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RE: Flyash

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The strength issue with fly ash is more to do with the rate of strength gain than with the actual strength.  Fly ash slows the strength gain (and thus the heat of hydration also), but at 50% replacement, strengths of 4000 and 5000 psi are easily attainable (particularly if you can use 56-day strength as your benchmark). 
 
As you point out, curing and finishing are the sticking points.  Literally in the case of finshing; high fly ash makes the concrete a lot stickier during troweling.  This can be mitigated by using an evaporation retarder (ie Master Builders Confilm) after floating.  In our office we specify a 14-day wet cure for concrete with more than 30% fly ash (usually reduced to 7 days when the contractor complains.)  The cure is critical since so much of the strength gain occurs late.  The combination of curing and finishing problems has led us to call for 30% fly ash for flatwork on jobs where we are otherwise using 50% replacement.
 
The biggest gain you get is workability, which allows you to use less water, which of course means less shrinkage.  As an example, during some informal research during grad school, a friend and I mixed up a batch of concrete with 75% fly ash and a 0.33 w/c+f ratio--don't ever try this at home, in 56 days it never got above 2500 psi--and it had a slump of more than 6 inches...without any super, or any WRA at all for that matter.  The typical 50% fly ash mix we have used in our office has a 0.30 w/c+f ratio with just a little bit of super-P.  I haven't seen this one poured, but I've heard it's extremely workable, and the finished product I have seen is beautiful.
 
As for references, there is an article in the August issue of Concrete International about the use of high-volume fly ash concrete in the retrofit of a building at UC Berkeley.
 
Regards,
Chris Willcox, SE
 
 
 -----Original Message-----
From: Peder Golberg [mailto:peder(--nospam--at)jgpierson.com]
Sent: Wednesday, August 28, 2002 3:09 PM
To: seaint(--nospam--at)seaint.org
Subject: Flyash

I have an architectural client who wants to use between 35% and 50% flyash to replace the cement content in a slab on grade (with a w/c ratio of 0.40).   The slab will be covered with flooring that is very moisture sensitive and this amount of flyash is being recommended to the architects by the "green" designers.    I have always thought 20% was the maximum recommended and any more than this makes the finish work and curing not practical along with strength issues.
 
Any have any good or bad experiences (or references) using this high amount of flyash content?   
 
Peder Golberg
Portland, Oregon