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RE: Estimating f'c beyond 28 days

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William-
"Composition and Properties of Concrete" by Troxell and Davis gives
information on the strength gain of concrete with age from some very long
tests at the University of California at Berkeley.  Concrete does increase
in strength with age but usually pretty slowly beyond about 1-2 months.

That said, it appears you are splitting hairs that should not be split.
Concrete tests give an overall indication of the strength of the concrete
mass but the variability is usually fairly wide.  To rely for some design
reason on a different age of comparison also puts you outside the basis of
most codes.  

Remember that concrete design is not a precision exercise.  Rather, it is an
attempt to understand the gross behavior of  a variable media with a
simplified model.  I remember reading an old ACI technical note back in my
younger days (and that was a while ago!) that suggested considering your
concrete design no more accurate than about 1 significant figure (i.e.,
using a strength value of 3 ksi or 4 ksi not 3.250 ksi is what you are
after).  The fact your computer will calculate it on some basis to a
bizillion figures doesn't make the answer more accurate.   

Regards,
Bill Cain, S.E.
Oakland  CA  


	-----Original Message-----
	From:	William Keil [SMTP:WKeil(--nospam--at)TheStellarGroup.com]
	Sent:	Thursday, September 21, 2000 2:37 PM
	To:	'seaint(--nospam--at)seaint.org'
	Subject:	Estimating f'c beyond 28 days

	Assuming that concrete continues to cure indefinitely under certain
	circumstances.  Does anyone have a reference or a rule of thumb for
	estimating the concrete compressive strength based on the 28 day
strength?
	So, based on a test yielding 4000 psi concrete at 28 days, what can
the
	estimated value at 56 days and 90 days be calculated at?

	TIA,
	William J. Keil, P.E.



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