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# RE: Increase of f'c over time

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: RE: Increase of f'c over time
• From: "Barclay, George" <GBarclay(--nospam--at)lgt.lg.com>
• Date: Fri, 21 Dec 2001 09:02:41 -0500

```Thank your Lucas.

I have the McGregor book and have looked at that formula.  I cannot be sure,
but I believe that this formula is for relatively short periods of time.
The grad school professor I spoke of has been on several ACI committies and
has performed dozens of research projects in conjunction with ACI and other
clients over a period of about 35 years.  (Ed Burdette, maybe some of you
have heard of him.)  He has seen core samples of 50 and 60 year old
structures in which f'c did double, assuming the concrete was poured at the
design document required strength.  This is the order of magnitude period at
which I am looking.

I worked construction for several years before going back to school.  I once
had to jack hammer out a 45 year old gas station pump island.  It took me
all day long, and compared to fairly new concrete, which I have also jack
hammered, it was a whole lot harder.

> -----Original Message-----
> From:	Lucas Jolly [SMTP:Lucas(--nospam--at)lbdg.com]
> Sent:	Thursday, December 20, 2001 7:09 PM
> To:	'seaint(--nospam--at)seaint.org'
> Subject:	RE: Increase of f'c over time
>
> 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.
> 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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Lockwood Greene Technologies business information intended only for the use
of the individual or entities named above.  If the reader of this message is
not the intended recipient you are hereby notified that any dissemination,
distribution or copying of this communication is strictly prohibited.  If
by e-mail at the originating address or at postmaster(--nospam--at)lgt.lg.com.

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