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RE: Vapor Barrier

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We use a 4" crushed gravel bed choked with clean sand to reduce the drag 
between the gravel and the underside of the slab.

We also get calls from about half of the concrete contractors telling us our 
plans are wrong and that the plastic should be on top of the gravel.

-----Original Message-----
From: Jim Kestner On Behalf Of Jim Kestner
Sent: Tuesday, October 13, 1998 1:06 PM
To: mail@ih {seaint(--nospam--at)seaint.org}
Subject: Vapor Barrier



I would like to get opinions on what I consider conflicting requirements
regarding vapor barrier under slab-on-grade. ACI 302.1-R, 2.4.1 requires
that if a vapor barrier be used that it be covered with 3" of sand
before pouring the slab. This minimizes plastic shrinkage cracking and
curling. It also controls bleed water coming to the surface. This is
logical and well documented. However, this procedure is risky when
pouring a slab not under cover. What are the chances of rain occuring
after the sand is placed over the vapor barrier but before the slab is
poured? Now, you may have water trapped which could be very expensive to
deal with or if ignored, worse than having no vapor barrier at all.

It seems like ACI requirements address the finishing and performance
aspects of the slab, but have ignored the practical aspects of what the
contactor must deal with. How do you other Engineers and Contractors
deal with this?

Jim Kestner
Green Bay, Wi