Slab edges that curl upward and centers of slabs slightly depressed from their original elevation are common in slabs on grade. Curling is caused by drying shrinkage and by moisture or temperature differences across the thickness of the slab. The most common positive temperature gradient with its downward edge curling is that caused by heat from the sun on the upper slab surface. Upward edge curling already caused by negative moisture gradients can be increased by cold slab surface temperatures. A permeable (porous) dry subgrade reduces curling. It allows some of the water to leave the bottom of the slab before the concrete takes its final set. This results in denser, lower-shrinkage concrete at the slab bottom. Serious cracking may occur when concrete is cast on an impervious base because excess water cannot leave the bottom of the slab before final set. Floor slabs on grade should be placed over a 1/2-inch layer of sand on top of a dry subgrade. Since curling and drying shrinkage are both a function of excess "workability" water in the concrete, curing methods that retain that excess water will only delay the shrinkage and curling of enclosed slabs on grade. Chemical means, such as high-range water reducers do not necessarily reduce shrinkage. Curing does not have the same beneficial effect as long-term high ambient relative humidity.