Q.: We would like to resolve the question of proper placement of vapor barriers under concrete slabs on grade. We have a system that has worked well for us in the past. In nonproblem areas we have traditionally used 6-mil polyethylene film directly under concrete slabs and over a granular fill on the subgrade. This produces a very slick surface on the underside of the slab; some people have claimed that the polyethylene eventually disintegrates but that this slick surface itself continues to function as a water barrier. In only one instance have we experienced slab curling and this may have been the result of improper rebar placement. Puncturing of the polyethylene film hasn't been too serious a problem since most granular fills were plate-compacted crushed rock passing the 3/4-inch screen and minor punctures didn't affect the overall slickness.

Our problem is that the local code now requires a minimum of 2 inches of sand cushion over the granular fill (or subgrade if there is no granular fill) with the polyethylene film over this sand. The sand cushion is easily indented by traffic and the result is uneven slab thickness. Some contractors prefer to place the sand cushion on top of the polyethylene. This does not solve the indenting problem. The sand base is watered just prior to placing of the concrete; the sand supposedly serves to protect the polyethylene and also allows some moisture absorption from the concrete and quicker finishing. It seems to us to be a potentially serious problem that this sand could either absorb too much water out of the slab or allow cement paste to disperse out of the slab into the sand. Is there a recommended method for placement of vapor barriers under slabs?

A.: It is true that the sand base under polyethylene could cause an indenting problem. If this kind of placement is required perhaps some other means of preventing indentation from foot traffic must be worked out.

It is also possible that too much water or cement paste would be absorbed from the concrete by the sand, but probably not under most circumstances.

ACI 302.1R-11 "Guide for Concrete Floor and Slab Construction," doesn't advocate universal use of vapor barriers but does make recommendations for their proper use. This guide says, in Section 2.4.1:

"On well-drained home sites, in areas where the water table consistently remains well below the ground surface, a coarse granular fill below the slab, at least 4 inches (100 millimeters) in depth, has performed satisfactorily in place of a vapor barrier when the floor covering or its adhesive is of a type which is not affected by moisture.

"Where no drainage or soil problems exist and in arid regions where irrigation and heavy sprinkling are not done a vapor barrier may not be necessary." Earlier in the same section the guide comments:

"Vapor barriers aggravate the problems of plastic and shrinkage cracking. . . . Their use should be avoided where ground moisture conditions permit. If ground conditions require their use, a 3-inch (75-millimeter) layer of sand over the vapor barrier (and under the concrete) has been shown to reduce these problems. The sand should be compacted before concrete is placed. Wetting the sand the day before is a simple way of compacting it; however, the sand should be free of 'drainable' water at the time of concrete placement so the sand can act as a blotter. When sand is used for this purpose, the subgrade must be excavated deeper to accommodate the sand and still maintain the desired concrete slab thickness."

This comment of the guide regarding location of the sand cushion is in agreement with the preference that you mention of "some contractors," and not in agreement with your local code. The guide suggests a 3-inch layer instead of 2-inch, and reports at least some degree of success in compacting the sand by wetting the day before concreting.