Balanced Mix Design for Surface Mixtures: 2021 and 2022 Plant Mix Schedule Pilots

About the report:

Many state highway agencies are currently exploring a new approach for designing and accepting asphalt mixtures, known as the balanced mix design (BMD) method (hereinafter “BMD”).  The Virginia Department of Transportation (VDOT) has made a commitment to adopt BMD to enhance the performance of asphalt mixtures. Since 2018, VDOT has taken incremental steps to phase in the use of BMD in production while addressing concerns expressed by VDOT and the industry.  The use of BMD line items in selected 2021and 2022 contracts has been an important learning opportunity for VDOT and the industry along the path to implementation. This study documented and assessed the 2021 and 2022 maintenance plant mix schedule pilots.  The objectives of the study were to provide information on quality control, acceptance testing, and independent assurance testing; analyze the performance properties of reheated mixtures and extracted and recovered binders; and document lessons learned from the process.  The analysis addressed several topics, including variability in production and testing and binder characterization and implications for performance. 

The pilot projects were developed by the VDOT districts using maintenance plant mix schedule contracts.  In 2021, approximately 72,000 tons of BMD mixtures were paved on selected routes in 10 maintenance schedules across five districts.  In 2022, approximately 222,000 tons of BMD mixtures were paved in 13 maintenance schedules distributed across all nine VDOT districts, with at least one BMD contract executed per district.  The Cantabro mass loss test, the indirect tensile cracking test (IDT-CT), the Asphalt Pavement Analyzer rut test, and the indirect tensile test at high temperature (IDT-HT) were performed on laboratory-produced design specimens and non-reheated and reheated plant-produced, laboratory-compacted specimens. Basic and advanced binder testing and analysis were conducted on extracted and recovered binders from selected samples of the mixtures. 

Based on the test results, the successful outcomes observed during the pilot projects in 2021 and 2022 clearly demonstrated the efficacy of applying BMD to surface mixtures (SMs) with A and D designations.  Moreover, there was a decrease in variability in test results from 2021 to 2022, showcasing the benefits derived from training and experience in BMD testing performance.  Notably, it was found that the source and formulation of the virgin binder used, along with the properties of the aged binder in reclaimed asphalt pavement (RAP) stockpiles, contributed to substantial variations in binder properties and affected the expected performance of the produced mixtures.

The study recommends that (1) efforts be pursued toward full implementation of BMD in Virginia for SMs with A and D designations, as supported by VDOT’s 2024 BMD special provision; (2) a comprehensive ruggedness study focusing on the refinement of specimen preparation and test methods for the IDT-CT and IDT-HT be initiated to assess key factors that demand stricter control and additional guidance during specimen preparation for the IDT-CT and IDT-HT; and (3) a comprehensive study to assess the relationships between the properties of virgin and RAP asphalt binders and those of the corresponding asphalt mixtures be conducted.

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