360R-06 Design of Slabs-on-Ground
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| 2.2—Slab types
There are four basic design choices for construction of slabs-on-ground: • Unreinforced concrete slab; • Slabs reinforced to limit crack widths due to shrinkage and temperature restraint and applied loads. These slabs consist of the following: –Mild steel bar, wire reinforcement, or fiber reinforce- ment, all with closely spaced joints; and –Continuously reinforced (sawcut contraction joint- free floors); • Slabs reinforced to prevent cracking due to shrinkage and temperature restraint and applied loads. These slabs consist of the following: –Shrinkage-compensating concrete; and –Post-tensioned; • Structural slabs (ACI 318). 2.2.1 Unreinforced concrete slab—The design of this type of slab involves determining its thickness as a plain concrete slab without reinforcement; however, it may have joints strengthened with steel dowels. It is designed to remain uncracked between joints due to loads on the slab surface and restraint to concrete volumetric changes. Unreinforced concrete slabs do not contain high-volume macropolymeric fibers, wire fabric, steel fibers, plain or deformed bars, post- tensioning, or any other type of steel reinforcement. The cement normally used is portland cement Type I or II (ASTM C 150). The effects of drying shrinkage and uniform subgrade support on slab cracking are critical to the perfor- mance of unreinforced concrete slabs. Design methods for unreinforced slabs are provided in Chapter 6 . 2.2.2 Slabs reinforced for crack width control—Thickness design can be the same as for unreinforced concrete slabs, and the slab is assumed to remain uncracked due to loads placed on its surface. Shrinkage crack width (if cracking occurs) for slabs constructed with portland cement is controlled by a nominal quantity of distributed reinforcement placed in the upper 1/3 of the slab. The primary purpose of the reinforcement is to limit the width of any cracks that may form between the joints. Bar or wire reinforcement should be stiff enough so that it can be accurately located in the upper 1/3 of the slab. Slabs may be reinforced with reinforcing bars, welded wire reinforcement sheets, steel fibers, or macropolymeric fibers. Bars or welded wire reinforcement can be used to provide moment capacity at a cracked section. In this case and for slabs of insufficient thickness to carry the applied loads as an unreinforced slab, the reinforcement required for strength should be sized by conventional reinforced concrete theory as described in ACI 318. Using the methods in ACI 318 with high steel reinforcement stresses, however, may lead to unacceptably wide crack widths. Currently, building codes do not support the use of fiber reinforcement to provide moment capacity in cracked sections. Reinforcement, other than post-tensioning or the reinforce- ment used in a shrinkage-compensating slab, does not prevent cracking. Typically, the most economical way to obtain increased load-carrying capacity is to increase the thickness of the slab. Design methods for slabs reinforced for limiting crack width can be found in Chapters 6 , 7 , and 10 . 2.2.3 Slabs reinforced to prevent cracking—Post- tensioned slabs and shrinkage-compensating slabs are typically designed not to crack. Some incidental minor cracking may still occur, however. The reinforcement is used to prevent the slab from cracking. For shrinkage-compensating slabs, the slab is designed unreinforced, and the reinforcement is designed to prestress the expanding slab to resist the later shrinkage and temperature restraint. For post-tensioned slabs, the reinforcement is typically designed to resist the shrinkage and temperature restraint and the applied loads. Shrinkage-compensating concrete slabs are produced either with a separate component admixture or with ASTM C 845 Type K cement, which is expansive. This concrete does shrink, but first expands to an amount intended to be slightly greater than its drying shrinkage. Distributed reinforce- ment is used in the upper 1/3 of the slab to limit the initial slab expansion and to prestress the concrete. Reinforcement should be rigid and supported so that it can be positively positioned in the upper 1/3 of the slab. The slab should be isolated from fixed portions of the structure, such as columns |