BACKGROUND: Fly ash--a byproduct of coal combustion--is widely used as a cementitious and pozzolanic ingredient in hydraulic cement concrete The use of fly ash in concrete is increasing because it improves some properties of concrete and often results in lower cost of concrete. However, the chemical and physical compositions of fly ash influence constructibility, performance, and durability and may contribute to problems, such as cracking and alkali-silica reactivity in concrete pavements, bridge decks, and other highway structures. Regulatory requirements have also contributed to changes in fly ash properties that may adversely affect concrete performance. In addition, current specifications and test methods do not adequately characterize fly ash properties, address the effects of fly ash characteristics on fresh and hardened concrete properties, or consider the alkali content of the cement. For example, carbon content of fly ash is not usually determined directly, but is often assumed to be approximately equal to the loss on ignition (LOI). Such inadequate characterization may lead to unwarranted restrictions on the use of suitable materials.
Although a great deal of research has been performed on the effects of fly ash characteristics on concrete properties, the research has not dealt with the applicability of current specifications to the fly ashes that currently are produced. In addition, existing test methods for sampling and testing fly ash used in concrete do not adequately address the characterization of fly ash or the performance aspects of highway concrete. Further research is needed to develop recommendations for improving fly ash specifications and test protocols and thus help highway agencies better evaluate and use fly ash that will provide acceptable structural performance and durability.
OBJECTIVE: The objective of this research is to recommend potential improvements to specifications and test protocols to determine the acceptability of fly ash for use in highway concrete.
Status: The project is complete. The final report has been piblished as NCHRP Report 749, available at