Study On Mechanical Properties Of Thin Overlay Asphalt Mixture Based On Discrete Element Method

The mechanical properties of thin-layer mixture is the key to ensure the pavement performance of thin-layer overlay.However,the evaluation of the mechanical properties of thin-layer mixture mainly depends on the laboratory test method based on phenomenology,which is difficult to reasonably,efficiently and accurately predict the service performance of thin-layer mixture,resulting in rutting,cracking and other diseases in the service life of thin-layer overlay.The engineering quality of thin layer overlay is uneven.In order to improve the pavement performance of thin layer overlay,the mechanical properties of thin layer mixture should be strictly controlled,which requires designers to master the methods to improve the mechanical properties of thin layer mixture.In view of the above research needs,this paper uses MATLAB and PFC3D discrete element software to establish a three-dimensional discrete element model of thin-layer mixture considering the irregular shape characteristics of coarse aggregate,and tests the relative errors of gradation and voids of the virtual specimen;carries out the virtual triaxial shear test,and systematically analyzes the skeleton structure of thin-layer mixture,the thickness of mixture and the coarse aggregate particle accuracy D_min.Through the simulation of SCB semicircle bending test,the fracture toughness difference of different thin-layer mixture is studied,and the anti-cracking ability of thin-layer mixture is analyzed;through the trabecular bending fatigue test,the fatigue characteristics and fatigue failure mechanism of thin-layer mixture are analyzed from the micro perspective,which has great theoretical value and application prospect.Firstly,based on Newton’s second law and the relationship between force and displacement of particles,the motion equation and boundary conditions of particles are derived,and the constitutive relationship of particles in discrete element software PFC3D is analyzed.Burgers model is used to characterize the viscoelastic properties of asphalt mortar,and the analytical solution of Burgers model is derived.Based on the strength theory and volume characteristics of asphalt mixture,the meso contact model of asphalt mixture is determined.Secondly,a three-dimensional discrete element model considering the irregular shape of coarse aggregate is established,and the rationality of the coarse aggregate precision control index D_min is discussed.Based on the actual indoor grading information,the distribution of coarse aggregate is statistically analyzed,and the rationality of the model grading and porosity is verified.The dynamic modulus test of asphalt mixture is carried out.Based on the time temperature equivalent principle and sigmoidal function,the main curves of dynamic modulus of asphalt mixture under five temperature conditions of 0℃,15℃,30℃,45℃and 60℃are obtained by fitting.The conversion relationship between macro test parameters and micro parameters is deduced.Combined with the existing research results and empirical values,the micro model parameters are calibrated and compared with the indoor test results The results show that the accuracy of the model is more than 85.5%.Thirdly,the virtual triaxial shear test and SCB semicircle bending test are carried out to analyze the mechanical properties of the thin-layer mixture at high and low temperature.Triaxial shear test systematically analyzes the influence of various factors on high temperature shear resistance of thin-layer mixture from three aspects of skeleton structure type,grading thickness and coarse aggregate precision D_min.The results show that:the skeleton structure type is the key factor to determine the shear strength of the mixture,the skeleton dense structure SMA has the best high-temperature shear performance,while the skeleton void structure OGFC has the poor high-temperature shear performance;and for the same skeleton structure type of thin-layer mixture,the finer gradation design can improve the cohesion of the mixture,and the reasonable choice of the skeleton structure type and structure of the thin-layer mixture Gradation can effectively improve the shear performance of mixture.Through virtual SCB test,the low-temperature crack resistance of three kinds of medium and thin-layer mixture was studied,and the rationality of the model was verified by comparing the test results with the indoor test.It is found that the fracture energy required for the three kinds of mixture decreases with the increase of the slit depth.The fracture toughness J_IC of SMA-13 mixture is about twice that of OGFC-13,which indicates that the mixture with dense skeleton structure has better low temperature crack resistance,while the fracture toughness of AC-13 and OGFC-13 has no significant difference.Finally,the bending fatigue test model of thin-layer mixture trabeculae is established to analyze the influence of stress level and gradation on the bending fatigue life of thin-layer mixture trabeculae.The results show that the higher the fatigue stress level is,the shorter the fatigue life of the thin layer mixture is.Taking SMA-13 mixture as an example,the fatigue life under 1.2MPa stress level is 87.81%less than that under0.8MPa stress level.In addition,the stress fatigue equation results show that AC-13and OGFC-13 are more sensitive to fatigue load.The mechanism of bending fatigue failure of the trabeculae is explained from the micro perspective,and the location characteristics and development law of the cracks are analyzed.On this basis,the particle displacement of the mixture in the trabeculae is analyzed.