Mechanistic-Based Evaluation of Performance Thresholds for Balanced Mix Design Asphalt Surface Mixtures

About the report:

As part of the Virginia Department of Transportation (VDOT) balanced mix design (BMD) implementation process, VDOT selected three quick, simple, and practical empirical tests to evaluate asphalt mixtures against different modes of distress for design and acceptance. These tests include the Cantabro test, with a mass loss limit of 7.5%; the indirect tensile cracking test, requiring a minimum cracking tolerance index of 70; and the asphalt pavement analyzer (APA) rut test, with a maximum rut depth of 8.0 mm. These tests assess the potential performance in durability, cracking, and rutting of asphalt mixtures, respectively. In 2021, performance-based specifications were evaluated to fully implement BMD in Virginia. Additional mixture tests confirmed the reasonableness of the established performance criteria. However, further investigation was advised to assess correlations between the Cantabro, APA rut, and indirect tensile cracking, tests and fundamental tests, mechanistic-empirical (ME) simulations, and field performance, ensuring suitable threshold criteria for BMD implementation. This study’s objectives were the following:

1. Compare the performance of VDOT’s Superpave and BMD-designed mixtures through evaluation of empirical and advanced test results.
2. Assess the reliability of empirical tests with respect to advanced tests.
3. Validate and refine BMD performance thresholds by linking empirical and fundamental test results and incorporating asphalt mixture volumetrics into ME structural design, including establishing preliminary traffic-based performance thresholds for empirical tests.
4. Evaluate material properties’ effect on long-term performance and the oxidative aging potential of Virginia surface mixtures with various reclaimed asphalt pavement contents.

The scope of the work included testing 18 surface mixtures—containing various reclaimed asphalt pavement contents, binder grades, recycling agents, fibers, and warm-mix additives—and ME simulations. This study highlights persistent knowledge and practical gaps in BMD despite significant development. Results indicate that VDOT’s BMD tests effectively assess durability, rutting, and cracking performance, aligning with advanced tests measuring fundamental properties. The study showed that current BMD thresholds can be revised and refined, including incorporating traffic-based rutting and cracking thresholds. Specifically, ME simulations indicated that mixtures intended for moderate to heavy traffic pavement structures (D) should be designed to meet a maximum mass loss of 6.3% (instead of 7.5%), a minimum cracking tolerance index of 110 (instead of 70), while maintaining the same APA rut depth threshold of 8.0 mm. Mixtures intended for low-volume traffic pavement structures (A) should meet a maximum mass loss of 5.9% (instead of 7.5%), a minimum cracking tolerance index of 124, and a maximum APA rut depth of 10 mm.

The research team recommends that VDOT: (1) Continue using the Cantabro, APA rut, and indirect tensile cracking tests as part of the BMD framework for designing surface mixtures with A and D designations; (2) Continue efforts to validate and refine the BMD special provisions by incorporating traffic-based cracking and rutting performance thresholds; and (3) Consider benchmarking and establishing performance-based specifications specifically for mixtures intended for use on low-volume roads

_{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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