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Re: Increase of f'c over time

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I guess it is certainly possible and even probable that the original 28 day
strength was higher than the specified design strength however I agree with
Syed that I would doubt that it would be twice as much, but who is to say

I can provide the following from the petrographic analysis we performed:

*  The course aggregate was 1/2 inch to 3/4 inch maximum sized crushed

*  The water/cement ratio was estimated to average 0.40.

*  There was no air entrainment.

*  There is no evidence of pozzolans.

*  The slump is estimated to be 4 inch or less.

*  The paste proportion averages about 28%.

The petrographic report indicates "considerable strength gain over the
years" but does not provide any more details than this general comment.

Thomas Hunt, S.E.
Duke/Fluor Daniel

                    Richardson           To:     seaint(--nospam--at)                                                 
                    <h.d.richardso       cc:                                                                       
                    n(--nospam--at)>           Subject:     Re: Increase of f'c over time                                
                    12/22/01 11:18                                                                                 
                    Please respond                                                                                 
                    to seaint                                                                                      


           I respectfully disagree with Thomas Hunt's statement "This would
indicate that the compressive strength increased nearly 3 fold over 50
years."  I agree with everything else that Thomas has presented.  His
was an excellent response.

           It has been my experience that 30 plus years ago it was fairly
for owners of industrial type projects to specify such concrete mix
design parameters as minimum cement content, maximum water/cement ratio,
maximum slump more stringent than normal, no use of fly ash or other
additives to increase flowability or reduce cement content, etc. etc.
Following these specifications always produced concrete that was much
stronger than the minimum 3,000 p.s.i. specified.  If such was the case
at the time of construction of the water intake structure it's fairly
likely that the actual 28 day strength may well have been between 4,500
and 6,000 p.s.i.  This would reduce the apparent increase of f'c over
time to something closer to the generally accepted values.

           It would be very interesting to see some of the original 28 day
results if they were available; but I should be surprised if they still
existed.  It would also be interesting to know if the original owner's
spec. contained some of the mix design parameters I mentioned in the
previous paragraph; perhaps Thomas will be able to satisfy our (my)
curiosity on that.

                                          Best regards,

                                          H. Daryl Richardson

syed faiz ahmad wrote:
> Agreeg & concurred.
> >From: Tom.Hunt(--nospam--at)
> >Reply-To: <seaint(--nospam--at)>
> >To: <seaint(--nospam--at)>
> >Subject: RE: Increase of f'c over time
> >Date: Fri, 21 Dec 2001 14:31:26 -0800
> >
> >
> >George,
> >
> >We just finished rehabilitating a 50 year old seawater intake structure
> >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
> >the 90 four-inch diameter core samples we took, we compression tested
> >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
> >psi.
> >
> >I agree with the others that the only way to be sure is to take some
> >samples.  You really do not know if the original concrete started out
> >the desired strength.  Someone could have added a lot of water at the
> >it may not have been consolidated correctly, and etc.  Coring and
> >is not a big deal and is not very expensive considering the liabilities
> >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
> >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
> >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
> >
> >
> >Gentlemen,
> >
> >I am performing a job that involves the capacity of structural plain
> >concrete.  The new (heavy equipment) demand exceeds the flexural
> >of
> >the member using the original concrete strength given in the 1960
> >construction documents.  The client obviously wants to avoid
> >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
> >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
> >
> >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
> >3000 when it was originally poured.  (Most of the break strengths that
> >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
> >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
> >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,
> >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
> >new equipment to his superiors.  Does anyone know of such a documented
> >study?
> >
> >Thanks,
> >George Barclay
> >
> >

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