Study On Meso-structure And Fatigue Performance Of Cement Stabilized Macadam Based On Discrete Element Method

Cement stabilized macadam material has been widely used in semi-rigid base road structure in China.Its performance is closely related to the internal composition structure such as porosity,distribution of aggregate particles and contact state between aggregates.At the same time,the fatigue performance of cement stabilized macadam directly affects its durability.For this reason,scholars have carried out a large number of theoretical and experimental studies on the structure and mechanical properties of cement stabilized crushed stone,but most of them adopt the method of macroscopic test,which is not only time-consuming and laborious,but also has poor reproducibility,and cannot deeply explore the internal structure of the mixture.At the same time,cement stabilized crushed stone is a three-phase heterogeneous material composed of graded crushed stone,cement mortar and pores.Its structure and mechanical properties are relatively complex.It is usually assumed to be homogeneous material in macroscopic tests,which has certain limitations.In recent years,domestic scholars have begun to study the discrete element method and gradually introduced it into the study of road materials.While making up for the lack of indoor test methods,it also provides a new way for the microscopic analysis of the internal structure of materials and the study of mechanical damage mechanism.Based on the discrete element method,combined with CT scanning technology and Avizo three-dimensional reconstruction method,this paper makes a preliminary exploration on the internal structure and fatigue durability of cement stabilized macadam from model construction to simulation test.Firstly,the CT tomography images of cement stabilized macadam specimens were imported into the three-dimensional reconstruction software Avizo to obtain the true morphology of aggregates with different particle sizes in cement stabilized macadam,and the particle cluster templates corresponding to the true contour of aggregates were generated in the discrete element software.Considering the reduction degree and operation efficiency,the parameters of the discrete element aggregate template used in this paper are given,which improves the similarity between the discrete element model of cement stabilized macadam and the indoor specimen.Secondly,a discrete element model composed of irregular particles was established,and the indoor test and simulation test of unconfined compressive strength of cement stabilized macadam were carried out to verify the reliability of the discrete element model.The obtained stress-strain curve was compared with the actual test results for parameter calibration,and the mesoscopic contact parameters of the model were determined,which provided a basis for the parameter setting of the subsequent simulation test of cement stabilized macadam.Thirdly,on the basis of the obtained true morphology and mesoscopic contact parameters of cement stabilized macadam aggregate,a cement stabilized macadam forming test model considering aggregate morphology is established,and the rationality of mesoscopic parameters is verified by comparing with the change of mixture height during the indoor heavy compaction forming test.The forming effect and the evolution law of the force chain of the mixture of the two methods of heavy compaction forming and vibration forming are studied.The influence of different forming methods on the internal composition of cement stabilized macadam is explained from the perspective of mesoscopic particle motion and interaction.The specimens formed by vibration method have better uniformity and aggregate embedding effect.Finally,the middle beam discrete element model of cement stabilized macadam is established,and the four-point bending test and fatigue test are simulated.By comparing the ultimate failure load of the simulated bending test with the indoor test results,the calibrated parameters are corrected,and finally the reasonable two-dimensional contact parameters are obtained.By monitoring the particle motion state,crack development and force chain evolution process inside the mixture during the simulation process,the failure mechanism of cement stabilized macadam and the occurrence and development law of fatigue damage are explored from a microscopic perspective.The results show that the flexural failure of cement stabilized macadam first appears at the bottom of the mid-span of the specimen,and then develops from bottom to top along the aggregate contour.Fatigue failure is a problem of damage accumulation from quantitative change to qualitative change and from continuous to discontinuous.It is caused by concentrated tensile stress.Firstly,the weak connection between cement mortar and aggregate occurs.Fatigue failure can be divided into four stages according to the development state of cracks.Energy analysis can explain the changes of specimens during fatigue failure to a certain extent.