Fatigue cracking of bituminous paving mixtures
This dissertation is concerned with the fatigue cracking of bituminous paving mixtures. It considers both the life to crack initiation and the life for crack propagation, including the development of a method for calculating the number of wheel load applications to either critical or failure condition. The development and subsequent validation of the Indirect Tensile Fatigue Test (ITFT) are described. Both the repeatability and reproducibility of the method are examined as well as its correlation with two other fatigue test methods. The test is shown to be a suitable method for measuring the life to crack initiation of bituminous paving mixtures as well as being an economically viable test. Poisson's ratio for bituminous paving mixtures is examined showing that, provided at least 500 conditioning pulses are used in order to achieve steady state conditions, 0.35 is an appropriate value. Prior to these early load applications Poisson's ratio is shown to be variable and often in excess of 0.50, an explanation of which is given in detail. Crack propagation was experimentally simulated using beams of material with a crack initiated on the underside. The work shows that the rate of crack propagation can be described by a power relationship between the stiffness of the mixture and the number of cycles to failure, which is mixture and binder dependent. A general equation is developed which allows the critical and failure lives of bituminous pavements to be calculated and these are compared to two traditional pavement design methods, giving equivalent results for unmodified mixtures but, giving more realistic results for polymer modified mixtures. Image analysis of the cracks demonstrates that they propagate around coarse aggregate trying to separate it from the matrix and that they travel in the straightest line possible between the point of crack initiation and the point of applied load.