Q&A: Slab Over an Existing Slab

A.The American Society of Concrete Construction (ASCC) replies: The required surface preparation depends on whether you’re placing a fully bonded topping or an unbonded topping. For a fully bonded application, the topping becomes an integral part of the repaired floor slab. This requires placing the topping on a clean, rough surface for maximum bond development.

Sometimes dry preparation methods such as abrasive blasting are specified to remove grease, oil, paint, and weak concrete. These methods also produce a textured surface that provides mechanical bonding. Another method is to roughen the existing floor to produce 1/4-inch-high ridges. Abrasive blasting won’t produce a profile this deep, so high-pressure water-blasting or impact-type mechanical devices are needed.

For an unbonded topping, the topping and base slab function as separate structural elements. Surface preparation for an unbonded topping consists of first sweeping the old base slab and filling badly worn areas, spalls, and holes with a cement-sand mortar to bring the surface to a reasonably flat plane. Next, a bond-breaker (separation layer) is applied. This may be layers of plastic sheeting, roofing felt, or waterproof building paper, or it can be a wax-based curing compound. The topping concrete is then placed directly on the bond-breaker.

A pamphlet titled "Resurfacing Concrete Floors" (IS144) is available from the Portland Cement Association (P.O. Box 726, Skokie, IL 60076-0726; 800/868-6733).

Reprinted with permission from the ASCC Trouble Shooting Newsletter #34.