360R-06 Design of Slabs-on-Ground
—Distributed reinforcement to reduce curling
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- 13.10—Thickened edges to reduce curling
| 13.9—Distributed reinforcement to reduce curling
and number of joints Because it is the upper part of a floor slab that has the greatest shrinkage, the reinforcement should be in the upper half of that slab so that the steel will restrain shrinkage of the concrete. One and one-half to 2 in. (38 to 51 mm) of concrete cover is preferred. Reinforcement in the lower part of the slab may actually increase upward slab curling for slabs under roof and not subject to surface heating by the sun. To avoid being pushed down by the feet of construction workers, reinforcing wire or bars should preferably be spaced a minimum of 14 in. (360 mm) in each direction. The deformed wire or bar should have a minimum diameter of 3/ 8 in. (9 mm) to provide sufficient stiffness to prevent bending during concrete placement. For unreinforced slabs, joint spacings of 24 to 36 times the slab thickness up to 18 ft (5.5 m) have generally produced acceptable results. A closer spacing, however, is more likely to accommodate the higher shrinkage concrete mixtures often encountered (refer to Fig. 5.6 for recommendations). If greater joint spacings than these are desirable to reduce maintenance, the designer should consider a continuously reinforced, a post-tensioned, or a shrinkage-compensating concrete slab as a means to reduce the number of joints in slabs-on-ground. The specified steel should be stiff enough and have a large enough spacing so that it is practical to expect the steel to be placed (and remain) in the upper half of the slab. Joint locations should be detailed on the slab construction drawings. 13.10—Thickened edges to reduce curling Curling is greatest at corners of slabs, and corner curling is reduced as slab thickness increases (Child and Kapernick 1958). For example, corner curling vertical deflections of 0.05 and 0.11 in. (1.3 and 2.8 mm) were measured for 8 and 6 in. (200 and 150 mm) thick slabs, respectively, after 15 days of surface drying. Thickening free edges subjected to loading is a design strategy that accounts for the difference in mid-panel and edge load response in slabs of constant thickness. Edge curling may be reduced by thickening slab edges. The thickened edge contributes added weight and also reduces the surface area exposed to drying relative to the volume of concrete, both of which help to reduce upward curling. Free slab edges and edges at construction joints where positive load-transfer devices, such as dowels, are not provided should be thickened 50% with a gradual 1-in-10 slope. |