Stuck on fly ash

Concrete Construction, June, 2003 by Douglas Yeggy

The From the Trenches piece, "Stuck on Tilt-Up?" by Michael Linn in your March issue (p. 90) has some problems with the lack of investigation into the use of fly ash in concrete construction. Concrete designed using 20 to 30% replacement of cement with Class F fly ash typically has water demands of 5 to 15% less if properly designed. Normal concrete technology (that is, based on w/c ratio) tells us that the lower the water, the more impermeable the concrete. Am I missing something here? Mr Linn's response is typical of someone trying to cover his behind, or worse, trying to be an expert when not sufficiently knowledgeable in simple concrete technology. Fly ash has gotten a bad reputation over the years in tilt-up construction because there was no one who would dispute the so-called experts (concrete contractors who want to get on to the next job). The use of fly ash is an acceptable part of every type of concrete and some people ought to wake up to the fact that the owners of these projects are possibly not getting the best possible product when the uneducated contractors and suppliers think that throwing more cement at the concrete is the best thing that you could do when in fact it is quite possibly the worst thing you can do, since it leads to excessive cracking due to thermal expansion and contraction and higher water demands (oh yes, Mr. Linn, more cement typically means more water).

A simple examination of the photo seems to indicate that there were no strength problems with the fly ash concrete in the tilt-up panels since the concrete seems to have pulled from the slab and it doesn't appear that any of the pick points were damaged. In addition, the tilt-up panels were cast on waste slabs, which are often made of concrete with very limited strength characteristics. Also, the author's acknowledgment halfway through the article that it rained between applications tells me that the bondbreaker was probably applied to wet concrete, which would dilute the concentration of the bondbreaker. And finally the statement three quarters of the way through the article, that most, if not all of the bondbreaker was washed off the floor, "which was most likely the primary cause of failure" makes me wonder why he even brought up fly ash at all.

Education and getting all the facts prior to a hasty conclusion can sometimes save us a lot of headaches. I am now going to have to spend the next couple years dealing with the ramifications of these comments with the engineers, contractors, and ready-mix concrete suppliers who use this magazine to learn about concrete.

--Douglas Yeggy

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