Role of adhesion and thin film tackiness of asphalt binders in moisture damage of HMA

Moisture damage is well known to be an aggregate-asphalt problem affected by many factors among which asphalt chemistry, asphalt rheology, aggregate surface chemistry and physical properties play important roles. For many years the focus of moisture damage research has been on testing the complete mixture as used in the field. This approach, although realistic and logical in terms of simulating the total HMA system, is too complex to allow differentiating between the role of asphalt binders and the role of aggregates in damage. Thus, this approach is too difficult to use to determine fundamental bonding properties of asphalt binders and how they change with chemistry or with modification. Although the asphalt research community accepts asphalts as adhesives, and although we keep referring to stickness of asphalts, we have not done enough to treat asphalts as adhesives and measure the stickness of different asphalts. No doubt there are many difficulties in differentiating between adhesion and cohesion of asphalts, but we can only start understanding these two mechanisms by acknowledging these as two possibly different properties and by focusing on developing systems to measure each. This study is an attempt to treat adhesion separately from cohesion and to propose two systems to measure them independently. To measure adhesion characteristics of selected asphalts to various mineral surfaces, using a modified pull-off test (PATTI) is proposed. To measure cohesion, the Dynamic Shear Rheometer was used to develop a test protocol to measure thin film tackiness of asphalts. The Thin Film Tackiness is proposed as a relevant measure of cohesion under thin film conditions typically seen in asphalt mixtures. The results offer evidence that we need to differentiate between adhesive failures and thin film cohesive failures. The results show that additives, including anti-strip and polymer additives, can affect adhesion differently than thin film cohesion. The study suggests that, although there is no replacement to testing the total HMA mixture, designing binders with improved adhesion properties and/or improved cohesion properties could be an essential step in improving resistance of HMA to moisture damage.