The key takeaways are that reinforcement in slabs on grade can control cracking, increase joint spacing, span soft spots in the subgrade, restrain curling, add structural strength after cracking, add impact resistance, reduce maintenance of slabs and joints, eliminate the need for contraction joints and thickened edges, and add confidence in the slab's performance.
The advantages of using reinforcement in slabs on grade include controlling cracking, increasing joint spacing, spanning soft spots in the subgrade, restraining curling, adding structural strength after cracking, adding impact resistance, reducing slab and joint maintenance, eliminating the need for contraction joints, working with shrinkage-compensating concrete, and eliminating the need for thickened slabs at joints and edges.
Considerations for positioning reinforcement in slabs on grade include placing it at or above the mid-depth of the slab, within 2 inches of the top surface, or one-third of the depth from the top surface. Support devices must be used to keep the reinforcement in the correct position during construction. For slabs 5 inches thick or less, position the reinforcement at mid-depth.
W
hen properly placed, rebar
and welded wire fabric (WWF) in a slab on grade pro- vide many advantages, some of which can be achieved in no oth- er way. Reinforcing steel can: C o n t rol cracking. Steel rein- forcement within a slab holds cracks caused by drying shrink- age, temperature changes, or ap- plied loadings, tightly closed. Keeping cracks closed preserves aggregate interlock and prevents faulting. I n c rease joint spacing. I n- creases in control joint spacing when rebar or WWF is used can range 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 subbase or subgrade can occur due to moisture, last-minute excava- tion for a drain, or similar situa- tions. A reinforced slab on grade will span soft spots by providing enough structural ca- pacity to bridge the weaker sup- porting areas. Restrain curling. Reinforcing the up- per half of a slab on grade restrains drying shrinkage, thus reducing curling. The closer the steel is to the top of the slab and the more steel used, the more curling will be reduced. Add structural strength after crack- i n g . When overloading occurs and the cracking moment limit of the slab has been 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 or WWF reduces strain to slabs on grade caused by impact loading. This helps pre- vent premature cracking. Reduce slab and joint maintenance. By keeping cracks tight and reducing curling, reinforcing steel substantially re- duces crack and joint maintenance. Eliminate the need for con- traction joints. When using re- inforcement, construction joints can be spaced according to the planned size of a days p o u r. This could be a strip or a l a rge rectangular panel. No contraction joints are needed because the distributed rein- forcement allows acceptable hairline cracking due to drying shrinkage. Work with shrinkage-com- pensating concre t e . S h r i n k a g e - compensating concrete demands the use of reinforcing steel so the concrete can expand and con- tract as planned. Eliminate the need for thickened slabs at joints and e d g e s . Reinforcing steel gives slabs on grade adequate strength at the joints. This allows the de- signer to maintain a constant slab depth without thickening at joints or edges. Thickened edges add more restraint to drying 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 to perform adequately, even though doubt may exist concerning the subgrades support capability. Reinfor cing st eel in slabs on gr ade Use welded-wir e fabr ic or r ebar t o cont r ol cr acking and incr ease st r engt h Reinforcing steel must be posi- tioned at or above mid-slab depth to be effective. Here workers carefully set wire on chair supports. General design process Though many details must be includ- ed in the design of a slab on grade, three important components are slab thick- ness, reinforcement requirements, and joint spacing. The designer determines the required slab thickness after determining the con- trolling loads, the appropriate safety fac- t o r, and the subgrade modulus appropriate for the base and fill materials. References at the end of the article discuss thickness determination in more detail. The next step is to select the reinforc- ing steel area (bar or wire size and their spacing) along with an acceptable joint spacing. Both of these involve knowing the slabs performance requirements, such as lanes of traffic, aisle and storage rack placement, flatness requirements, joint details, and dowel recommendations. WWF or rebar are then selected for crack control, using the subgrade drag equation, or for structural strength, using common reinforced concrete design procedures. Regardless of the intended purpose of the reinforcement, it must be structurally s t i ff (to support workers placing concrete) or widely spaced (so workers can step be- tween bars or wires). Furthermore, the steel must be supported at the proper po- sition in the slab. Design drawings should clearly show all these details. Design examples The following design examples for two d i fferent joint spacings illustrate how to select reinforcing steel for controlling cracks caused by shrinkage and tempera- ture effects. Both use the subgrade drag equation 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 wire and bars. Each example is for an 8-inch-thick in- dustrial floor slab. Column spacings are 48 feet center to center. Construction joints also have this spacing for strip placement of the concrete. 