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Re: Concrete Slabs-on-Grade

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A 4 sack mix probly will shrink less than a 5 sack mix if everything else
is equal, but the 5 sack mix will have much more strength to resist the
shrinkage forces and therefore will crack less, I believe.

Stan Scholl, P.E.
Laguna Beach, CA

On Wed, 31 Jul 2002 10:33:51 -0500 "Sprague, Harold O."
<SpragueHO(--nospam--at)bv.com> writes:
> 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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