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Increase of f'c over time

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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

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

George Barclay

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