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Re: Concrete set acceleration leading to cracking

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Daryl,

Yes, Schmidt hammer testing would be inexpensive and would give us an
idea as to whether the in-place strength of the concrete is seriously
different from the cylinder tests.  We are also thinking of a
petrographic study, which could reveal an excessively high water-cement
ratio, leading to excessive shrinkage.  Really though, these tests would
only help us eliminate more typical causes of widespread cracking. 

 It could also be that the mix design was fundamentally wrong,
inappropriate use of a set accelerator notwithstanding.  The ready-mix
supplier was relatively new to our market, and I don't believe the
design had had a large number of previous uses.  I believe "laboratory
trials" were used to justify the design.

We are also thinking of trying to see whether any curing was performed
by searching for traces of membrane-type curing agent in protected
places like beneath partition wall stud tracks.  We have information
from the accelerator manufacturer that indicates that good curing
practices would be critical when using an accelerator in hot weather. 
Perhaps even spray-on membrane curing would be inadequate and continuous
water sprinkling or moist blankets would be required for several days
with immediate application after finishing.

Tom


Thomas B. Higgins, P.E., S.E.

Group Mackenzie
0690 S.W. Bancroft Street
Portland, OR 97239-0039
Phone (503) 224-9560
Fax (503) 228-1285
<http://www.groupmackenzie.com>
This e-mail is confidential, may be legally privileged, and is intended
solely for the addressee.  If you are not the intended recipient, access
is prohibited.  As e-mail can be altered, its integrity is not
guaranteed.


>>> h.d.richardson(--nospam--at)shaw.ca 02/06/03 11:31AM >>>
Tom,

        I have had two experiences which might be applicable to your
situation.

        The first was with cold weather concreting about 25 years ago. 
It
involved site mixed concrete which included a batch mix plant and
concrete
mixer trucks to hall the concrete about 50 yards (or meters) to the
pour.
With the first load the water was too hot; the cement hydrated but the
mixing action in the truck did not permit the concrete to actually set
up.
The result looked like a load of wet aggregate with an excessive amount
of
fines; it was completely useless; the aggregate couldn't even be
reused.

        The second experience, which seems to be more closely related
to
your problem, was with hot weather concreting (no accelerators or hot
water
used, of course) about 10 years ago.  The construction was a
proprietary
system consisting of  2" or 2.5" concrete topping over flat sheet metal
over
cold formed (sheet metal) channels which were spaced at 16" or 24"
(I've
forgotten which).  This was a composite system with ash tray shaped
shear
connectors screwed through the flat sheet metal into the channels. 
The
cracking pattern was similar to what you describe.

        The client/owner was a church and all construction was being
done by
volunteer labour.  They really didn't want me to reject the floor out
of
hand; besides, with all of the cracks in compression I believed that
it
would work.  The floor has now been in use for about 10 years with no
problems whatever.

        Your case seems to me to be much more extreme with regards to
the
quality of the concrete.  You might actually want to do some "Schmidt
Hammer" testing to assure yourself that you actually have acceptable
quality
concrete.

        Hope this is helpful.

Regards,

H. Daryl Richardson

Tom Higgins wrote:

> Colleagues:
>
> Here is the scenario:  A contractor had a concrete mix design
reviewed
> and accepted for cold-weather concreting.  The mix design utilized
hot
> water and a liquid, non CaCl2, accelerating admixture.  Using this
mix
> design he placed some footings and slabs in the winter and
everything
> went acceptably well.  Spring came and it was time to place a
composite
> slab for a mezzanine (20 gage, 1-1/2" composite deck with 2" normal
> weight concrete cover over the flutes, 3 1/2" total thickness, f'c =
> 4000 psi, 6X6 - W1.4xW1.4 WWF 3/4" clear top, supposedly securely
> chaired, supported by open-web joists at about 4'-9" O.C.).  The
> contractor found it convenient to use the same concrete mix design,
> perhaps reasoning that the weather was still on the cool side.  The
day
> of the pour, however, turned out to be unseasonably warm and sunny. 
At
> the time of placement, about 9 to 10 am, ambient temperature in the
> building was around 50 degrees F.  Per NOAA, the outdoor high was in
the
> low 80's that day.  Outdoor temperature probably peaked around 3 pm.
> Concrete temperature was about 66 degrees F at the time of
placement.
> Temperatures inside the half-finished building may have been as high
as
> 90 degrees F by mid-afternoon, but measurements are lacking.  The
> contractor used the accelerated mix, even to the extent of using hot
mix
> water, despite the hot weather.  The slab ended up with a network of
> random cracks, not particularly wide, dividing most of the slab into
> rough octagons and hexagons perhaps 18" to 24".
>
> Question:  Did acceleration contribute heavily to the unusual
cracking
> due to excessively rapid hydration, excessively high concrete
> temperature, thermal expansion and subsequent contraction (in
addition
> to normal shrinkage).  Has anyone experienced problem like this?  Is
> using an accelerator in hot weather for thin slabs a known formula
for
> excessive cracking?
>
> Tom
>
> Thomas B. Higgins, P.E., S.E.
>
> Group Mackenzie
> 0690 S.W. Bancroft Street
> Portland, OR 97239-0039
> Phone (503) 224-9560
> Fax (503) 228-1285
> <http://www.groupmackenzie.com>
> This e-mail is confidential, may be legally privileged, and is
intended
> solely for the addressee.  If you are not the intended recipient,
access
> is prohibited.  As e-mail can be altered, its integrity is not
> guaranteed.
>
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