Prediction of fatigue cracking in asphalt pavements - Do we follow the right approach?

The fatigue performance of asphalt concrete pavements is difficult to predict not only because many of the input parameters needed for the analyses are difficult to obtain but also because the fatigue phenomenon itself is not well understood. Most analyses, for example, take the tensile strain at the bottom of the asphalt layer as the factor that explains fatigue, although many pavements exhibit top-down cracking that has nothing to do with the tensile strain at the bottom of the asphalt layer. This approach implies that calibrating fatigue predictions on the basis of tensile strains at the bottom of the asphalt layer with the amount of cracking observed at the pavement surface is rather unrealistic. Furthermore, it is unlikely that fatigue cracks, if initiated at the bottom of the asphalt layer, show up at the pavement surface as clearly defined cracks. There are many indications that fatigue at the bottom of the asphalt layer is a matter of the development of a deteriorated zone with microcracks rather than development of clearly defined, discrete cracks. It is evident that large shift factors are needed to apply laboratory fatigue relations to field predictions. The magnitude of these shift factors depends, among other determinants, on the type of fatigue test and mode of loading. As discussed in this paper, only the slope of the fatigue relation can be estimated with confidence from laboratory fatigue tests. Finally, this paper shows that the existence of an endurance limit can be debated; in any case it is not a constant value of about 70 mu m/m. Experiments have shown that the suggested value is too high.