DEM Simulations And Study Of Fracture And Rutting Behaviors For Asphalt Mixture

As the most widely used pavement material,asphalt mixture is affected by the conditions such as overloading,sudden temperature changes and rain erosion in its long term service.Then common damage phenomena including cracks and rutting arise easily,which severely affect the comfortability and safety of driving,the maintenance and repair of roads will be harder and bring big expenses at the same time.Considering the complexity of composition and internal structure in asphalt mixture,it is of great significance in preventing and controlling pavement distress to build a heterogeneous model composed of aggregates and asphalt mastic and to predict the cracking and rutting mechanical behavior of pavement structure under loading.The discrete element method(DEM)is receiving more and more attention as a rising numerical method in recent years.One important reason is that it can avoid the element distortion in FEM when dealing with large deformation problem.In this article the cross-section image of a sample was dealt with the digital image processing technique and the two-dimension(2D)heterogeneous model was established in DEM.In view of the different mechanical properties of asphalt mixture when cracking at low temprature and rutting at high temprature,corresponding different constitutive models were adopted to exhibit material performance of the matrix,interface and aggregate,and mechanical behavior simulations were conducted in contrast to the experimental result.The main creative achievements are shown as follows.(1)In consideration of the complexity of aggregate shape and the important role it plays in mechanical behavior of asphalt mixture,the method to establish a heterogeneous model was proposed based on the image processing technique in this article.Through this method,aggregate edges in the cross-section image were detected and refined to get aggregate informations,which were combined with the packing algorithm to locate the aggregates one by one in the DEM software.Thus,the 2D heterogeneous model of asphalt mixture was built in DEM.The information of gradation,shape and distribution for aggregates are all reserved,which provides an excellent pathway for comparion between the experimental result and numerical simulation.In order to improve the computational efficiency,asphalt mixture was simplified as a two phase composite composed of coarse aggregates and asphalt matrix.(2)Three-point bending fracture of pre-notched asphalt mixture beam was conducted at low temprature in experiment,and both the mode I and mixed-mode fracture behaviors were studied when pre-notches were located at the mid-span or an off-center site respectively.Considering the quasi-brittle characteristics of asphalt mixture at low temprature,the linear elastic constitutive model was adopted to describe the material performance of aggregares and asphalt matrix.A 2D model for the beam in DEM was established by use of the image processing technique,and the influence of both the pre-notch location and aggregate distribution on fracture mechanical behavior was studied.The distribution of force and fracturing process inside the specimen were analyzed when both mode I and mixed-mode fracture occurred from the viewpoint of mesomechanics.(3)Semi-circular bending(SCB)fracture of asphalt mixture with different pre-notch orientations was conducted at low temprature experimentally,and the relation between fracture behaviour and pre-notch orientations was analyzed.In order to reveal the intrinsic mechanism behind these phenomena,a 2D numerical model owning the same aggregare information with a specimen was established in DEM.The material performance of asphalt matrix and interface was characterized with a bilinear cohesive model.The influences of pre-notch orientations on peak value load and the corresponding displacement,and the deviation angle of crack were evaluated.The function of both normal tensile and tangential contact forces in the fracture process was analyzed,it is evident that crack propagation is carried out in mixed-mode.(4)The laboratory data from short-term creep tests of asphaltic sand was used to acquire Burgers model parameters of different temperatures through curve fitting.Then the parameters were transformed with the time-temperature superposition principle and their reasonability was also verified by comparing with the laboratory rutting result at the same temprature.The 2D DEM numerical model for rutting test was built through the digital image processing technique,and the viscoelastic properties of asphalt matrix and interface were characterized with the Burgers model.The rutting deformations under different temperatures and overloading ratios were simulated.The effects of temperature and overloading on rutting deformation and rutting deformation increment were evaluated.The existence of asphalt softening point was confirmed from another angle and it has an enormous influence on the rutting mechanical behavior.