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RE: Increase of f'c over time

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Never, ever, ever rely on "reasonable assumptions of strength" when
evaluating an existing structure. If you check the model building codes, for
example, you will find that existing structures MUST be evaluated using
empirical measurements. That's why, whenever I'm doing a renovation/retrofit
job, and the contractor/owner tells me "oh, and I have the drawings right
here," I tell him "that's nice, they'll serve as a good reference. But they
will NOT be relied upon as the final word. I will take measurements
including if need be assessments of the actual strength of the in situ

What you did at the outset was fine, for a first approximation. But you're
going to need material testing to back up your assumptions if you want the
existing structure to sustain an appreciable increase in service loading.

This is where this game gets dicey, because contractors/owners don't want to
hear that; they think we ought to know these things, I suppose by osmosis.

William L. Polhemus, Jr. P.E.
Polhemus Engineering Company
Katy, TX, USA
Phone (281) 492-2251
FAX (281) 492-8203
email bill(--nospam--at)

-----Original Message-----
From: Barclay, George [mailto:GBarclay(--nospam--at)]
Sent: Thursday, December 20, 2001 5:35 PM
To: 'seaint(--nospam--at)'
Subject: Increase of f'c over time
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.

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