Great input, Harold. I have a related (I think) question regarding
A couple of days ago I went to a GREEN seminar. The speaker told us that
using a great amount of fly ash (replacing part of the PC) will greatly
reduce the CO2 into the air. I know that fly ash will also act as a retarder
(for the concrete strength). Question: Is the strength vs. the modulus of
elasticity curve the same as for using portland cement ?
Antonio S. "Tony" Luisoni
Granada Hills, CA
In a message dated 9/2/2009 6:54:06 A.M. Pacific Daylight Time,
leave it arbitrary, or a rule of thumb. Calculations can be developed
for concrete at various ages and temperatures.
There are many
good guides out there. I would suggest that you look at the Concrete
Manual, US Dept of Interior, Figure 10 as a guide. It is a graph
showing concrete age on the horizontal axis and percent of strength on the
vertical axis. The chart contains various temperatures of concrete so
that you can anticipate strength gain at various times.
For example a 70 degree F ambient will have about 35% of the 28
day strength in 3 days. If your target is 1,500 psf for a slab on grade
for construction, your 28 day target should be about 4,300 psi. In 7 days you
will get about 65% of your 28 day target at 70 degrees.
it is important, you can develop your mix for strengths at various
times. The example cited above is subject to other aspects of the mix
matrix like polycarboxolates and flyash to get that LEED point.
Regards, Harold Sprague
Date: Wed, 2 Sep 2009
Subject: RE: Concrete Cure Time
Thanks for everyone’s
input on this one. As far as I know the 3 day rule has just been
something of a rule of thumb in this area. It appears to me that even at
an early stage concrete will have enough strength to sustain a day or so worth
of hollow block work – that’s not really all that much pressure. I’m
going to suggest that we allow the placement at 24 hours, but not allow the
cells to be poured until a yet to be determined time frame.
Sent: Wednesday, September 02, 2009 7:23
Re: Concrete Cure Time
I concur with Harold and Jordan. We transfer prestress at 12-16 hours,
depending on how long the crew worked the day before. Our minimum is 3,500
psi, and can be as high as 4,200 psi with our normal mix. We break two
cylinders (three, soon, I guess) for a full test before cutting strand. We
used to use high-early, but cannot purchase it anymore. I doubt our normal mix
would resemble the average slab-on-grade normal mix other than rocks, water
and cement, but let them prove they can proceed. This is not your
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