Multiscale Analysis Of Viscoelastic Mechanical Response Of Asphalt Pavements Based On Random Aggregate Model

The damage mechanism of asphalt pavements has always been the critical subject in road engineering.Due to the complicated structural feature of asphalt mixture which contains air voids,aggregates,mineral fillers and asphalt bitumen,it is hard to effectively consider the characteristics within the mesostructure of asphalt concrete nor present the damage mechanism of asphalt pavements by using the pavement mechanics analysis method at the single macroscopic scale,and to perform reasonable multiscale analysis of damage mechanism of asphalt pavement by the mechanical response at microscale.Therefore,this paper proposed a novel random aggregate model suitable for simulating two-dimensional asphalt mixture microstructure which can be quickly obtained,and based on the established microstructure,a multi-scale finite element model of asphalt pavement which simultaneously considers the characteristics of asphalt layer at both macro and micro scales can be built.This paper involves the following researches:(1)Proposed a random aggregate model which contains two sub-methods: the “convexconcave surface method” of generating aggregate particles and the “double background grid method” of placing aggregates.These two methods can effectively solve the problem of the inconformity for virtual aggregate shape compared to actual aggregate and the low efficiency of aggregate placement.The effectiveness of the random aggregate model was verified by comparing the preset area of aggregates based on experimental aggregate gradation and the area of the generated aggregates within the microstructure,the superiority of the “double background grid” method was proved by calculating the running time for generating the microstructure with same volume of aggregates under double background grid with different sizes.(2)Three representative volume elements(RVE)were randomly selected from the mesostructure generated by random aggregate model,the corresponding numerical simulation tests of dynamic modulus,thermal contraction and heat conduction were carried out on the RVEs,and then the thermodynamic parameters of asphalt concrete were predicted by means of homogenization principle.The homogenization formulas suitable for finite element analysis were derived.The results showed that the calculated macroscopic performance parameters of asphalt mixture were close to the classical values which proved the effectiveness of the derived homogenization formulas,and this settles a foundation for material consistency of asphalt layer at both macro and micro scales in the following multiscale models.(3)In order to investigate the relation of the micro-parameters and macro-properties of asphalt mixture in a more accurate and efficient way,the RVEs of asphalt mixture were generated by the proposed random aggregate model,and the variables of the volume of aggregates and air voids were precisely controlled.The corresponding numerical simulation tests of dynamic modulus,thermal contraction and heat conduction were performed on these RVEs,and then the thermodynamic parameters of asphalt mixture were obtained by the derived homogenization formulas.The results showed that with the increase of aggregate content,the dynamic modulus of the mixture increased,and the coefficient of thermal contraction and thermal conductivity decreased.With the increase of air void fraction,the dynamic modulus and thermal conductivity of mixture decreased,however,this effect would weaken or even be reverse as the void fraction increasing to a certain extent.The influence of void fraction on the coefficient of thermal contraction of asphalt mixture at medium temperature was not obvious when the void fraction varied between 3% and 6%.(4)The mesostructure of asphalt mixture generated by the random aggregate model was embedded into the homogeneous asphalt surface layer at macroscale to form the multiscale finite element model of asphalt pavements.The viscoelastic mechanical behavior of cementtreated based(CTB)and granular based(GB)asphalt pavement were simulated under different conditions,and investigate damage mechanism of asphalt pavement by statistical analysis of the first principal stress within asphalt layer,and the influence of three thicknesses of asphalt layer(10,15 and 20cm)and two temperature(5℃ and 35℃)on the mechanical behavior of pavement structure are also considered.The results showed that the proposed multiscale model can effectively characterize the complicated mechanical behavior of asphalt pavement.By the statistical analysis of the first principal stress within mesostructure of asphalt layer,the mechanical behavior of asphalt pavements at macroscale can be reasonably reflected,and the damage mechanism of asphalt pavement can be revealed from the point of view of material tensile damage.The “top-down” crack was more likely to occur in CTB pavements,and the “bottom-up” crack would more easily occur in GB pavements while the “top-down” crack cannot be neglected as well.The increase of the thickness of asphalt layer can significantly reduce the stress level of asphalt layer for CTB pavement under low temperature,but mostly have an increase effect for GB pavement.When the deformation was small,the area of high tensile stress around air voids is more likely to be the cause of road surface damage.