Digitalized Classification Of Coarse Aggregate In Asphalt Mixture And Its Application

In the asphalt mixture,the coarse aggregates occupy a considerable proportion of the volume,have strong graininess,and their morphological characteristics are quite different,resulting in greater variability in material selection and mixture design.Obviously,it has great practical significance to reasonably distinguish coarse aggregates with complex morphological features,which could be used to reduce the variability of engineering design and improve the road performances of asphalt mixtures.In recent years,for the characterization of coarse aggregates,lots of research work through image-based processing,laboratory tests,and numerical-simulation calculation have been carried out by researchers at home and abroad.Many calculation and analysis methods have been proposed and used to evaluate the shape,angularity,texture,fractured face,and the content of flat and elongated particles,etc.In the processes of mixture design,such as Superpave and Marshall,the evaluation indexes and testing methods for morphological characteristics of coarse aggregates have been also put forward.In addition,with the gradually matured development of modern computer,image-based,3D scanning and printing,wireless sensing,and virtual-simulation technologies,some researchers have begun to explore the digital and intelligent issues of coarse aggregates.However,regardless of the inheritance and development of the traditional mixture design methods,or the integration and innovation of modern science and technologies,the problem to be solved urgently is how to rationally classify and accurately recognize the morphological characteristics of coarse aggregates? Only by solving this problem,the coarse aggregates could become the measurable,controllable,and designable parameters.Therefore,for the coarse aggregates,the morphological characteristics were taken as the main object and shapes were regarded as entry point to proposed a “five types-four parameters-five degrees” classification approach,and the corresponding studies on performances of asphalt mixtures were carried out.The main work has been done to achieve the objectives:(1)Develop a new digitalized classification system of coarse aggregates with the combination of morphological features,laboratory tests,evaluation parameters,and image-based technologies.(2)Collect and record the information of coarse aggregates with different places of origin,lithologies,sieve sizes,shape types,physical and mechanical properties.(3)Verify the feasibility and accuracy of 3D scanner and make recommendation on how to choose an appropriate image technology through a comparative study of the optical scanning and X-ray CT results.(4)Build the digital library through X-ray CT and 3D scanner and propose a coding system to name each data in the library.(5)Develop the virtual-simulation SGC(Superpave Gyration Compactor)experiment based on the utilization of PFC5.0(Particle Flow Code Version 5)and digital library.(6)Verify the feasibility and accuracy of the virtual-simulation SGC experiment through tracking particles with different shapes through the comparison between testing and simulated results of asphalt mixtures.(7)Analyze the impacts of different aggregate shapes on asphalt mixture compaction through laboratory tests and numerical simulations.(8)Explore the movement characteristics of coarse aggregates during asphalt mixture compaction through tracking particles with different shapes.(9)Prefabricate cubic and spherical particles(angular and rounded coarse aggregates)with the specific colors,sizes,and shapes.(10)Verify the feasibility of concrete particles through the comparison on DCT(Disk-shaped Compact Tension)test results with the concrete and realistic particles testing groups.(11)Investigate the impacts of particles sizes and shapes on asphalt mechanical performances with cubic or spherical particles based on Hamburg wheel track,Disk-shaped compact tension,and four-point bending beam fatigue tests.This study adds new content to the traditional research on coarse aggregate morphologies in highway engineering through the study on digitalized classification of coarse aggregate and its application in asphalt mixture.The application of digital library in discrete element numerical simulation could make the virtual specimen become closer to the testing specimen and it could be used to replace the traditional method of sample forming in virtual-simulation experiments.This lays a foundation to narrow the test range,shorten the test period,improve the test efficiency,and save costs of the mixture design based on trial and error method.At the same time,the creation of digital library of coarse aggregate and the development of virtualsimulation experiment of asphalt mixture make the coarse aggregate as a computable,analyzable,and designable test parameter for the further study of the meso-structure analysis of asphalt mixture and provide the data support to the proportion design of coarse aggregate morphologies.Based on the comparison,laboratory tests,and numerical simulations in this study,it was found that:(1)For the classification of coarse aggregates,visual inspection or measurement were used in conventional methods.Because of the susceptibly to human factors,some random errors easily appeared in these methods.In this study,with the combination of laboratory test,parameter calculation,and image-based technology,the “five types-four parameters-five degrees” classification approach was developed to categorize morphological features of coarse aggregate quickly and accurately.The proposed classification approach realized the analysis of coarse aggregate in three-dimensional scale and provided basis for digital library building of coarse aggregates.