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Re: sand layer under slabs - was ACI or CRSI question-answer center

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Ken,

First of all, the most current recommendation on the "blotter" layer (if I
recall correctly) was to use a layer of "crushed fines" or something
similar.  The idea was to have a layer of something that would allow the
moisture from the bottom side of the slab to migrate into that layer
allowing the slab to cure.  Initially, the recommendation was sand (which
allows movement of moisture relatively well).  The problem with sand is
that it gets "mushed" during construction (i.e. placement of rebar,
placement of concrete, etc) which can destroy the nice uniform surface.
Thus, the idea of using something like a well-draining crushed fines that
can compact relatively well and not be distrubed during the rest of the
construction process.

The recent change has been with the use of the "blotter" layer with a
vapor barrier.  Recent information indicates that this formerly
recommended system (a "blotter" layer under the slab but on top of the
vapor barrier) can create problems with some moisture susceptible flooring
materials.  The problem arrises from the fact that any moisture that gets
trapped between the vapor barrier and the bottom of the slab in the
"blotter" layer (i.e. rains after placement of the vapor barrier and the
"blotter" layer but before the placement of the slab and not enough time
is allowed for the "blotter" layer to dry; "excessive" moist cure that
causes moisture to get in to this layer; wetting of the "blotter"
layer prior to placement of concrete...a common/required practice; etc)
can only go in one direction...up through the slab.  The potential result
is that the moisture/vapor emmision rate could be high enough to cause
some flooring materials to fail.

The result is that ACI 302 and 360 issued a revised flow chart for
determining when to use vapor barriers and what other conditions should be
required with or without the vapor barrier.  This was issued in the spring
of 2001 in an issue of Concrete Construction.  Additionally, the revision
should be included with any new purchases of the main ACI 302 or 360
documents and likely the MCP.  This subject was discussed on the list a
couple of months ago...take a look at the archive.

HTH,

Scott
Ypsilanti, MI


On Tue, 5 Nov 2002, Ken Peoples wrote:

> Is this true?  Is the sand layer no longer recommended?
>
> Kenneth S. Peoples, P. E.
> Lehigh Valley Technical Associates
> 1584 Weaversville Road
> Northampton, PA 18067-9039
> Phone: (610) 262-6345
> Fax: (610) 262-8188
> e-mail: kpeoples(--nospam--at)lvta.net
>
> ----- Original Message -----
> From: "Roger Davis" <rdavis(--nospam--at)sdsarch.com>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Tuesday, November 05, 2002 8:06 AM
> Subject: RE: ACI or CRSI question-answer center
>
>
> > Scott,
> > That warm fuzzy feeling is what I wanted.  Questions seldom come up until
> > you need an answer.  I want immediate feedback so I can continue with the
> > work that brought up the question.  The answers that you and others
> > contribute on this list have confirmed some of my opinions and made me
> > change my mind on some things and educated me on many others - they seldom
> > come with any authority like a ruling from an ACI committee  but that
> > doesn't bother me. I judge for myself whether or not to accept them - just
> > like I did with the old ACI Committee recommendation to place a sand lift
> > under a concrete slab on grade. I think it was only appropriate under
> > special circumstances even though the committee recommended it for years
> > until they recently reversed themselves.
> >
> > Roger C. Davis
> > Architect
> > SDS Architects, Inc.
> >
> > -----Original Message-----
> > From: Scott Maxwell [SMTP:smaxwell(--nospam--at)engin.umich.edu]
> > Sent: Monday, November 04, 2002 5:05 PM
> > To: seaint(--nospam--at)seaint.org
> > Subject: RE: ACI or CRSI question-answer center
> >
> > To clarify what Jay wrote below...
> >
> > What he states is absolutely correct if you want an "official"
> > interpretation of something in an ACI document.
> >
> > On the other hands, if you want the opinion of an INDIVIDUAL who was
> > involved with the creation of that document, then members of committee's
> > can give their own opinion.  The key is that it is the opinion of that one
> > individual, NOT an official opinion in any way.  It may be that opinion is
> > correct and co-insides with what the official interpretation would be if
> > rendered.  The end result is that getting the opinion from an individual
> > committee member or an ACI staff member (or staff member from CRSI, PCA,
> > PCI, etc) may give you a warm fuzzy feeling, but it is more or less worth
> > what you paid for it (i.e. nothing) if you want to use it in any form of a
> > legal or substantive arguement.
> >
> > So, to use Roger's example, the answer he got was only useful to give him
> > a warm fuzzy feeling.  Even though it came from a member of the 318
> > committee, it was not in anyway an official answer or a respresentation of
> > what was actually meant other than that individual's opinion.
> >
> > FYI, ACI staff member when they do respond to technical inquiries will
> > sometimes contact committee members to see if one is willing to get an
> > opinion.  Not only does this potentially get a more authoritative (notice
> > the adj.) for the person asking the question, it can also point out to the
> > committee where there are some items of confusion that may need
> > clarification in the next iteration of the document.
> >
> > HTH,
> >
> > Scott
> > Ypsilanti, MI
> >
> >
> >
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