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RE: Admixtures for Concrete Clarifier Tank - fly ash

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I would have to disagree with using 15% as the "maximum" for fly ash
content.  Actually, I use 15% as a "minimum" when fly ash is used and
25% as the maximum.  In some tests, fly ash has had a detrimental effect
on sulfate resistance of concrete when less than 15% fly ash was used,
but sulfate resistance was improved when the content was in excess of
15%.  (I've recently heard that the lower limit should be 20%.) 

I normally use fly ash as a 1:1 replacement for cement.  Although I have
not yet adopted changes in curing requirements (from 7 days curing time)
when fly ash is used vs straight cement, I have sometimes toyed with the
idea of extending curing time to 10 days or more due to the slower
strength gain of fly ash. 

William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

> -----Original Message-----
> From: Kevin Elbury [mailto:kelbury(--nospam--at)] 
> Sent: Monday, May 01, 2006 5:21 PM
> To: seaint(--nospam--at)
> Subject: RE: Admixtures for Concrete Clarifier Tank - further thoughts
> One other thing to note with the fly ash is you should take 
> enough test cylinders to do a 56 day break also. I have had 
> some tests fail on 28 day breaks but did fine after 56 days. 
> . I agree with 15% as a maximum limit although I thought that 
> was a 1:1 replacment by mass. I will have to check with the 
> supplier next time...
> Kevin 
> -----Original Message-----
> From: Harold Sprague [mailto:spraguehope(--nospam--at)]
> Sent: Thursday, April 27, 2006 3:55 PM
> To: seaint(--nospam--at)
> Subject: Re: Admixtures for Concrete Clarifier Tank - further thoughts
> Dave,
> By "Old School", I mean that there are many that trade out F 
> ash at a 1 to 1 and don't limit the maximum to 15%.  I have 
> just had good luck with the 1.5 to 1 and 15% max.
> I think that if you try these limitations with F ash, you 
> will find that you have better performance in the plastic 
> state, setting, and the hardened state.
> Regards,
> Harold Sprague

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