Study On Micromechanical Properties And Evolution Behavior Of Asphalt Mixture

At present,the research on the mechanical behavior of asphalt mixture mostly ignores or simplifies its micro structure characteristics,but uses the continuous medium theory and macro phenomenological test methods to establish its mechanical performance model.However,these research methods have great limitations in establishing the relationship between the internal structure,composition and macro mechanical performance,and revealing the mechanism of local damage caused by the material heterogeneity.Based on the background of granular matter nature and multi-scale research,this paper uses granular mechanics theory,image processing technology,random algorithm technology,discrete element numerical simulation method and relevant indoor tests to simulate the particle packing characteristics,and three important macro mechanical properties of viscoelasticity,plasticity and fracture behavior of asphalt mixture in different temperature range.After realizing the effective prediction of the performance of asphalt mixture,the micro mechanical properties and evolution behavior of asphalt mixture are studied from the interaction between particles of the non-cohesive particles system and the cohesive particles system.Finally,this paper initially constructs the multi-scale mechanical system of asphalt mixture in the field of discontinuous medium mechanics.The main work and conclusions are as follows:(1)Based on the image processing technology of brightness conversion,watershed segmentation,morphological operation,canny boundary recognition and pixel recognition,the real microstructure model of asphalt mixture is established.Monte Carlo method and random polyhedral aggregate particle model generation method are proposed to reconstruct the random micro structure models,which provide a full-automatic modeling scheme for the mechanical simulation of asphalt mixture.(2)After studying the effects of friction,interference,gradation and vibration on the aggregate packing structure of asphalt mixture,it is found that: the packing density and coordination number decrease continuously and then tend to be stable with the friction coefficient.The RDF presents the characteristics of disordered fluctuation,and the peak value of the second peak of RDF is related to the packing density.The evolution of binary particle packing density is related to the particle size ratio and volume,and the interaction between particles is the strongest near the transformation point of the dominant system.The number,proportion,average force of the contact force chain and key particle size are closely related to the grading type and the maximum nominal particle size in the grading packing system.An evaluation method based on the strong and weak force chain and participation rate of main skeleton particles(PRMSP)index are proposed to evaluate the skeleton framework of mixtures.The significance of OGFC skeleton is greater than that of SMA and AC.With the decrease of the nominal maximum particle size,the PRMSP value of each mixture increases.The vibration stress amplitude and frequency have a comprehensive effect on the change speed,fluctuation degree and the stable packing density,which should be controlled in a reasonable range.(3)The micro viscoelastic mechanical model of asphalt mixture is established by assigning the linear model,Burgess model and modified Burgess model to the internal contact of aggregate,the internal contact of mortar and the interface contact of aggregate/mortar respectively,and effectively predict the dynamic modulus and phase angle of the mixture.The development of the internal force chain of the mixture under viscoelastic state is discontinuous,while the internal contact force chain of aggregate,asphalt mortar and the interface of aggregate/mortar is all compression chain,and mainly compression chain but with some tension chains respectively.The maximum contact force in aggregate is the largest,followed by the interface and the mortar.The micromechanical response speed of aggregate is greater than that of asphalt mortar.The length diameter ratio and orientation of aggregate have comprehensive effect on the dynamic modulus and phase angle of mixture.With the increase of volume fraction of flake aggregate and void,the dynamic modulus of asphalt mixture decreases.(4)The virtual creep and rutting test model are established by adopting the aggregate clump model and the time-temperature equivalent principle as to effectively predict the high-temperature plastic mechanical properties.The micro displacement field has a trend of diffusion to both sides of the horizontal direction,and an obvious upward movement trend in the load edge,and the maximum displacement of the mortar is larger than that of coarse aggregate,which reveals the mechanism of local uplift deformation and extensive oil.The horizontal diffusion range and maximum displacement of AC-13 aggregate particle are greater than SMA-13.The skeleton mainly plays a role in the shear deformation of material in the later stage of load action.Under the action of local wheel load,the mixture is dominated by compression force chain,and the tension chain is mainly concentrated in the edge area,but has opening upward arc distribution in the middle of the specimen.The XY plane translation angle ?,the XZ plane translation angle ? and the YZ plane translation angle ? of aggregate particles is mainly concentrated in the range of 0?40°,70?90° and 80?90°.The development law of the average rotation angle of aggregate is consistent with that of the permanent deformation.The aggregate particles form the main force transfer framework of the mixture at high temperature.The high-temperature performance of asphalt mixture with polyhedral coarse aggregate is better than that with ellipsoid and spherical coarse aggregate.The dynamic stability of the mixture increases first and then decreases with the increase of the content of flake coarse aggregate.(5)Based on SCB test,a new evaluation index of benchmark critical strain energy mixture is put forward.It is found that the impact of nominal maximum particle size on the crack resistance of mixture is more significant than that of gradation type,and the smaller the nominal maximum particle size is,the better the crack resistance of mixture is.Micro cracking mechanical model is established by using bilinear cohesion model for modeling initiation and propagation of the crack.The compression and tension contact force are mainly concentrated on the top and bottom,and the notch tip of the specimen respectively.The microcracks mainly occur in the asphalt mortar and the interface,and usually develop along the aggregate boundary.Because of the concentration of tensile stress,the aggregate at the tip of notch will also crack.With the decrease of temperature,the location of microcrack initiation will be transferred from the interface to the aggregate.The tensile strength of coarse aggregate has significant influence on peak load and failure mode of mixture.