Performance of Fly Ash as a Base Material in the Atlanta District

Project Description

The Atlanta District first began evaluating crushed fly ash, or hydrated fly ash, as a base material in 1990. Hydrated fly ash is produced by allowing a Class C powder fly ash (ASTM C 618) from coal-fired power plants to cure with moisture. The hydrated (cured) fly ash becomes a stiff material that can be crushed to form a synthetic aggregate. When properly processed and compacted to optimum moisture content, the hydrated fly ash continues to gain strength after placement as a base material. While district personnel were pleased with early performance characteristics of the test sections, its long-term performance was in question. In addition, some problems occurred on some of the pavements soon after placement of a surface treatment whereby the surface treatment delaminated from the underlying fly-ash base material. TTI researchers (1) evaluated and monitored the performance and changes in material properties for six pavements over a five-year period and (2) performed a laboratory investigation into the cause and cure for the failure of surface treatments on the hydrated fly-ash base courses. Based on five years of visual documentation, falling-weight deflectometer tests, ground-penetrating radar, and compressive strengths of field cores, researchers concluded that the hydrated fly-ash test pavements have performed well. In the laboratory study, the bond between the base and surface treatment was evaluated using a torsional shear test. In addition, the South African durability test was performed, in which the samples were subjected to a water bath and trafficked under a loaded wheel for 5000 repetitions. Researchers concluded that the curing time of the base prior to application of the surface treatment may be the key to achieving a good bond.

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