Evaluation of adhesive properties of mineral-bitumen interfaces in cold asphalt mixtures

The performance of asphalt mixtures is strongly influenced by the physical and chemical properties of the minerals and binders used, at various micro to macro scales. In cold asphalt mixtures a process that particularly strongly influences adherence between the minerals and binders (and thus performance) is the wetting of bitumen on the minerals' surfaces. Their adhesion is influenced by numerous factors and parameters, such as surface free energies of both binders and aggregates in the presence of moisture or dust on the surface of aggregates, mixing temperatures, surface textures including open porosity, nature of the minerals and their surface chemical composition, as well as additives present in the binder phase. However, the relationships involved are not fully understood. Thus, in this study the surface free energies of both minerals/aggregates and binders were characterized using two approaches, one based on contact angles and the other on vapor sorption methods. Precise specific surface areas of four aggregates and seven minerals were determined using BET (Brunauer, Emmett and Teller) theory, by measuring the physical adsorption of selected gas vapors on their surfaces, and calculating amounts of adsorbed vapors corresponding to monolayer occupancy on the surfaces. Interfacial bond strengths between bitumen and aggregates were also calculated, based on measured surface free energy components of minerals/aggregates and binders, in both dry and wet conditions. The adhesive bond strength for the binder with each mineral/aggregate combination in wet condition has been improved by using additives. The presented study has highlighted the need for accurate measurements of aggregates' and minerals' specific surface areas and (hence) requirements to develop new approaches to resolve problems associated with BET-based methods. (C) 2016 Elsevier Ltd. All rights reserved.