Q.: There is a floor in an old building which we intend to convert to a warehouse. There are some differences in floor surface elevation due to random cracking and settlement. We want to put in a new floor. Since increase in elevation would be no problem can we simply cast the new floor over the top of the old one? Would this be a place where we could use an unbonded topping? If so, would a layer of sand be suitable to prevent bonding?

A.: Yes, you could cast an unbonded topping of 2 1/2 inches or greater thickness over the old floor. A sand layer would be a suitable bond breaker provided it were of sufficient thickness to cover the highest areas of the old slab and were uniformly compacted. It would be wise to study the recommendations of "Guide for Concrete Floor and Slab Construction," ACI 302.1R-80 with respect to unbonded toppings.