24-foot joint spacing. A slab with this joint 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 (A s ), use the subgrade drag equation: For Grade 60 rebar A s = F L w/2 f s where F (the subgrade friction factor) = 1.5 (a commonly used value), L = 24 feet, w = 100 psf (weight of slab), and f s (working stress in steel) = 2 3 f y where f y (yield strength of steel) is 60,000 psi. A s = 0.045 square inches per foot of slab width, required each way. Use #3 bars at 29 inches center to cen- ter in each direction, A s = 0.046 square inches per foot. If following the American Concrete Institute (ACI) structural slab limitation of 5h or 18 inches, whichever is less, then use #3 bars at 18 inches, A s = 0.073 square inches per foot. For ASTM A 185 plain WWF Values in the subgrade drag equation are the same except for f y , which is 65,000 psi. A s = 0.042 square inches per foot of slab width, required each way. Use W4.5 wire at 12-inch spacings in each direction, designated as: 12 x 12W4.5 x W4.5 48-foot joint spacing. A floor slab with 48-foot-wide joint spacing could be considered a joint-free slab. No sawcut contraction joints are used longitudinally. H o w e v e r, if using strip placement of the slab, then cut a contraction joint at 48-foot spacings transversely along each strip. In the subgrade drag equation, the length L is now 48 feet and yield strength of steel, f y , is either 60,000 psi (bars) or 70,000 psi (deformed wire): For Grade 60 rebar A s = F L w/2 f s where F = 1.5, L = 48 feet, w = 100 psf, and f s is 2 3 f y = 2 3 x 60,000 psi = 40,000 psi. A s = 0.090 square inches per foot of slab width, required each way. Use #4 bars at 25-inch spacings center to center each way, A s = 0.096 square inches per foot (or #4 bars at 18 inches center to center to meet ACI criteria, A s = 0.133 square inches per foot). For ASTM A 497 deformed welded wire fabric Values in the subgrade drag equation are the same as above except for f y , which is 70,000 psi. A s = 0.077 square inches per foot of slab width, required each way. Use D8 wire at 12-inch spacings in each direction, designated as: 12 x 12 D8 x D8 (A s = 0.080 square inches per foot) The steel areas selected using the sub- grade drag equation are for shrinkage and temperature effects. If the reinforcement is intended to be structurally active to resist bending stresses produced by loads on the slab then the subgrade drag equation is not appropriate (Ref. 2). Positioning reinforcement To be effective, reinforcing steel must be positioned at or above the mid-depth of the slab. Some authorities recommend placing the steel 2 inches below the top surface of the slab. Others recommend placing the steel one-third of the depth down from the top of the slab. Any of these locations can be appropriate, de- pending on reinforcement needs, such as for crack control or structural strength. Never position a single reinforcement layer below mid-depth. Steel in two direc- tions with the bars or wires in contact with one another is considered one lay- e r. For most slabs on grade, position the reinforcement at one-third the depth from the top surface. If the slab is 5 inches thick or less, then position the steel at mid-depth. Supporting reinforcement Since positioning of reinforcement is critical, support devices must be used to keep the steel at the correct position dur- ing the construction process, especially during concrete placement. When rebar are specified, they should be placed in two layers (one layer directly contacting the other), with bars in each layer perpendicular to the other. When WWF (deformed or plain) is specified, its wire diameters should be large enough so the fabric has enough stiffness to remain in proper position during slab construc- tion. If bars or wires are not stiff enough to 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 joints A question that often arises is what to do with reinforcing steel at joints, par- ticularly contraction joints. The answer depends on the purpose of the joint. If the joint is to be a working joint that must open and provide relief for stresses due to shrinkage and temperature ef- fects, then its best to discontinue all steel at the joint. Any steel that contin- ues through a contraction joint will re- strain joint movement. If the joint is to remain closed, then the steel may con- tinue through the joint. If load transfer is required at the joint, but the distributed steel is interrupted, use dowels across the joint (see article on p. 532). References 1. Guide for Concrete Floor and Slab Construction, ACI 302.1R-89, Ameri- can Concrete Institute (ACI), Detroit, 1989. 2. The Structurally Reinforced Slab on Grade, 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, 2nd Edition, Portland Cement Association, Skokie, Ill., rev. 1990. 6. Design of Slabs on Grade, ACI 360R-92, ACI, 1992. Acknowledgment This article is adapted with permission from Reinforcing Steel in Slabs on Grade (Engineering Data Report No. 37), published by the Concrete Rein- forcing Steel Institute in a joint effort with the Wire Reinforcement Institute (Tech Facts 701). For a copy of the re- port, call CRSI (708-517-1200) or WRI (202-429-5125). P U B L I C ATION #C920538 Copyright 1992, The Aberdeen Group All rights reserved