When properly placed, rebarand welded wire fabric

(WWF) in a slab on grade pro-vide many advantages, some ofwhich can be achieved in no oth-er way. Reinforcing steel can:

C o n t rol cracking. Steel rein-forcement within a slab holdscracks caused by drying shrink-age, temperature changes, or ap-plied loadings, tightly closed.Keeping cracks closed preservesaggregate interlock and preventsfaulting.

I n c rease joint spacing. I n-creases in control joint spacingwhen rebar or WWF is used canrange from slight to substantial,depending on the design and in-tended performance of the slab.

Span soft spots in the sub-g r a d e . Soft spots in the subbaseor subgrade can occur due tomoisture, last-minute excava-tion for a drain, or similar situa-tions. A reinforced slab ongrade will span soft spots byproviding enough structural ca-pacity to bridge the weaker sup-porting areas.

Restrain curling. Reinforcing the up-per half of a slab on grade restrains dryingshrinkage, thus reducing curling. Thecloser the steel is to the top of the slab andthe more steel used, the more curling willbe reduced.

Add structural strength after crack-i n g . When overloading occurs and thecracking moment limit of the slab hasbeen exceeded, structural cracks can oc-

c u r. The steel acts as structural reinforce-ment and provides moment capacity.

Add impact re s i s t a n c e . Rebar orWWF reduces strain to slabs on gradecaused by impact loading. This helps pre-vent premature cracking.

Reduce slab and joint maintenance.By keeping cracks tight and reducingcurling, reinforcing steel substantially re-duces crack and joint maintenance.

Eliminate the need for con-traction joints. When using re-inforcement, constructionjoints can be spaced accordingto the planned size of a day’sp o u r. This could be a strip or al a rge rectangular panel. Nocontraction joints are neededbecause the distributed rein-forcement allows acceptablehairline cracking due to dryingshrinkage.

Work with shrinkage-com-pensating concre t e . S h r i n k a g e -compensating concrete demandsthe use of reinforcing steel so theconcrete can expand and con-tract as planned.

Eliminate the need forthickened slabs at joints ande d g e s . Reinforcing steel givesslabs on grade adequate strengthat the joints. This allows the de-

signer to maintain a constant slab depthwithout thickening at joints or edges.Thickened edges add more restraint todrying shrinkage and cost more.

Add confidence. The presence of re-bar or WWF in a slab on grade increas-es confidence in the ability of the slab toperform adequately, even though doubtmay exist concerning the subgrade’ssupport capability.

Reinforcing steel in slabs on gradeUse welded-wire fabric or rebar to control cracking and increase strength

Reinforcing steel must be posi-tioned at or above mid-slabdepth to be effective. Hereworkers carefully set wire onchair supports.

General design processThough many details must be includ-

ed in the design of a slab on grade, threeimportant components are slab thick-ness, reinforcement requirements, andjoint spacing.

The designer determines the requiredslab thickness after determining the con-trolling loads, the appropriate safety fac-t o r, and the subgrade modulus appropriatefor the base and fill materials. Referencesat the end of the article discuss thicknessdetermination in more detail.

The next step is to select the reinforc-ing steel area (bar or wire size and theirspacing) along with an acceptable jointspacing. Both of these involve knowingthe slab’s performance requirements, suchas lanes of traffic, aisle and storage rackplacement, flatness requirements, jointdetails, and dowel recommendations.WWF or rebar are then selected for crackcontrol, using the subgrade drag equation,or for structural strength, using commonreinforced concrete design procedures.

Regardless of the intended purpose ofthe reinforcement, it must be structurallys t i ff (to support workers placing concrete)or widely spaced (so workers can step be-tween bars or wires). Furthermore, thesteel must be supported at the proper po-sition in the slab. Design drawings shouldclearly show all these details.

Design examplesThe following design examples for two

d i fferent joint spacings illustrate how toselect reinforcing steel for controllingcracks caused by shrinkage and tempera-ture effects. Both use the subgrade dragequation for calculating the steel areas.The reinforcing steel is not to be continu-ous through any of the contraction or con-struction joints. This holds for both wireand bars.

Each example is for an 8-inch-thick in-dustrial floor slab. Column spacings are48 feet center to center. Constructionjoints also have this spacing for stripplacement of the concrete.

24-foot joint spacing. A slab with thisjoint spacing may be reinforced with bars(ASTM A 615, A 616, A 617, or A 706)or WWF (ASTM A 185 or A 497). To se-lect the appropriate steel area (As), use thesubgrade drag equation:

For Grade 60 rebarAs = F L w/2 fs where F (the subgrade

friction factor) = 1.5 (a commonly usedvalue), L = 24 feet, w = 100 psf (weightof slab), and fs (working stress in steel) =2⁄3 fy where fy (yield strength of steel) is60,000 psi.

As = 0.045 square inches per foot ofslab width, required each way.

