Research On The Morphology Reconstruction And Mechanical Performance Of Graded Gravel Based On Discrete Element Modeling

(School of Transportation,Southeast University,Jiangsu Nanjing 210096,China)The discrete element method is widely used in the Geotechnical Engineering and Highway Engineering because of simulating the granular materials’ micro contact state precisely.The PFC2D,known as the discrete-element software,is adopted to reconstruct the realistic shape of the stone and to study the graded crushed stone’s mechanical properties.The existing methods fail to simulate precisely.Some of them can’t reconstruct the realistic shape,and the other generate a lot of disks in software which leads a low computation efficiency.The graded crushed stone’s realistic shape has a significant influence on its mechanical properties based on related research.So it is meaningful to reconstruct the stone’s realistic shape and improve the computation efficiency at the same time when conducting the numerical simulation.The main work of the paper includes the following aspects:(1)Firstly,the aggregates were divided by standard sieve and the shape index of different size particle was measured by Aggregate Image Measurement System(AIMS)equipment,including the particle outline image,angularity index,texture index,flat ratio and so on.27 samples were selected then by AIMS based on angularity index to prepare for reconstruction.The shape data distribution was analyzed to study the relations between the morphological characteristics and the particle size.(2)An image processing method was proposed to filter the redundant pixels of aggregate images and rebuilt the digital shape contour of aggregates.By using discrete-element software named as Particle Flow Code in two dimensions(PFC2D),a contour-filling algorithm was developed to model the two-dimensional shape contours of aggregates.By linking the shape to the angularity index for each aggregate,a database was built for numerical simulation of granular aggregates within PFC2D and a random-distribution modeling algorithm was introduced to generate granular aggregate samples according to required shape indices and gradations.(3)It is hard for specimen to reach a compacted state when the numerical particles just fell under gravity.So a boundary-judgment algorithm was developed to simulate the compacted specimen.By judging the left and right boundary of each position,the outer region of coarse aggregate skeleton could be determined.This algorithm can help to generate the fine aggregates fast by distinguishing the irregular and hollowed-out shape of the coarse aggregate skeleton.Based on the micro contact state,the escaping disks due to the high initial speed can be filtered.By analyzing the actual example,the differences between the generated aggregate amount in the numerical experiment and the required goals was obtained to prove the practicability of this method.(4)According to the PFC2D’s principle,two loading algorithms were completed which were controlled by constant stress and constant strain respectively.Based on the density differences and study requirements,the generated number of particles could be calculated fast by the void ratio and also by the dry density.Virtual penetration test was built by PFC2D to study the microscopic parameters’ influence on simulation results.By conducting the virtual penetration test with different material composition,including different single size particles,different gradation,SMA-13,AC-13,the best microscopic parameters could be obtained for coarse aggregates.The advantages of the contour-filling algorithm could be obviously seen compared with the conventional discrete-element method from the point of void ratio,micro contact state and computational efficiency.By conducting the virtual penetration test with different angular particles,the influence on micro contact state due to the different angularity index could be concluded.(5)Based on the contour-filling algorithm and the boundary-judgment algorithm,the numerical optimization design was made to improve the continuous gradation of graded crushed stone.By changing the n value and the biggest particle size,9 numerical specimens were developed for measuring CBR and shear strength.Based on the simulation results,the best value of n and the biggest particle size could be deterimined.