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RE: Fly ash in concrete

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Fly ash in concrete is typically used to reduce the per-yard cost of the concrete by reducing the quantity of cement while still producing the desired strength at 28days.  ACI recommends at least 15%replacement with an upper limit of 25% for normal mixtures.  Strength-gain for fly-ash containing concrete may be somewhat slower than for more conventional mixes depending on the dosage.  This could be seen as a problem if a specific finish is desired and that that finish might get degraded by the construction processes going on during the first 28 days.  I cannot conceive of this kind of problem for foundations. 
We have had some architectural resistance to the use of fly ash in slab-on-grade mixes in parking structures because of the perceived hardening issues.  However, it has been my experience the possible negative affects on the sweated swirl finish do not materialize and are far outweighed by the durability benefits.
Fly ash can also increase the finishability and pumpability of the concrete as it helps to fill in the grading sequence of the fine aggregates.
Nicholas Blackburn

 -----Original Message-----
From: DWILLIAMSSEI(--nospam--at) [mailto:DWILLIAMSSEI(--nospam--at)]
Sent: Thursday, May 24, 2001 9:25 AM
To: seaint(--nospam--at)
Subject: Fly ash in concrete

Has anyone had negative experiences with using fly ash in concrete mixes?

I'm working with an architect who doesn't want it used, but I'm not sure why.
 The spec's call for 3,500 psi 28 day strength concrete.  Fly ash is not
mentioned in the spec's, but a contractor has asked if it's permissible to
add it to the mix.  I understand that fly ash is used to improve flowability
of dry mixes.  I couldn't find much other information regarding the use of
fly ash in ACI 318 or any of my materials and concrete references.

The application for the concrete in question is spread footings for steel
columns and perimeter footings for slab/brick masonry support for a 2-story


David Williams, P.E.
Snyder Engineering, Inc.
Columbia, MO