The effect of bitumen polar groups adsorption on mastics properties at low temperatures

The current design standard for asphalt mixtures provides guidance on selection of aggregates and asphalt binder and includes requirements for the amount of mineral filler to be added in the mix. In the SuperPave system, the amount of filler that can be included is limited to a ratio in the range of 0.6–1.2 by mass of the filler to the binder (i.e. approximately 20–40% by volume of mastic). However, this range is based on experience rather than on scientific evaluation of the interaction between filler and binder. The experimental results collected in this study for mastics produced with two bitumens and a set of commonly used fillers indicate that the physico-chemical interaction effect is the result of adsorption of asphaltenes and resins (i.e. polar groups in asphalt) on the surface of the filler particles. This adsorption is partially responsible for the changes in the mechanical properties (e.g. stiffness and viscosity) and thermo-volumetric properties (e.g. coefficients of thermal contraction and glass transition temperature – T g) of the mastic. The amount of polar groups adsorbed was found to depend on the Braunauer–Emmett–Teller (BET) surface area of the filler, but not on the chemical composition of the filler or the bitumen. The higher the BET surface area, the greater the amount of polar groups adsorbed, and the lower the glass transition temperature. The significant reduction in the glass transition temperature indicates that this adsorption phenomenon results in softening the bitumen in the mastics.