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Re: Increase of f'c over time

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I don't think you mean "plain " concrete. Plain concrete is unreinforced.
Most concrete texts show a curve of strength increase but not as much as
you are estimating. Testing it is the only sure way because strength
increase is dependent on type and length of cure. If the moisture was
allowed to escape quickly the reaction stops and strength increase stops
for practical purposes.

Stan Scholl, P.E.
Laguna Beach, CA

On Thu, 20 Dec 2001 18:34:59 -0500 "Barclay, George"
<GBarclay(--nospam--at)lgt.lg.com> writes:
> 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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