Fatigue Crack Growth in Pavements

Abstract

A common laboratory fatigue test for life estimation of pavements uses a repeated load flexural device with nonlayered beam specimens. As a result of such factors as traffic wander, and also due to different crack growth rates between such beam specimens and layered media, highway pavements have been found to sustain from 10 to 100 times the number of load applications estimated by this type of laboratory test. This study focuses on identifying the differences in crack propagation between beam specimens and layered pavements. A 2D finite-element program with specialized fracture mechanics capabilities is used in the analysis. An isotropic linear elastic material is assumed. Noncohesive cracks are introduced in the mesh as geometrical discontinuities. Cracks are propagated through the material under cyclic loading. Stress intensity factors for mixed mode are calculated for each crack length. Two different configurations are analyzed: laboratory beam specimens and layered pavements. It will be shown that predicted crack growth rates in pavements for the same maximum longitudinal strain are much lower than in beam specimens, which is consistent with laboratory and field observations. Two different crack growth rates are identified in pavements. The first is associated with tensile stresses at the bottom of the surface course. The second one is caused by tensile stresses at the top of the surface course, when the load is ahead of or behind the crack. The growth rate for surface cracks (growing downward) is much smaller than the internal crack growth rate (growing upward).