Q.: Once you remove the forms how long will it take for concrete to dry too much for hydration to continue?
A.: In flatwork most of the drying occurs through the top surface and little through the edges where the forms have been removed, although some drying does take place in that location. Good practice would suggest that if the side forms are removed while the slab is less than 7 days old the newly exposed surface ought to be sprayed with a curing compound or covered with polyethylene sheet. Hydration stops when the internal relative humidity of the concrete drops below about 80 percent. The length of time required for the internal humidity to drop this low is not a definite figure that can be applied to all concretes. It depends upon a number of factors such as the permeability of the concrete, the water-cement ratio, the temperature, the relative humidity of the air, the wind and whether a liquid membrane curing compound has been applied to the surface. Actually the surface will ordinarily become too dry to hydrate before the interior of the concrete because a moisture gradient develops during the drying process.