Tests performed conclusively show that in concrete with a dosage rate of 1 gallon per cubic yard, vapor transmission is reduced over one half. In addition, the concrete has an effective perm rating of 0.04, which exceeds the recommendations of ASTM C-755 by a factor of 10. It is interesting to note that the samples tested were only one inch thick. In field use, where concrete thicknesses are typically 5 inches or greater, the transmission of water and vapor is less due to the increase in mass. In no instance was there any penetration of cement paste or fines of concrete, or water from the concrete in the porous fill.
Plastic sheets are subject to improper placement and puncturing during concrete placement.
Plastic sheeting also deteriorates in only a short period of time. It is often impossible to obtain a fully vapor resistant under-slab system at drains, drain trenches, electrical ductwork, conduit, and plumbing when plastic sheets are used. The American Concrete Institute recognizes the detrimental effects when plastic sheets are used directly under concrete slabs. Slab-curl, surface map cracking, soft friable surfaces, reduced strengths, permeability, and excessive drying shrinkage are some typical results when plastic sheets are used. The ACI does recommend however, a three inch pervious sand bed should be spread on top of the plastic film to reduce cracking, This is neither practical nor cost effective for most projects.