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RE: Concrete Slabs-on-Grade

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Stan,

I would agree if the cracks were flexural, but with all things equal, a 4
sack mix will shrink less than a 5 sack mix.  Yitterberg from Kalman did
both the lab work (shrinkage test) and the field work to confirm it.  There
is nothing else in the concrete matrix that shrinks other than the cement
paste.  You can reach a point where you have inadequate cement for aggregate
bonding, but that will not be a problem with a 4 sack mix.

You are correct in pumped mixes unless you stick to your guns on your mix
which will force them to use the correct pump line sizes.  Pumpers like to
use the smaller line sizes for either horizontal or boom pumps.  The vast
majority of slabs on grade are placed using horizontal line pumps (often
called slick line).  

Big industrial floor companies like Kalman tailgate everything and eliminate
the pumping problem.  They also use laser screeds, and enough finishers to
do the job right.  I developed a difficult mix on one occasion due to some
other constraints and consulted with a Kalman superintendent (Charlie with a
cowboy hard hat) to see if he could place it.  He told me that if I gave him
dog ****, he could place it and finish it.  The concrete was placed
perfectly, finished perfectly, cured perfectly, and there were no cracks.

Personally, I like a pre-construction meeting with the cement supplier,
testing lab, finishing contractor, and the pumping contractor.  On big jobs,
I like to have a test pour to simulate the job conditions as much as
possible.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Stanley E Scholl [SMTP:sscholl2(--nospam--at)juno.com]
> Sent:	Tuesday, July 30, 2002 8:30 PM
> To:	seaint(--nospam--at)seaint.org
> Cc:	seaint(--nospam--at)seaint.org
> Subject:	Re: Concrete Slabs-on-Grade
> 
> 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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