Balanced Mix Design for Surface Asphalt Mixtures: 2019 Field Trials

About the report:

In 2017,the Virginia Department of Transportation (VDOT) began to evaluate the feasibility of introducing performance requirements into mix design using the balanced mix design (BMD) method.  VDOT has since committed to the implementation of the BMD method in an effort to improve asphalt mixture performance. Through a collaborative effort with industry, VDOT developed two special provisions for use with field trials that use the BMD method to specify as-designed mixture performance.  This study documented and evaluated the field trials that were the first applications of these specifications in 2019 to design, produce, and place BMD asphalt mixtures in Virginia.  The analyses addressed the application of the BMD concept, production variability, comparisons of mixtures, and differences in specimen test response with and without reheating of the loose mixture for fabrication.

Nine mixtures were evaluated from the two field trials; the mixtures incorporated combinations of different reclaimed asphalt pavement (RAP) contents, two binder grades, two recycling agents, and two warm mix asphalt (WMA) additives. 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 a softer binder, WMA additives, 40% RAP, and recycling agents may be designed and produced to meet current BMD performance thresholds and current volumetric properties, gradation, and asphalt content requirements.  The laboratory performance test results indicated that for some mixtures, WMA additives and a recycling agent may be expected to provide performance that is equal to or better than the performance of their counterpart mixtures containing only a WMA additive.  In addition, results indicated that the evaluated asphalt mixtures containing 40% RAP and softer binders or a recycling agent can be expected to provide performance that is equal to or better than the performance of their counterpart mixtures with 30% RAP.  The long-term field and laboratory performance of all of these mixtures needs to be evaluated to verify these early findings.  The relationships between performance test results and mixture properties need further study, and improvements in test methods and criteria should be pursued.

Based on the outcomes of the study, it is necessary to determine the precision estimates for the Cantabro and APA tests. In addition, the development of different performance criteria for the Cantabro and IDT-CT tests to be applied to non-reheated specimen testing may be necessary.  The performance of multiple Cantabro and IDT-CT performance tests for each lot during production may be necessary such that an average test value can be used in BMD quality control, quality assurance, or acceptance practices, because of the variability in individual test results.  In light of the lack of failing test results among the mixtures, APA tests may not need to be performed as frequently as Cantabro and IDT-CT tests during production, although this needs to be investigated further.

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