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In recent years, increased use of recycled asphalt materials (RAP) has created a need for softer binders to compensate stiffer binder coming from RAP. Economic alternatives, like recycled oils and proprietary bio-based oils, can be potential modifiers that will reduce the dependence on petroleum-based alternatives. However, there is limited information on the long-term rheological performance of binders modified with proprietary modifiers. These modifiers are chemically complex and their interaction with binders further complicates the binder chemistry. Therefore, the objective of this study was to evaluate the impact of modifier chemistry on modified binders’ long-term cracking potential. A base binder of Superpave Performance Grade (PG) 64-22 was used to develop PG 58-28 binder using six different modifiers. An unmodified PG 58-28 was included for a comparative analysis. A few modified binders rheologically outperformed the base binder and others performed similarly. The modifier derived from recycled engine oil showed the worst performance. Chemical analysis indicated that the best performing modified binders had significant amounts of nitrogen in the form of amines. On the other hand, poor performing modified binder had traces of sulfur. Additionally, modifiers with lower average molecular weights appeared to have a positive impact on the performance of aged binders.

Fatigue cracking caused by repeated heavy traffic loading is a critical distress in asphalt concrete pavements and is significantly affected by the selected binder. In recent years, the growing use of recycled asphalt materials has increased the need for the production of softer asphalt binder. Various modifiers/additives are marketed to adjust the grade and/or enhance the binder performance at high and low temperatures. The modifiers are expected to alter the rheological and chemical characteristics of binders and, therefore, their performance. In this study, the damage characteristics of modified and unmodified binders, at standard long-term and extended aging conditions, were tested using the linear amplitude sweep (LAS) test. Current data-interpretation methods for LAS measurements (including AASHTO TP 101-12, T 391-20, and recent literature) showed inconsistent results for modified binders. An alternative method to interpret LAS results was developed in this study. The method considers the data until peak shear-stress is reached because complex stress states and failure patterns are observed in the specimens after that point. The proposed parameter (Δ| G*|peak τ) quantifies the reduction in complex shear modulus measured at the peak shear-stress. The parameter successfully captures the effect of aging and modification of binders.

The increased use of softer binders in Illinois over the past decade is primarily attributed to the increased use of recycled materials in asphalt pavement construction. The shift in demand of using PG 58-28 over PG 64-22 has resulted in potential alternative methods to produce softer binders more economically using proprietary products. However, there are challenges in using these proprietary products for asphalt modification because of uncertainty in their long-term performance and significant variability in binder chemistry. The current SuperPave performance grading specification for asphalt binders is insufficient in differentiating binders produced from these modifiers. Therefore, the objective of this study is to evaluate the performance of various softener-type asphalt binder modifiers using a wide array of rheological and chemistry tests for their integration into the Illinois Department of Transportation’s material specifications. The small-strain rheological tests and their parameters allowed for consistent grouping of modified binders and can be used as surrogates to identify performing and nonperforming asphalt binders. A new parameter, Δ|G*|peak τ, was developed from the linear amplitude sweep test and showed potential to discriminate binders based on their large-strain behavior. Chemistry-based parameters were shown to track aging and formulation changes. The modifier sources were identified using fingerprint testing and were manifested in the modified binder chemical and compositional characteristics. The two sources of base binders blended with the modifiers governed the aging rate of the modified binders. Mixture performance testing using the Illinois Flexibility Index Test and the Hamburg Wheel-Track Test were consistent with the rheological and chemical findings, except for the glycol amine-based modified binder, which showed the worst cracking performance with the lowest flexibility index among the studied modifiers. This was contrary to its superior rheological performance, which may be attributed to lower thermal stability, resulting in high mass loss during mixing. According to the characterization of field-aged binders, laboratory aging of two pressurized aging vessel cycles or more may represent realistic field aging of 10 to 15 years at the pavement surface and is able to distinguish modified binders. Therefore, an extended aging method of two pressurized aging vessel cycles was recommended for modified binders. Two different testing suites were recommended for product approval protocol with preliminary thresholds for acceptable performance validated with field-aged data.

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