Q.: A customer has wood block flooring in a pitch mastic over a concrete subbase about 4 inches thick. He wants to take out the wood and put in a 4-inch-thick concrete floor in its place. Will it be necessary to take out the mastic, and if so, how thoroughly must the base slab be cleaned?

A.: The proposed floor can be considered a Class 7 unbonded topping described in ACI 302, "Recommended Practice for Concrete Floor and Slab Construction." It would probably be wise to cover the base slab with polyethylene to be sure the new floor is completely free of the base. The polyethylene would minimize bleeding of mastic into the new floor, but such bleeding through good quality concrete does not seem likely.

Joints in the new floor should match those in the base if convenient, but joints in the floor should be located at about 15-foot intervals, if possible, whether joints in the subbase were that close or not.