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Fwd: Re: Location of Vapor Barrier Below Concrete Slab on Grade

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We typically see 2" sand over membrane over 2" sand in many soils reports.
 As I understand it, the 2" sand below the membrane serves as a capillary
break with the membrane serving as imprevious moisture barrier.  The sand
below the membrane also helps to prevent punctures due to small rocks, etc.
in the soil, especially if a layer of gravel is required below the sand
(membrane over sand over gravel). 

The sand on top of the membrane serves to prevent punctures in the membrane
from people walking on it and the wire chairs/adobe concrete blocks used to
support the slab reinforcing. I have been told that sand helps with the slab
curing and reducing curling since air can get in to the underside of the slab
at the edges.  But,  you have to be carefull when placing the concrete on top
so as not to get sand pockets in the slab.  I would think the differential
shrinkage across the slab is going to best be controlled by closely spaced
control joints, which probably need to be cut in the slab as soon as you can
walk on it (less than 24 hours after pouring).  The mix design (low slump,
low water/cement ratio) will obviously effect the shrinkage, and therefore
cracking and permeability of the slab to moisture.

My understanding is that you want to provide a capillary break under the slab
to prevent moisture from moving upward, but this does not necessarily mean
that it has to be a membrane.  Since the floor manufacturer/installer will
not warrent the work, it seems to me that you are stuck putting a membrane,
especially since if you do not place the membrane and the floor covering
buckles, then everyone is going to blame it on the lack of the membrane.  By
placing the sand over the membrane, I don't think you will have to be as
concerned about the concrete mix design for the reasons that you listed (low
slump, low water/cement ratio, etc.) since this will serve as an additonal
capillary break.

Question,  If the flooring is so moisture sensitive, won't the installer also
be sealing the concrete surface before installing the floor as added

Michael Cochran
Brian L. Cochran Associates
Los Angeles, CA
In a message dated 97-02-18 17:13:52 EST, you write:

<< Subj:	Location of Vapor Barrier Below Concrete Slab on Grade
 Date:	97-02-18 17:13:52 EST
 From:	Mcsherrygp(--nospam--at)
 Reply-to:	seaoc(--nospam--at)
 To:	seaoc(--nospam--at)
 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
 as low a water-cement ratio as practical, low slump, and wet curing to
 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)

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