Verification of Engineered Frameworks for Evaluating the Use of Recycling Agents in Surface Asphalt Mixtures

About the report:

The use of reclaimed asphalt pavement (RAP) in asphalt mixtures has been steadily increasing for years, nationally and internationally, given the economic and associated environmental benefits of using such material. However, using RAP in asphalt mixtures poses some challenges regarding durability and performance that are often associated with the highly aged binder in RAP. The balanced mix design (BMD) that the Virginia Department of Transportation (VDOT) adopted for 9.5- and 12.5-mm nominal maximum aggregate size dense-graded surface mixtures with A and D designations addresses some of these challenges. Nonetheless, the gradual statewide implementation of the BMD framework and the lack of BMD specifications for certain types of asphalt mixtures that may contain RAP encourage the evaluation of other solutions to potentially mitigate some of the aforementioned challenges in the meantime. Softer binders or additives such as recycling agents (RAs) have been gaining considerable attention in the asphalt industry as a potential solution, but no specifications exist for the performance evaluation of RAs and their acceptance in Virginia. The Virginia Transportation Research Council in collaboration with North Carolina State University conducted VDOT Research Project No. 117566, Engineered Frameworks for Evaluating the Use of Recycling Agents in Surface Asphalt Mixtures for Virginia, resulting in two performance-based frameworks: one for the assessment of RA products for inclusion into VDOT’s Approved Product List and the second for designing asphalt mixtures with high RAP content and RA in accordance with VDOT’s BMD specifications.

The objective of this study was to verify the engineered frameworks proposed in VDOT Project No. 117566. Six virgin asphalt binders, 5 RAP sources, and 8 RA products were evaluated and combined to produce 12 asphalt binder blends and 11 asphalt mixtures in combination with the binder blends and mixtures evaluated in Project No. 117566. The binder blends were subjected to short- and long-term aging, and the corresponding rheological properties and performance grades were obtained. The asphalt mixtures were subjected to short- and long-term oven aging and further tested for durability (Cantabro), cracking resistance (indirect tensile cracking test), and rutting resistance (Asphalt Pavement Analyzer).

The asphalt binder results showed that the proposed framework in VDOT Project No. 117566 yielded an optimized RA dosage consistently less than the supplier-recommended dosage to restore the low-temperature performance grade of the recycled binder blend. The rejuvenation path index was confirmed to be independent of the RA dosage and could be used for acceptance of other RAs into VDOT’s Approved Product List. Lower asphalt content showed lower cracking tolerance index and higher Cantabro mass loss values, with some mixtures failing the BMD thresholds. The Asphalt Pavement Analyzer rutting test could not discriminate between the mixtures’ rutting susceptibility. The new binder blends and mixtures confirmed the negative relationship between cracking tolerance index and Glover-Rowe parameter, highlighting the importance of binder properties on the mixtures’ cracking performance. A preliminary cracking tolerance index aging sensitivity of 45% was verified and proposed. Based on the findings, this study proposes two recommendations: (1) adopting the performance-based framework verified in this project, accompanied by a Virginia Test Method and an automated Excel-based program for the RA assessment, and (2) validating the framework with field trials and under production conditions in future research projects.

_{Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.}

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