Many well-constructed flexible pavements with a thick HMA structure have been in service for 40 or more years without any evidence of fatigue cracking. This field experience suggests that an endurance limit, that is, a level of strain below which fatigue damage does not accumulate for any number of load repetitions, is a valid concept for HMA mixtures. NCHRP Project 9-38, “Endurance Limit of Hot Mix Asphalt Mixtures to Prevent Fatigue Cracking in Flexible Pavements,” completed in 2009, confirmed the existence of an HMA fatigue endurance limit through an extensive program of laboratory testing, which further suggested that the endurance limit is influenced by HMA mixture and binder properties. Based on these results, a practical definition of the endurance limit was developed, along with a method to estimate its value in the laboratory. NCHRP Project 9-44A was designed to extend the results and findings of Project 9-38, with particular attention to the influence of asphalt binder and mixture properties on the endurance limit and to the relationship of the endurance limit to the phenomenon of healing hypothesized to occur in asphalt mixtures during the rest period between load applications in the laboratory and in pavements. The specific objectives of the project were to (1) carry out a laboratory experiment to identify the mixture and pavement layer design features related to an endurance limit for bottom-initiated fatigue cracking of HMA and (2) develop an algorithm to incorporate this endurance limit into the Pavement ME Design software and other selected pavement design methods. The research was performed by Arizona State University, Tempe, Arizona, in association with AMEC (formerly MACTEC), Phoenix, Arizona. The research investigated the relationship of the fatigue endurance limit to factors such as asphalt binder rheology, air voids, asphalt content, temperature, strain level, number of load cycles, and rest period between load cycles. Both beam fatigue (AASHTO T321) and uniaxial compression-tension testing were conducted according to a factorial design that permitted statistical analysis of the main factor and up to three-factor interactions. Robust regression models were developed that described the effect of the main factors and factor interactions on the stiffness ratio, SR, which is defined as the ratio of the stiffness measured at any load cycle during beam fatigue or uniaxial fatigue testing to the initial stiffness of the specimen. Testing was conducted with rest periods of 0, 1, 5, and 10s between load cycles. The endurance limit can then be determined for any mixture initial stiffness as the strain at SR = 1, i.e., for the condition in which complete healing of the fatigue damage takes place after each load cycle. Thus, this research reaffirmed the existence of the HMA fatigue endurance limit and demonstrated that the endurance limit is the result of a balance between loading damage and the healing, i.e., damage recovery, that happens during rest periods and that the value of the limit varies with the mixture initial stiffness (acting as a surrogate for binder rheology, air voids, asphalt content, and temperature) and the duration of the rest period. It was also found that for a load cycle of 0.1s, a rest period greater than 5 to 10s (from beam fatigue testing) or greater than 3s (from uniaxial testing) will not produce additional healing of the fatigue damage in the laboratory. Finally, the report recommends that the beam fatigue model be used for future study of the endurance limit and its implementation. The beam fatigue test is better established than the uniaxial test and has a larger database of results in the literature. This report fully documents the research and discusses incorporation of the endurance limit derived from the SR regression model formalism as an algorithm in Pavement ME Design software and other design methods. The report includes three appendixes, also, available online at http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=2518: Appendix 1, Integrated Predictive Model for Healing and Fatigue Endurance Limit for Asphalt Concrete; Appendix 2, Endurance Limit for HMA Based on Healing Phenomena Using Viscoelastic Continuum Damage Analysis; and Appendix 3, Project Lab Test Results Inserted into the Mechanistic Empirical Distress Prediction Models (M-E_DPM) Database. (Author/publisher)
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