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RE: Increase of f'c over time

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Testing is definitely the key.  For contrast of the story below, I worked on
a job a few years ago where we cored some 30 year old columns with concrete
originally specified at 5,000 psi.  Most of them came back at around 7,000
psi with one or two as high as 8,000 psi, but several were as under 6,000
psi and I seem to remember one was even at about 4,800 psi.  Based on this
small sample of 20 - 30 columns there was essentially no pattern and no rule
of thumb regarding strength gain.  They were all in an enclosed basement, so
the climate and humidity should have been essentially the same for all of
them.  They were all precast columns, so I would have guessed that the
controls on the mix would have been fairly good.  Without any more project
history than was available 30 years after the fact, I have nothing but vague
guesses why there was so much variation.  I was glad that we opted for such
extensive testing, testing every column, rather than just taking a few
sample cores.  

Paul Crocker, P.E.


-----Original Message-----
From: Tom.Hunt(--nospam--at)d-fd.com [mailto:Tom.Hunt(--nospam--at)d-fd.com]
Sent: Friday, December 21, 2001 2:31 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Increase of f'c over time



George,

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
psi.

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)lbdg.co       To:     "'seaint(--nospam--at)seaint.org'"
<seaint(--nospam--at)seaint.org>                         
                    m>                   cc:

                                         Subject:     RE: Increase of f'c
over time                                
                    12/20/01 04:08

                    PM

                    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
2.3
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)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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