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The existence of a fatigue endurance limit has been postulated for a considerable time. With the increasing emphasis on extended-life hot-mix asphalt pavement, or perpetual pavement, verification of the existence of this endurance limit, a strain below which none or very little fatigue damage develops, has become a substantial consideration in the design of these new multilayered full-depth pavements. Fatigue data are presented that were collected on a surface mix and a binder mixture tested for an extended period from 5 million to 48 million load repetitions at strain levels down to 70 microstrain. The fatigue results are analyzed in the traditional manner and using the dissipated energy ratio. This analysis shows that there is a difference in the data at normal strain levels recommended for fatigue testing and at the low strain levels. This difference cannot substantiate an endurance limit using traditional analysis procedures, but the dissipated energy approach clearly shows a distinct change in material behavior at low flexural strain levels, which supports the fact that at low strain levels the damage accumulated from each load cycle is disproportionately less than what is predicted from extrapolations of fatigue testing at normal strain levels. This reduced damage may be attributed to the healing process. The conclusion of this study is that laboratory testing can verify the existence of a fatigue endurance limit in the range of 90 to 70 microstrain below which the fatigue life of the mixture is significantly extended relative to normal design considerations.

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The healing phenomenon has been noted by pavement engineers for years, but its relation to hot-mix asphalt (HMA) fatigue behavior is still far from clear. This study conducted an analysis of healing and HMA fatigue behavior by introducing a specifically designed fatigue-healing test. These results help explain the differences in fatigue behavior at normal and low strain levels. An approach using the ratio of dissipated energy change, which is based on energy concepts, is used in this study. The results show that healing does exist, and its effect on fatigue life can be indicated by an energy recovery per second of rest period. The effect of healing is more prominent at low strain levels or in very long rest periods. At low strain conditions, the dominance of healing compared with the very low external load damage, considering the energy equilibrium, can result in full damage recovery. This full recovery of energy explains the existence of a fatigue endurance limit, below which HMA materials tend to have extraordinarily long fatigue lives that, as is shown, can be related to healing. The testing conducted clearly shows why polymer modification may extend the fatigue life in the field even though laboratory testing may show minimal differences compared with the neat binder test results.