Mesoscopic Evaluation Of The Balance Design Method Of Asphalt Mixture

The balance design method of asphalt mixtures is a new design method proposed in recent years to cope with the complex and variable requirements of asphalt pavement.It focuses on the tightness and density of the asphalt mixture characterized by macro design parameters,such as the percent voids in coarse mineral aggregate in bituminous mixtures(VCAmix),the percent voids in mineral aggregate that are filled with asphalt in bituminous mixtures(VFA)and the percent air voids in bituminous mixtures(VV).But these parameters are difficult to reflect the internal state of the asphalt mixture,so it is necessary to use the meso-parameters for description.Therefore,a detailed study on the relationship between mesostructure and macro-performance is needed.Based on the characteristics of the volume parameter of the Marshall specimen,determine the coarse aggregate content through the balance design method,and then adjust the different ratios of the aggregates of 4.75～9.5mm and 9.5～13.2mm(4:1,3:1 and 2:1)and use the rutting test,water-immersed Marshall test and low-temperature bending test.The test results show that the best content of coarse aggregate is 70% and the best ratio of13.2～9.5mm aggregate to 9.5～4.75 mm aggregate used in the asphalt mixture is 3:1.The digital image processing technology is used to identify the three-phase material components separately,carry out three-dimensional modeling and extract the required mesostructure parameters.Among the parameters,the distribution of the two-dimensional voids of the asphalt mixture is characterized by the cross-sectional void ratio and the number of cross-sectional voids whereas the distribution of the three-dimensional voids of the asphalt mixture is characterized by the gradation of voids.As the depth of the asphalt mixture increases,the voids first decrease,then slightly fluctuate in a small range,and finally increase,which conforms to the "bathtub" distribution.As the aggregate gradation changes from coarse to fine,the number of voids in the section gradually increases and the void gradation becomes diminutive.The thickness spectrum of asphalt mortar is used to evaluate the asphalt mortar thickness,and the log-normal distribution model is used for fitting.The results show that the fitting effect achieved the desired performance.As the gradation changes from coarse to thin,the fitting curve becomes thicker,the peak value decreases and the peak value shifts to the right,the expected value of the asphalt mortar thickness spectrum increases,and the mode is almost the same.The state of the coarse aggregate is mainly evaluated by shape characteristics,contact properties,uniform properties and compaction effect.Due to the same batch of aggregates used,the shape characteristics are the same.The contact in asphalt mixture is composed of point contact and surface contact,which could form interlock strength.The asphalt mixture with13.2～9.5mm aggregate and 9.5～4.75 mm aggregate at a ratio of 3:1 had more uniform distributions of voids and aggregates with better compaction effect.Through the gray relational analysis,the angularity,the surface texture characteristics,and the main axis concentration of the aggregate are strongly correlated with the road performance of the asphalt mixture.Meanwhile,the VCAmix and the VFA are strongly correlated with the Meso parameters of the asphalt mixture.Therefore,it is necessary to pay attention to controlling the VCAmix and the VFA to improve the stability of the asphalt mixture and enhance its road performance.the gray correlation analysis method is used to analyze the correlation between the microscopic parameters and the road performance of the asphalt mixture.The results showed that angularity,the surface texture characteristics,and the main axis concentration of the aggregate have the highest correlation with road performance.Meanwhile,the correlation between the volume parameters of the asphalt mixture and these three meso parameters is analyzed,and the results show that the VCAmix and the VFA have the highest correlation with meso parameters.Based on these research results,it is recommended to control the VCAmix and the VFA to effectively improve the comprehensive performance of asphalt mixture and enhance its road performance.