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RE: Separating slab on grade

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Bill,

Be a little careful with this.  The location of the vaper barrier (and
even the use of one) has been re-opened as a "can of worms" in the last
several years.

ACI 302 and 360 currently have in them a recommendation that the vapor
"barrier" (if required) be placed below "...a minimum of 4" of trimable,
compactable, granular fill (not sand)."  Recent events (i.e. floor
material failures) have indicated that this may not be the best thing in
many cases.  As a result, in 2001 ACI committees 302 and 360 issued an
"update" (in the April 2001 Concrete International) that adjusts this
recommendation.  Presumably, this "update" or something very similar with
some minor modifications will appear in the new versions of 302 and 360
when they are released.

The problems seen were that sometimes the vapor barrier with the granular
fill would sit on the site prior to placement of the concrete slab and
collect moisture from rain, wet-curing, wet-grinding or cutting, and
cleaning.  This moisture would then be trapped in the granular fill
between the vapor barrier and the slab.  Since the moisture could not go
down through the vapor barrier, it had to go up through the slab.  As a
result, the vapor trasmission rate up through the slab exceeded what many
floor materials could handle and you then had floor material failures.

Thus, while having the granular fill might help the concrete itself in
many ways (such as minimizing curling, etc), it might not be the best
thing in the world for the flooring material that gets placed on the slab.

HTH,

Scott
Ypsilanti, MI


On Wed, 16 Jul 2003, Bill Marczewski wrote:

> Andy,
>
>
>
> I don't think pouring concrete over a vapor barrier is a good idea from
> what I have heard and read about from seminars.  I usually specify a
> vapor barrier with sand on top of this barrier to absorb the bleedwater
> from the concrete.  Temperature differentials due to base material
> preparation, curing methods, etc. may have initiated some slab curling,
> or center heaving.  The science behind slab on grade construction is
> more involved than most people believe exist.  Without the use of any
> reinforcement, there is little to control shrinkage.  If the slab was
> poured directly on the vapor barrier you may also have a weak layer on
> the bottom due to the concrete sitting in the bleedwater.  Curing
> process is important too.  I typically specify control joints on the
> order of 10 to 12 foot on center and at corners, and use diamond
> block-outs at columns.  There is a nice document by Kim Basham out there
> titled "Control of Shrinkage and Curling."  You may try to find this or
> I can send a copy to you.
>
>
>
> Bill S. Marczewski
>
> BSM Structural, LLC
>
> Astoria, Oregon
>
>
>
>
>
>
>
> -----Original Message-----
> From: Andy Richardson [mailto:arichardson(--nospam--at)IngeniumUSA.com]
> Sent: Wednesday, July 16, 2003 3:42 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Separating slab on grade
>
>
>
> We had a structural evaluation where the slab on grade is separating
> about 1/8" to 3/16" along the control joint and causing problems with
> the ceramic tile floor.  The plans call out for 4" reinforced concrete
> slab over 6 mil poly over 4" sandy fill.  The subgrade is assumed
> predominately noncohesive soil.  It is apparent that the foundation sub
> interpreted "reinforced" as fiberglass fibermesh reinforced.  There is
> no other reinforcement in the slab.  The slab is about 34'x34' with
> control joints in the center.  Any ideas what may be causing this
> excessive cracking (assuming this is excessive), and how to rectify the
> situation.  My primary concern is that the floor will continue to
> separate after the floor is repaired, and hence more cracks in the tile.
>
>
>
>
> Another interesting point is that 6 months ago another homeowner called
> about his house which was also placed on a similar system.  A similar
> crack in the slab caused termite damage in his hardwood floors.
>
>
>
>
>
> Andy Richardson, PE
>
> Bluffton, SC
>  <mailto:arichardson(--nospam--at)IngeniumUSA.com> arichardson(--nospam--at)IngeniumUSA.com
>  <http://www.ingeniumusa.com/> www.IngeniumUSA.com
>
>
>
>
>
>


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