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Vapor Barriers

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From: "Dennis S. Wish PE" <wish(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: RE: Vapor Barriers

I might be missing something here, but I have a couple observations
1) Generally, the soil is saturated before pouring a slab. I understand that
the water can percolate out - but:
2) The concrete is poured wet - some water must settle below the concrete
whether the sand between barrier and slab is wet or dry.
3) How much water is will remain once the slab dries? Doesn't the heat
generated in the curing process help draw the water up to the surface? Won't
this help to wick the excess moisture from below into the slab?
4) Assuming the sand is compacted well prior to placing the slab, Isn't the
moisture retained rather negligible?

Dennis Wish PE

The American Concrete Institute recommends that the sand lift above the
vapor barrier be dampened prior to casting the concrete.  The ACI recommends
sand over the vapor barrier because of a concern about slab curling.  I
disagree with this policy because most of the work we do requires an adhered
finish over the concrete slab.  The slab has to reach a low level of
moisture content prior to installing the finish if the adhesive is going to
be effective.  There have been articles written about how much greater the
drying time is following the ACI procedure, but I can't find any now.  I
can't remember the last time we had a project when the drying time of the
concrete slab was not a concern.  On the few occasions when we have had a
curling problem, it has been easily taken care of with a little grinding.
However, according to William Perenchio, a senior consultant with Wiss,
Janney, Elsttner Associates as far as he can determine there doesn't appear
to be any test data that shows that slabs placed directly over impermeable
surfaces curl any more than slabs placed over absorptive surfaces. (Concret
Construction, April 1997, p. 383)  In the same issue Ward Malisch,
Engineering Editor of Concrete Construction says that "In many cases,
grinding costs less than the design measures needed to limit curl to the
flooring installer's requirements."
I have seen floors where the adhesive has turned to a goey mess long after
the floor finish had been installed.  The top inch of concrete may have
dried out sufficiently to pass a moisture test because it was well
ventilated.  But when the moisture in the bottom of the slab and in the sand
lift got the the top of the slab, the adhesive was re-hydrated and stayed
that way because the floor finish was impermeable.
I think there are times when following the ACI procedure is reasonable, but
most of the time I think it is unreasonable.

Roger Davis
SDS Architects