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