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Fwd: Location of Vapor Barrier Below Concrete Slab on Grade

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Can anyone provide comments on the specific advantages/disadvantages of
placing a slab on grade vapor barrier directly underneath the concrete slab
or below a 3" sand cushion?

We are designing a slab on grade system for a office/warehouse facility for
which the Geotechnical report does not require the installation of a vapor
barrier.  However, the flooring system the tenant requires will only
install/warranty their product if a vapor barrier is provided.  The vapor
barrier system they would like to use ("Premoulded Membrane Vapor Seal" from
W.R. Meadows or "Moistop" from Fortifiber) recommends placing the vapor
barrier directly below the concrete slab while specifying a concrete mix with
as low a water-cement ratio as practical, low slump, and wet curing to reduce
differential shrinkage and slab edge curling.  ACI and PCA recommend a 3"
sand cushion to reduce differential shrinkage and curling regardless of the
type of vapor barrier used.

Please respond ASAP.  Thank you.

John Carroll, Structural Engineer
The McSherry Group, Inc.
email:  mcsherrygp(--nospam--at)aol.com


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Richard Lewis, P.E.
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rlewis(--nospam--at)techteam.org

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