Q.: We are concerned with the maintenance of a cast-in-place school building built in our town in 1937--one of four local schools built by the Works Progress Administration (WPA). In its lifetime it has had several coats of paint. Finally it has become necessary to remove paint. We planned to remove it by sandblasting but we were told that this would remove a surface layer of cement and sand, making the reinforcing steel vulnerable to corrosion. Do you think it is safe to sandblast?
A.: The amount of concrete that you remove by sandblasting the surface to remove old paint is not likely to leave the concrete vulnerable to corrosion. It is true that the thickness of concrete covering the steel is a determining factor in whether or not the steel will corrode. Consequently there are standard specifications giving the minimum amount of cover required for various kinds of service. The required cover depends on the type of exposure and the type and size of the steel reinforcement. You can find the standard requirements in ACI 318, "Building Code Requirements for Reinforced Concrete."
Removing up to 1/8 inch of concrete by sandblasting should not affect the amount of concrete cover substantially. Of course if the building did not originally have the required concrete cover or if you are still concerned about having enough corrosion protection after sandblasting, you could apply a sealer or new paint coating.