The use of asphalt mixtures containing high Recycled Binder Replacements (RBRs) is increasing. Recycled binders are oxidized and thus, harder and more susceptible to cracking than virgin binders. Consequently, the use of higher recycled content mixtures has prompted heightened interest in recycling agents and necessitated the use of asphalt extenders to produce softer virgin binder grades. Recycling agents include a wide-range of both softening agents and rejuvenators that are intended to restore the physical and chemical properties of aged asphalt binders. Petroleum-based extender products have been in existence for a long time (e.g., Re-refined Engine Oil Bottom (REOB)). Nonpetroleum based products have been more recently introduced (e.g., bio-oils).
The AASHTO M320 Performance-Graded (PG) specification for was developed on the basis of unmodified, petroleum-based asphalts. Consequently, it has been demonstrated that the specification parameters and aging procedures often fail to discriminate the performance of modified asphalt binders. This shortcoming has been overcome with respect to rutting resistance and corresponding high-temperature grading with the introduction of AASHTO M332. However, the shortcomings of the current PG specification with respect to cracking performance have not been addressed. The characterization of binder cracking resistance requires an appropriate aging procedure to simulate long-term oxidative aging coupled with a test method to capture cracking resistance.
An in-depth study is needed to identify aging and test methods to better quantify binder cracking resistance to inform the development of an improved binder specification framework; this framework would enable improved, unified procedures to qualify recycling agent and extender products and establish dosage requirements. The study should rely on the use of relatively simple experiments that make use of standard binder laboratory equipment to ensure that the research results can be easily implemented. To address this need, the proposed research will achieve two objectives: (1) identify an appropriate aging procedure to simulate long-term oxidative aging coupled with a test method to characterize the cracking resistance of asphalt binders that uses Superpave binder equipment; (2) develop proposed improvements to binder specifications with corresponding recommendations for extender and recycling agent product approval and dosage selection.
The research results will lead to improved binder specifications that better capture cracking resistance. This framework will enable the approval and dosage selection of recycling agents and extenders. These specifications will improve material selection, consequently increasing pavement service life and decreasing life-cycle costs.