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Unformatted text preview: I ndustrial slabs are not covered with car- pet or tile. Since the concrete surface is the wearing surface, cracking and curling seriously reduce their productive and aes- thetic value. This article deals primarily with shrinkage and upward curling of these industrial slabs on grade that are un- der roofs and inside buildings where they are not exposed to the sun. Most of the principles, however, also apply to other flatwork. For example, highway and air- port pavements, exposed to the sun, are subject to both moisture and temperature gradients. For simplicity, the moisture gradients are expressed in equivalent de- grees per unit of slab thickness. Concepts developed here do not apply to slabs on grade for tilt-up construction until after the panels have been raised and the roof constructed. To improve floor slabs for tilt-up buildings, cast the wall panels on waste slabs rather than on the finished floor slab. Shrinkage cracking and upward slab edge curling are common in enclosed in- dustrial floor slabs on grade. These prob- lems can be caused by: Moist subgrades Dry air on the upper slab surface Excess water needed to make con- crete workable but not needed to hy- drate the cement Evaporation of moisture from the up- per surface of the slab causes drying shrinkage. Slab edges curl upward be- cause the top of the slab dries to a lower moisture content than the bottom of the slab, and therefore shrinks more than the b o t t o m . Designers often overlook the effects of shrinkage and curling due to moisture loss from slab surfaces because of the great emphasis placed on compressive strength and slump testing and because of the lack of information on curling. But owners expect floor slabs to be relatively free of shrinkage cracks and free of curled edges at control and construction joints. Designers specify control joints at clos- er spacings today mostly because of the Portland Cement Association (PCA) rec- ommendations given in Reference 1, first published in 1978. The additional joints recommended by that publication reduce shrinkage cracking. But curling and break- down of joint edges, at the increased num- ber of joints, is often a bigger maintenance problem than the shrinkage cracks elimi- nated by the extra joints. U n f o r t u n a t e l y, enclosed slabs on grade made with portland cement concrete have worse shrinkage and curling problems to- day than 25 years ago for several reasons: Shrinkage is neglected. Basic recom- mendations in References 1 and 2 fail to emphasize the need for low-shrinkage concrete for floor slabs on grade. They imply that if slump is low, then almost everything possible has been done to minimize shrinkage. Shrinkage testing should be every bit as important as com- pressive strength testing for enclosed slabs on grade....
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