Improved Hirsch model for estimating the modulus of hot-mix asphalt

This paper presents an improved version of the Hirsch model for estimating the modulus of hot-mix asphalt (HMA) from asphalt binder modulus and mixture composition. The original Hirsch model was developed in 2002 and has been shown by several independent researchers to be reasonably accurate. However, the authors believed that the model could be improved by addressing several issues: (1) simplifying the Hirsch equation mathematically; (2) including aggregate specific gravity as a predictor, which indirectly accounts for variations in aggregate modulus; (3) including strain level as a factor; (4) recalibrating the model using a data set gathered using the asphalt mixture performance tester following current standard protocols; and (5) addressing steric hardening during testing. These goals were achieved, although it appears that under normal testing conditions strain sensitivity and steric hardening are not significant factors affecting HMA dynamic modulus. Verification of the model using a limited amount of independently collected data suggests that the improved model eliminates or reduces a tendency of the original model to underestimate HMA dynamic modulus values.