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Re: Increase of f'c over time

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Thomas,

	Syed's level of knowledge, as evidenced by his postings, makes it very
hard to ever disagree with him; and I certainly can't argue with your
test results; the testing company has done a very thorough job.  Not
that this is any surprise; my own years at Fluor make me aware that
Fluor engineers virtually always insist upon an excellent job.

	I do, however, have an ancient copy of a textbook, "Concrete", by
Troxell and Davis (both from University of California), copyright 1956,
which makes it almost the same age as your intake structure.  On page
107 there is a chart of concrete strength vs. water/cement ratio
indicating a w/c of 4 gallons per sack indicates 6,000 p.s.i concrete
and 5 gallons per sack indicates 5,000 p.s.i concrete.  I assume this
referrers to U.S. gallons at 8.33 pounds per gallon and 94 pounds per
sack of cement, therefore a w/c ratio of 0.4 would be about 4.5 gallons
per sack which might indicate something like 5,500 p.s.i concrete.  That
said, there is a lot more to concrete strength than just w/c ratio;
also, whether the site conditions at the time your concrete was made
were close enough to ideal to actually produce concrete of this quality
is a different matter.

	All this is moot, however.  What we are all agreeing upon is that it is
necessary to do some testing and use the results that are actually
measured today and not to rely on what the drawings say about concrete
strength.

			Best regards, Thomas, and Merry Christmas.

			H. Daryl Richardson

Tom.Hunt(--nospam--at)d-fd.com wrote:
> 
> Daryl,
> 
> 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
> otherwise.
> 
> 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
> diorite
> 
> *  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
> 
> 
>                     Daryl
>                     Richardson           To:     seaint(--nospam--at)seaint.org
>                     <h.d.richardso       cc:
>                     n(--nospam--at)shaw.ca>           Subject:     Re: Increase of f'c over time
> 
>                     12/22/01 11:18
>                     AM
>                     Please respond
>                     to seaint
> 
> 
> 
> 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
>

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