Predictive Equations to Evaluate Thermal Fracture of Asphalt Rubber Mixtures



Thermal cracking is a serious type of pavement distress and its prevention is a critical issue for many transportation agencies around the world. The indirect tensile (IDT) creep and strength tests were developed during the Strategic Highway Research Program (SHRP) to characterize the resistance of Hot Mix Asphalt concrete (HMA) to low-temperature cracking. Currently, the IDT creep and strength tests are considered one of the most promising tests for predicting the low-temperature performance of asphalt concrete mixtures. The IDT tests are used as the material characterization tests for thermal fracture in the Mechanistic-Empirical Pavement Design Guide (MEPDG) of New and rehabilitated Pavement Structures (also known as the 2002 Design Guide) developed during the National Cooperative Highway Research Program (NCHRP) Project 1-37A. The existing Thermal Cracking (TC) Model that is currently an integral part of the MEPDG has proved to adequately predict low temperature cracking of asphalt concrete mixtures utilizing conventional binders. Thermal fracture characterization studies conducted at Arizona State University (ASU) concluded that the IDT procedures need certain refinements and revisions. In addition, experience from testing several asphalt rubber mixtures have shown that the existing TC Model in the MEPDG falls short in properly characterizing the exceptional thermal cracking resistance of the asphalt rubber mixtures in the field. The objective of this research was to develop a new method for evaluation of the thermal cracking potential in HMA, with focus on asphalt rubber mixtures. Necessary revisions and refinements of the existing IDT test protocol were made, and an IDT test results database was created and used in the development of the fracture energy prediction model. The new development utilizes the fracture energy parameter instead of the tensile strength maximum limit of the material and the slope of the creep compliance – “m” parameter. The results of the total fracture energy measured during the IDT strength test in the lab were correlated with volumetric properties of the mixtures and a regression model was developed.

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