Q.: Our finishers complain whenever we're required to place concrete directly on a vapor barrier. The concrete bleeds longer, delaying finishing, and seems more likely to crack. We've been told we can solve this problem by ordering concrete with an admixture that makes the walls of internal concrete pores water-repellent. Is this an accepted alternative for a vapor barrier or vapor retarder?

A.: We don't think so. Though such dampproofing admixtures may limit liquid moisture movement caused by capillary flow, they have little effect on the movement of water vapor through the pores of unsaturated concrete. Water-vapor emissions cause many of the problems that vapor barriers are designed to prevent. Also, if the concrete slab cracks or is jointed, water vapor can still move through these openings despite the presence of a dampproofing admixture.

A special advisory committee to the Building Research Advisory Board reached the following conclusion based on data from tests of moisture transmission through unsaturated concrete slabs: "The Committee does not find adequate data to demonstrate the effectiveness of any admixture to reduce the transmission of moisture through concrete slabs-on-ground in a manner sufficient to replace either a vapor barrier or granular base, or both, under conditions where such protection would be needed."

Reference

"Effectiveness of Concrete Admixtures on Controlling the Transmission of Moisture Through Slabs-on-Ground," Building Research Advisory Board, Publication No. 596, National Research Council, Washington, D.C., July 1958.