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RE: Cracking in Concrete Slabs on Grade

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>>> SpragueHO(--nospam--at)bv.com 06/28/00 01:13PM >>>

Another important consideration is developing a slip plane with a well
graded sub course and a slip sheet of polyurethane.  <snip>

Harold,
I have a question concerning the use of the poly sheet. I understand the slip sheet principal for allowing the concrete to slip, (i.e. shrink), without developing tensile stresses due to subgrade friction, but does the poly below the slab promote curl?.....or are you placing it below the sub-coarse to allow for a more uniform moisture loss? If so, how thick of a sub-coarse are you spec'ing to get good compaction of the base? 

I've always wanted to try your method but was leary having gone thru some curling problems. I know many things contribute to curling (i.e. mix design, curing, ect, but poly directly below the slab is always listed as a contributor in the literature. 

Thanx,
Jim T