where there is an abrupt change in stiffness of adjoining elements. Two-way slab cracking at corners that are tied to walls may result from gravity loading as well as from restrained shrinkage and thermal actions. The wall will restrain the

Fig.4.2a—Typical restrained-shrinkage cracks in two-way slabs.

Table 4.2—Contributions to typical slab shortening (Martin and Perry 2004)

ItemPercentage contribution of total

shorteningShrinkage 70

Creep 12Temperature 18

Total 100

Fig.4.2b—Probable restraint crack formation near rein-forcement termination or location of maximum combined tensile stresses.

Fig. 4.2.1—Illustration of possible restraint cracking in two-way slab.





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tendency of the slab corner to lift when subjected to out-of-plane (gravity) loading, producing localized moments, and cracking in the slab.

4.3—Crack mitigation and controlThe principal approaches to mitigate and control restraint

cracking in two-way slabs are the judicious arrangement of supporting members that may restrain the slab, and the use of structural separations and joints, releases between the slab and supporting elements, and special reinforcement details.

4.3.1 Restraining member layout—One of the most effective methods of restraint crack mitigation in two-way slabs is through the selection of favorable column and wall locations. The tendency of the slab is to shrink toward its geometric center. A symmetric positioning and selection of an equal number of walls located at a point of zero move-ment is most favorable for crack mitigation purposes. Figure 4.3.1a contrasts the more favorable to less favorable arrange-ment of walls to mitigate slab cracking. Figure 4.3.1b shows differences between a more favorable and less favorable column layout for mitigation of slab cracking. In general, it is best to have the center of rigidity of the two-way system coincide with the center of mass of the slab. This is illus-trated by each of the more favorable wall arrangements in Fig. 4.3.1a. Uniform column layouts with equal column

spacing will also tend to locate the center of rigidity with the slab geometric center. It is understood that, in most cases, architectural functionality dictates the location of vertical supporting elements. It is important, however, for the engineer to inform the architect and owner of improved performance (from a cracking point of view) if walls and other restraining elements are placed favorably. Less favor-able arrangement of restraining elements, however, can be accommodated by providing temporary wall releases or through the installation of pour strips, which will allow a significant amount of shortening to take place before the slab is locked into place.

4.3.2 Structural separation and expansion joint loca-tion—Slabs of irregular geometry are more susceptible to restraint cracking. Figure 4.3.2 shows a logical location to separate slab areas to avoid cracking at the reentrant corner. This separation can be detailed as an expansion joint. By separating slab areas, the centers of rigidity and mass for each area coincide, thus providing two smaller and regular slab areas instead of one irregular large slab area.

4.3.3 Structural element releases—Providing mecha-nisms that allow relative movement between the slab and supporting elements can be an effective means of crack mitigation, particularly when a favorable arrangement of columns and walls is not fully possible. The relative move-ments can be accomplished through appropriate design of slab-to-wall connections, column-slab connections, slab joints, or wall joints.

4.3.3.1 Slab-to-wall connections—Figure 4.3.3.1 shows a fixed or rigid slab-to-wall connection and three types of commonly used connections to facilitate slippage at slab and wall interfaces. These types of slab-to-wall releases are applicable to both exterior and interior locations. It is recom-mended that some form of tie be made between the slab and wall to allow for shear transfer. This provides stability for intended or unintended lateral loads and overall structural integrity.

A temporary release allows the two-way slab system to experience the majority of its shortening before being perma-nently fixed. A temporary release is typically provided for a

Fig. 4.2.2—Localized cracking in two-way slabs.

Fig.4.3.1a—Influence of restraining wall location on cracking.





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duration of approximately 20 to 50 days during construc-tion to accommodate the initial shortening. For example, this duration can reduce the overall effects of shortening due to creep and shrinkage of a post-tensioned slab by 35 to 50 percent. The temporary release is achieved by disen-gaging the vertical reinforcement extending from the lower wall into the slab using a 3 in. (75 mm) diameter pocket at the dowel bar tail locations, as shown in Fig. 4.3.3.1(b) for a concrete wall. The pocket is empty during the release period, and is later grouted to achieve continuity once the release duration has ended. The pocket is normally made the full depth of the slab thickness to facilitate grouting. Tempo-rary releases are primarily used in single-story structures, as illustrated in Fig. 4.3.3.1(b). Temporary releases are rarely used in multistory buildings because the time delay between the upper wall and slab pour is typically minimal (7 days or less), thus requiring complex details to facilitate grouting of the pocket at the end of the release period (20 to 50 days) if the upper wall is already in place.

Fig. 4.3.3.1(c) and (d) show permanent release details for concrete and masonry wall systems, respectively. The permanent release is created by filling the pocket at the dowel bar locations with a compressible material (for example, closed-cell extruded polystyrene foam) at the time of construction, thus permanently disengaging the dowel bar between the lower wall and the slab. Note the L-shaped, or hooked, reinforcing bars that provide continuity between the slab and the upper wall are located beyond, not aligned with, the foam-filled pocket.

The performance of the release is dependent on the mate-rials selected and workmanship. The top of the walls must have a smooth trowelled surface, as a concrete surface that is simply struck off provides too much interlock. A single sheet of polyethylene plastic is inadequate to provide the neces-sary slip plane. A 1/8 to 1/4 in. (3 to 6 mm) thick tempered wood particle board or two layers of heavy polyethylene sheeting with a thickness of at least 20 mils (0.5 mm) is generally adequate. For the case of masonry (concrete-filled block) construction, there is a need to provide a compress-ible material that will allow for relatively free movement of the concrete slab, as the interfacial concrete and block surface could be too rough to allow for movement (refer to Fig. 4.3.3.1(d)).

