Study On Crack Initiation Mechanism And Propagation Behavior Of Asphalt Pavements

Fatigue cracking is one of the major modes of premature distresses in asphalt pavements. Cracks of asphalt pavements start as microcracks and later propagate, densify, and coalesce due to repeated traffic loading or low temperature thermal stresses or combinations of both to form macrocracks, and result in structural distresses of asphalt pavements. Research on initiation mechanism and propagation behavior of cracks in asphalt pavements is necessary for a sound evaluation of mechanistic behavior of pavements. It is a great help to us to knowledge the state of the stress field and crack propagation path, to improve understanding of the mechanism of pavement damage and premature failure, and to provide guidance for pavement engineers in designing and maintaining asphalt pavements. The knowledge and understanding of the mechanism of crack initiation and propagation processes are very important to enhance the resistance cracking performance of asphalt pavement structure, to select optimum asphalt mixtures and to improve the present design methods.Firstly, based on literature review of a large number of relevant domestic and international research findings, the mechanism of premature distresses and identification of cracks in asphalt pavements were investigated and summarized in detail. As there are many complicated factors that have influence on premature cracking of asphalt pavements, a schematic of influencing factors having interaction relationship is given to help identify crack initiation mechanism. Using linear elastic fracture mechanics and finite element method and window moving technique, a two-dimensional asphalt pavement crack propagation path simulator (APCPPS2D) program has been developed with a combination of 8-node quadrilateral isoparametric element and 6-node triangular collapsed singular isoparametric element. APCPPS2D program can consider traffic loading and temperature variance simultaneously, and can simulate the propagation paths of multiple cracks at the same time. Verification and validation studies were conducted and results were compared to three examples published in literature that have analytical solutions and experimental results. The results indicate that theprogram is capable of accurately calculating the stress intensity factor and modeling the propagation paths of mode I crack, mode II crack and mixed mode I-II crack.In order to model the mixed mode crack propagation behavior of asphalt concrete, a three point bending beam with mixed mode notched has been designed. A series of mixed mode three point bending beam tests and numerical simulations have been performed to simulate mixed mode crack initiation and growth. The results showed that the crack initiation angle and the peak load increased as the distance of the preexisting crack from the midpoint of the beam increased. The crack propagation path follows some regularity in general. The experimental results showed that the crack propagation path followed, mainly, the boundaries of the grains and rarely cut through the grains with differences attributed to the grain size effect in asphalt concrete.It is the fist time that crack propagation path in asphalt pavements is modeled explicitly by the mesh in domestic. Two kinds of cracked pavement structures have been simulated using APCPPS2D program. One is a cracked pavement with a near surface crack; another is a cracked pavement with a near surface crack and a reflective crack at the bottom of the base. A comprehensive parametric study of the cracked pavement has been performed to evaluate effects of various combinations of traffic load and low temperature load, effects of various combinations of pavement structure, effects of non-uniform distributed tire pressures and overload and load spectra (magnitude and positioning with respect to crack). Load positioning was shown to have the most influence on crack propagation, along with asphalt and base layer stiffness. The direction of crack growth was computed and was shown to change with increasing crack length. The near surface cra...