Relationship Between Surface Free Energy and Total Work of Fracture of Asphalt Binder and Asphalt Binder-Aggregate Interfaces

Performance of asphalt mixtures depends on the properties of its constituent materials, mixture volumetrics, and external factors such as load and environment. An important material property that influences the performance of an asphalt mixture is the surface free energy of the asphalt binder and the aggregate. Surface free energy, which is a thermodynamic material property, is directly related to the adhesive bond energy between the asphalt binder and the aggregate as well as the cohesive bond energy of the asphalt binder. This thermodynamic material property has been successfully used to select asphalt binders and aggregates that have the necessary compatibility to form strong bonds and resist fracture. Surface free energy, being based on thermodynamics, assumes the asphalt binder is a brittle elastic material. In reality, the asphalt binder is not brittle and dissipates energy during loading and unloading. The total work of fracture is the culmination of all energy inputted into the sample to create two new surfaces of unit area and is dependent on the test geometry and testing conditions (e.g., temperature, loading rate, specimen size, etc.). The magnitude of the bond energy (either adhesive or cohesive) can be much smaller in magnitude when compared to the total work of fracture measured using mechanical tests (i.e., peel test, pull-off test, etc.). Despite the large difference in magnitude, there exists evidence in the literature supporting the use of the bond energy to characterize the resistance of composite systems to cohesive and/or adhesive failures. If the bond energy is to be recognized as a useful screening tool by the paving industry, the relationship between the bond energy and total work of fracture needs to be understood and verified. The effect of different types of modifications (addition of polymers, addition of anti-strip agents, and aging) on the surface free energy components of various asphalt binders was explored in order to understand how changes in the surface free energy components are related to the performance of the asphalt mixtures. After the asphalt binder-aggregate combination was explored, the next step was to study how the surface free energy of water was affected by contact with the asphalt binder-aggregate interface. Aggregates, which have a pH of greater than seven, will cause the pH of water that contacts them to increase. A change in the pH of the contacting water could indicate a change in its overall surface free energy, which might subsequently increase or decrease the water's moisture damage potential. With surface free energy fully explored, the total work of fracture was measured using pull-off tests for asphalt binder-aggregate combinations with known surface free energy components. In order to fully explore the relationship between bond energy and total work of fracture, temperature, loading rate, specimen geometry, and moisture content were varied in the experiments. The results of this work found that modifications made to the asphalt binder can have significant positive or negative effects on its surface free energy components and bond energy. Moreover, the results from the pull-off tests demonstrated that a relationship exists between bond energy (from surface free energy) and total work of fracture (from pull-off tests), and that surface free energy can be used to estimate the performance of asphalt binder-aggregate combinations