Field Validation of Balanced Mix Design Initial Criteria

About the report:

The Virginia Department of Transportation (VDOT) initiated efforts to implement balanced mix design (BMD) in 2017, using 11 mixtures from a well-documented previous study in a benchmarking effort to select performance-related tests to use within the BMD framework and develop criteria for those tests. When these mixtures were produced and paved in 2015, sampling for extensive characterization was performed, and mixture locations were documented, providing important information to assess the in-service performance of the mixtures. The field projects associated with these benchmarking mixtures reached 8 years in service in 2023, providing the opportunity to assess performance and evaluate the suitability of the initial BMD cracking test criteria for design and production of 9.5-mm and 12.5-mm surface mixtures having A and D binder designations.

The purpose of this study was to validate the suitability of the initial BMD cracking test criteria for design and reheated production specimens determined from the laboratory benchmark testing of 11 surface mixtures placed in 2015. Pavement condition surveys, falling weight deflectometer testing, field visits, and core testing were conducted to assess the mixture properties and pavement conditions after 8 years in service. Results were analyzed relative to the initial mixture properties and results of BMD benchmark testing to validate VDOT’s initial BMD cracking criteria.

The study showed that higher cracking resistance of the asphalt surface mixture, indicated by higher cracking tolerance index values, is associated with a reduced percent cracking in the field. The complex interactions of as-constructed density, pavement structural capacity, traffic loading, and climate complicate the relationship. Mixture quality, construction quality, and traffic loading have a significant influence on the pavement cracking performance, and trends were shown to align with expectations. Asphalt Pavement Analyzer rutting test results were determined to generally indicate field rutting trends. In this study, mixtures with higher laboratory Asphalt Pavement Analyzer rut depths tended to exhibit greater rut depths in service, confirming that the Asphalt Pavement Analyzer remains a useful indicator of rutting susceptibility within BMD projects. Overall, it was concluded that achieving a balanced asphalt mixture, as characterized by VDOT’s BMD framework, is critical to optimizing asphalt surface mixture longevity in Virginia. Rutting has generally not been a major concern in Virginia. However, mixtures with increased cracking tolerance index values in this study tended to show greater rut depths, highlighting that improving cracking resistance must be balanced with evaluating rutting potential during mixture design and production to preserve long‐term pavement durability.

The study recommends that VDOT’s Materials Division continue to implement BMD. The results of this study demonstrate that VDOT’s BMD test outcomes align with observed field performance trends. Mixtures meeting the established thresholds exhibited improved cracking resistance and acceptable rut depths when other variables were comparable. Consequently, these findings validate the effectiveness of the current BMD tests and thresholds in indicating in-service asphalt mixture performance and support continued implementation. In addition, it is recommended that the Virginia Transportation Research Council continue monitoring the performance of 

_{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.}

Authors