Evaluation of synthetic aggregates using off-ASTM specification ashes as road base course materials

Abstract

In recent years, there has been renewed interest in synthetic aggregate for flexible or rigid base manufactured with coal combustion by-products because of a scarcity of dependable sources of aggregate and high transportation cost. This creates the need for alternative cost-effective sources for material that are available locally. Although successful use of hydrated ponded Class C fly ash as road-base material has been reported, the utilization of off-ASTM specification ash such as fluidized bed combustion (FBC) ash as flexible or rigid base material is still challenging due to inherent ash properties (high sulfate and carbon contents, angular shape, and free lime). Therefore, this research was performed to identify and develop the potential of FBC ash as both a synthetic aggregate and a stabilizer in flexible base construction.To assess the performance characteristics of FBC aggregate in a base-course mix design, the laboratory tests including the determination of optimum moisture content, unconfined compressive strength, evaluation of the moisture susceptibility using the tube suction test, and 3-dimensional swelling test of the fabricated specimens were carried out.The results have shown that a usable aggregate using FBC fly ash can be manufactured with virtually no potential for sulfate attack in base course application. Furthermore, FBC fly ash as a stabilizer from either naturally stockpiled or manufactured fly ash aggregate blends could be effectively used in flexible base construction without sacrificing its ability to develop high early strength and without causing sulfate swelling due to ettringite formation.