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Re: Concrete set acceleration leading to cracking

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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>
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