360R-06 Design of Slabs-on-Ground
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- CHAPTER 8—DESIGN OF SHRINKAGE- COMPENSATING CONCRETE SLABS 8.1—Introduction
| 7.5—Reinforcement location
Reinforcement for crack-width control only should be at or above mid-depth of the slab-on-ground, never below mid- depth. A common practice is to specify that the steel have 1.5 to 2 in. (38 to 51 mm) cover below the top surface of the concrete. Reinforcement for moment capacity should be at the centroid of the tensile area of the uncracked concrete section. CHAPTER 8—DESIGN OF SHRINKAGE- COMPENSATING CONCRETE SLABS 8.1—Introduction This chapter deals with concrete slabs-on-ground constructed with shrinkage-compensating concrete made with cement conforming to ASTM C 845. The design procedure differs significantly from that for conventional concrete with ASTM C 150 portland cement and blends conforming to ASTM C 595. When concrete dries, it contracts or shrinks, and when it is wetted again, it expands. These volume changes with changes in moisture content are an inherent characteristic of hydraulic-cement concrete. ACI 224R discusses this phenomenon in detail. Volume changes also occur with temperature changes. Shrinkage-compensating concrete is an expansive cement concrete that, when restrained by the proper amount of rein- forcement or other means, will expand an amount equal to or slightly greater than the anticipated drying shrinkage. Subse- quent drying shrinkage will reduce the expansion strains, but ideally, a residual compressive stress will remain in the concrete, thereby minimizing shrinkage cracking and curling. How shrinkage-compensating concrete differs from conventional concrete with respect to these volume changes is explained in Sections 8.1.1 and 8.1.2 . 8.1.1 Portland-cement and blended-cement concrete— The shortening of portland-cement and blended-cement concrete due to shrinkage is restrained by reinforcement and friction between the ground and the slab. This shortening may occur at an early age with the friction restraint stressing A s 4.4 MOR h × × f s ------------------------------------ = A s 370 MOR h × × f s -------------------------------------- = f c ′ f c ′ f c ′ f c ′ |