The purpose of this activity is to use fracture mechanics principles to assess both asphalt binder and asphalt pavement fatigue performance. Completed activity 2.030, "Completion Of 67-80 ALF Mixture Fatigue and Rutting Characterization," focused on continuum mechanics, which only describes damage to the point in time a crack initiates. Once cracks have been initiated, the theoretical constraints must shift from continuum mechanics to fracture mechanics. This activity complements the completed continuum mechanics work with fracture mechanics. For example, there are some mechanistic-empirical pavement cracking models that have both a stand-alone crack initiation component and a crack propagation component. Many variables, such as physical and chemical aging, pavement structure, thickness and specimen size effects, healing phenomena, interface effects, and probably others, make it difficult to develop all-inclusive models to predict fatigue life from simple laboratory tests. The goal of this activity is to develop a practical fracture mechanics (crack propagation) based method to determine the fatigue performance of asphalt pavements that can be implemented in any asphalt laboratory, namely in the asphalt mix performance tester. It is known that asphalt binders undergo large plastic deformations before failure initiation (surface separation), while others display high separation energies with less plastic deformation. This is why a small strain linear viscoelastic parameter, the "loss modulus," has been unable to capture the binder fatigue performance. The essential work of fracture method is a thermodynamic approach to separate the total failure energy into two energies, one essential for the surface separation (essential work of fracture) and another responsible for the plastic deformation (plastic work of fracture). It is believed that essential work of fracture has a critical role in fatigue performance. The method evaluates the entire failure process that leads to the end of a pavement life. The procedure involves direct tension testing of double-edge notched samples in conditions representative for the material fatigue life. The critical tip opening displacement is a derived material parameter that uses the essential work of fracture energy and the yield stress to evaluate the critical strain tolerance of a binder or mixture material. Calculated this way, the critical tip opening displacement is a potential fatigue grading parameter. The current Accelerated Pavement Testing Facility sections and materials give the opportunity to investigate both the binder and the mixture critical tip opening displacement as possible fatigue parameters. Cored and laboratory-made Accelerated Pavement Testing Facility mix samples will be subjected to direct tension tests to determine the critical strain tolerance. Binder testing will include laboratory-aged and extracted material from the Accelerated Pavement Testing Facility sections. Binders and mixtures at different aging stages will be evaluated using this testing procedure and the results will be correlated with the existing fatigue performance database provided by the Accelerated Pavement Testing Facility lanes.
Binder Critical Tip Opening Displacement is recommended as a discriminating specification parameter for asphalt binders. However, this was not the case for mixtures. Although the test provides the same rank as binders, the variability was not acceptable for a performance test or specification.