Q.: Our company is renovating a blacktop warehouse floor. The new floor may be used for lift truck traffic. We would like to place about 3 inches of concrete over the existing blacktop but there is grease on some parts of the surface. Would the grease harm the concrete? Should we use welded wire fabric?

A.: The blacktop can be used as a base for the overlay of concrete. You should use at least 4 inches of concrete if there is to be little or no lift truck traffic. If there will be any significant amount of lift truck traffic, you should check the thickness requirement against wheel loads in Figure of ACI 302.1R-80.

Welded wire fabric is optional, because much can be done to minimize cracking by properly controlling the joint spacing. The greasy areas on the blacktop will not harm the concrete, provided the grease is not in puddles. Any excess oil or grease should be mopped up prior to concreting. Note that placing concrete on top of blacktop is similar to placing concrete on top of a vapor barrier; it will increase the amount of bleeding.