U.S. Federal Highway Administration published “Use of Industrial Byproducts in Concrete Paving Applications,” outlining considerations to be taken into account by highway agency and contractor engineers in evaluating the beneficial use of such products, including off-spec fly ash and bottom ash, in concrete paving projects.
Historically, bottom ash, as well as fly ash that does not meet AASHTO M 295 or ASTM C618 standards for use in concrete, has been placed into storage. However, as coal power plants continue to be retired, limiting the availability of fresh production ash, landfilled/impounded fly ash and bottom ash (frequently comingled) are increasingly being harvested and beneficiated for use in concrete construction.
FHWA’s report covers the basic physical and chemical characteristics of fly ash and bottom ash and their potential for use in a variety of bound (concrete) and unbound (fill and base material) applications, concluding: “Used alone or blended with other materials, off-spec coal ash can possess the chemical and physical characteristics needed to provide benefits for concrete and stabilized bases. Off-spec coal ash has been found to be a suitable material for use in stabilizing soils, with high-CaO off-spec ash showing better performance than fly ash meeting AASHTO M 295 (ASTM C618) in stabilizing some soils.”
Once a material—whether coal ash or other byproduct—has been identified with the potential for beneficial use in a specific concrete paving project, FHWA says that the technical performance, economic feasibility, and environmental impacts of the material’s beneficial use should be analyzed. “Tests should be performed on the material itself and on the application product (e.g., base, fill, or concrete material) during the qualification/preconstruction phase and upon delivery or during construction,” FHWA says. The economic benefits can be quantified using a life-cycle cost analysis, while the environmental impacts and benefits can be evaluated using a life-cycle assessment, the agency adds.