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Concrete Slabs-on-Grade-vapor barrier

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Harold, I am curious as to why concrete shouldn't be placed directly on the
vapor barrier.
Could you please explain why?

TIA

Greg Meyer

-----Original Message-----
From: Sprague, Harold O. [mailto:SpragueHO(--nospam--at)bv.com]
Sent: Tuesday, July 30, 2002 12:47 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Concrete Slabs-on-Grade


Jim,

Several things have happened.
Aggregate in general is stable and has been stable since it was created
millions of years ago.  It is the cement paste that shrinks.  The more
cement, the more shrinkage.

Aggregate:
Years ago aggregates were uniformly graded.  Since then, we have stopped
requiring it and the asphalt industry has routinely pulled the 3/8" for
asphalt.  Therefor we have more gaps in the aggregate, and there is more
paste required.

Labor:
Labor is expensive.  The lower the slump, the more laborers are required.

Water:
Water is cheap; super P is expensive.  When you need concrete to flow, the
contractors reach for the hose.

Time:
To get on the concrete early, contractors have added more cement.

Curing:
Today it is a myth.  It takes time.  Time is money.  You can only get wet
curing at gun point, and spray on membranes are rarely put on correctly.

Joints:
They used to be tooled in the plastic concrete.  Now we use an abrasive saw.
Contractors can cut the joints days after the concrete is placed, and they
do.  Unfortunately the joints have formed themselves at about age 4 to 6
hours.  Properly design joints to allow the concrete to shrink to the center
of the area.  Eliminate all thickened slabs at columns and under walls.
Create a uniformly level sub base.  Use square dowels to allow the slab to
shrink in plan, but resist differential vertical movement.

Subgrade:
Properly compact the subgrade which should be something like compacted road
base or crusher fines.  Concrete should never be paced directly on a vapor
barrier.  The subgrade should be damp, but not wet.

There is a remedy for all of these maladies.  The contractor on my house was
amazed when he came back to the basement of my house after 4 years and there
were no visible cracks in the floor or walls.  Kalman flooring does acres of
floors every month, and they don't have many cracks.  I have worked on
concrete projects placed in August in Phoenix, and had great results.  It
all really doesn't cost that much.  Uniformly graded aggregate costs about a
dollar extra per ton.  In some markets, they don't charge extra.  Once a
contractor uses polycarbonate super P, he will use it everywhere.  Curing
just has to be forced.  Spray on membranes (if I have to) should be placed
with 2 passes sprayed 90 degrees to one another.  The best cure is fog,
which I require when using hot mixes like silica fume.  Any wet cure is
better than the spray on membrane.  But cure for sure.

I do like to back off to a 3500 psi for general use slabs on grade just to
minimize the cement, but if you adopt the practices above, most of your
problems go away.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Jim Kestner [SMTP:jkestner(--nospam--at)somervilleinc.com]
> Sent:	Tuesday, July 30, 2002 12:01 PM
> To:	'SEAINT'
> Subject:	Concrete Slabs-on-Grade
>
> It seems today that there are more problems (curling, shrinkage cracks,
> etc.) with concrete slabs-on-grade then in years past. Years ago when we
> used 3000 psi and few if any additives, there were fewer problems. What
> has
> changed?
>
> Today, we use 4000 psi concrete, more admixtures, faster schedules, less
> skilled workers, etc. I seemed to recall that we use 4000 psi in slabs for
> more durability but we really don't need much durability in schools,
> offices, clinics, etc. I could see using 4000 psi for industy or
> warehouses
> and sidewalks...perhaps 3500 psi for retail. Should we reconsider where we
> are using 4000 psi?
>
> Jim K.
>
>
>
>
>
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