4.3.3.2 Column-slab connection—In reinforced concrete two-way slab construction, the column-slab connections are generally not released. In some cases where a relatively stiff system of shear walls is used, columns should be released, as shown in Fig. 4.3.3.2, to allow for slab shortening. A release is provided at the base of the end column and is typically also provided at each upper level for very long slabs where large lateral movement of end columns can occur due to shortening of the slab, or for structural systems having very stiff, oversized end columns. The slab/column/wall system should be analyzed to ensure overall structural stability, and the bearing pad should have sufficient bearing and rotational capacity.

4.3.3.3 Slab joints—Slab joints provide a separation in the slab between openings and other discontinuity locations. A slab joint extends only along the slab, and does not extend into the supporting structure. These joints are neither strong enough to resist the forces due to shortening nor flexible

Fig. 4.3.2—Structural separation of irregular slab area.

Fig.4.3.1b—Influence of column arrangement on cracking.





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enough to articulate or deform without distress. In essence, as illustrated in Fig. 4.3.3.3, these slab joints are contraction joints that provide for a preferred location of cracking.

4.3.3.4 Wall joints—Wall joints provide vertical separa-tions between adjacent walls that accommodate displace-ment of slabs supported by walls. These wall joints help to mitigate cracking in the slabs as well as the supporting walls. Figure 4.3.3.4 shows an example of where wall joints can be

used. Wall No. 1 is positioned in its weak axis in relation to the direction of slab movement, and will be flexible enough to allow the slab to shorten. Wall No. 2 is positioned parallel to the direction of movement, and would greatly restrain the slab from shortening. Hence, stiff Wall No. 2 should be released using a slip plane between the wall and slab to avoid slab or wall cracking. The vertical joint between the walls allows movement of Wall No. 1 without restraint from Wall

Fig. 4.3.3.1—Slab-to-wall releases.





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No. 2. The joint between Wall No. 1 and No. 2 may contain filler material ranging from an easily compressible material to a more rigid neoprene material, depending on the structure and analysis assumptions. The licensed design professional should select the filler material best suited for the structural condition. Although this example describes unidirectional slab movement, the restrained volume changes in each direc-tion should be considered and treated independently.

4.3.4 Reinforcement detailing—Detailing reinforcement used in areas of abrupt changes in stiffness and discontinui-ties is also beneficial for crack control. It is important to note that reinforcement detailing alone is generally not sufficient to limit all cracking and must be done in conjunction with a well-planned layout of columns, walls, and other restraining elements.

Figures 4.3.4a through 4.3.4c illustrate good reinforce-ment detailing practices to control cracking at walls and discontinuities. In Fig. 4.3.4a, the crack control reinforce-ment is placed parallel to the wall and should be distributed from the slab edge to the center of the first span, but not less than 10 ft (3 m) from the slab edge. No. 3 or 4 (No. 10M or 13M) reinforcing bars alternately placed at the top and bottom of the slab at approximately 12 to 16 in. (300 to 400 mm) on center are generally sufficient for this purpose.

Fig. 4.3.3.2—Column-to-slab release.

Fig. 4.3.3.3—Slab joints to isolate regions of potential distress.

Fig. 4.3.3.4—Wall joint release.

Fig. 4.3.4a—Slab crack control reinforcement parallel to wall.





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Figure 4.3.4b illustrates reinforcement details at interior and exterior wall locations. Crack control reinforcement is placed parallel to the wall and at the wall ends to control restraint cracking. Reinforcement details for three slab corner condi-tions are shown in Fig. 4.3.4c. For the case of a cantilevered slab edge, reinforcement is placed along the slab to control the potential for cracking along the edge, and reinforcement extending from the cantilevered slab is extended into the main slab along with diagonal bars to control cracking at reentrant corners. For the case of a supported corner (Fig. 4.3.4c), the orthogonal reinforcement pattern shown, as well as that shown in ACI 318-11, Fig. R13.3.6, Option 2, will provide crack control for restraint inherent at a slab corner.

Another good detailing practice to control discontinuity cracking in two-way slab systems is illustrated in Fig. 4.3.4d. Although calculations may indicate that negative moment reinforcement can be terminated, it is good practice to continue one-fourth to one-third of the reinforcing steel, not to exceed 18 in. (450 mm) spacing, across the span to avoid formation of restraint cracks at reinforcement termina-tion locations.

CHAPTER 5—DETAILING OF ONE-WAY REINFORCED CONCRETE SLAB SYSTEMS

5.1—GeneralSection 10.6 of ACI 318-11 prescribes the minimum

requirements for the control of flexural cracking in one-way slabs and beams. Moreover, Section 7.12 of ACI 318-11 prescribes the minimum reinforcement required in one-way slabs to control cracking in the transverse direction to the main flexural reinforcement that may be induced by temper-ature and shrinkage stresses. Section 7.12.1.2 of ACI 318-11 cautions that the minimum reinforcement prescribed might not be enough when there is significant restraint of move-ments from temperature and shrinkage. In this case, ACI 318 requires consideration of the forces introduced to the slab due to the restraint.

This chapter reviews restraint cracking and mitigation strategies for one-way slab systems. One-way slab systems are generally characterized by slabs that span between beams or girders in structural frames. In the principal flexural direction (even though reinforcing bars or post-tensioning tendons are provided to meet code requirements for strength and crack control), frame stiffness due to beams, columns, and walls may restrain shortening of the one-way slab to a degree that temperature and shrinkage results in stresses that

Fig. 4.3.4b—Slab crack control reinforcement at walls.

Fig. 4.3.4c—Crack control reinforcement at slab corners.

Fig. 4.3.4d—Continuous negative moment slab reinforcement.





