Articles

President's Message - July 2012

As most of our Branch knows, our July meeting will be a special meeting for two reasons. First, it is a joint meeting with our friends in the Fort Worth Branch, and second we will be offering six different technical seminars (plus an ethics seminar) following the luncheon. More information is available in this newsletter and on our website. Registration is open, so please sign up early for both the luncheon and the technical sessions.

Since July will have a technical focus, I thought that this month’s President’s Message should have the same flavor. I have been asked by a few people lately about fly ash, its benefits, and current availability so I thought this would be a good topic.

Fly ash is the fine particulate matter that rises with flue gasses at a coal-fired power plant. It is one of the most commonly used supplementary cementitious materials in concrete production because it is a pozzolan (has cementitious properties when combined with calcium hydroxide).  There are two main classes of fly ash, defined by ASTM C618 as Class F and Class C, and categorized depending on chemical composition. The burning of harder, older anthracite, bituminous coal, and some lignite coals (like we have in Texas) typically produces Class F fly ash.  This is the most commonly specified class of fly ash, as concrete made with Class F fly ash is more sulfate resistant and more resistant to harmful alkali-silica reaction (ASR). Class C fly ash is produced from the burning of some types of lignite and subbituminous coals. In addition to having pozzolanic properties, it also has some self-cementing properties, but lacks some of the benefits of Class F fly ash.

Fly ash particles are spherical and tend to make fresh concrete more workable, reducing the amount of water needed for mixing and placing.  While the short term contribution to concrete strength is less than that from the Portland cement, mixes containing fly ash have a higher long term strength (strength gain is slower) than a comparable concrete without it. Also, its use generally reduces permeability and can improve finish and durability. Typically, fly ash can be used to replace up to about 30% of the Portland cement, although some high-volume fly ash mixes contain up to 60% fly ash. So, all in all, the use of fly ash in concrete is a good thing. As with all construction materials it can be misused or over used. Also, due to the inherent variability, using a reliable fly ash source from a supplier with a good QC program is needed.

The history of fly ash production in the U.S. is interesting.  While first used in the late 1930s, its use did not take off until the EPA mandated the collection of fly ash and classified it as a “solid waste.”  Because of its pozzolanic properties, there has been considerable research to see how this “waste” could be used in the concrete industry. In fact, I worked on one such research project in college in the 1980s. Over the last 30 years the use of fly ash in Texas has become commonplace (or even required) and can contribute points to the U.S. Green Building Council’s LEED program.
  
Now the EPA is considering reclassifying fly ash as a “special waste” with an allowance for “beneficial use” or even as a “hazardous waste” because of some of the toxic constituents. There doesn’t seem to be much concern when used in concrete as the bigger concern is some accidental spill or other release of bulk fly ash. However, the concrete industry (and TxDOT) is nervous that end users will not want concrete that contains a “hazardous” material.

To make matters worse for the construction industry, the EPA is changing emissions standards for coal-fired plants. While no one would argue with the need for cleaner air, the EPA changes will likely change the properties of some fly ash and force some power plant operators to change fuel sources and not produce fly ash. In addition to environmental regulations that may impact the availability of fly ash, our mild Texas winter this last year resulted in a low power demand and less fly ash production.  Also, with natural gas becoming less expensive, some power plant operators have switched from coal for economic reasons. All this has had an impact on the availability of Class F fly ash in Texas.

So when specifying the use of Class F fly ash for projects, particularly concrete civil structures (bridge decks, columns, and precast bridge beams) where ASR may be a concern, be prepared to face availability issues. For more information, the TxDOT website has some guidelines for using alternate fly ash sources and the use of non-traditional concrete mixes.    

See you at our next meeting on Monday, July 9, 2012.


Douglas L. Smith, P.E.
President
ASCE Dallas Branch, 2011 - 2012