Fatigue endurance limit for HMA based on healing

A long lasting flexible pavement has been the goal of the pavement community for many years. Designing perpetual pavements requires knowledge of the stresses or strains below which damage does not occur or can be healed during unloading. The endurance limit, as applied to hot mix asphalt (HMA) and flexible pavement design, is the strain or stress level below which the HMA would endure indefinite load repetitions and the pavement would not experience bottom-up fatigue cracking. The purpose of this study is to determine the endurance limit for HMA under different conditions using laboratory beam fatigue tests. The approach used in this study assumes that if stresses or strains are kept below a certain level, fatigue damage may not occur or can be healed during unloading. Relating healing to the endurance limit makes this approach unique compared to previous studies that investigated these concepts separately. An extensive laboratory beam fatigue testing program covering a wide range of asphalt mixtures was conducted according to the AASHTO T 321-03 test procedure as a part of the NCHRP 9-44A project. The endurance limit was determined when no accumulated damage occurred indicating complete healing during the rest period after each load application. The endurance limit varied between 22 and 264 micro-strains (μs) depending on binder grade, binder content, air void, temperature and the rest period between load applications when 0.1 sec loading cycles are used. The results of this study can be used to design perpetual pavements that can sustain a large number of load applications if traffic volume and vehicle weights are controlled.