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exceed the tensile strength of concrete, resulting in through-thickness cracks. In the direction transverse to the prin-cipal flexural direction, reinforcing bars or post-tensioning tendons meeting minimum code requirements should be adequate to control cracking unless there are transverse framing elements or walls that provide restraint.

5.2—Causes and types of restraint crackingThe factors that lead to restraint cracks in one-way rein-

forced concrete slabs are similar to those discussed in Chapter 4 for two-way slab systems. Concrete slabs tend to shorten, and structurally stiff elements such as walls, elevator and stairwell cores, columns, and beams restrain the slab. Depending on the stiffness of the restraining elements and length of the slab spans, multiple restrained-shrinkage cracks can form in the slab. Because the orientation and loca-tion of restraint cracks are primarily dependent on the prox-imity of the restraining elements, these restraint cracks may be oriented transverse to the beams, parallel to the girders, or both. The characteristic feature of shrinkage cracking is that it is usually full-depth in contrast to partial-depth flexural cracking. Figure 5.2a illustrates typical restraint cracking that occurs in one-way slab systems. In this case, restraint stresses accumulate at midspan and cracks form there. A factor that can cause a restraint crack to form in a different location in a one-way slab system is the termination (cutoff) of reinforcement, as illustrated in Fig. 5.2b, because of the resulting discontinuity. Section 12.12 of ACI 318-11 prescribes requirements for termination of negative-moment reinforcing steel in members. Although there is a theoretical point in which reinforcement is no longer needed for strength to control the effects of restrained shrinkage and temperature cracks, it is recommended to provide some continuous top

and bottom reinforcement. The percentage of continuous reinforcement depends on the amount of potential restraint. Continuous reinforcement should be in the range of 25 to 50 percent.

5.2.1 Overall slab cracks—The primary cause of overall restraint cracking in one-way slab systems is the layout of gravity- and lateral-load structural elements, the manner in which the slab is tied into those elements, and irregularities in the slab geometry. Figure 5.2.1 illustrates the probable locations of restraint cracks in a one-way slab system rela-tive to the location of structural elements and discontinui-ties. The extent and location of restraint cracking in one-way slab systems will depend on the type of lateral-load-resisting system used. As illustrated in Fig. 5.2.1, the cracks trans-verse to the main flexural direction, or span direction, of the one-way slab are induced by the moment-resisting frames restraining horizontal movement in the principal flex-ural direction of the slab. In addition, the diagonal cracks emanating from the lower moment-resisting frame may result because of the stiffness effects in the transverse direc-tion. Some slab cracks shown in Fig. 5.2.1 are parallel to the principal flexural direction or span direction of the slab, and may result from the restraint caused by the framing beam and columns in the orthogonal direction. For shear wall systems, the cracking will be similar to that illustrated for two-way systems (Fig. 4.2.1) because the wall stiffness is significantly greater than that of the girders. In contrast,

Fig. 5.2a—Typical restrained-shrinkage cracks in a one-way slab.

Fig. 5.2b—Restraint crack predisposed to form at location of termination of flexural reinforcement in one-way slab.

Fig. 5.2.1—Illustration of restraint cracking in one-way slab.





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restraint cracking in a moment-resisting frame will be more a function of overall beam/column frame stiffness.

5.2.2 Localized slab cracks—Figure 5.2.2 presents exam-ples of localized cracks in slabs. This type of cracking is asso-ciated with slabs tied to edge girders, and with slab openings and reentrant corners where there is an abrupt change in stiff-ness, such as at the intersection of beams and columns.

5.3—Crack mitigation and controlThe principal procedures for mitigating and controlling

restraint cracking in one-way slabs are the careful posi-tioning of restraining members and the use of structural separations and joints, release connections, and special rein-forcement details.

5.3.1 Restraining member layout—One of the most effec-tive methods of crack mitigation in one-way slabs is judi-cious location of columns and lateral-load-resisting systems, such as shear walls and moment-resisting frames. Symmetric positioning and selection of an equal number of frames or walls located at a point of zero movement is most favor-able for crack mitigation purposes. Figure 4.3.1a illustrates the most favorable arrangement for walls. Figure 5.3.1a contrasts the more favorable to less favorable arrangement of frames to mitigate slab cracking. The concept illustrated by this figure is that frames located toward the center of the building frame are less problematic with respect to restraint because potential movement caused by temperature and shrinkage is toward the center. In contrast, frames near the exterior of the building restrain movement toward the center

of the structure, resulting in a less favorable arrangement. Figure 5.3.1b shows the preferred column layout for slab cracking mitigation. In general, it is best to have the center of rigidity of the one-way system coincide with the center of mass of the slab. It is common to find cracking in thinner slab sections in floor systems where varying slab thickness is used due to changes in one-way span length.

5.3.2 Structural separation and expansion joint loca-tion—As discussed in 4.3.2, continuous slabs of irregular geometry are susceptible to restraint cracking. Figure 5.3.2 shows the logical location to separate one large, irregular slab area into two smaller slabs so that the centers of rigidity and mass coincide for each newly selected area. The separa-tion between the slab areas can be detailed as an expansion joint. This option could be more costly because columns and foundations may have to be used on each side of the joint, but clearly the delineation of a joint at this location will reduce restraint that could lead to cracking. It is possible to use one column and foundation by detailing a column with corbels to provide for a joint and beam support on each side of the joint. It may also be possible to add more top and bottom reinforcement across this slab area to avoid the expansion joint and addition of columns. The designer needs to eval-uate the benefit of less restraint and more regular framing to the potential cost increase.

5.3.3 Structural element releases—Providing mecha-nisms that allow relative movement between the slab and supporting elements can be an effective means of crack mitigation. This is particularly the case when a favor-able arrangement of column and beam frames is not fully possible. Section 4.3.3 addresses the types of releases that are helpful for crack mitigation.

