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RE: Estimating f'c beyond 28 days

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The PCI Manual for Structural Design of Architectural Precast Concrete, 1977
edition has a couple of charts that indicate 90 day concrete to have about
125% of the strength of 28 day concrete provided it is continuously moist
cured at 55 degrees.  At 73 degrees, continuous temperature and continuous
moist curing, there is only a 10 % gain.  If you have some "suspect"
concrete cured under normal conditions, you probably should not consider
trying to justify using a higher value than the 28 day strength.

Roger Davis
SDS Architects, Inc
205 N. Dewey Street
Eau Claire, WI 54703

-----Original Message-----
From: seaint-return(--nospam--at) [mailto:seaint-return(--nospam--at)]On
Behalf Of William Keil
Sent: Thursday, September 21, 2000 2:37 PM
To: 'seaint(--nospam--at)'
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?

William J. Keil, P.E.

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