Following the initial lay-down of a new pavement, the asphalt binder cures and solidifies. Specific components (molecules) of the asphalt adhere to the interfacial surfaces of aggregate, polymer, and fillers such as lime, and harden in place. This solidification process may be caused by the fluxes of heat and matter that occur at the interfaces. Oxidation and steric effects cause further age hardening of the binder. We use thermally controlled atomic force microscopy (AFM) imaging techniques to study these phenomena. Non-equilibrium thermodynamic and statistical mechanics models allow us to interpret and quantify asphalt–aggregate solidification processes at the microstructure scale.
Adhesion may be measured using atomic force microscopy techniques to quantify the surface energy of asphalt binder thin films. These adhesion measurements are derived from force curve measurements based on continuum–contact mechanics models, including the Johnson-Kendell-Roberts (JKR) contact theory and the DMT (Derjaguin-Muller-Toporov) theory. WRI uses atomic force microscopy to measure force curves and quantify the surface energy of asphalts as a function of contact load, temperature, and rate of loading; this constitutes a measurement of the micro/nano-rheological properties of the interface. A minimum contact load then corresponds to the equilibrium surface energy at a given temperature for a given asphalt–substrate system to quantify the adhesive strength of the asphalt–aggregate interface.
AFM imaging techniques also allow us to characterize the roughness (friction) and morphology of aggregate surfaces as they pertain to adhesion hysteresis (cause/effect). Quantifying the adhesive properties of materials used to construct pavements provides a basis for predicting the bond strength of the asphalt–aggregate interface and thus, the performance of the highway over time.