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

Abstract: The Virginia Department of Transportation (VDOT) selected three quick, simple, and practical empirical tests, as part of its balanced mix design (BMD) implementation process, to evaluate asphalt mixtures against different modes of distress for design and acceptance. These tests include the Cantabro test, with a mass loss (ML) limit of 7.5%; the indirect tensile cracking test (IDT-CT), requiring a minimum cracking tolerance (CT) index of 70; and the asphalt pavement analyzer (APA) rut test, with a maximum rut depth (RD) of 8.0 mm. These tests aim to 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 Cantabro, IDT-CT, and APA tests, and fundamental tests, mechanistic-empirical (ME) simulations, and field performance, ensuring suitable threshold criteria for BMD implementation.
This study aimed to: 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/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' impact on long-term performance and oxidative aging potential of Virginia surface mixtures (SMs) with various reclaimed asphalt pavement (RAP) contents. The scope of the work included testing eighteen SMs with various RAP contents, binder grades, recycling agents, fibers, and warm-mix additives, and ME simulations.
This study highlighted persistent knowledge and practical gaps in BMD despite significant development. Results indicated 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 incorporation of 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 ML of 6.3% (instead of 7.5%), a minimum CT index of 110 (instead of 70), while maintaining the same APA RD threshold of 8.0 mm. Mixtures intended for low-volume traffic pavement structures (“A”) should meet a maximum ML of 5.9% (instead of 7.5%), a minimum CT index of 124, and a maximum APA RD of 10 mm.
The study recommended that VDOT: 1) Continue using the Cantabro test, APA rut test, and IDT-CT as part of its BMD framework for designing SMs 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. 3) Consider benchmarking and establishing performance-based specifications specifically for mixtures intended for use on low-volume roads.