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Re: Concrete Freezing

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Dan,

	Stan and Nels both have good advice regarding frozen concrete.  I have
been involved with two cases of failure of frozen concrete which I will
relate below.

1.)	On a 16 story building, the roof slab (flat plate, approx. 6" thick)
was cast New Years Eve, about 1970 (party time, insufficient attention
to hoarding and heating).  This was a government owned project; the
owner had a full time inspector on site to monitor construction.  The
temperature went down below -20 F; and stayed cold until mid February. 
The forms were removed and everything seemed fine.  When it thawed out
in February the slab collapsed!  Totally and completely!  It was mainly
ice which had held it together for 6 weeks.  The concrete behavior was
not unlike soft coal; with effort some of the pieces could be broken BY
HAND!  Needless to say the inspector and the site superintendent were
both removed from the project; one summarily, and the other discretely.

	Of interest to structural engineers, after the cleanup was complete we
were only able to find two hairline cracks on the underside of the 16th
floor.  This was somewhat astonishing since the roof slab (75 psf) had
actually fallen 8 feet onto the floor which was designed for a live load
of only 40 psf.  At that time we were using the ACI 318 code.

2.)	The second building, a few stories shorter, similar time period,
suffered a similar fate.  In this case evidence of possible collapse was
evident at the start of form removal.  The roof slab was immediately
hoarded in and heated (to room temperature or a bit more) for something
like 56 days.  After this time it was subjected to a load test in
accordance with the then applicable engineering standards; and IT
PASSED!

	My understanding (and I am NOT a concrete material specialist) is that
if concrete achieves its initial set (which takes something like 12 to
24 hours after casting) BEFORE it freezes it will continue curing and
gaining strength after it thaws out; if it DOES NOT achieve its initial
set before it freezes it will not continue curing and gaining strength
after it thaws out.

	If this were my project and if I were in serious doubt about the
quality of concrete because of possible freezing: I would determine the
minimum f'c which will allow continuation of the construction and/or be
acceptable for final service (this will probably be quite a bit less
than the specified 28 day f'c); I would then ensure heated curing
(certainly above 35 - 40 F but as high as practical since heat enhances
curing) as protection against future freezing; and I would have cores
taken and tested to monitor progress of the during.  These rather
stringent measures could be stopped at any time if it becomes apparent
that freezing has not been detrimental to the concrete quality.  All of
this would be at the contractor's cost; plus you might require a
reduction of the contract price if the material which is finally
accepted does not meet the contract (specified) quality.  The
alternative, of course, is removal of the inferior concrete and
reconstruction; don't accept anything you can't live with.

	For concrete subject to cycles of freezing and thawing you should have
specified 4% - 6% entrained air (or some similar amount based on
aggregate size).  If you did this you have a much better chance of
salvaging the concrete construction.  I would suggest that you do one,
or perhaps two, microscopic examinations of the tested cylinders for
entrained air content (it is certainly not necessary to test every
core).

	This is probably not what you asked for but I thought it might be
helpful.

				Regards,

				H. Daryl Richardson
				Calgary, Canada

Nels Roselund, SE wrote:
> 
> Dan,
> 
> ACI 201.2R, Guide to Durable of Concrete, as well as ACI 306 documents on
> cold-weather concreting, deal with how to protect concrete from freezing --
> not with the effects, nor the means of evaluating concrete potentially
> damaged by freezing.  ACI 201.2R has one reference to a general discussion
> on the subject of frost damage in concrete by Cordon (1966).  William A
> Cordon was a member of ACI Committee 201.  Maybe you could search out
> Cordon's document on the web.
> 
> My guess is:
> Water in concrete that is curing but that has not yet been incorporated into
> the matrix as water of hydration is subject to freezing.  Water forms
> crystals and expands as it freezes.  That kind of action will form
> irreparable micro-cracks and voids that will weaken the concrete.  In the
> absence of any other reference, I would take core-drilled specimens for
> testing and compare the results with the expected strength to determine if
> the concrete has been damaged.
> 
> Nels Roselund
> Structural Engineer
> South San Gabriel, CA
> njineer(--nospam--at)att.net
> 
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