Studying the Propagation of Top-Down Cracking (TDC) in Asphalt Concrete pavements by Means of Fracture Mechanics Theory

Fatigue can be expressed as the failure of asphalt concrete under repeated loading in such a way that stress induced in the asphalt layer could be less than ultimate strength of the material. Fatigue cracking in asphalt concrete layer can be classified into two types regarding the place of crack initiation and the type of crack propagation. The first type the cracks initiated in the bottom of asphalt layer and propagates upward. This type of cracking is produced in the thin asphalt layer. The second type of cracking which is produced in thick asphalt layer is initiated in surface area then propagates downward which is called Top-Down cracking. The second type of cracking is studied less compared to the first one. One type of fatigue cracking, is fracture mechanics approach in which the failure in asphalt mixture is related to number of repeated loading. In this study the fracture mechanics approach was implemented to understand the effective parameters in propagation of top-down cracking. as well as developing a model to interpret the crack propagation in pavement also some graphs were produced for different types of material and loading which gives the length of cracks versus the number of repeated loading. the results showed that with increasing the load distance from the initial place which is initiated the stress intensity factor is increased in mode Ι and mode ΙΙ. Also increasing the stiffness gradient in asphalt concrete due to temperature differential in top and bottom of asphalt layer or differential increasing between stiffness of asphalt surface and asphalt base course will be increased the stress intensity factor.