Q.: What would cause tiny craters to form on the surface of parking decks made with concrete containing a calcium-nitrite corrosion inhibitor? The craters are about 1 inch in diameter and 1/4 inch high. They primarily occur over deep beams, and sometimes cracks pass through them after the concrete hardens.

A.: We consulted Ken Rear of Grace Construction Products, a manufacturer of a calcium-nitrite based corrosion inhibitor. One of Ken's field engineers has observed cratering in very deep placements but not ones containing calcium nitrite. In one case, he said the water came to the surface so rapidly that the surface looked like a mass of crayfish mounds. He believes the cause was water in the bottom of the form that wasn't removed before concrete placement. Another field person recalled a job where the contractor placed concrete on aluminum decking. The aluminum reacted with calcium hydroxide (not calcium nitrite) in the concrete and produced hydrogen gas and bubbles on the surface.

Ken has seen similar eruptions in concrete reinforced with steel fibers. Huge bleeding channels appeared at certain spots, with holes as large as 1/4 inch at the end of each channel. This, again, wasn't related to a calcium-nitrite corrosion inhibitor being used in the concrete, nor did it occur over the whole slab surface.