Application of the dissipated energy concept to fatigue cracking in asphalt pavements
Following a description of the general properties of asphaltic materials, a review is presented on fatigue damage. Fatigue element tests have been carried out using two types of procedures; uniaxial tension-compression and a trapezoidal cantilever beam. The data from the Trapezoidal test has been used to develop relationships between dissipated energy and the number of load cycles to crack initiation. A method has been developed which enables the stiffness loss during a fatigue test to be quantified in terms of the initial mixture rheology. In addition, an improved method for defining the crack initiation point, N1, has been developed along with the definition of an energy ratio to enable determination of fatigue life for intermediate modes of loading. An assessment of two tests involving indirect tension has been made. A series of tests were conducted in the Slab Test Facility to determine the performance of various asphaltic mixtures with respect to fatigue. These have been used to validate the results from the element tests and assess the suitability of different shift factors. A 2-dimensional Finite Element visco-elastic analysis method has been used to calculate dissipated energy in pavement structures. This method has been compared to an elastic analysis method. It was observed that the F.E. method is less sensitive to pavement thickness. The F.E. method has some potential for prediction of surface cracking and fatigue life but further work is needed to implement a 3 dimensional model. Finally, based upon an assessment of the results obtained, recommendations have been made for additional work involving materials testing, model development and pavement design.