Study On The Aggregate Abrasion Resistance Using Multi-scale Test And Simulation

The composition,particle shape and micromorphology of aggregates are important factors affecting the properties of aggregates and can be named as material genes.These factors also affect their wearing properties and the pavement performance of asphalt mixtures and have a great influence on aggregate service performance.In this study,five types of aggregates,namely quartzite,basalt,limestone,gneiss and red sandstone,are selected for multi-scale tests and simulation to assess their wearing properties from macro to micro scales.Tools including Transmission Electron Microscopy(TEM),Scanning Electron Microscopy(SEM),X-ray Diffraction(XRD).X-CT Scanning(X-CT),Micro-Deval Test,Polished Stone Value Test and Crushing Test have been used in conjunction with molecular dynamics simulation and finite element simulation to achieve the objectives of the research.The main research activities and discoveries are summarized as follows.(1)On the macro-scale,the improved Micro-Deval test is adopted to measure the aggregate abrasion resistance following the American Society for Testing and Materials(ASTM)standards.It was found that the abrasion value-number of revolution relationships of different aggregates follow a power-law relationship,which may provide the basis and reference for the aggregate wear resistance evaluation.Image-Pro Plus software(IPP)was used to analyze the gravel images acquired after the aggregates were subjected to the Micro-Deval testing.The shape,roundness,circumcircle diameter and other information for gravel particles were obtained.The changes of these parameters have revealed the process of aggregate being gradually broken into sands in the Micro-Deval test,which might provide a new method for rapid analysis of coarse aggregate shape characteristics.(2)X-CT scanning technology was used to reconstruct the 3D models of aggregates,a database for the 3D models was built using the Mimics software.These models serve as basis for the discrete element and finite element simulation of aggregate properties.The MATLAB software was used to optimize the profiling of gray values of aggregate,and the watershed transform function was used to realize the automatic segmentation of the aggregate images.The discriminations among different particles were improved.On this basis,the best fitting models of the aggregate 3D surfaces were obtained using the characteristics analysis functions carried on by Geomagic Studio software.Six evaluation indices were presented for the evaluation of aggregate 3D shape,which can give more accurate and comprehensive evaluation.These indices provide the basis for the optimal selection of aggregate.(3)The bulk densities of five aggregates were measured by a CoreLok multifunction aggregate density tester.The elastic modulus and Poisson’s ratio of quartzite aggregate were obtained by uniaxial compression test,which provided data support for the virtual abrasion test of aggregates.The constructed roller,ball,water and air models,and the real 3D model of aggregate were used in the ABAQUS software for virtual abrasion test.The Mohr Coulomb plasticity failure criterion was used to simulate the process of virtual Micro-Deval test.It was shown that it is feasible to use finite element method to simulate the aggregate Micro-Deval abrasion test.The change of aggregate stress,plastic strain and 3D shape was analyzed in the virtual abrasion test process.The virtual abrasion value was found to be in line with the real test results.It is found that repetitive virtual abrasion testing on single influence factor is feasible,which may provide a new way to evaluate aggregate wearing properties.(4)At the micro-scale diffraction data of the five aggregates were obtained using the X-ray diffraction(XRD)technique,and the mineral compositions of the aggregates were analyzed by the JADE 6.0 software.In conjunction with with energy spectrum analysis,the mineral compositions of different aggregates were obtained,which were then used for constructing complex aggregate molecular dynamics models.The difference of aggregate mineral composition is the mainly influencing factor which led to the different wearing properties of aggregates.It was found that higher contents of quartz lead higher wearing resistances of aggregates.The aggregate scanning images with 200X,1 000X,2 000X,5 000X and 20 000X multiples were obtained using the Zeiss Auriga focused ion beam field emission scanning electron microscope,and the microscopic morphology of aggregate samples was analyzed.The aggregate surface fractal dimensions under different magnifications were analyzed using the box counting method.The aggregate surface morphology characteristics were compared before and after Micro-Deval testing.It was found that the fractal dimension of different aggregate surfaces increases first and then decreases as magnification increases.The fractal dimension of red sandstone increases significantly after Micro-Deval test,which indicates that its surface morphology became rougher and its wearing resistance increases significantly.(5)Direct shear tests were performed on the samples of the five different sands.The digital micrometer and its data collection system were adopted in the direct shear test for collecting testing data automatically.It improved the accuracy and efficiency of the test.The direct shear stresses under different pressure conditions for the five different sands and powdered samples were analyzed.The cohesion and the internal friction angle of different sands and powdered samples were obtained.It was found that the interaction among sands and powdered particles also existed in dry conditions.Water had different effects on different aggregates but overall it decreased the interaction among quartz sand particles while increased the cohesion of gneiss sand.The granular program in LAMMPS software was used to simulate the behavior of non-cohesive materials.The natural repose angle and the flow process of sand particles under gravity were simulated.The direct shear test for sand was simulated under the vertical stress loading and with uniform motion of atoms.It was found that the increase of pressure will decrease the friction coefficient and the wearing resistance of sands under micro-scale.(6)Single aggregate molecular models(including SiO2,CaO,Na2O,MgO)were built using the Materials Studio software.According to the XRD analysis result,the limestone model was built with three main mineral composition named dolomite(CaMg(CO3)2),quartz(SiO2),and talc(Mg2SiO4)for molecular dynamics simulation.The periodic boundary conditions,the COMPASS force field and Tersoff potential function,and the NVT ensemble were used to simulate the friction between an Fe ball and an aggregate particle using LAMMPS software.It is found that the wearing resistance of different oxides follows the order SiO2>MgO>Na2O>CaO.The structure of SiO2 is more stable and has the highest wearing resistance.CaO has the least wearing resistance.It is also found that SiO2 has less wearing resistance at a higher temperature.The significant difference in aggregate mineral composition is the essential factor contributing to the differences in aggregate wearing resistance.