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RE: Cracking in Concrete Slabs on Grade

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I never put the concrete slab in direct contact with polyurethane.  It will
cause curling.  Some recommend punching holes in the polyurethane.  I don't.
I put crusher fines between the concrete slab and the polyurethane.

The surface of the bottom of the concrete slab is one horizontal plane.
There are no thickened slabs or drops at the columns.  These areas will
result in constraints.  If additional foundation support is required for
walls or small point loads, it is a separate pour and the horizontal slip
plane is maintained.  Columns are treated as penetrations, and are wrapped
in closed cell foam to isolate the column.  Penetrations and re-entrant
corners are given extra rebar at the corners.

My slabs use the following construction:
With vapor barrier:
	Concrete slab reinforced with rebar
	3" layer of compacted crusher fines
	Vapor barrier
	3" layer of open graded and compacted 3/8" gravel to serve as a
capillary break
	prepared base

Without vapor barrier:
	Concrete slab reinforced with rebar
	3" layer of compacted crusher fines
	6 mil polyurethane slip sheet
	2" layer of compacted crusher fines
	prepared base

It is also good to keep in mind that the slab shrinks to the center.

Harold Sprague

> -----Original Message-----
> From:	Jim Todd [SMTP:JIMT(--nospam--at)]
> Sent:	Thursday, June 29, 2000 12:56 PM
> To:	seaint(--nospam--at)
> Subject:	RE: Cracking in Concrete Slabs on Grade
> >>> SpragueHO(--nospam--at) 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