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Re: Increase of f'c over time

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George, Panos,

	I've just been through this regarding an addition to an historic
building.  In my case I happen to have a long term colleague and friend
who is a retired Ph.D. specializing is concrete materials.  I discussed
your current problem with my colleague and his comments are the
following:

1.)	Expecting the F'c to double is excessively optimistic; a 25% to 50%
is much more realistic.  The actual value should be based on core tests.

2.)	Impact tests will give unrealistically high values due to
carbonation of the surface which would make the surface hardness not
typical of the concrete mass.

	One thing I did not discuss with my colleague was whether or not other
concrete properties increased with age at the same ratio as the
compressive strength.  You may be wise to have some splitting tests, or
other tests performed to satisfy yourself in this regard.

	Hope I haven't poured water on your campfire.

			Compliments of the season to all of you,

			H. Daryl Richardson

Panos Trochalakis wrote:
> 
> An option you may want to look at is a rebound hammer test. This test does
> not require any coring or damage to the concrete. However, I believe it is
> not extremely accurate. (+/-25%) Almost any testing/inspection firm should
> be able to perform this test for a very reasonable cost.
> 
> -----Original Message-----
> From: Barclay, George [mailto:GBarclay(--nospam--at)lgt.lg.com]
> Sent: Thursday, December 20, 2001 3:35 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: Increase of f'c over time
> 
> Gentlemen,
> 
> I am performing a job that involves the capacity of structural plain
> concrete.  The new (heavy equipment) demand exceeds the flexural capacity of
> the member using the original concrete strength given in the 1960
> construction documents.  The client obviously wants to avoid modification of
> the structure.  He has good reasons which I will not go into here, for
> length of the discussion.  I know that concrete strength increases over
> time.  I spoke with one of my grad school professors from whom I originally
> heard of this increase.  He said that he wouldn't be surprised if the
> strength of the concrete had as much as doubled over that amount of time.
> 
> I passed this along to my client, but tempered it with the statement that I
> would not want to count on a doubling of strength from 3000 psi to 6000 psi.
> I told him that there is a good chance that the concrete was something over
> 3000 when it was originally poured.  (Most of the break strengths that I've
> seen in my 9 years are somewhat over the required f'c in the design
> documents)  I told him that this fact, plus the strength increase might get
> us to as much as 5000 psi.  We decided to use this in the evaluation
> calculations, but I told him that my report would state that this strength
> would have to be documented by testing prior to his new equipment being
> placed.
> 
> The analysis is complete and the structure's capacity is now adequate, but
> now my client would like to see a study or some type of documentation of
> this strength increase phenomenon before he commits to the placing of the
> new equipment to his superiors.  Does anyone know of such a documented
> study?
> 
> Thanks,
> George Barclay
> 
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