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# Re: Concrete Cure Time

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Concrete Cure Time
• From: "Gerard Madden, SE" <gmse4603(--nospam--at)gmail.com>
• Date: Thu, 10 Sep 2009 16:16:23 -0700

Contractors will want you to get rid of the flyash completely and use Slag. The concrete apparently stays too wet for too long using high amounts fly ash.

-gm

On Thu, Sep 10, 2009 at 3:17 PM, wrote:
Great input, Harold. I have a related (I think) question regarding strength.
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 ?
Thanks
Antonio S. "Tony" Luisoni
Consulting SE

In a message dated 9/2/2009 6:54:06 A.M. Pacific Daylight Time, spraguehope(--nospam--at)hotmail.com writes:
Don't 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.

If 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

From: RichardC(--nospam--at)lbbe.com
To: seaint(--nospam--at)seaint.org
Date: Wed, 2 Sep 2009 08:15:53 -0400
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.

Again, thanks.

From: Jim Getaz [mailto:jgetaz(--nospam--at)shockeyprecast.com]
Sent: Wednesday, September 02, 2009 7:23 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Concrete Cure Time

Richard,

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 call.

Jim Getaz

Precast Concrete Engineer