If you haven't heard of permanent upward curling of concrete floor slabs on grade perhaps it is because people with such trouble either prefer not to talk about it or do not seek information from others because they think nothing can be done about it. Corner curling of up on one-half inch has been reported to occur in heated and unheated areas in both plain and reinforced slabs. Curling usually develops within about 30 days after construction while the excess water provided for workability evaporates, resulting in concrete shrinkage. In slabs on grade the drying and shrinkage occur more rapidly at the exposed surface, especially in heated building, where the humidity is low. This trends to cause upward curling at the corners and edges. The greater the difference in moisture content between top and bottom of the slab the greater the tendency to curl. Maximum curling occurs at corners; at the edges it amounts to about one-third to one-half as much. Observation of standard recommended procedures in the ACI 302 Standard is the first necessity in avoiding curling. This particularly includes: keeping the water content low; waiting for bleed water to disappear; retaining curing water; and ensuring bond in two course floors. One practice that has reduced curling and permitted reduction of the number of control joints in floors is the use of shrinkage-compensating cement. During the initial stages of hydration of such cement the concrete tends to expand. If this expansion is restrained by reinforcing steel, residual compressive stress builds up which counteracts the normal tendency for tensile stresses to develop as the concrete dries.