Q.: Many years ago I started filling industrial floor joints with lead strips, which seemed better than anything else available. In the past several years I've used mostly epoxies, because they bond to the sides of the joints. But they aren't the best answer, either, because they frequently cause a crack in the concrete parallel to the joint. What's the latest material, if any, that's better? Don't tell me "elastomers"; they're too soft.

A.: Some experts have recently been advocating semirigid epoxies for sealing joints in industrial floors. Generally an epoxy designed for the purpose, with a Shore A hardness of about 80, is said to be about right. These have a reasonable degree of hardness, yet also a small amount of extensibility. ACI Committee 302, in its forthcoming "Guide for Concrete Floor and Slab Construction," will be recommending that such joint materials be 100 percent solids epoxy resins with a minimum Shore D hardness of 50 (ASTM D 676) and elongation of 6 percent (ASTM D 2240).