Q.: Is use of plastic sheeting a good curing method for concrete or does this produce a solar still effect—drawing the moisture out of freshly placed concrete too quickly?

A.: Plastic sheeting does produce a miniature solar still; that is, a system in which moisture evaporates from the concrete and condenses on the bottom surface of the polyethylene sheet. (The phenomenon has also been called the "greenhouse effect.") Nevertheless, the condensed water tends to run back onto the surface to be reabsorbed or reevaporated. This means that although a certain amount of water leaves the concrete to circulate through this evaporation-condensation system, the amount of water involved is rather small and little water is lost from the concrete. The solar still effect does not significantly inhibit the curing process.