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Visqueen under slab in Arizona

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George,

The concern about visqueen under a slab here in Arizona is that with the 
*low* humidity (frequently less than 10 percent) and high temperatures (very 
frequently well over 100 degrees), all of the free water after hydration has 
to exit the concrete thru the top of the slab.  This can lead to cracking and 
curling, however, a conscientious program of water curing can prevent both.  
With a layer of sand over the visqueen, care also has to be taken not to over 
wet the sand just prior to placing the concrete as the same condition will 
exist.

The question of using a vapor barrier or not is a damned if you do, damned if 
you don't situation.  I was called out to look at a house that had "all this 
white stuff coming up between the tiles."  The "white stuff" was, of course, 
efflorescence, and it was at the joints in vinyl tile.  Moisture was 
migrating up thru the concrete, leaching the salts and when it evaporated in 
the house, it left the "white stuff."

If you do decide to go with a vapor barrier, I would suggest a layer of clean 
concrete sand over the vapor barrier that is only lightly sprinkled prior to 
placing the concrete and a good program of water curing the concrete.

Hope this helps.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

George Richards wrote:

>> We are being told that it construction practice in Arizona along the
Colorado river is to not place Visqueen under residential floor slabs.
Would anyone please comment.<<