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RE: Concrete Slabs-on-Grade-vapor barrier

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There are 2 reasons not to place the slab directly over the vapor barrier.

1st - Many of the commonly used materials for vapor barriers degrade with
time due to the concrete alkalinity.
2nd - Concrete cast directly on the vapor barrier will curl differentially
(Nicholson, How to Minimize Cracking ..., Concrete Construction, Sept.

If you in fact need a vapor barrier, I prefer the following:
	1.	Properly prepare a relatively smooth damp (not wet)
	2.	Place and compact a 3" layer of 3/8" open graded crushed
	This will have no fines and will serve as a capillary break, and
preclude perched water on the subgrade.  The crushed rock will lock together
to form a good working surface for construction traffic.    
	3.	Place a vapor barrier.  
	This is a manufactured product like Mirafi, not visqueen.  Flash all
penetrations, lap joints, caulk as required by manufacturer.
	4.	Place and compact 2" or 3" of road base or crusher fines.
The surface should be a smooth horizontal plane. 
	This is to break contact with concrete to minimize curling and to
avoid vapor barrier degradation due to concrete alkalinity.  The road base
or crusher fines will compact to form a good working surface that will not
be dislocated by construction traffic placing the rebar or dragging a slick
line over the surface.  Pea gravel or sand will shift during construction
when placed on the vapor barrier.
	5.	Set rebar or WWR on dobies or chairs.  Either use rebar
spaced 12" or 18" so that construction workers can step in the spaces or go
to a 4" spaced WWR so that the workers can walk on the reinforcing.  Use
sheet WWR not rolls.  Do not allow the contractor to "hook" the bar into
	6.	Place concrete slab on grade.
	7.	Tool control joints in plastic concrete, or use early entry
saw with anti-ravel skid plate (Soff-Cut Saw) to cut control joints within 4
hours of concrete set.  If you wait longer the joints will have formed
themselves.  You won't see them right away, but they will open themselves up
in time as the concrete continues to shrink..

Harold O. Sprague

> -----Original Message-----
> From:	Greg Meyer [SMTP:gmeyer(--nospam--at)]
> Sent:	Friday, August 02, 2002 7:55 AM
> To:	seaint(--nospam--at)
> 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?
> Greg Meyer
> -----Original Message-----
> From: Sprague, Harold O. [mailto:SpragueHO(--nospam--at)]
> Sent: Tuesday, July 30, 2002 12:47 PM
> To: 'seaint(--nospam--at)'
> 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)]
> > 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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