Q.: Is it cheaper to pave with concrete over existing blacktop than to do a complete new concrete parking lot? The existing blacktop is somewhat wavy and not uniform in elevation.

A.: Easy Street Construction Company, of Naperville, Illinois, has done both kinds of paving and prefers to work on new ground if it is hard clay. They say that if the job is on new ground and can be prepared with a trimmer, calculating the concrete costs is straightforward. But on an uneven blacktop base it is uncertain how much concrete will be needed. Hence the contractor needs to give both a square-foot and a cubic-foot price. Even if an engineer has drawn up the whole design the contractor cannot know exactly how much concrete is needed. On all rebuild work the contractor needs a protection clause for quantity of concrete. On new work, on the other hand, the contractor can inspect the subgrade and figure concrete quantities based on the company's known tolerances for trimming the subgrade and placing the concrete. Easy Street says that when they estimate 6 inches thickness on a trimmed subgrade they get 6 inches.