HVFAC
HVFAC
(OP)
I am wondering if anyone here has used high volume Class F fly ash concrete (>50% fly ash replacement) for a structural application and what the results were (compressive strength deviation from target at 28 days, w/c ratio used, etc.). Would you have any problem specifying it again and what was the application?





RE: HVFAC
RE: HVFAC
RE: HVFAC
One reason to limit it is that it takes more time to gain strength. You might not be able to strip forms or backfill as quickly if you use too much flyash. Since most contractors would like to finish sooner rather than later, it isn't generally specified.
RE: HVFAC
RE: HVFAC
Concerning your questions, typical replacement values for 28 days performance are 15 to 25%. With strength requirement delayed to 56 days, 90 days, or later, dosage rates can be increased to 30 to 50% or more.
RE: HVFAC
RE: HVFAC
In 1994 I used HFVAC commercially in a green building I designed, and I presented several papers at various conferences on it at the time. Mitchelon is correct - use ONLY Class F. There have been a number of lawsuits associated with HVFA Class C use. In my experience, form-stripping delay was not a major problem. It hardens quite quickly - in less than two days it was impossible to drive a masonry nail into our footings. One great thing about HVFAC is the continuing gaining of strength (as long as hydration continues, of course) for long, long after the 28-day design value. It is necessary to use super-plasticizers with it, however. This is because you can reduce the w/c ratio (to around 25%, as I remember) due to fly ash's ability to act as tiny spherical lubricators (this low w/c is one key to its superior strength). One other word of caution: it is not particularly recommended for a floor slab (at least in my experience), for it is hard to float. Good luck!
Municipal engineering. Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.
RE: HVFAC
CANMETS's Scientist Emeritus, VM Malhotra has done a lot of work in this area.
Check out this slide show he put together
http:
RE: HVFAC
Yes, I am very familiar with Mohan Malhotra's work at CanMet. Several years before I became an engineer, as I was designing that green building - researching materials for such attributes as low lifetime embodied energy, use of waste products, reduction of greenhouse gas, durability, passive solar potential, etc. - I had called Dr. Malhotra in Ottawa. I spoke to him at length, and he was most helpful, was kind enough to send me copies of various research studies he had done, some with G. Carette and W. Langley, an engineer from Nova Scotia - including an extremely good text he had written on the subject. Without this early pioneering research I would never have attempted what I did in the field. I used a mix very similar to some in the research (specified by Gordon Leaman, P.Eng, an associate of Langley), and was extremely satisfied with the result. In fact, I may be wrong, but I believe that Langley was the first engineer to my knowledge to use HVFAC in these kinds of proportions actually in the field, designing structural beams and piles for a large commercial building in downtown Halifax around 1988, as I remember. Later I had the great privilege of meeting him. These guys are still some of my heroes.
RE: HVFAC
http: