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Reinforcing Steel in Slabs-On-Grade

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Reinforcing Steel in Slabs-On-Grade REINFORCING STEEL CRSI IN ENGINEERING SLABS-ON-GRADE DATA REPORT A SERVICE OF THE CONCRETE REINFORCING STEEL INSTITUTE NUMBER 37 933 N. Plum Grove Rd., Schaumburg, Illinois 60173-4758 WELDED WIRE FADRIC REINFORCING BARS INTRODUCTION The phrase slab-on-grade, as commonly used in When welded wire fabric (deformed or plain) is practice, covers a number of practical slab specified, it should have sufficiently large wire applications. Included are industrial and commercial diameters so as to be structurally stiff enough to floors, residential floors, parking lots, pavements, remain in its proper position during construction, with airport slabs and other practical uses of ground- a wire spacing selected for construction stability and supported concrete slabs. This report applies to all of convenience, and the WWF must be supported. these applications. Reinforcing steel, in the form of welded wire fabric ADVANTAGES or deformed bars in both directions, may be placed in There are numerous advantages that result from the slab-on-grade for a number of reasons. This form placing reinforcing bars or welded wire fabric in a of reinforcement adds to the strength of the slab, acts slab-on-grade. The following list shows what as crack control including the well-known restraint of reinforcement can do for the slab. Some of these can shrinkage (as well as temperature) effects, reduces be achieved in no other way: future maintenance, and generally helps produce a higher quality concrete slab. A complete listing of • Controls Cracking advantages is included elsewhere in this report. Reinforcement within a slab prevents cracks from opening and becoming wide and Reinforcing bars or welded wire fabric (WWF) objectionable. These cracks may be due to functions as reinforcement because such steel is drying shrinkage or due to externally applied supported and thus placed in a specific position in loadings. When these cracks are kept tight, the slab prior to the placement of the concrete. Visual aggregate interlock will still exist and faulting inspection verifies that the reinforcement is properly will not occur. Cracks are frequently kept so positioned and will have the positive effects desired. tight as to be difficult to see. When reinforcing bars are specified, they are to be • Allows Increased Joint Spacings placed in two layers (one layer directly in contact with The spacings of construction and control the other), with bars running in two perpendicular joints may be increased when distributed directions, tied together so as to maintain the reinforcement is used. The increase can vary spacing requirements, and they must be supported. from ‘slight’ to ‘substantial’ depending upon © Copyright 1998 by the Concrete Reinforcing Steel Institute the concept of design and the intended maintenance costs should be substantially performance of the slab. The subgrade drag reduced. equation (explained in design example) has • Allows Construction of a “No—Joint” Slab been used to relate wider joint spacings with With reinforcement, construction joints may given areas of reinforcing bars or welded be spaced according to the planned size of a wire fabric. single daVs concrete placement. This can be in strip shape or large panel shape. In such • Spans Soft Spots in Subgrade a slab, no contraction joints are used. The During the construction process of grading, distributed reinforcement allows acceptable filling, and compacting of the subgrade hairline cracking due to drying shrinkage. system, it is not uncommon to encounter or produce soft spots. These can occur due to • Required for Shrinkage Compensating Concrete local moisture, last-minute excavation for a The use of shrinkage-compensating drain line, or similar occurrences. The concrete demands the use of reinforcing reinforced slab-on-grade will span these so- steel. All the advantages of the steel will called soft spots by providing enough exist even though the steel is required for the structural capacity to bridge the weaker concrete to expand and contract as planned. supporting areas. • Avoids Use of Thickened Slabs at Joints and • Restraint to Curling Edges Reinforcement in the upper half of the Reinforcing steel can be selected to give the concrete slab will act as a restraint to concrete slab adequate strength at the joints. concrete shrinkage. When the shrinkage in This will allow the slab to maintain a constant the upper portion of the slab is restrained, thickness without thickening at joints or and thereby lessened, curling will be edges. This thickening adds more restraint to reduced. The closer the steel is to the top drying shrinkage as well as requiring and the more steel area there is, the more additional construction effort and costs. the curling will be reduced. • Gives Confidence when Conditions of Support • Structural Strength After Cracking