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Don't be too quick to discount potential problems with vapor transmission.
There have been problems with delamination of adhesively applied flooring
due to vapor pressure.  In this region, it has been observed in private,
state, and federal projects.

Apparently, the adhesives used in modern times is more susceptible to vapor
transmission.  The flooring industry has adopted moisture emission
standards that can be difficult to meet in a timely manner.

The sand or fine-grain aggregate layer can potentially contribute to this
by accepting water from the freshly placed concrete (which is why it would
be placed there).  The potential benefit of this reservoir of moisture to
aid slab curing may also have a disadvantageous long term effect on
flooring.  There are various products, such as floor sealers designed to
limit vapor transmission, that are available.

You might consider discussing  this issue and the sand layer with your
architect, and let him make the call after consideration of the potential
benefits and disadvantages.  It's not solely a structural decision.

Gary Chock

At 12:35 PM 4/25/00 EDT, you wrote:
>Just hoping I can get a basic question clarified. I  have a client  I'm
>a series of homes for, and he wants me to specify that the slab on grade be 
>placed directly over the vapor barrier, (no sand between) He claims that
>sand over the vapor barrier is moistened prior to pouring concrete, sand
> never dry out,  the moisture from the sand will travel through the slab for 
>a very long time causing bubbleing in the vinyl flooring, prevents  wood 
>flooring from sticking to concrete, etc. I've always believed that the sand 
>aids the curing of the slab.  The caveat is that vapor barrier is installed 
>properly and not punctured during placement of concrete. Any help is 
>Andy Vidikan


Martin, Bravo, & Chock, Inc.
1132 Bishop Street, Suite 1550
Honolulu, HI   96813
808-521-4513 (voice)
808-531-4508 (fax)