Use #3 bars at 29 inches center to cen-ter in each direction, As = 0.046 squareinches per foot. If following the AmericanConcrete Institute (ACI) structural slablimitation of 5h or 18 inches, whichever isless, then use #3 bars at 18 inches, As =0.073 square inches per foot.

For ASTM A 185 plain WWFValues in the subgrade drag equation

are the same except for fy, which is65,000 psi.

As = 0.042 square inches per foot ofslab width, required each way.

Use W4.5 wire at 12-inch spacings ineach direction, designated as:

12 x 12—W4.5 x W4.5

48-foot joint spacing. A floor slabwith 48-foot-wide joint spacing could beconsidered a joint-free slab. No sawcutcontraction joints are used longitudinally.H o w e v e r, if using strip placement of theslab, then cut a contraction joint at 48-footspacings transversely along each strip. Inthe subgrade drag equation, the length Lis now 48 feet and yield strength of steel,fy, is either 60,000 psi (bars) or 70,000 psi(deformed wire):

For Grade 60 rebarAs = F L w/2 fs where F = 1.5, L = 48

feet, w = 100 psf, and fs is 2⁄3 fy = 2⁄3 x60,000 psi = 40,000 psi.

As = 0.090 square inches per foot ofslab width, required each way.

Use #4 bars at 25-inch spacings centerto center each way, As = 0.096 squareinches per foot (or #4 bars at 18 inchescenter to center to meet ACI criteria, As =0.133 square inches per foot).

For ASTM A 497 deformed weldedwire fabric

Values in the subgrade drag equationare the same as above except for fy, whichis 70,000 psi.

As = 0.077 square inches per foot ofslab width, required each way.

Use D8 wire at 12-inch spacings ineach direction, designated as: 12 x 12—D8 x D8 (As = 0.080 square inches perfoot)

The steel areas selected using the sub-grade drag equation are for shrinkage andtemperature effects. If the reinforcementis intended to be structurally active — toresist bending stresses produced by loadson the slab — then the subgrade dragequation is not appropriate (Ref. 2).

Positioning reinforcementTo be effective, reinforcing steel must

be positioned at or above the mid-depthof the slab. Some authorities recommendplacing the steel 2 inches below the topsurface of the slab. Others recommendplacing the steel one-third of the depthdown from the top of the slab. Any ofthese locations can be appropriate, de-pending on reinforcement needs, such asfor crack control or structural strength.

Never position a single reinforcementlayer below mid-depth. Steel in two direc-tions with the bars or wires in contactwith one another is considered “one lay-e r.” For most slabs on grade, position thereinforcement at one-third the depth fromthe top surface. If the slab is 5 inchesthick or less, then position the steel atmid-depth.

Supporting reinforcementSince positioning of reinforcement is

critical, support devices must be used tokeep the steel at the correct position dur-ing the construction process, especiallyduring concrete placement.

When rebar are specified, they shouldbe placed in two layers (one layer directlycontacting the other), with bars in eachlayer perpendicular to the other. WhenWWF (deformed or plain) is specified, itswire diameters should be large enough sothe fabric has enough stiffness to remainin proper position during slab construc-

tion. If bars or wires are not stiff enoughto support workers standing on the steel,then their spacings must be wide enough— at least 12 inches center to center —for workers to stand between them.

What to do at jointsA question that often arises is what to

do with reinforcing steel at joints, par-ticularly contraction joints. The answerdepends on the purpose of the joint. Ifthe joint is to be a working joint thatmust open and provide relief for stressesdue to shrinkage and temperature ef-fects, then it’s best to discontinue allsteel at the joint. Any steel that contin-ues through a contraction joint will re-strain joint movement. If the joint is toremain closed, then the steel may con-tinue through the joint. If load transfer isrequired at the joint, but the distributedsteel is interrupted, use dowels acrossthe joint (see article on p. 532).

References1. Guide for Concrete Floor and SlabConstruction, ACI 302.1R-89, Ameri-can Concrete Institute (ACI), Detroit,1989.

2. The Structurally Reinforced Slab onGrade, Engineering Data Report No.33, Concrete Reinforcing Steel Insti-tute, Schaumburg, Ill., 1989.

3. Building Code Requirements for Re-inforced Concrete and Commentary,ACI 318-89/ACI 318R-89, ACI, 1989.

4. Structural Welded Wire Fabric De-tailing Manual, Part I (1983) and Part II(1989), Wire Reinforcement Institute,Washington, DC.

5. Concrete Floors on Ground, 2ndEdition, Portland Cement Association,Skokie, Ill., rev. 1990.

6. Design of Slabs on Grade, ACI360R-92, ACI, 1992.

AcknowledgmentThis article is adapted with permissionfrom Reinforcing Steel in Slabs onGrade (Engineering Data Report No.37), published by the Concrete Rein-forcing Steel Institute in a joint effortwith the Wire Reinforcement Institute(Tech Facts 701). For a copy of the re-port, call CRSI (708-517-1200) or WRI(202-429-5125).

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