Adhesion between bitumen and aggregate: implementation of spectroscopic ellipsometry characterisation and estimation of Hamaker's constant

Refractive indices of seven bitumen samples and three aggregates (typical components in flexible asphalt pavement) were determined by ellipsometry in order to predict dispersive bitumen-aggregate adhesion and dispersive bitumen-bitumen cohesion using Hamaker's constant. Hamaker's constant according to Lifshitz was introduced to the asphalt field by two of the authors to describe and estimate van der Waal's interaction and bitumen-aggregate adhesion. Lifshitz used the refractive index to estimate the dispersive non-polar van der Waal's interaction component of adhesion, the predominant component in adhesion between minerals and bituminous binder. The impact of an intervening thin medium such as air or water on the adhesion can be estimated using Hamaker's coefficient, which in turn can be related to stripping potential. The bitumen binders studied were delivered as a paving grade 70/100 according to EN 12591:2009 and came from different bitumen suppliers. The three aggregates studied were two types of granite and one diabase. It is concluded from the measurement of refractive indices and calculations of the dispersive component of Hamaker's constant that there was a larger spread in refractive index among the three aggregate samples studied than among the seven bitumen samples.