## 1 Introduction
[PERSON] [10] established the interference coefficient of the particle gradation evaluation index based on the particle packing theory, obtained the relationship between the spatial distribution of voids and particle contact in the asphalt mixture, and studied the influence of gradation on the meso-structure of the asphalt mixture. A discrete element model of an asphalt mixture with different coarse aggregate morphologies and particle sizes has been established using the Partical Flow Code in Two Dimensions (PFC 2D) software. The virtual test results of the aggregate skeleton mesoscopic analysis show that the discrete element method (DEM) can rapidly and accurately predict the stress-strain characteristics of the coarse aggregate; the results are consistent with those obtained from laboratory tests [11]. A three-dimensional DEM model has also been established using the Particle Flow Code in Three Dimensions (PFC 3D) software to track the rutting deformation of the asphalt mixture. From a mesoscopic mechanical view, the aggregate shape has a significant influence on the horizontal, vertical, and rotational movement characteristics of the asphalt mixture. There is good interlocking at the contact points among aggregates that restricts the movement of coarse aggregates, and there is sufficient contact within the aggregate skeleton to bear the contact force. The foregoing is important to guarantee the rutting resistance of an asphalt mixture [12; 13]. Meanwhile, research findings show that there is a strong correlation between the internal skeleton structure indices of contact points, the aggregate size, the inclination angle,and the rutting performance of the asphalt mixture [14; 15; 16; 17]. In relation to the voids in the mineral aggregate (VMA), the aggregate skeleton of the asphalt mixture is a significant factor that affects critical asphalt mixture properties, such as durability, workability, permeability, rutting resistance, and cracking resistance. An analytical approach for estimating changes in the VMA because of gradation variation and determining the relevant aggregate skeleton characteristics of asphalt mixtures is the linear-mixture packing model, which is an analytical packing model that considers the mechanisms of particle packing, filling, and occupation [18]. Furthermore, a skeleton penetration test is developed to evaluate the skeleton strength of porous asphalt mixtures; the results show that skeleton strength correlates well with the rutting resistance [19]. The investigations on the skeleton structure from a mesoscopic perspective mainly include the statistics and analysis of the main skeleton's meso-structural characteristics, the motion tracking of coarse aggregates subjected to load, and the numerical simulation and validation of the skeleton structure. However, based on the DIP, the gradual meso-structural response behaviour of characteristics of the asphalt mixture main skeleton subjected to load has not been quantitatively analysed, and quantitative mesoscopic evaluation indices and criteria for the main skeleton have not been established. Based on the above background and through the design of unique experiments, this study applies the DIP to analyse the aforementioned gradual meso-structural response behaviour from a mesoscopic perspective. According to the evaluation of the meso-performance of the designed main skeleton of the asphalt mixture, the gradation optimisation measures and experimental criteria for the quantitative mesoscopic evaluation indices for the main skeleton are proposed. It is anticipated that these can be used as references for the design and gradation optimisation of DSAM.