Laboratory Validation of Healing-Based Fatigue Endurance Limit for Hot-Mix Asphalt

One of the main requirements of designing perpetual pavements is to determine the endurance limit of hot-mix asphalt (HMA). The endurance limit, as applied to HMA and flexible pavement design, is the strain or stress level below which the HMA would endure indefinite fatigue load repetitions and the pavement would not experience bottom-up fatigue cracking. The purpose of this study is to validate the endurance limit for HMA by using laboratory beam fatigue tests. A rational procedure was developed to determine the endurance limit of HMA attributable to healing that occurs during the rest periods between loading cycles. Relating healing to endurance limit makes this procedure unique compared with previous research projects that investigated these concepts separately. An extensive laboratory testing program was conducted according to AASHTO T 321-03 test procedure as a part of NCHRP Project 9-44A. Six factors that affected the fatigue response of HMA were evaluated: binder grade, binder content, air voids, test temperature, rest period, and applied strain. The endurance limit was determined when no accumulated damage occurred and indicated complete healing during the rest period after each load application. A threshold rest period of about 5 s for a load duration of 0.1 s—beyond which no more healing was gained—was obtained. HMA exhibits endurance limits ranging from 37 to 246 microstrains, depending on mixture properties and environmental conditions. The results of this study can be used to design perpetual pavements that can sustain a large number of loads if traffic volumes and vehicle weights are controlled.