Comparison of fatigue damage, healing, and endurance limit with beam and uniaxial fatigue tests

The concept of an endurance limit assumes a strain value below which the net fatigue damage that occurs during a load cycle is zero. The fact that real traffic loads are separated by rest periods may allow for partial or full healing of the microcracks, which can affect this endurance limit. If the asphalt layer thickness is controlled to keep strains below the endurance limit, the fatigue life of the pavement can be extended considerably. In the study reported in this paper, it was hypothesized that the endurance limit in asphalt concrete developed from the interaction and balance of damage and healing during a load cycle. This hypothesis formed the basis of the testing and analysis program, which evaluated the effects of air voids, asphalt content, rest periods, and temperature on the endurance limit. Two types of fatigue tests were conducted: beam (flexural) and uniaxial. A regression model also was developed on the basis of the results of each test and used to obtain the endurance limit values. This paper compares fatigue damage, healing, and endurance limit results from the two tests under similar conditions. The comparison shows that the beam fatigue test yields less overall fatigue damage and less healing than the uniaxial fatigue test. Beam fatigue yields 8 to 14 times longer fatigue lives, while uniaxial fatigue yields higher healing (10.4 times for the only available case). Because damage and healing combined to govern the endurance limit, the two tests produced close values in which the overall uniaxial endurance limit values were 12% less than the beam fatigue endurance limit values.