Reflective Crack Propagation And Fatigue Life For Asphalt Concrete Pavement On Semi-Rigid Base Course

The asphalt concrete pavement on semi-rigid base course is the main form of high-grade road in China, but various kinds of damages will happen to it for crack in their early service. Reflective cracking is one of major forms of early destruction in asphalt overlay on semi-rigid base. So how to effectively control its initiation, propagation and reduce its influence on fatigue life in pavement structures still remains a huge problem in road engineering. Traditional continuum mechanics and fatigue strength theory are difficulty to describe the essence of the cracking performance. On the basis of the improved Williams series, isoperimetric element and the method of finite element with generalized degrees of freedom (GDOFs), Williams element with GDOFs was proposed for analyzing stress intensity factor (SIF) of reflective cracking under surface loading. The generalized parameters which were from the solution of govern equation were introduced into the model of prediction the fatigue life, and determined the fatigue life for the reflective cracking propagation on pavement structure. As fatigue life is affected by many factors, such as crack geometry, extending direction, expanding law, materials, structure geometry, environment conditions and loading procession. These factors usually have some randomness, so fatigue life is also a random variable. For these reasons, the study will focus on predicting random fatigue life caused by reflective cracking propagation on semi-rigid base course, and using Monte Carlo Simulation Method (MCS) to value the reliability of the pavement structure. Main works are as follows:Part I:the layered linear elastic plane strain fracture mechanics analysis model was constructed for semi-rigid pavement structure, and some reasonable assumptions and simplifications were made. By using the reflective cracking fracture modes, they led to some basic theory and concepts within the linear elastic fracture mechanics. Based on Westergaard stress function, study gave the stress and displacement field expression of three basic modes cracking, and then raised the conception of SIR Since reflective cracking usually acted as mixed mode I-II, this thesis deduced the more flexible Williams series characteristic expansion of plane crack.Part II:the analysis model was discretized the singular region and regular region. And the singular region was disposed by employing the refined mesh of traditional finite element method and quarter-node singular element, respectively. Basic concepts and construction principles of isoperimetric element and quarter-node singular element were introduced in detail, and then their shape functions were derived. The two have a same defect from the examples:they cannot directly obtain SIF, but need use linear extrapolation method to acquire it. The process was very complex and large amount of calculation. Part III:it was focused on the defect that researchers mostly use finite element method analysis software or the traditional finite element method to predict the SIF at crack tip, this study presented the Williams element with GDOFs for SIF of the pavement on semi-rigid base course. In this element model which has a parameter that is related to SIF, so that we can directly determine the SIF at crack tip, and effectively avoid the defects of the traditional finite element method. From the construction principles of Williams element, we found that this kind element no longer treated node parameters as unknown quantities but denoted as undetermined parameter in Williams series. So the number of elements of singular region had no effect on total degrees of freedom, it was only determined by how many number of terms the Williams series have. With these virtues, the Williams element greatly reduced the discrete degrees of freedom and increased the calculation efficiency. We just need know coordinate information of boundary nodes in singular region and region regular, then according to geometric series summation, we can form Williams element stiffness matrix in singular region easily.Part IV:combining the Williams element with GDOFs and the traditional theory and formula for predicting fatigue life, the analysis model of fatigue life for reflective cracking propagation was constructed by the generalized parameters. It will avoid the two-step thinking that determined the SIF by the structural response, and soon the fatigue life was analyzed by the SIF. The relationship between surface modulus and layer thickness was analyzed in detail. The results show that when increasing surface layer thickness or decreasing surface modulus can improve the fatigue life, but modulus reduce will bring in some new pavement damage in their service. So the suggestion that was from decreasing surface modulus just for the aim of improving the fatigue life need deep consideration for choosing.Part Ⅴ:Paris formula from traditional fatigue life prediction was turned into a revised flexible series form, and then introducing generalized parameter to predict the fatigue life of pavement structure, this format is simple and can easily be accepted by the engineering staff. In examples, we took into account the randomness of single variable, and the fatigue life was calculated by Monte Carlo simulation method from the crack extension through the entire surface. The reliability analysis model was established by the fatigue life and design life. Moreover, the reliability index was obtained for pavement structure.