The Beneficial Use of Fly Ash in Concrete
Editorial by W. R. "Randy" Cox Manager, ASBI
In December 2008, a dike failure at a Tennessee Valley Authority Coal Combustion Waste Containment Area resulted in a spill of over 1 billion gallons of ash slurry into Kingston, Tennessee. Clean up costs are estimated to be over $1 billion and this accident spurred the Environmental Protection Agency (EPA) to evaluate the status of coal combustion waste storage and disposal. In the U.S., 131 million tons of waste is produced yearly.
At present, the EPA does not regulate coal combustion waste but provides recommendations for its storage and disposal. In 2000, the EPA published a regulatory determination that these wastes are not hazardous and has encouraged its beneficial use. The benefits provided by fly ash to the concrete segmental bridge industry are significant:
• It makes concrete denser and less permeable to water and chloride intrusion.
• It mitigates against the deterioration of concrete from alkali-silica reactivity where certain types of aggregate react with the alkalis in the cement.
• It slows the rate of temperature gain due to cement hydration and minimizes thermal cracking in concrete.
• It increases the workability of concrete resulting in fewer voids in the placed concrete.
These benefits all contribute to stronger and longer lasting concrete bridges.
The use of fly ash as a partial cement replacement establishes the construction of concrete bridges as a potential key component of a sustainable transportation system. The principals of “green” construction are achieved through the recycling of this beneficial waste material, eliminating millions of tons of greenhouse gases produced annually from the production of portland cement.
The EPA’s decision on regulating coal combustion waste, including fly ash, is expected to be announced soon. Should these materials be reclassified as hazardous waste, the beneficial use will be impacted due to negative perceptions and potential legal liabilities of incorporating “hazardous waste” into public works projects. We will all have an opportunity to voice our concerns during the open comment period when the proposed rules are issued. For more information on the proposed coal ash rule, visit the EPA Rulemaking Gateway (http://yosemite.epa.gov/opei/RuleGate.nsf/).