Recommendations for Revision of AASHTO M 295 Standard Specification to Include Marginal and Unconventional Source Coal Fly Ashes

Project Details









Cement, Concrete, Evaluation and assessment, Fly ash, Highways, Pozzolan, Recommendations, Standards

Project description

Fly ash is the most common supplementary cementitious material used to help improve the durability and sustainability of concrete and mitigate alkali-silica reactivity (ASR). While the demand for fly ash for use in concrete remains steady, the supply of high quality fly ash has been dwindling or, at least, becoming unpredictable. This is partly due to electric power plants changing their fuel sources (e.g., from coal to natural gas) and partly because of tighter environmental and air pollution regulations that have resulted in increased carbon, calcium, sulfur, alkali, and ammonia contents of the fly ash, which adversely affect its quality for use in concrete. In recent years, there has been a growing concern among concrete users and producers about the quality and the quantity of fly ash presently available. In a recent American Association of State and Highway Transportation Officials (AASHTO) survey of the state departments of transportation, respondents noted significant issues with the supply of coal fly ash. This supply and demand imbalance for quality fly ash has already resulted in regional and seasonal shortages, which is expected to further exacerbate with time. Given this restriction on supply, coal fly ash that was once deemed marginal is now being used in concrete. Marginal fly ash is defined as the fly ash that is of lower quality [i.e., with high loss-on-ignition (LOI), fineness, moisture content, etc.] or otherwise unusable in concrete. To address the shortage, even unconventional sources of fly ash are now being considered, i.e., primarily the ash disposed of in landfills or impoundments. To help meet the growing need for fly ash while maintaining the quality and performance of concrete, it becomes imperative for the concrete industry and the transportation agencies to look for and evaluate marginal and unconventional source fly ashes. To that end, it is also imperative that fly ash specifications measure properties that affect performance and not reject materials that, although suitable for use in concrete, just do not come up to the historical specifications adopted when high quality fly ash was plentiful.