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RE: Vapor Barriers

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raranous(--nospam--at) wrote:

>Now I think we are getting into the realm of theory vs. practicality.  It
>has been my experience that a slab poured in contact with the barrier is
>subject to curl because of the differential migration of the the water.  If
>a contractor were to cure a slab like the specifications called--keep it
>moist at all times, there should not be any curl and there would be no
>need to use sand or gravel.  Unfortunately, to cure a slab like the
>specifications call is expensive.  Contractors want to pour the slab and
>get on with the rest of construction.  This is one reason why they want
>to sawcut joints instead of using zip strips or similar products.

Another reason to put the vapor barrier under the sand or gravel layer is
to reduce the amount of bleed water on the surface of the slab.  If there
is a small amount of bleed, the contractor can usually wait until it
evaporates off before finishing the surface.  If there is too much bleed,
the contractor either has to wait much longer to finish the surface or
some of the bleed water gets worked into the surface during the
finishing, resulting in a weaker surface and usually surface pattern

>This is a very interesting concept, Thor.  I would pose the general
>question--would a slab perform better if it were cured properly and
>used a barrier directly under the slab?  Secondly, in the overall scheme
>of things, would this system be more economical than using sand or

>Any takers on this one?

The ACI recommends that the water-cement ratio be 0.45 or less if the
vapor barrier is directly under the slab.  This may be too stiff of a mix for
some contractors, although a superplastizer could be added to the mix,
thereby increasing the cost a bit more.  Also, how much quality control is
used in placing basement slabs for residential construction?  If the mix is
too stiff, typically water is added to the mix at the site.    

Even with good quality control, if curling, warping, or surface shrinkage
cracks occur in even a fraction of the slabs, then it makes more
economic sense to place the barrier underneath the sand or gravel layer,

Michael S. Davis, P.E.