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Re: Concrete Slabs-on-Grade

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I agree with most of your comments but a 4 sack mix will crack more than
a 5 sack mix if everything else is constrant. The low cement mix doesn't
have enough strength ot resist the shrinkage forces. And if it is a pump
mix, it will shrink much more than if not a pump mix.

Stan Scholl, P.E.
Laguna Beach, CA

On Tue, 30 Jul 2002 12:46:48 -0500 "Sprague, Harold O."
<SpragueHO(--nospam--at)bv.com> writes:
> 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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