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Re: slabs on grade (sorry it is long)

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ACI 302R-96 _DOES_ tell you what type of material to use and the
thickness.  It is in section 4.1.5 in the first paragraph.  It starts out

"If a vapor barrier or
vapor retarder is required due to local conditions, these prod-ucts
should be placed under a minimum of 4 in. (100 mm) of
trimable, compactible, granular fill (not sand). A so-called
crusher run material, usually graded from 1 1 / 2 in. to 2 in.
(38 mm to 50 mm) down to rock dust, is suitable. Following
compaction, the surface can be choked off with a fine-grade
material (Section 4.1.4) to reduce friction between the base
material and the slab."

Now, again I will point out that this recommedation by ACI committee 302
(and committee 360) has been modified due to PROVEN floor material
failures.  The update has not been incorporated directly into the ACI 302
or 360 documents as new versions have not yet been approved and released
for them.  But an "update" has been approved and released by both
committees, which means in effect the documents HAVE been changed.  The
update was published in the April 2001 Concrete International.  I also
believe that if you order ACI 302R-96, ACI sends a copy of the two pages
of the update along with it.  I don't know if they do the same for the MCP
or not (they should if they are doing it for the 302 document).

The point again is that purely from a structural engineering/concrete
slab point of view (i.e. ignoring the architectural floor finish) the
method recommended in the 302 document is rather good.  It should in
theory minimize curling and produce the best results.  BUT it does have
the potential to trap significant moisture below the slab in the granular
material level just above the vapor barrier/retarder, which then means
that eventually as that slab fully "dries out" that moisture has to go
somewhere and that somewhere is UP through the slab.  Thus, if you have an
architectural floor finish material that is potentially damaged by
moisture migration (i.e. many tile floors with thin set mortars/adhesives
and some VCT systems), then you can expect that your floor system could
have some failures, which means those not so rare beasties called lawyers
may pay you a visit.  If you are going to use a floor material system on
the slab that is not subject to moisture migration issues (i.e. carpet or
tile set in tradition thick set portland cement based mortar), then you
maybe fine with the recommendation that is in the ACI 302 document.  The
end result is that you need to be aware of what type of flooring material
will go on the slab-on-grade and what are its limitations.

As to your other questions, Harold already seems to have answered all of
them.  Since he is smarter than about 100 of me put together, I don't
really have anything to add.


Ypsilanti, MI

On Thu, 17 Jul 2003, Andrew Kester wrote:

> We have the complete set of 2002 "ACI Manual of Concrete Practice" which has
> ACI 302 in it. Now it says place a layer of compactable fill OVER the vaper
> RETARDER (not barrier..). It does not tell you what this type of material
> may be , or the thickness. What Scott said about the layer sitting on site,
> possibly getting rained on, then trapping moisture between the retarder and
> the slab makes sense. But what the ACI says makes sense too, about the
> ability of the water to bleed into a sub base rather then getting trapped by
> the retarder. I guess it is an in the field timing issue. Ideally, place the
> retarder, place the base over top, then pour the slab in a short amount of
> time.

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