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RE: Increase of f'c over time

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We just finished rehabilitating a 50 year old seawater intake structure and
I can give you our actual results for comparison.  The original drawings
showed a mix of 2500 psi and 3000 psi specified design strengths.  Out of
the 90 four-inch diameter core samples we took, we compression tested 53.
Out of the 53 compression tests the lowest strength was 6220 psi and the
highest 12,840 psi with an average strength of 8635 psi.  This would
indicate that the compressive strength increased nearly 3 fold over 50
years.  The split tensile strengths were very consistent and averaged 828

I agree with the others that the only way to be sure is to take some core
samples.  You really do not know if the original concrete started out with
the desired strength.  Someone could have added a lot of water at the site,
it may not have been consolidated correctly, and etc.  Coring and testing
is not a big deal and is not very expensive considering the liabilities you
may get into.

Thomas Hunt, S.E.
Duke/Fluor Daniel

                    Lucas Jolly                                                                                    
                    <Lucas(--nospam--at)       To:     "'seaint(--nospam--at)'" <seaint(--nospam--at)>                         
                    m>                   cc:                                                                       
                                         Subject:     RE: Increase of f'c over time                                
                    12/20/01 04:08                                                                                 
                    Please respond                                                                                 
                    to seaint                                                                                      

Per MacGregor's book, "Reinforced Concrete - Mechanics & Design":
f'c(t) = f'c(28)*[t/(4+0.85*t)]
t is in days
According to the text, this formula is for Type I cement (normally used
prior to 1975).  For Type III cement, the coefficients 4 and 0.85 become
and 0.92, respectively.
This also assumes 70 degree temperatures.
The rest of this chapter address other factors, such as time of loading,
varying moisture conditions, etc.

No matter how you slice it,  this formula suggests you aren't getting much
over a 20% increase.

Lucas Jolly

-----Original Message-----
From: Barclay, George [mailto:GBarclay(--nospam--at)]
Sent: Thursday, December 20, 2001 3:35 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
the member using the original concrete strength given in the 1960
construction documents.  The client obviously wants to avoid modification
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
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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