(2)Based on the classification approach,digital building system with “sample selection-particle classification-information record-image acquisition-data codingdata call” was proposed,and the digital library of coarse aggregates was built in this study.The digital library could be utilized to both characterize the features of particles and provide data for asphalt mixture analysis in numerical simulation.Additionally,the developed coding system of digital library could improve the library management effectively and made its application conveniently.Therefore,digitalized classification could not only realize the digitalization and information of coarse aggregate classification,but also could contribute to the development of morphological characterization of coarse aggregate and numerical simulation of asphalt mixture.(3)The feasibility of 3D optical scanner used in image acquisition was verified with the comparison of X-ray CT.With advantages such as economy,safety,portability,and energy conservation,3D optical scanner was recommended to obtain the information of coarse aggregates under normal conditions.The use of X-ray CT was recommended,when some special aggregates(flat or elongated aggregates)or the aggregate particles with sharp angles or rich surface features would induce larger differences.The optimized selection of the image technology could not only save costs,but also facilitated the increase of speed and accuracy of digital library building.(4)Based on the discrete element method and digital library of coarse aggregates,the virtual-simulation SGC experiment of asphalt mixture was developed.It turned out to be feasible and accurate,through the comparison between testing and simulated results of asphalt mixture base on the classification approach and the digital library.The application of digital library in DEM(Discrete Element Method)simulation made the numerical model be closer to the real testing situation and could be used to analyze effects of coarse shape on asphalt mixture compaction.It made a significant contribution to analyzing on asphalt mixture compaction and mechanical performances.(5)Different types of aggregate shapes significantly impacted mixture compaction.In the SGC testing processing,comparison on the results of the asphalt mixtures with hybrid,100%angular,and 100%fractured coarse aggregates,the rounded,elongated,and flat coarse aggregates had adverse impact on compaction.The influences of flat coarse aggregates were greater than that of elongated ones on asphalt mixtures under a certain degree of asphalt mixture compaction.It was also found that the larger percentage of elongated and flat aggregates,with a 3:1 ratio,resulted in worse compactability of asphalt mixtures under laboratory conditions.Special attention should be paid to asphalt mixtures with a large percentage of elongated or flat coarse aggregates.(6)Movement characteristics of coarse aggregates in SGC test could be divided into three main stages;the ratio of vertical displacement was obviously larger than that of horizontal displacement,on the contrary,the variation of vertical rotation was obviously smaller than that of horizontal rotation in asphalt mixtures with differently-shaped compositions;the rounded,elongated,and flat coarse aggregates had greater influences on horizontal displacement compared with vertical displacement,but had adverse effects on particle rotations during the compaction process;the effects of elongated coarse aggregates on particle movement were larger than those of flat coarse aggregates,while the flat coarse aggregates had more influences on particle rotation for the variations of horizontal and vertical rotational angles.(7)Through the comparison between the DCT results of asphalt mixtures with concrete particles and mineral aggregates,the results of concrete particles were corrected and using concrete particles to replace mineral aggregates for analyzing impacts of coarse particles on asphalt mixture performances was feasible.Prefabricated particles could provide some reference to analyze the effects of realistic aggregate morphology on asphalt mixture mechanical performances.(8)According to comprehensive testing results of asphalt mixture with different type combination proportions,for the coarse aggregates,the larger size range was recommended to use in asphalt mixtures with satisfied performances of rutting resistance,anti-stripping resistance,fatigue cracking resistance,and fracture resistance.Special attention should be also paid to the coarse aggregate with large proportion of spherical particles(rounded shapes).