Q.: How can carbonation hardening by air occur when a slab is sealed by a curing compound?

A.: A curing compound can be expected to seal the concrete against reaction with carbon dioxide. Even when carefully applied, however, there are likely to be some thin spots, pinholes and holidays in the film which provide little or no barrier to contact between carbon dioxide and the concrete. Furthermore, as time goes on most curing compounds dissipate and leave the surface of the concrete exposed to the air, which normally contains about 0.03 percent carbon dioxide by volume. At the same time some of the curing compounds may become worn away by traffic. The reaction with carbon dioxide then continues indefinitely and probably stops when the pores have become sufficiently plugged with reaction products to a sufficient depth to choke off the reaction.