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Re: proper placement of vapor retarder below slab-on-grade (redux)

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Cliff,

The addendum that you found in ACI 302.1R-96 was not added just before
going to print.  It is a result of more recent information since ACI
302.1R-96 was originally released.  It was added after approval from the
two slab-on-grade committees (302 and 360) sometime last year.  It was
originally printed in Concrete International as an "emergency" update.

The reason for the addendum is that there have been appearently a number
of cases when the original guidelines in the 302 and 360 documents were
followed where significant moisture was trapped in the "blotter" level
that could not travel downward because of the moisture barrier and was
forced to travel upward through the slab.  This resulted in higher than
expected moisture transmition rates through the slab which causes some
floor finishes to fail (i.e. pop up from the concrete).  The obvious
result is lawsuits and such because many floor finishes place limits on
the permitted moisture transmition rate for the warranty to stay in
effect.  Thus, the change in position to prevent future projects from
having similar problems.

The end result (if I recall correctly) is the recommendation now is to NOT
provide a moisture barrier unless you have a high water table or some
other moisture problem and will be using moisture sensative flooring.  If
so, then the DO use a moisture/vapor barrier, but place it directly below
the slab.  Since I don't have the flow chart in front of me and my memory
is going (what was I talking about? <grin>), I would suggest that you look
at the chart yourself.

As you have pointed out, the reason that the "blotter" layer was
introduced was to minimize curling (supposedly it would allow the moisture
from the bottom of the slab during curing somewhere to go which would
allow for more uniform curing through the cross section).  Curling can
still be minimized if placed directly below the slab, but it requires the
contractor to make a serious effort at curing the slab slowly and
carefully (i.e. a good moist cure) so that the top of the slab doesn't
loose moisture too quickly.

HTH,

Scott
Ypsilanti, MI


On Wed, 25 Sep 2002, Cliff Schwinger wrote:

> This subject has already been beat to death on this forum, so I figured
> why not beat it some more.
>
> I've been vacillating on my opinion as to the need to install a 4" thick
> "blotter" layer of compacted granular fill over vapor retarders below
> slabs-on-grade. I have been designing building structures for over 20
> years, have never seen or specified such blotter layers below slabs on
> grade, and have never had any problems with curling (or anything else)
> on my slabs.  Some engineers I've spoken to insist that the blotter
> layer over the vapor retarder is necessary and should be standard
> practice.
>
> I was reading ACI 302.1R - 96 today and upon reading the addendum that
> was added at the very end of that publication (perhaps as a last minute
> addition before going to print) there was a cautionary note that seems
> to indicate that these blotter layers may cause problems with vapor
> sensitive floor coverings.  A flow chart was provided in this addendum
> that basically said the following:
>
> Exterior slabs-on-grade: no vapor retarder
>
> All slabs-on-grade with moisture sensitive floor coverings: Install
> vapor retarder directly below the slab.
>
> Slabs-on-grade that will be constructed inside a building below a roof
> where the roofing system has been installed:  Install blotter layer over
> vapor retarder.
>
> All other slabs on grade: Install vapor retarder directly below slab.
>
> It seems that ACI 302.1R recommends that the vapor retarder (when
> needed) should always be installed directly below the slab except in the
> (rare) case where both of the following conditions are met:
>
> Slab is constructed below the protective cover of a roof.
>
> Vapor sensitive floor coverings will not be installed on the floor slab.
>
> Am I wrong with in my understanding that the purpose of the blotter
> layer is solely to prevent curling of the slab-on-grade?
>
> Couldn't curling be prevented by properly curing the freshly placed
> concrete?
>
> Are we engineers attempting to prevent a curling problem that might be
> caused due to some contractors' inattention to properly curing the
> concrete, only to create other (potential) problems related to floor
> coverings and flooring adhesive failures due to vapor transmission
> through the slab (for which fingers will be rapidly pointed in our
> direction)?
>
> Instead of going to all the trouble of installing the blotter layer over
> the vapor retarder, why not just make the contractor properly cure the
> slab - like they are suppose to do anyway?
>
> Maybe the need for blotter layers is more important in certain parts of
> the country where there is very low humidity year round and the moisture
> rapidly wicks out the top of the slab - but it does not seem to be as
> much of an issue in the Mid-Atlantic States where most of our projects
> are located.
>
> Like I said before - 20 years, no blotter layers, no problems.  Why
> start now?
>
> Thanks in advance for any comments, opinions, etc. I sincerely
> appreciate all of the knowledge that is so freely shared on this forum.
>
> Clifford Schwinger, P.E.
>
>
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