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RE: Concrete Slabs-on-Grade-vapor barrier

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I would be inclined not to use open graded "gravel", because of its tendency
to shift during construction.  I would still want to use the open graded
crushed rock above the subgrade and place the vapor barrier over the open
graded crushed rock.  Its purpose is to serve as a capillary break to avoid
the problem of perched water migrating through any holidays in the vapor
barrier.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Tom Bayne [SMTP:Tom(--nospam--at)soilsengineering.com]
> Sent:	Friday, August 02, 2002 12:41 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	RE: Concrete Slabs-on-Grade-vapor barrier
> 
> FYI:
> 
> At a recent seminar by an expert discussing vapor barriers and problems
> with
> slabs such as "mold" and soil salts
> the audience was informed that the use of a vapor barrier was good and
> that
> the use a layer of open-graded gravel was not good because us provided a
> place for vapors to accumulate and enter the slab.  The practice of
> shading
> the vapor barrier with sand was discussed as a measure to absorb excess
> moisture from concrete and it was stated that the moisture added to the
> sand
> shading before the concrete arrives is a critical item.
> 
> T Bayne
> 
> -----Original Message-----
> From: Eric Green [mailto:EGreen(--nospam--at)walterpmoore.com]
> Sent: Friday, August 02, 2002 6:34 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Concrete Slabs-on-Grade-vapor barrier
> 
> 
> I would simply add that you may want to careful confer with your E&O
> carrier
> before placing the slab without a vapor barrier directly underneath. While
> as a structural engineers you may recognize certain benefits of the sand
> bed
> b/t the slab and vapor barrier, many juries and arbitrators believe that
> this practice contributes to flooring failures. Many are the ones whom
> have
> paid for this design at the courthouse.
> 
> I will not discuss my personal views on the subject, just want to point
> out
> that this is a controversial issue.
> 
> eric green
> -speaking only for myself, not my employer.
> 
> -----Original Message-----
> From: Greg Meyer [mailto:gmeyer(--nospam--at)scahouston.com]
> Sent: Friday, August 02, 2002 7:55 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Concrete Slabs-on-Grade-vapor barrier
> 
> 
> 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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