5.3.4 Reinforcement detailing—Reinforcing steel can be detailed in areas of abrupt changes in stiffness and disconti-nuities to be beneficial for crack control, and is implemented

Fig. 5.2.2—Localized cracking in one-way slab.

Fig. 5.3.1a—Location of restraining frames.





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in addition to a well-planned layout of columns, beams, and other restricting elements. Section 4.3.4 illustrates the type of reinforcement detailing that assists in crack control.

For typical one-way slabs, 10.6.4 of ACI 318-11 specifies the maximum spacing of reinforcing bars or post-tensioning tendons in the principal flexural direction depending on the amount of concrete cover and exposure conditions. Typi-cally, this results in a maximum spacing of 10 to 12 in. (250 to 300 mm). This maximum spacing should be reduced in more aggressive environments, such as coastal areas or loca-tions where chemical deicers are regularly used.

Beyond the flexural reinforcement requirements, the minimum required reinforcement for temperature and shrinkage is determined by 7.12.2.1 of ACI 318-11. For a typical 6 in. (150 mm) reinforced concrete slab and GR 60 (415 MPa) reinforcement, this translates into a No. 3 (No. 10M) reinforcing bar at approximately 10 in. (250 mm) spacing. For post-tensioned slabs, 7.12.3 of ACI 318-11 requires temperature tendons to precompress the gross area of concrete to a minimum 100 psi (0.7 MPa) with a maximum 6 ft (1.8 m) tendon spacing.

In the transverse direction, 7.12.2.2 of ACI 318-11 requires that the maximum spacing of reinforcing bars be five times the slab thickness, but not greater than 18 in. (450 mm) to control cracking caused by temperature and shrinkage stresses. Post-tensioning tendons used for temperature and shrinkage control should not exceed 6 ft (1.8 m) according to 7.12.3.4 of ACI 318. For a typical one-way structural

slab, the 18 in. (450 mm) spacing limit will likely control the reinforcement detailing. Whereas 18 in. (450 mm) is the maximum permissible ACI 318 spacing, this spacing should be limited to 12 in. (305 mm).Where the minimum amount of temperature and shrinkage reinforcement is used, the reinforcement stress can exceed 40 ksi (276 MPa) at first cracking, resulting in crack widths greater than 0.016 in. (0.41 mm). Tests results and analytical modeling indicate that a maximum spacing of 12 in. (305 mm) will improve crack control (Blackman and Frosch 2005; Frosch 1999).

CHAPTER 6—DETAILING OF COLUMNS

6.1—GeneralACI 318 does not specifically provide requirements for

structural and restraint crack control in structural concrete columns. This chapter discusses several column conditions that may lead to restraint cracking and presents appropriate mitigation and control strategies.

6.2—Short columnsColumns that are less than one story in height may be

particularly prone to both restraint cracking and severe shear cracking due to their increased flexural stiffness. For example, short columns at split levels in parking structures, as illustrated in Fig. 6.2, can develop severe cracks and spalling of concrete due to shortening of the parking decks or levels immediately above and below, and from the short-ening of the floor slab or beams framing into the columns. In post-tensioned parking structures, the tendon anchorages terminate at the columns, further increasing the horizontal forces exerted on the short column. The short column cannot accommodate the large deformation demand applied by the two different sections of the parking structure without unex-pected, and possibly excessive, cracking. The use of releases can mitigate short column cracking in these situations. One option is to provide a full release of the column using central dowel column alignment bars, as shown in Fig. 6.2, in essence creating a pinned column-to-slab/beam connection. This option minimizes the short column end moments due to relative horizontal deformations of the structure. For the

Fig. 5.3.1b—Column and beam arrangement.

Fig. 5.3.2—Structural separation of irregular slab area.





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proper sizing of the dowel column alignment bars, a pinned column end condition is modeled to calculate the shear forces across this plane. The addition of closely spaced column ties is often not sufficient to control the column cracking and can result in excessive steel congestion at the column and slab or beam intersection. A more detailed analysis of the stresses introduced into the short columns is needed to deter-mine the effectiveness and required quantity of confinement reinforcing needed.

A second option is to maintain a rigid or fixed connec-tion between the base of the short column and the slab while providing a slip release at the top of the column (refer to Fig. 6.2). The release is designed similar to that shown previously for slab-wall connections (refer to 4.3.3.1 and Fig. 4.3.3.1). The release must be detailed to create a horizontal slip plane between the top of the column and bottom of the slab, avoiding a pinned connection and the development of a large base moment in the short column. The use of a compress-ible material (for example, closed-cell extruded polystyrene foam pocket) in the upper slab will allow relative movement between the slab and column, minimizing restraint (refer to Fig. 4.3.3.1 and 6.2). Regardless of the type of release used, the slab/column system should be analyzed to ensure overall structural stability.

6.3—Columns between wallsColumns tied to half-height walls represent conditions

similar to those described for short columns in 6.2, and may develop similar restraint cracks, as shown in Fig. 6.3. Crack formation is especially severe in beam-slab floor construc-tion where the short remaining section of the column above the wall is stiffened due to restraint provided by the beam. Provisions of full-height or partial-height joints between the walls and the columns, illustrated in Fig. 6.3, are effective methods for mitigating such cracks.

6.4—Corner and exterior columnsCorner or exterior columns supporting slabs of 150 ft (45

m) or more in length are particularly susceptible to cracks,

as illustrated in Fig. 6.4. The design of such columns should account for the moment generated in the column due to shortening of the slab in addition to all load combinations of gravity and lateral load application.