are in Doubt When overloading occurs, such that the It is common for a slab-on-grade to be cracking moment limit of the concrete slab planned and designed without a complete has been exceeded, structural cracks may report on the soil, fill material, or the occur. The steel will then act as structural subgrade support capability. The presence reinforcement and provide moment capacity of reinforcing bars or welded wire fabric according to normal, cracked-section, creates a confidence in the ability of the slab reinforced concrete theory. This concept may to perform adequately even though some also be intentionally used in the original doubt may exist concerning the subgrade design concept of the slab; that is, designing support. the slab to have structurally-active reinforcement under externally applied THE GENERAL PROCESS OF DESIGN loadings. The design of a slab-on-grade includes all decisions and details made before the job is bid and • Increases Resistance to Impact constructed. Three of the more obvious items In numerous slab applications, loading is included in the design process are slab thickness, such that impact is caused on the slab. Such reinforcement requirements and joint spacings. After loading increases the stress level in the determining the controlling loadings, the appropriate concrete. Reinforcement will reduce the safety factor, and the subgrade modulus appropriate strains caused by the impact loading, for the base and fill materials, the designer thereby preventing premature cracking, and determines the required thickness. provide structural strength to the slab if the impact is so much that cracking does References for thickness determination include actually occur. publications of the Portland Cement Association, the American Concrete Institute, the Concrete Reinforcing • Reduces Slab and Joint Maintenance Steel Institute, and the Wire Reinforcement Institute. When cracks are maintained in a tighter condition and when curling is reduced, both The next most common step is the selection of the by the presence of reinforcing steel, reinforcing steel area (bar or wire size and their maintenance of these items is reduced. With spacing) along with an acceptable joint spacing. Both supported steel in the top half of the slab, the of these involve knowing the slab’s performance • For ASTM A 185 plain welded wire fabric : requirements such as lanes of traffic, aisle and storage rack placements, flatness requirements, joint The subgrade drag equation is the same details, dowel recommendations and the like. except for the use of fy which is 65,000 psi. Reinforcing steel, in the form of welded wire fabric or patterns of deformed bars, is then selected for As = 0.042 sq.in./ft of slab width, required purposes of either crack control, using the subgrade each way. drag formula, or for structural strength, using common reinforced concrete design procedures. Use W4.5 wire at 12-in, spacings in each direction, designated as: Regardless of the intended purpose of the reinforcement, it must be structurally stiff (strong 12 x 12 - W4.5 x W4.5. enough to support working loads) and/or widely spaced (individual elements spaced wide enough so 2. 48—ft joint spacings: that workers can walk between them). Further, it This is a wide joint spacing and could be must be supported at the proper position, so as to considered as a “no joint” slab design. No saw-cut provide its intended advantages. All requirements for contraction joints are used longitudinally; however, if a quality slab-on-grade should be detailed and strip placement of the slab is used, then a clearly shown on the design drawings. contraction loint would be cut at 48 ft spacings transversely along the pour strip. In the subgrade DESIGN EXAMPLES drag equation, the length L is now 48 ft and the yield strength, fy, is either 60,000 psi (bars) or 70,000 psi These design examples illustrate the selection of (deformed wire), as follows: reinforcing steel for the purposes of controlling cracks caused by shrinkage. The subgrade drag • For Grade 60 reinforcing bars : equation is used for calculation of the steel areas, which are determined here for both a common loint As = F Lw / 2 fs where F = 1.5, L = 48 ft, spacing as well as for a wide joint spacing. The w = 100 psf, and fs is 2/3 fy = 2/3 x 60000 reinforcing steel is not to be continuous through any psi = 40,000 psi. of the contraction or construction joints. This holds for both wires and bars. 0.090 sq.in./ft of slab width, each way. For these design examples, assume that an 8-in. Use #4 bars at 25 in. spacings c.-c., each slab is required. Column spacings are 48 ft center-to- way (or #4 bars at 18 in. c.-c.) center and construction joints will have this spacing for strip placement of the concrete. • For ASTM A 497 deformed welded wire fabric : 1. 24—ft loint spacings: The subgrade drag equation is the same This joint spacing, commonly encountered in the except for the use of fy which is 70,000 psi.
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