Evaluating Impact of Corrected Optimum Asphalt Content and Benchmarking Cracking Resistance of Georgia Mixtures for Balanced Mix Design Implementation

Abstract

The Georgia Department of Transportation (GDOT) implemented the corrected optimum asphalt content (COAC) approach to reducing the allowable reclaimed asphalt pavement (RAP) binder contribution for improving mixture durability. The COAC of 60:40 implemented in 2019 allows 60% of the RAP binder to be credited to the total mix, requiring additional virgin asphalt binder equivalent to 40% of the RAP binder to be added to the volumetric optimum binder content. To further enhance the durability of asphalt mixtures, GDOT plans to implement the indirect tensile asphalt cracking test (IDEAL-CT) in the balanced mix design (BMD) method in conjunction with the Hamburg wheel tracking test that is currently specified in the GDOT specifications. To assist GDOT in the implementation effort, this study was conducted to (1) evaluate the effect of the COAC approach on the cracking and rutting resistance of asphalt mixtures using the proposed BMD test methods; and (2) benchmark the cracking resistance (i.e., CT_index) of plant-produced mixtures being produced in the state of Georgia for developing CT_index thresholds for future implementation. The results of this study suggest that the 60:40 COAC helps improve the cracking resistance of asphalt mixtures containing high RAP contents with minimum effect on their rutting resistance when compared with the corresponding virgin mixtures. In addition, because of statistical differences in the CT_index results between stone mastic asphalt (SMA), surface, and intermediate/base mixtures tested in this study, different CT_index thresholds are proposed for these three mixture groups for pilot projects. Further research is anticipated to refine these thresholds and verify with field performance for future implementation in mix design approval and acceptance testing.