| Asphalt pavement with a semi-rigid base is a widely used structure for high-grade highways due to a number of advantages,including high smoothness and water stability,and good fatigue performance.However,under the action of temperature and vehicle loading,the crack would appear at the bottom of the surface layer in asphalt pavement.In the subsequent service process,the crack tips will be more critical to bearing the stress concentration and prone to make the existing crack propagate upward and form the reflective crack ultimately.The reflective crack is a serious disease of pavement and so called as bottom-up crack(BUC).The crack not only affects the pavement performance,but also makes the surface water permeate into the pavement and greatly reduce the service life of the pavement.The analysis of crack propagation with numerical simulation will provide a useful insight into the pavement maintenance strategy.It is difficult to simulate the whole propagation process of BUC in pavement under periodic load through finite element(FE)method,because it requires huge time and space resources of computer.In addition,the asphalt mixture is a temperaturesensitive material,then the influence of temperature on its fracture propertied need to be further researched.Based on the above-mentioned problem,the following research work would be carried out in this paper.(1)As a viscoelastic material,the asphalt mixture would exhibit relaxation behavior after a long period under low temperature.Then the asphalt mixture could be assumed as elastic material under this condition.But its viscoelastic behavior could not be ignored under medium temperature.In order to provide material parameters for subsequent simulation model,its viscoelastic properties at medium temperature were obtained through uniaxial compression test.Furthermore,the semicircular bending test(SCB)under different temperature was carried out on this type of asphalt mixture to achieve the load-displacement curve.The SCB results could supply a data base for calibrating the parameters of fracture properties of asphalt mixture under different temperature in subsequent simulation work.(2)The asphalt overlay over pavement with cracks is a major maintenance strategy to improve the pavement performance.However,the overlay cannot completely eliminate the crack disease in pavement,as the crack would propagate upwards eventually after several service years.For cracked pavement after overlayer,the asphalt layer is assumed to be elastic under low temperature.In this condition,a 3-D overlayed asphalt pavement finite element model with an initial transverse crack in the surface layer constructed by extended finite element model(XFEM)was established.With this model,the propagation potential of the crack with different geometric characteristics and different locations in overlaid pavement under vehicle load were analyzed to evaluate the overlay effectiveness.Considering the influence of existing crack’s length,the existing crack’s inclination,the existing crack’s location,the overlay thickness,and the loading position,the stress intensity factor(SIF)was selected as the index to analyze the crack propagation trends under different conditions.(3)Besides the vehicle load,the temperature is another main factor causing the crack propagation in asphalt pavement.For the overlayed cracked pavement model,taking Beijing area as example,the transient thermal analysis module was used to construct the 24-hour temperature field in winter according to local meteorological data including hourly variation of short-wave solar radiation and atmospheric temperature.Since the temperature was low in winter,the asphalt layer was still assumed to be elastic material.Then the temperature field at different time was applied to the simulation model as the body load to analyze the stress response of the structure under the combined action of vehicle and temperature.The SIFs at different times were extracted to analyze the differences of fracture properties after introducing the influence of temperature and to evaluate the overlay effectiveness under this condition.The results showed that,compared with the action of pure vehicle load,the structural fracture mode varied from type Ⅰ to compound type Ⅰ and Ⅱ.Moreover,the most dangerous load position was closer to the crack plane.(4)The asphalt mixture behaves obvious viscoelasticity under medium temperature and it is not suitable to simulate the crack propagation with XFEM.The cohesive zone model(CZM)was introduced in this study to simulate the crack propagation in viscoelastic asphalt mixture under medium temperature.The CZM could reflect the fracture strength and fracture energy without SIF and the key issue for CZM lies in the parameter’s determination.The parameter of CZM is generally determined through manually adjustment to match the numerical simulation to the experimental measurement.However,the method is time consuming and precision uncontrolled.In this study,a novel method combined the efficiency of Kriging surrogate model plus the global optimization advantage of the genetic algorithm was proposed to predict the CZM parameters of asphalt mixture under 25℃.Then,the influence of each parameter on the response curve was analyzed.Finally,a pre-select strategy for the optimization method was proposed to improve the computational efficiency.Furthermore,the CZM parameters in SCB simulation model at 15℃ and 35℃ were determined by this method,and then the influence of temperature on the fracture properties of asphalt mixture could be analyzed.The proposed method could obtain the CZM parameters with high accuracy,and the efficiency of parameters calibration is related to the selection strategy of initial sample points.The closer the initial sample points are to the optimal point,the higher the calibration efficiency will be.Furthermore,the increase of temperature mainly reduces the fracture strength of asphalt mixture,which leads to the decrease of its fracture energy.(5)The Mode Ⅰ dominated bilinear CZM after calibration was introduced into FE model to simulate the BUC propagation in asphalt pavement under displacement load.The mechanism of crack propagation under different types of periodic load was analyzed based on two indexes from Mode Ⅰ dominated bilinear CZM.The simulation model and loading condition was modified to ensure only BUC propagation occurs in the asphalt layer.Then,the propagation rate and the propagation life of BUC were predicted by combined method of simulation and regression based on the modified model.Moreover,the predicted value of propagation life had a certain rationality in referring to the engineering design standards.The method obtained in this research could predict the propagation life of bottom-up crack in asphalt layer after a few times of numerical simulation.It is sufficient to demonstrate the feasibility and high efficiency of this method.In this study,a numerical simulation model for analyzing fracture properties of three-dimensional low-temperature overlayed cracked asphalt pavement structure was established.And the propagation life prediction method of bottom-up crack in cracked asphalt pavement under medium temperature was proposed.The results of this paper provide a good reference for related research. |
PDF Full Text Request