Think of asphalt as the world’s largest source of glue.  Just as a typical adhesive forms a thin film that bonds two surfaces, asphalt in roads exists as thin films that bond particles of aggregate.  In asphalt pavements, adhesion is what bonds the asphalt to an aggregate surface, while cohesion is how well the asphalt holds together.
 
The interfacial bonds between asphalt and aggregate, as well as between asphalt and polymer, fillers, and other additives, directly affect the adhesive, cohesive and mechanical strength of the pavement. The adhesive and cohesive properties of asphalts determine how well roads hold up to traffic, weather and other forces working against their integrity. Yet, a critical hole in the current Superpave^® specification is the failure to distinguish the adhesive and cohesive properties of asphalts originating from different sources. 

WRI uses techniques such as temperature-controlled atomic force microscopy (AFM), nuclear magnetic resonance (NMR) imaging, and precision sliding plate viscometry to quantify asphalt–aggregate adhesion and better understand how the thin-film properties of asphalts affect their performance as the binder, or glue, in asphalt pavements.  The results of this work are forming a basis of knowledge leading to improved specifications, products and test methods for the improved performance of future pavements.

Physics and Chemistry of Asphalt Surfaces and Interfaces
Viscoelastic Properties of Asphalt Thin-film Interfaces
Water Interactions at Asphalt Interfaces