Introducing the critical aging point (CAP) of asphalt based on its restoration capacity

This study introduces a critical aging point (CAP) for asphalt binder beyond which rejuvenation of aged asphalt binder is not effective. To do so, we determined the effect of aging on the restoration capacity of asphalt binder via both computational modeling and laboratory experiments. Evaluation was done based on the extent of change in the thermo-mechanical properties of asphalt binder as it goes through the processes of aging and rejuvenation. Our molecular-level analysis showed that as aging progressed, the binding energy of asphaltene monomers increased, leading to an increase in the size of asphaltene nanoaggregates. The latter increase in intermolecular interactions was supported by our laboratory experiments showing an increase in shear thinning as aging progressed. It was also found that aging continuously increased the crossover modulus, Glover-Rowe parameter, stiffness, and critical cracking temperature; aging continuously decreased the stress relaxation capacity, healing index, and thermal conductivity. The rate of change in binder properties was high at the beginning of aging and slowed down as aging progressed. Initially, oxidation mechanism dominates, however in latter stages aromatization and carbonation dominate. The rejuvenator effectively restored aged asphalt; however, the rejuvenator's efficacy diminished as aging progressed, to a point that it had only a marginal effect on asphalt binder aged beyond 80 h (equivalent to 2nd or 3rd generation RAP depending on pavement location and sun intensity). This is especially critical since with the increasing use of RAP in new pavements, road authorities soon will be dealing with second, third, fourth, and older generations of RAP. The study outcomes further highlight the importance of accounting for the age of as received RAP; for instance, RAP in states with high temperature and high sun intensity are aged more than RAP in milder climates. The results emphasize that adjusting the age of the RAP using a rejuvenator before each recycling stage is critical to avoid reaching and passing the CAP beyond which rejuvenation is not effective. Such adjustment also allows for implementation of a uniform RAP application guideline regardless of the location and source of the RAP.