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Re: Vapor Barriers

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Roger Davis wrote:
> In response to:
> 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
> Architect
> SDS Architects




donn wooldridge  RA/SE
Phoenix AZ