6.5—Slender columnsDesign of slender columns typically requires consid-

eration of possible secondary (P-delta) effects and other stability factors. If the structural system includes stiff elements such as large walls, slab shortening may subject flexible slender columns to relative lateral displacements, resulting in cracking of the column. In addition, the moment caused by the lateral displacement of the column due to slab shortening should be considered in the design (second-order or P-delta effects). To provide crack control and sufficient ductility and to preserve column integrity, slender columns should have increased confining reinforcement. Good prac-tice follows 21.13 of ACI 318-11.

6.6—Multistory columnsMultistory towers extending above rigid podium or plaza

levels, as illustrated in Fig. 6.6, generate potential distress locations at the junction of the tower to the plaza level due to concrete shortening effects. The same figure identifies the lowest level column of the tower over the footing as another distress location with a great likelihood of crack formation due to the differential shortening of the rigid foundation and the first elevated concrete slab. Typically, the shortening at the plaza level columns is most critical because the founda-tion represents a rigid base and the first elevated level will shorten. The plaza or podium level slab in this example is the elevated transfer slab before a typical office or hotel floor starts to repeat. On levels above the plaza slab, the designer should be concerned about the construction time period between levels. The difference in relative shortening between levels is equal to the total shortening of one level during the time the next level is completed. Construction schedules of multistory buildings that call for minimal time delay between different level slab placements limit the rela-

Fig. 6.2—Cracking in short column at split level of parking structure.





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tive shortening between levels and, hence, minimize the concern of distress due to shortening from level to level.

6.7—Column/slab jointsColumn/slab joints within a concrete frame may be prone

to construction-related problems that lead to serviceability and safety concerns. Inadequate information on the struc-tural drawings regarding the top-of-column cold joint range in relation to slab soffit may result in questionable construc-

tion execution. Columns where the concrete placement has stopped (for example, construction joint) below the slab soffit, drop cap, or drop panel may result in paste bleeding out from the formwork, resulting in honeycombed concrete and the potential for future cracking at the joints below the slab soffit. Conversely, columns where the concrete place-ment has stopped such that the column extends beyond slab formwork into the slab section, drop cap, or drop panel may no longer satisfy punching shear demand, alter slab rein-forcement placement, and result in the formation of cracks. This is especially common in podium slabs with complex slopes for drainage, where every column has a different height. Variations in column elevation and slab thickness should not exceed tolerances provided in ACI 117.

6.8—Oversized or unique-shaped columns (architectural columns)

Oversized architectural columns are another source of distress. During time-dependent concrete member short-ening of the floor diaphragm, the larger stiffness of over-sized architectural columns in relation to adjacent standard columns results in undue stress on the oversized columns, especially if such columns are located along the perimeter of a structure. The difference in relative column stiffness can result in increased restraint forces and moments. Figure 6.8 illustrates a detail developed for oversized architectural columns. The use of a compressible material around the

Fig. 6.4—Cracks in end columns of long buildings.

Fig. 6.6—Locations of potential distress due to shortening of slabs in multistory buildings.

Fig. 6.8—Release detail for slab/architectural column connections.

Fig. 6.3—Full- or partial-height wall-column release.





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slab-column dowel and neoprene pad between the slab and column concrete will allow relative movement between the slab and column, minimizing restraint. Note that this type of detail is only appropriate if slab-column frame action is not required to resist lateral loads in the structure.

CHAPTER 7—DETAILING OF WALLS

7.1—Gravity load-bearing wallsWall cracks may be grouped into global and local catego-

ries. Figure 7.1a illustrates the most common crack forma-tion due to the global behavior of walls tied to floor slabs. The diagonal tension cracks form at and near the ends of the walls due to the movement of the slab and extend over a region with a length of approximately one to two wall heights from the wall end. Such cracks can be reduced or elimi-nated by using temporary or permanent wall/slab releases. In transition regions where temporary or permanent releases change to permanent connections, it is advisable to increase wall and slab reinforcement over a distance of two times the wall height from the end of the wall. This can be achieved by increasing the minimum temperature and shrinkage rein-forcing requirement from 0.0018 to 0.0036 over a distance of two times the wall height from the end of the wall with a maximum spacing of 10 in. (250 mm) (Aalami and Barth 1989).

Local distress may occur at the ends of inadequately detailed concrete walls, as shown in Fig. 7.1b, due to the concentrated loading produced by the slab end reaction on the wall, concentrated loading at edge due to slab deflection, or both. Such distress locations need to be identified during the design phase and structural drawings detailed to with-stand the anticipated movement by adding reinforcement at the location of load transfer. One approach is to provide corner reinforcement, as shown in Fig. 7.1b, in the form of No. 4 bars (No. 13M) (maximum size) with a 10 in. (250 mm) maximum spacing.

7.2—Non-load-bearing wallsNon-load-bearing walls are typically slender and need

lateral support at the top of the wall to resist design lateral load forces, especially in seismically active regions. It is important to detail the lateral connection at the top of the wall to allow for slab movement and to prevent gravity load transfer to the wall. Slab movement relative to the wall may be facilitated by providing the top of the wall with a steel trowel finish and using a bond breaker to minimize friction and bond between the slab and wall concrete. Alternatively, neoprene bearing pads or frictionless embedded plates may be provided on top of the wall to facilitate slab movement. Details similar to those shown in Fig. 4.3.3.1 may also be used. Non-load-bearing walls not designed as vertical canti-levers can also be supported at the top of wall without using a fixed connection. This is accomplished by installing steel keeper angles on each side of the wall, attached to the slab soffit only, for lateral alignment of walls.

