The North Carolina Department of Transportation (NCDOT) modified the procedures for asphalt mixture design in 2018 in part, to increase the asphalt content and address observed cracking issues with asphalt mixtures. These changes have reduced the number of asphalt mixture types, changed compaction levels and the volumetric limits for some mixtures, and adjusted how recycled materials are considered. Given the complexity of the interactions between material parameters, the procedural changes do not guarantee that the resultant asphalt mixture designs have actually achieved the intended goal of improved durability.
This study investigates how these recent changes have affected asphalt mixture designs with respect to composition and performance. Direct statistical analysis of mixture volumetric properties (voids in mineral aggregates, voids filled with asphalt, recycled binder replacement, asphalt content, etc.) were first assessed to identify systematic changes in mixture composition resulting from the specification changes. Then, several performance prediction models identified from the literature were utilized to predict performance differences between mixtures before and after the changes based on compositional changes. Visual as well as statistical (Student's t-test) approaches were utilized to evaluate the differences in composition and performance. Some statistically significant differences in the composition and predicted performance of past and present asphalt mixtures were identified. However, the magnitudes of the differences and number of cases where statistically significant differences were identified versus not suggest no systematic differences of practically significance resulted from the mixture design specification changes.
Recommendations are included in the final
report. The NCDOT should continue
monitoring pavements built after the changes to mix-design were made for any
improvements in durability performance, declines in rutting performance, and
changes in functional performance and consider integrating a durability related
performance test into the mixture design process.
