The use of reclaimed asphalt pavement (RAP) in neat asphalt mixtures has received significant attention because of its economic and environmental advantages. However, bitumen contained in RAP is severely aged; when RAP is blended with virgin asphalt at a high percentage, it can negatively affect the properties of the resulting blend. Therefore, recycling agents have been used to improve the properties of aged asphalt and consequently the performance of the mixture. This paper uses computational modeling and laboratory experiments to examine the role of chemical structure and the composition of recycling agents on their interactions with oxidized bitumen. To do so, rheometry as well as Fourier transform infrared spectroscopy, thin-layer chromatography with flame ionization detection, gel permeation chromatography, and contact-angle measurements were used to track changes in physicochemical and surface properties of oxidized bitumen in the presence of four recycling agents with widely different compositions. The study results show that oxidized bitumen became softer after adding recycling agents (regardless of their source) and that all recycling agents were able to increase the phase angle and the colloidal stability index of aged bitumen. However, not all the recycling agents restored molecular conformation. The most effective recycling agent was found to be recycling agent A, which led to a concurrent increase in the crossover modulus and crossover frequency of aged bitumen. In contrast, recycling agent C had the least effect on crossover values. Molecular dynamics simulation showed that recycling agent A restored molecular conformation and reduced the size of asphaltene nanoaggregates, while recycling agent C promoted agglomeration, causing an increase in the size of nanoaggregates. Recycling agent A was also effective in reducing the moisture susceptibility of aged bitumen, as shown by a smaller change in contact angle of aged bitumen between wet and dry, compared to other recycling agents. Based on the study results, the efficacy of a recycling agent in the deagglomeration of oxidized asphaltenes is shown by a concurrent increase in both the crossover modulus and the crossover frequency; these two rheology-based indicators can be used to distinguish recycling agents that can accurately be called rejuvenators.
