Q.: We're supplying ready mixed concrete for a floor slab that will be placed in freezing weather. We warned the contractor about placing concrete on a frozen subgrade. He says he'll throw flake calcium chloride on the subgrade surface to thaw it before he places the concrete. Is there any code or document that prohibits this?

A.: We don't know of a document that specifically prohibits this practice, but it's not an effective way to thaw the subgrade to any depth. Some contractors put up a tent or other enclosure and use space heaters inside it to thaw the subgrade (see Concrete Construction, November 1989, page 956). Others use electrically heated concrete curing blankets that are laid on the ground. One such product comes in 36-foot-long sections that are 4 1/2 feet wide. The blankets require a 220-volt power source and can be covered with insulated curing blankets to retain the heat. They reportedly maintain a 60° F temperature beneath the blanket even when the air temperature is at 0° F. The cost is estimated at less than $2.50 per square foot.