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Re: Increase of f'c over time/statistics in concrete mix design

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Daryl,

You have a point there, right. But you should also give allowance to the fact that in good old days the mix design was based on 'volume batching' & was based on minimum design strength with no relevance to statistical aspect of it. This could hardly ensure the actual tested strengths to always remain above the design value.

Whereas now, we have more scientific approach to the design of mixes. Today, the statistic plays a critical role & the design is based on 'Characteristic Stength' which has a 'margin' over & above the design strength. And, this 'margin' is based on the statistical history of the kind of concrete that is intended to be produced. All these have a built-in mechanism that sort of ensure that the actual tested strengths of concrete continue to remain above the design value; or if fall below, they still remain within the acceptable percentage of failures.

In short, there is every likelyhood & strong supports to believe that old days mix design would tend to produce inconsistent, erratic concrete that may or may not ensure consistent progressive increase of concrete strengths over a period of time as compared to the modern day concrete mixes.

You may opiniate on this issue & i would like to hear some thing from you Daryl.

Regards,

SYED FAIZ AHMAD; MENGG, MASCE
SENIOR STRUCTURAL ENGINEER
SAUDI OGER LTD,
RIYADH, SAUDI ARABIA.




From: Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: Re: Increase of f'c over time
Date: Sat, 22 Dec 2001 12:18:55 -0700

Gentlemen,

	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 common
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 test
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.
>
> SYED FAIZ AHMAD
>
> >From: Tom.Hunt(--nospam--at)d-fd.com
> >Reply-To: <seaint(--nospam--at)seaint.org>
> >To: <seaint(--nospam--at)seaint.org>
> >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 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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> SYED FAIZ AHMAD; MENGG, MASCE
> SENIOR STRUCTURAL ENGINEER
> SAUDI OGER LTD
> RIYADH, SAUDI ARABIA.
>
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SYED FAIZ AHMAD; MENGG, MASCE
SENIOR STRUCTURAL ENGINEER
SAUDI OGER LTD
RIYADH, SAUDI ARABIA.


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