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RE: Concrete set acceleration leading to cracking

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The mix design sure didn't help. In addition to the above normal temperature
shrinkage, you've got the restraint created by the composite deck working
against it. Under normal placement conditions, I figure the shrinkage
associated with temperature change is probably the same order of magnitude
as shrinkage associated with drying of the concrete over time. If you've got
a much hotter mix, it wouldn't be very hard to aggravate the situation. 

The thin section probably didn't help you much either. You would tend to
lose both moisture and heat relatively quickly compared to a thicker slab.
While the concrete modulus of elasticity is lower at an early age, so is the
strength, so you don't have much tensile strength in the slab to resist
cracking while all this fairly rapid shrinkage strain is going on. 

-----Original Message-----
From: Tom Higgins [mailto:thiggins(--nospam--at)grpmack.com]
Sent: Thursday, February 06, 2003 10:06 AM
To: seaint(--nospam--at)seaint.org
Subject: Concrete set acceleration leading to cracking


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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