7.3—Shear wallsShear walls that are not fully separated from adjoining

walls may transfer lateral load to these adjacent walls. Walls not designated as shear walls may not be reinforced for the unintended loads, and may experience cracking as a result. It is imperative to isolate shear walls from adjoining members to ensure that the intended load path is developed and to prevent cracking of adjoining members. The location of shear walls that are fixed and permanently connected to the slab should be carefully selected. The best location of shear walls, from a frame shortening perspective, is near the center of mass or point of minimal slab movement, such as the central region of a slab. Figure 4.3.1a offers guidance in planning the shear wall layout to mitigate slab cracking. Figure 7.3 shows that shear walls are located near the center of no movement in their stiff axis, but capable of following the slab movement in their weak axis, which minimizes possible restraint.

7.4—Walls intended to contain liquidReinforced concrete walls that are intended to contain

liquid require special consideration to minimize cracking that could be detrimental to the reinforcement and cause leakage. Information on the design of water-retaining and environmental structures can be found in ACI 350 and ACI 350.4R.

Fig. 7.1a—Cracks in wall due to slab movement.

Fig. 7.1b—Spalling of concrete walls at slip joints and recommended added reinforcement.





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CHAPTER 8—DETAILING ON SLABS-ON-GROUND

8.1—GeneralSlabs-on-ground are considered nonstructural elements

where applied loads are passed directly though the slab to the ground. In way of comparison, structural slabs transmit vertical loads or lateral forces from other portions of the structure to the soil, and these slabs should be designed in accordance with ACI 318. Cracking in nonstructural slabs-on-ground can be mitigated and controlled to a signifi-cant extent by the proper use of contraction, construction, expansion, and isolation joints. Other factors can influence cracking, such as subgrade support, friction between slab and subgrade, reinforcement quantity and details, concrete mate-rials, mixture proportions, and curing and ambient weather during construction. This chapter discusses the use of joints and details related to joints to mitigate and control cracking. Properly designed joints allow slabs-on-ground to contract and, thus, minimize crack formation from thermal contrac-tion and drying shrinkage behavior of concrete. Curling from non-uniform contraction or shrinkage through the slab thickness is an important mechanism of slab behavior that can contribute to cracking. Any restraint to contraction and shrinkage, or curling, can lead to the development of tensile stresses that can initiate cracking of the slab. If sections of a concrete slab are free to contract, shrink, or curl, then tensile stresses are minimal and cracking is mitigated. Curling, however, can cause the corners and edges of slab sections to lift off the ground; therefore, these locations are more vulnerable to cracking by static and dynamic live loads than at other slab locations. Other approaches to mitigating or controlling cracks, such as expansive cement concrete, post-tensioning, or steel and synthetic fiber reinforcement, are not covered in this chapter, but are covered in ACI 360R.

Guidance provided in this section is intended to summa-rize recommendations needed for minimizing cracks in slabs-on-ground. More detailed guidance can be found in ACI 224.3R, ACI 302.1R, and ACI 360R. The reader is directed to ACI 360R for comprehensive coverage of the topic.

8.2—Contraction jointsContraction joints are used to promote crack formation in

a slab-on-ground at predetermined locations by creating a weakened section in the slab. Of the numerous factors that influence slab cracking, including material behavior, design details, and construction related effects, the most influential and economical method of minimizing random cracks is a close spacing of contraction joints. Scoring with a grooving tool, inserting a plastic strip in fresh concrete, or saw-cutting the slab are methods of weakening a section of slab to create a contraction joint. It is desirable to install contraction joints early, before volume changes of the concrete occur due to contraction from cooling or shrinkage from moisture loss. Contraction joints should be in place before the cool ambient temperatures of night occur on the day the concrete is cast.

Traditionally, designers specify the depth of contraction joints as one-fourth the depth of the slab, regardless of the manner in which the joint was formed. ACI 360R states that one-fourth the depth of the slab is appropriate for joints saw-cut by conventional wet-cut saws. Also, for early-entry dry-cut saws, the depth can be a minimum of 1 in. (25 mm) for slabs up to 9 in. (225 mm) thick. Some slab designers require the slab to be cut the following day to one-fourth the depth of slab by deepening the 1 in. (25 mm) saw-cut. The early-entry saw-cuts typically need to be placed within 1 hour in hot weather and 4 hours in cold weather. Experi-ence has shown that when creating a weakened cross section in fresh concrete by tooling joints, inserting plastic strips, or saw-cutting with early-entry saws, the resulting joints

Fig. 7.3—Favorable shear wall locations to minimize restraint.





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can function successfully at depths less than one-fourth of the slab depth. For example, in the initial years of early-entry saw-cutting, equipment was available that only cut to a depth of 3.4 in. (19 mm) and cutting contraction joints in 4 in. (100 mm) slabs was successful in mitigating random cracks. It is good practice from a design standpoint, however, to specify contrac-tion joints having a depth of one-fourth the slab thickness.

The purpose of reinforcing bars in slabs-on-ground is to control crack widths at the top surface of the slab. As such, the desired location of the bars is above the neutral axis, but below the depth of the contraction joint. Reinforcement does not prevent cracks, and too much reinforcement can actu-ally cause cracking. Continuous reinforcing bars that cross contraction joints influence the effectiveness of the joint by affecting the magnitude of the joint opening (crack width at the joint) and relief of stresses in the slab. Too much rein-forcement crossing a joint can cause problems, as described in the following.

General guidance for using contraction joints in unrein-forced and reinforced concrete slabs-on-ground are given in Fig. 8.2a. Figure 8.2a(a) shows a contraction joint for unre-inforced to lightly reinforced concrete, where the amount of reinforcement ranges from 0 to 0.10 percent. Slabs where no reinforcement crosses the contraction joints are accept-able. The spacing of joints, however, needs to be especially close so that the crack width at the joints remains narrow for effective load transfer by aggregate interlock. ACI 360R discusses this topic in more depth. A relatively small amount of reinforcement crossing the joint, at approximately 0.10 percent or less, has the desirable features of not providing excessive restraint at the joint, yet providing sufficient rein-forcement to assure load transfer by aggregate interlock and prevent vertical offsets at cracks.

