Fly ash 2

[engineerfin]

It wasn't that long ago that fly ash in the concrete mix design was a bad thing. Today it seems like its the "green" thing to do. Walmart is specifying 15-25% in their mix design to reduce off gassing. So how to structural engineers feel about the use of fly ash today?

 

[cvg]

as far as I know, its never really been a "bad thing", in my opinion it is mostly a good thing.

 

[dik]

It was a 'bad thing' because of traces of mercury in the production of the cement powder. I don't know what has happened with that. I still spec it, up to 30%

Dik

 

[cvg]

as stated in this article, the US EPA concern has little to do with concrete production and it is commonly used in the US.

It should be noted that environmentalists’ concerns about potentially toxic water pollution (particularly mercury) have no relevance to the use of fly ash in concrete production. The mercury level in most fly ash is the same as that found in most virgin soils. Further, if CCPs are high in mercury (mercury has an affinity for carbon), its carbon content would make it unsuitable for concrete. EPA’s main concern with fly ash has been certain unlined landfills and specific massive earth fills. Failures of fly ash settling ponds, like the 2008 TVA spill, are extremely rare events, and in any case, only dry fly ash is used in concrete – not wet material in settling ponds. And lastly, the EPA has not indicated any desire to restrict fly ash use in concrete.

https://precast.org/2013/11/the-future-of-fly-ash-use-concrete/

 

[Ron]

Commonly used...just understand what it does to your concrete! It delays strength gain quite a bit....not a problem ultimately but can create timing problems for components and shoring.

 

[JedClampett]

This structural engineer considers fly ash like a magic elixir for concrete. However, there's actually a shortage of it. The less coal burned, the less fly ash. Plus, I think that the fly ash collected now is of a lower quality than the fly ash of yore. See

http://ftp.dot.state.tx.us/pub/txdot-info/cst/tips/fly_ash_0412.pdf.

Now there's literally lakes of fly ash polluting the state and waterways of North Carolina. But for whatever reasons, using it is not an option.

 

[BigH]

I've used fly ash (added) on a job - and we used up to 50% cementitious replacement. Had no issues with strength. But, we didn't use 28 days as the strength result date but used 90 days. Low cementitious (for mass concrete) we used 1 year - keeping in mind that for mass concrete dam, the strength, except in the exposed faces (durability) is not all that important.