Mechanical and environmental characteristics of bituminous mixtures with incinerator bottom ash aggregates

Incinerator bottom ash is a material removed from the grating at the bottom of energy-from-waste plants. This ash has traditionally been land-filled. However, this ash could be processed to produce incinerator bottom ash aggregate (IBAA). Due to the low density and high stiffness of IBAA, it has become widely used as a secondary aggregate material especially in road construction. It is chemically inert, with very low concentrations of soluble materials. Despite this increase in IBAA use, higher usage still is desirable. To achieve this aim, the mechanical and leaching properties of bituminous mixtures containing high levels of IBAA merit further exploration. In this paper, an experimental investigation was undertaken to explore the mechanical and environmental characteristics of bituminous mixtures containing high IBAA contents. Three different mixtures were studied containing 30, 60 and 80% IBAA. A limestone blend, used as a reference mix, was also included. To evaluate the mechanical properties, the effect of IBAA content on indirect tensile stiffness modulus was investigated. The sensitivity to moisture ingress was also studied. The British Board of Agrement regime was adopted and used for this purpose. In addition, an accelerated oven-ageing procedure was applied on samples to study the age-hardening effect on their stiffness. Subsequently, uniaxial compression tests were carried out to study the effect of IBAA on bituminous mixtures' rutting resistance. As regards to the environmental characteristics, mixtures were tested for their leaching potential and the constituent concentrations in their leachates were determined for compacted bituminous and loose non-bituminous blends. The agitated extraction and the tank leaching tests were used for this objective. Results showed that, generally, replacing limestone with IBAA in bituminous mixtures improved their stiffness, excluding 80% IBAA content mix. Moreover, it was found that water ingress had a limited effect on IBAA bituminous mixtures' properties while age hardening has a more significant effect. IBAA was, generally, found to improve the bituminous mixtures' rutting resistance. Leaching test results showed that most leachate constituents were found to be within allowable limits. Despite this, diffusion and diffusion coefficient results highlighted the importance of investigating the potential effects of sulphate leaching on the environment. Finally, a one diffusion dimensional model was used as the worst case scenario tool to predict cumulative release from IBAA bituminous mixtures.