Balanced Mix Design for Surface Mixtures: 2020 Field Trials

About the report:

The Virginia Department of Transportation (VDOT) has been working toward the implementation of balanced mix design (BMD) for several years.  During that time, special provisions have been developed to address dense-graded surface mixtures with reclaimed asphalt pavement (RAP) contents up to 30% and with RAP contents of 40% and above.  In 2020, five field trials encompassing 12 mixtures were constructed to evaluate BMD mixtures designed and produced in accordance with VDOT’s special provision for surface mixtures with high RAP contents.  Typical dense-graded Superpave surface mixtures were used as controls.  This study documented and assessed these trials to provide information to evaluate the impact of various RAP contents and additives, production variability, reheating, and binder properties on BMD performance test results.

Twelve mixtures were evaluated during the five field trials.  The mixtures included combinations of different RAP contents, two binder grades, four recycling agents, and fiber.  Volumetric and gradation analysis was performed on the mixtures.  The Cantabro mass loss test (Cantabro test),the indirect tensile cracking test (IDT-CT), and the Asphalt Pavement Analyzer (APA) test were performed on laboratory-produced design specimens and non-reheated and reheated plant-produced, laboratory-compacted specimens.  All findings and conclusions are limited to the mixtures evaluated.

Based on the test results, mixtures containing 35% or greater RAP contents, softer binders, recycling agents, and fiber may be designed and produced to meet current BMD performance thresholds and current volumetric properties, gradation, and asphalt content requirements.  It was found that some mixtures that were volumetrically designed under current VDOT specifications met BMD requirements.  In addition, the expected trends in mixture performance testing were not always observed, likely due to masking by variability due to specimen fabrication practices or by inherent test variability.  Results showed that modest relationships between non-reheated and reheated specimen results for the Cantabro test and IDT-CT were present. In addition, changes due to the use of a softer binder and/or recycling agents were seen in the BMD mixture binders as compared with the control mixture binder.  Finally, comparisons of extracted and recovered binders from control and BMD mixtures were found to depend on the binder test under consideration, with different tests indicating differences in expected performance.

Based on the outcomes of the study, a testing protocol capable of evaluating the performance of recycling agents used in BMD mixtures is needed.  This protocol would provide a means for VDOT to evaluate and accept these materials such that their use in innovative mixtures can be allowed in a manner that preserves the goals of sustainable, longer-lasting, and cost-effective pavements. In addition, efforts should be made to determine the effect of asphalt binder properties on the overall performance of asphalt mixtures with a primary focus on cracking and durability to allow VDOT to specify better-performing binders for use in asphalt mixtures.  This would allow for the further optimization of mixture properties that should result in improved mixture performance.

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