Sulfur-Modified Bottom Ash as Aggregate in Hot-Mix Asphalt Concrete: Field Demonstration Project

The use of bottom ash in road construction as an aggregate for both roadway surfaces and base courses has been limited because of its absorbency and friability. The former tends to increase asphalt binder demand, and the latter adversely affects its ability to withstand the crushing effects of traffic loads. Nevertheless, bottom ash is lighter in weight and generally much cheaper than conventional quality aggregates such as crushed limestone, sand, and gravel. Research was undertaken to upgrade the load-bearing characteristics of bottom ash and maximize its use in asphaltic concrete mixtures through sulfur modification. The process essentially coats the ash with liquid sulfur, which, upon cooling, fills the voids on the surface of the particles while increasing their crushing resistance. The results of laboratory investigations indicate that asphaltic concrete mixture designs in which bottom ash represents 50 to 100 percent of the aggregate fraction can be achieved. Sulfur-modified bottom ash (SMBA) mixtures compare favorably with typical Texas Department of Transportation surface course mixtures in terms of strength, durability, and asphalt demand. Results of a field demonstration project in which SMBA was incorporated into hot-mix asphalt concrete are presented. An asphalt batch plant was successfully used to incorporate the sulfur into the bottom ash and produce the hotmix asphalt concrete. The mixture was successfully placed as a 50-mm (2-in.) overlay on an existing asphalt surface at the entry to the asphalt plant facility, and adequate density was achieved with standard compaction equipment. The SMBA hot-mix overlay is currently under traffic by heavy haul trucks entering the plant facility, and early indications of performance are good.