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Re: Slabs on Ground

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In a message dated 7/19/2003 1:39:30 AM Eastern Daylight Time, chuckuc(--nospam--at) writes:

The biggest dispute seems to be over where to locate the vapor barrier. A couple of years back I heard a talk by Tom Butt who served (chaired?) on ASTM E 1745.  Their conclusion was that if  moisture coming up through the slab would be a problem (vinyl, wood floors,etc.)  Then put the barrier on top where you could verify it's integrity before the pour & then insist on a wet cure for the slab.  Tom does a lot of forensic work & has seen a lot of damage from  crummy, perforated, visqueen barriers. PCA  wants the barrier under the "blotter" layer which makes life easier for the finishers but increases the risks of later moisture problems.  I don't do a lot of slabs but I try to discuss the issues with the owner & contractor early on.

As I think others have pointed out,  ACI Committees 302 and 360 have issued a joint addendum to their reports that discusses the location of the vapor retarder.   (It's a vapor retarder, not a barrier, since some small amount of permeance is expected.  If a barrier is needed, something like a bituminous membrane is needed.)  The most recent PCA Slabs on Ground book cites this adendum.

One of the key points is whether a vapor retarder is needed in the first place.  If it's not needed, it shouldn't be used.  In addition to being an extra materials and labor expense, it's just one more thing to go wrong.  

With respect to some of the discussion on mesh,  if it is going to be used,  the specifications should require it be supplied as sheets, not as a roll.  Most of the larger ("big foot" wire sizes) are only available as sheets anyways.  The mesh might not end up in the right place, but at least it will be flat.

Gail Kelley

The Truth is rarely pure and never simple. - Oscar Wilde