Figure 8.2a(b) shows a contraction joint in moderately reinforced concrete, where the amount of reinforcement is approximately 0.18 to 0.20 percent (that is, the minimum reinforcement percentage according to ACI 318 for structural reinforced concrete slabs; at times, designers use this rein-forcement percentage for nonstructural slabs-on-ground). For these joints to effectively minimize random cracks in the slab, the reinforcement crossing the joint should be reduced by cutting alternate bars. The bars should be cut approxi-mately 3 in. (75 mm) away from the joint on each side of the joint. If alternate bars are not cut, the contraction joints will not open as wide as when less reinforcement crosses the joint. Possible consequences of this extra restraint are additional random cracking, or construction joints that may open considerably wider than expected. This latter problem has been observed with laser-screeded slabs or slabs cast as large areas as opposed to narrow strip-cast sections. The problem is related to the long distances between construc-tion joints that can be as much as 100 ft (30 m) or more in both directions. If the amount of reinforcement crossing contraction joints is large enough that the opening of the joint is restricted, the overall shortening of the slab between construction joints is not distributed among the contraction joints and can appear as a wide gap at the construction joints (up to 1 in. [25 mm] or more).

Heavily reinforced concrete slabs-on-ground with rein-forcement amounts of 0.50 percent or greater, shown in Fig. 8.2a(c), do not require contraction joints. Cracking in this type of slab, also called continuously reinforced concrete, is permitted to occur randomly. The increased reinforcement amount provides internal restraint to concrete shrinkage

Fig. 8.2a—Recommendations for reinforcement crossing contraction joints for slabs-on-ground. (Note: 1 in. = 25.4 mm.)

Fig. 8.2b—Joint layout and spacing for slabs-on-ground.





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and results in more frequent cracks (spacing of 3 to 8 ft [0.9 to 2.7 m]) as compared with slabs with less reinforcement (Federal Highway Administration 1990). The width of the cracks in heavily reinforced slabs, however, is narrower than in slabs with less reinforcement.

Figure 8.2b shows contraction joint spacing and layout for slabs-on-ground. The contraction joint spacing should be reduced next to a footing or similar thickened section, as illustrated in Fig. 8.2b. Contraction joint layout should be square or rectangular with the length of one side less than or equal to 1.5 times the length of the adjacent side. Contrac-tion joints should not be offset, as shown in Fig. 8.2c.

The spacing of contraction joints is the most important factor affecting the ability of contraction joints to control cracking in slabs. ACI 360R provides recommendations on spacing of joints, as shown in Fig. 8.2d. Appropriate joint layout and spacing can provide a more effective and economical approach to minimizing random cracks than all

other factors that affect slab cracking, including material behavior, design details, and construction-related effects.

8.3—Construction jointsConstruction joints are locations in the slab where concrete

placement is intentionally terminated and then resumed at a later time. In addition to being an important part of slab construction planning and execution, construction joints can be designed to function as contraction joints, and in most instances, this is desirable. Construction joints can also be designed for full restraint, as would occur for continuously reinforced concrete slabs. Figure 8.3a shows two examples of construction joints designed to function as contraction joints. Dowel configurations are preferable to tongue-and-grove designs. After contraction of a slab, the tongue-and-groove joint has opened a small distance and the top lip of the groove is then unsupported and vulnerable to breaking by wheeled traffic. Figure 8.3b shows a plan view of plate and bar dowels used to provide load transfer at the joint. Dowels

Fig. 8.2c—Undesirable contraction joint layout (not recommended).

Fig. 8.2d—Recommendations for contraction joint spacing (ACI 360R).

Fig. 8.3a—Construction joints designed to function as a contraction joint.

Fig. 8.3b—Dowel configurations at construction joints.





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may be smooth bars of circular or square cross section, or can consist of various shaped plates. Square and plate dowels may be cushioned on their vertical sides to allow movement of the slab parallel to the joint. The dowels should be coated to prevent bond and inserted perpendicular to the vertical face of the joint. The dowels should not restrain contrac-tion of the slab. ACI 360R discusses this topic more fully,

including illustration of various bar and plate dowel types, and provides recommendations on dowel size and spacing.

8.4—Expansion and isolation jointsExpansion and isolation joints are similar in detail, but

have different functions. Expansion joints have the primary function of permitting slab sections to expand when ambient temperatures significantly increase. Without expansion joints, some slabs may buckle or blow up. For instance, sidewalks, which are long and narrow, are susceptible to blowups. ACI 224.3R states that expansion joints are rarely placed less than 100 ft (30 m) apart. In contrast, on the subject of concrete parking lots, ACI 330R states that expan-sion joints are seldom needed to accommodate concrete expansion, especially with properly spaced contraction joints (because these joints open from thermal contraction and drying shrinkage, and later close with expansion).

Isolation joints are used to separate concrete slabs from fixed structures, such as light pole foundations, buildings, or machinery foundations. Isolation joints are typically composed of a crushable material, such as fiberboard or foam that extends through the full depth of the slab, as illus-trated in Fig. 8.4a.

Column posts that pass through slabs-on-ground should be isolated from the slab. ACI 360R provides two details at column posts in Fig. 8.4b and 8.4c. Note that in Fig. 8.4b, void space is provided around the steel column from the top of the slab to the foundation level. This space is to be filled

Fig. 8.4a—Isolation joint.

Fig. 8.4b—Typical isolation joint at tubular column (ACI 360R).

