Research On Formation Of Spatial Structure For Asphalt Mixture Using Discrete Element Method

Asphalt mixture which is composed of coarse aggregates,asphalt mastic and air voids can be considered as a multiphase material.Its mechanical behavior is directly related to spatial structure characteristics,such as the distribution of aggregate particles,the contact state between aggregates.Obviously,the spatial structure formation of asphalt mixture closely depends on compaction process.So,how to ensure adequate compaction of asphalt mixture becomes a focus topic on pavement researchers.The objective of this study is to develop 3D(3-dimensional)mixture compaction models,and discuss the formation of spatial structure for asphalt mixture with PFC3D(3-dimenisonal particle flow code).The main contents are stated as follows:Firstly,because the existing algorithm of irregular polyhedron can not represent the actual aggregate,an improved algorithm was developed on the basis of random cut method.The key step of this improved algorithm was to properly enlarge the radius of graded balls,thus avoiding decrease the size of aggregates.In order to further verify the accuracy of the aggregate size,according to the principle of aggregate screening test,a virtual vibrating screen model was constructed with discrete element method.In addition,based on the improved algorithm,take AC(asphalt concrete)-20 for example,a virtual assembly process of asphalt mixture specimen would be investigated.It can realize the visualization of meso-scale characteristics of asphalt mixture,and laid a foundation of compaction mechanism and mechanical response from meso-scale perspective in asphalt mixture.Secondly,according to the characteristics of a variety of meso-scale contact models in PFC3D,the suitable contact models were choose to describe interior structural characters of asphalt mixture.On the basis of analyzing mesoscopic parameters of the selected contact models systematically,the relationship between mesoscopic and macroscopic parameters were established.Based on the TTS(time-temperature superposition)principle,the mesoscopic parameters were determined with experimental test.It provided the material parameters for numerical simulation.Thirdly,based on the above stated aggregate generation and determined materials parameters,a static compacting model with PFC3D for coarse aggregate mixture was established.Moreover,this model was verified using a laboratory test.Then,combination of mesoscopic and macroscopic indicators,the migration and evolution of aggregates and their influence factors,such as the stiffness,maximum nominal size and gradation type were investigated in the compaction procedure.According to the fabricated digital samples with the static compaction,a triaxial virtual test was employed to evaluate the mechanical properties of the aggregates.It provided a basis for further exploration the compaction mechanism and the mechanical performance,and had a benefit reference on designing and other compacting methods in asphalt mixture.Fourthly,a static compacting model in asphalt mixture with PFC3D was presented to investigate the aggregates spatial distribution,rotational and translational velocity,contact force and contact number internal asphalt mixture.The formation of spatial structure for asphalt mixture from mesoscopic level was analyzed.It provided a meaningful reference on other laboratory compacting manners in asphalt mixture and field compaction in pavement construction.Finally,the loose SMA(stone matrix asphalt)-13 was prepared using the virtual specimen generation based on the PFC3D.Then,a 3D discrete element model for asphalt pavement was developed on the basis of the distributed load-time superposition principle.In terms of the average contact force,strain rate and energy,etc.,some factors influencing the evolution laws of asphalt mixture,such as initial voids ratio,pavement thickness,compaction method and compaction speed were employed to analyze the migration and movement laws of spatial structure for asphalt pavement in the compaction process.To a certain extent,it revealed the mesoscopic mechanism in pavement field compaction.