HVFAC 1

[haynewp]

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?

 

[JAE]

We've always limited it to 25%. So I don't have any application to refer to.

 

[haynewp]

I think that is what is required to get the LEED credit point so I figure that is where most engineers say that's enough.

 

[cvg]

local standard around here is to limit to 15% maximum replacement rate. Corps of Engineers requires between 15 - 35% replacement. No limit for mass concrete except that mix design must be approved. These specifications have been around a lot longer than LEED.

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.

 

[haynewp]

I figured it would be getting used in mass concrete applications, mat foundations and such.

 

[mitchelon]

Class F is definitely used in mass concrete. Class C can generate more heat; therefore it is not used for this purpose.
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.

 

[mporisky]

There has been more and more buzz about HVFA concrete since it drastically reduces the carbon footprint of the concrete. Personally I have not had any extensive firsthand experience but have done extensive research on the subject. Research revealed that there is no reson why HVFA concrete cannot be used in lue of conventional concrete if proportioned properly. Check out

http://www.ecosmartconcrete.com

for a plethora of case studies. Basically a few major trends emmerge; a low w/cm ratio below 0.38 , proper curing, and adequate reinforcing cover. Polycarboxylate based HRWR produce better early strength and have great water reducing abilities and would be my first choice as a water reducer. Check out the case study the york university which spec'd 30Mpa at 28 days.

 

[Aton]

heynewp, yes, I have had some excellent experience with HVFAC - 56% by portland replacement volume. So many people think that using fly ash is simply a way to get rid of a waste material from coal-burning generation plants. In fact, fly ash as a portland replacement can vastly increase the quality of the concrete, if done right. Interestingly, the Romans made concrete using volcanic ash (thus the word "pozzolan", from the Roman city of Pozzuoli). Many Roman civil works are still standing.

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.

 

[henri2]

Great points Aton.

CANMETS's Scientist Emeritus, VM Malhotra has done a lot of work in this area.

Check out this slide show he put together

http://www.hvfacprojectindia.com/resources/First Seminar Series Presentations/CANMET2.pdf

 

[Aton]

Thank you, henri2,

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.

 

[haynewp]

Here is a presentation I developed after researching the topic: Comments are welcome.

http://www.angelfire.com/linux/haynewp/presentation.pdf