Fig. 8.4c—Alternate isolation joint at wide flange column (ACI 360R).





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with concrete or by other methods after the slab has been cast and the column is carrying full dead load.

CHAPTER 9—REFERENCESAmerican Concrete InstituteACI 117-10—Specification for Tolerances for Concrete

Construction and Materials and CommentaryACI 209.2R-08—Guide for Modeling and Calculating

Shrinkage and Creep in Hardened ConcreteACI 224R-01—Control of Cracking in Concrete Struc-

tures (Reapproved 2008)ACI 224.2R-92—Cracking of Concrete Members in

Direct Tension (Reapproved 2004)ACI 224.3R-94—Joints in Concrete Construction (Reap-

proved 2008)ACI 302.1R-04—Guide for Concrete Floor and Slab

ConstructionACI 318-11—Building Code Requirements for Structural

Concrete and CommentaryACI 330R-08—Guide for the Design and Construction of

Concrete Parking LotsACI 350-06—Code Requirements for Environmental

Engineering Concrete StructuresACI 350.4R-04—Design Considerations for Environ-

mental Engineering Concrete StructuresACI 360R-10—Design of Slabs-on-Ground

Cited referencesAalami, B. O., and Barth, F. G., 1989, “Restraint Cracks

and Their Mitigation in Unbonded Post-Tensioned Building Structures,” Cracking in Prestressed Concrete Structures, SP-113, G. T. Halvorsen and N. H. Burns, eds., American Concrete Institute, Farmington Hills, MI, pp. 157-202.

ACI Committee 315, 2004, ACI Detailing Manual, SP-66, American Concrete Institute, Farmington Hills, MI, 212 pp.

Altoubat, S. A., and Lange, D. A., 2001, “Creep, Shrinkage, and Cracking of Restrained Concrete at Early Age,” ACI Materials Journal, V. 98, No. 4, July-Aug., pp. 323-331.

Blackman, D. T., and Frosch, R. J., 2005, “Epoxy Coated Reinforcement and Crack Control,” Serviceability of Concrete, SP-225, American Concrete Institute, Farmington Hills, MI, pp. 163-178.

Federal Highway Administration, 1990, “Continuously Reinforced Concrete Pavement,” Technical Advisory T 5080.14, 7 pp.

Frosch, R. J., 1999, “Another Look at Cracking and Crack Control in Reinforced Concrete,” ACI Structural Journal, V. 96, No. 3, May-June, pp. 437-442.

Kim, J. K., and Lee, C. S., 1998, “Prediction of Differen-tial Drying Shrinkage in Concrete,” Cement and Concrete Research, V. 28, No. 7, pp. 984-994.

Martin, L. D., and Perry, C. J., eds., 2004, PCI Design Handbook for Precast Prestressed Concrete, sixth edition, Precast/Prestressed Concrete Institute, Chicago, IL, 750 pp.

Nawy, E. G., 2005, Reinforced Concrete, fifth edition, Prentice Hall, Upper Saddle River, NJ, 864 pp.





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As ACI begins its second century of advancing concrete knowledge, its original chartered purpose remains “to provide a comradeship in finding the best ways to do concrete work of all kinds and in spreading knowledge.” In keeping with this purpose, ACI supports the following activities:

· Technical committees that produce consensus reports, guides, specifications, and codes.

· Spring and fall conventions to facilitate the work of its committees.

· Educational seminars that disseminate reliable information on concrete.

· Certification programs for personnel employed within the concrete industry.

· Student programs such as scholarships, internships, and competitions.

· Sponsoring and co-sponsoring international conferences and symposia.

· Formal coordination with several international concrete related societies.

· Periodicals: the ACI Structural Journal and the ACI Materials Journal, and Concrete International.

Benefits of membership include a subscription to Concrete International and to an ACI Journal. ACI members receive discounts of up to 40% on all ACI products and services, including documents, seminars and convention registration fees.

As a member of ACI, you join thousands of practitioners and professionals worldwide who share a commitment to maintain the highest industry standards for concrete technology, construction, and practices. In addition, ACI chapters provide opportunities for interaction of professionals and practitioners at a local level.

American Concrete Institute38800 Country Club DriveFarmington Hills, MI 48331U.S.A.Phone: 248-848-3700Fax: 248-848-3701

www.concrete.org




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The AMERICAN CONCRETE INSTITUTE

was founded in 1904 as a nonprofit membership organization dedicated to public service and representing the user interest in the field of concrete. ACI gathers and distributes information on the improvement of design, construction and maintenance of concrete products and structures. The work of ACI is conducted by individual ACI members and through volunteer committees composed of both members and non-members.

The committees, as well as ACI as a whole, operate under a consensus format, which assures all participants the right to have their views considered. Committee activities include the development of building codes and specifications; analysis of research and development results; presentation of construction and repair techniques; and education.

Individuals interested in the activities of ACI are encouraged to become a member. There are no educational or employment requirements. ACI’s membership is composed of engineers, architects, scientists, contractors, educators, and representatives from a variety of companies and organizations.

Members are encouraged to participate in committee activities that relate to their specific areas of interest. For more information, contact ACI.

www.concrete.org




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CONTENTSCHAPTER 1— INTRODUCTION AND SCOPE1.1—Introduction1.2—Objective1.3—Scope

CHAPTER 2— NOTATION AND DEFINITIONS2.1—Notation2.2—Definitions

CHAPTER 3— DESIGN-DETAILING CONSIDERATIONS3.1—Concrete member type and reinforcement3.2—Overall and local regions3.3—Framing compatibility

CHAPTER 4— DETAILING OF TWO-WAY REINFORCE

224.4r-13 guide to design detailing to mitigate cracking · to offer design and detailing...

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