A MECHANISTIC-EMPIRICAL MODEL FOR TOP-DOWN CRACKING IN ASPHALT PAVEMENT OVERLAY
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This study investigates top-down cracking in the wheel path in asphalt pavement overlay that has been observed increasingly in recent years. It focuses on developing a mechanistic-empirical model to predict top-down cracking in the asphalt pavement overlay. The research consisted of literature review, model evaluation, critical material property determination, development of new prediction model and analysis procedures, and model calibration and verification. Climate, pavement structure, traffic and material properties are identified and incorporated into the newly developed prediction model for top-down cracking. Horizontal failure strain which is a measure of the fracture property of asphalt mixtures is determined as the significant material property for top-down cracking. Both horizontal failure strain and dynamic modulus are incorporated in the newly developed model for characterizing the fatigue life of asphalt pavements. The shear strain at the edge of tire is considered as critical pavement structural response for top-down cracking, which is also considered in the fatigue life model. The model for prediction top-down cracking is calibrated and verified using the data collected from National Cooperative Highway Research Program (NCHRP) 9-49A project, and is recommended to supplement the current practice of the Mechanistic-Empirical Pavement Design Guide (MEPDG). Two levels of horizontal failure strain characterization are provided for the use of pavement designers. The protocol for obtaining the horizontal failure strain from laboratory experiment is provided. The estimation of horizontal failure strain values for Level II input required in MEPDG is also provided. To mitigate top-down cracking in the asphalt pavement overlay, it is recommended to design an asphalt mix with higher horizontal failure strain value to reduce the shear strain response along the edges of tire.