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RE: Fly Ash[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaoc(--nospam--at)seaoc.org>
- Subject: RE: Fly Ash
- From: "Brent Koch" <brentk(--nospam--at)tdl.com>
- Date: Wed, 15 Apr 1998 10:49:10 -0700
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.
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