The success of any patching, restoration or overlaying of concrete depends on proper preparation of the surface being repaired. Whether applying small patches, large toppings or overlays, the weak link in the system is the surface of the original concrete. Even with sound concrete, the surface is still the weakest part, whether it is formed or troweled. For example, direct tensile strength tests performed on 4000-psi concrete yielded an average surface tensile strength of only 75 psi. Sandblasting the surface of this concrete revealed a new surface with tensile strength of 275 psi. One can therefore see the importance of surface preparation even on sound concrete. The preparation of the surface assumes an even more important role in the repair of old, disintegrated concrete.
Concrete disintegration in most cases begins as a surface phenomenon. Where disintegration has progressed to the reinforcement, concrete should be removed from around the entire steel bar; the corrosion products on the reinforcing steel must also be removed. The steel can then be completely encapsulated with the repair material to exclude moisture and oxygen. The steel can continue to corrode if this procedure is not followed, and may force out the patch.
Inspect all concrete surfaces before repair or application of adhesive to be sure that the following requirements are met: Surfaces should be sound and have coarse aggregate exposed (coarse aggregate exposure may be waived when surface preparation is by acid etching); surfaces should be free of deleterious materials such as laitance, curing compounds, dust, dirt and oil loose materials remaining after specified preparation of concrete surfaces must be removed; surface temperatures must be at the proper level recommended for the particular patching material being used for epoxy adhesives, at least 40 degrees F is required; all surfaces should have a moisture content not exceeding saturated surface dry condition.