Q.: The surface of a 12-year-old slab has heaved above the embedded electrical conduit. Wherever the concrete has been disrupted enough to expose the conduit to view, a white-colored deposit can be seen. This looks much like the deposit that develops on the post of an automobile battery. What has caused the heaving? We think calcium chloride was used in the concrete mix and wonder if that could have had anything to do with the trouble.

A.: What you describe is typical of galvanic corrosion. This type of corrosion can occur wherever two dissimilar metals within a slab are in contact with one another. Salts of the conduit metal (perhaps zinc salts or aluminum salts) form on the metal surface, creating expansive forces that disrupt the concrete. The corrosion is likely to be even more severe if the concrete contains calcium chloride.