Comparison of Fatigue Analysis Approaches for Two Hot Mix Asphalt Concrete (HMAC) Mixtures (0-4468-2)

Over the past decade, the Texas Department of Transportation (TxDOT) focused research efforts on improving mixture design to preclude rutting in the early life of the pavement. However, these rut resistant stiff mixtures may be susceptible to long-term fatigue cracking in the pavement structure as the binder stiffens due to oxidative aging. To address this concern, TxDOT initiated a research study with the primary goal of evaluating and recommending a HMAC nrixture fatigue design and analysis system to ensure adequate mixture fatigue performance in a particular pavement structure under specific environmental and traffic loading conditions. A secondary goal of the research was to compare the fatigue resistance of commonly used TxDOT HMAC mixtures including investigating the effects of binder aging on fatigue performance. Four fatigue analysis approaches, the mechanistic empirical (ME), the calibrated mechanistic with (CMSE) and without (CM) surface energy measurements, and the proposed NCHRP l-37A Pavement Design Guide were investigated in this project to evaluate the fatigue resistance of two common TxDOT mixtures (Rut Resistant and Basic Type C) including the effects of aging. Based on the value engineering assessment including test results, statistical analysis, costs, and relative comparison of each analysis procedure, the continium micromechanics based CMSE fatigue analysis approach was recommended for predicting HMAC mixture fatigue life (Nj). While binder oxidative aging reduced the HMAC mixture resistance to fracture and its ability to heal, the Rut Resistant mixture exhibited better fatigue resistance in terms of Nf magnitude compared to the Basic Type C mixture possibly due to an increased polymer modified binder content. Test results also indicated that both binders and mixtures stiffen with oxidative aging, and that mixture aging correlated quantitatively with binder aging. From the binder shear properties and binder-mixture relationships, aging shift factors were developed and produced promising results. Nonetheless, more CMSE laboratory HMAC mixture fatigue characterization and field validation is recommended.