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George A. Everding, AIA, CSI, CCS, CCCA
Senior Member
Username: geverding

Post Number: 504
Registered: 11-2004


Posted on Wednesday, January 13, 2010 - 03:41 pm:   Edit PostDelete PostPrint Post

We had a discussion yesterday about how flyash can discolor the finish of concrete, and I was inadequate in my discription of what it looks like. Does anyone have an image of flyash discoloration at the surface of concrete that you can send me so I can pass it on to our design team? geverding@cannondesign.com

Thanks
George A. Everding AIA CSI CCS CCCA
Cannon Design - St. Louis, MO
(Unregistered Guest)
Unregistered guest
Posted on Wednesday, January 13, 2010 - 06:00 pm:   Edit PostDelete PostPrint Post

Fly ash meeting requirements of ASTM C 618 does not "discolor" concrete. I have been specifying high volume fly ash concrete for years, and the argument that this "discolors" concrete is old, tired, and inaccurate. Class F replacement of cement with fly ash in the 40% and higher range gives the concrete a slightly buff/gray color, but nothing that is so pronounced to be immediately obvious to anyone but a concrete expert.

Lots of CIP concrete mass pours use high volume fly ash mixes to reduce the heat of hydration during curing. This has been practiced for decades - look at your nearest cocnrete dam, chances are that you are looking at a HVFA concrete mix. You will not see discoloration caused by the fly ash.

I have specified mix designs with 50% fly ash (as a percentage of total cementitious content) for slabs on grade, columns, stem footings, and walls and have never had an issue with "discoloration." Prior to placing these mixes, I heard all manner of complaints and reasons why it would be a bad idea to use HVFA mexes - from the concrete ready mix produers and finishers - none of which turned out to be true. One scare tactic was to claim that fly ash would make the concrete an ugly streaky black. Total BS.

Ready mix producers in my part of the country ROUTINELY add anywhere from 15%-25% fly ash into EVERY mix that leaves the facility, specified or not. This is done for cost and performance reasons. They never get complaints about discoloration due to fly ash content, because it doesn't happen.
Mark Gilligan SE, CSI
Senior Member
Username: mark_gilligan

Post Number: 254
Registered: 10-2007
Posted on Wednesday, January 13, 2010 - 06:40 pm:   Edit PostDelete PostPrint Post

Aa web site that I found stated:

"...buff-colored ash can impart a tan color to the concrete. Although rare, some fly ash has a form of iron that can result in an orange or red color in the concrete. But as long as the ash or slag are uniformly distributed and all batches have equal amounts of the same ash or slag, there shouldn't be any problems with acid staining, Kosmatka says."

http://www.concretedecor.net/All_Access/601/CD601-Fly_Ash.cfm

Would suggest that you require that they not change sources of materials, for color sensitive concrete. Also would suggest that they provide a sample cast from the mix to be used so that color isues can be verified. Often it is feasible to use a batch of the visual concrete in the foundation, thus allowing time for the concrete sample to cure and for the Architect to verify acceptability.

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