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RE: Fly Ash

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You are finding opposing opinions regarding effects of fly ash on concrete
mixtures because properties and effects resulting from its use vary
significantly depending on the type and amount of fly ash used in
conjunction with the particular cement and aggregates being used.

Fly ash is a relatively common additive which typically replaces some cement
content, hence the lower concrete unit cost. "Proper" replacement
percentages are a function of the fly ash used and its effects on your
desired concrete properties.

In general, I view fly ash as a good constituent. Reduced water demand and
better finishability are several benefits of these type mixes. Generally, I
like the consitencies and workability of fly ash mixes over pure cement
mixes. I would avoid its use if rate of early strength gain is important. Of
course, the proposed mixes should be tested to properly evaluate the effects
of its addition.

A good resource for info on fly ash (and concrete mixtures in general) is
PCA's "Design and Control of Concrete Mixtures."

To quote from the PCA book, "Before such an admiture is accepted for use, it
should be tested in combination with the specific cement and aggregates
being used to ascertain its suitability with regard to water requirements,
strength development, shrinkage, heat of hydration, durability, or special
properties such as prevention of alkalaii-aggregate reaction or reduction of
sulfate attack."

Carefully weigh the value of technical bulletins provided by
suppliers/manufacturers as, amazingly, they always seem to support the use
of their product. When considering the use of various admixtures I always
pick the brain of the suppliers tech rep, including request for names of
contractors/projects/readi-mix companies where the proposed or similar
mixtures have been previously used.

Good luck with your research.

Brent Koch, P.E.



-----Original Message-----
From: Mike Brown [mailto:mike.brown(--nospam--at)cshqa.com]
Sent: Wednesday, April 15, 1998 9:36 AM
To: seaoc
Subject: Fly Ash


Can anyone tell me the pluses and minuses of fly ash?  I have text books
telling me one thing and technical bulletins telling me the opposite.

The text books are telling me that fly ash increases concrete shrinkage
cracks while the technical bulletin is telling me that fly ash reduces
concrete shrinkage.  Granted, the technical bulletin is from Pozzolanic
(which is fly ash).  I am trying to use a concrete mix design that will
limit the amount of shrinkage cracks in the exposed concrete.

The Concrete Contractor has proposed the use of fly ash and has sent
literature to back up his claims (25% fly ash compared to the cement
content).  I do know that by allowing fly ash the cost for the concrete
will be reduced.  My concern is that the literature may not be telling the
whole story and he is just trying to save himself some $$.

I would appreciate anyone's help in this matter.

Thank you.

Mike Brown, P.E.