## 5 Conclusions  
Based on the DIP, in this study, the gradual meso-structural response behaviour of the asphalt mixture main skeleton subjected to load is investigated; moreover, the design criteria of mesoscopic evaluation indices for the main skeleton are proposed in this paper. The main conclusions are as follows:  
(1) Aggregates with particle sizes of 2.36-4.75 mm can effectively increase the total quantity of contact points, but the stability of the main skeleton is insufficient. Coarse aggregates with a particle size larger than 4.75 mm provide the most significant contribution to the formation of a steady main skeleton; hence, it is the crucial particle size for the formation of such skeletons. Increasing the content of coarse aggregates with particle sizes larger than 4.75 mm in the gradation design process aids to stabilise the formation of the main skeleton, and the use of asphalt with a higher PG grade can effectively restrain the movement of coarse aggregates and improve the rutting resistance of the asphalt mixture.  
(2) For the mixture with a nominal maximum particle size of 13.2 mm, the maximum and minimum coordination numbers of coarse aggregates are 5 and 0, respectively. The coordination number peak position gradually moves to the direction of increase under load action; the peak coordination number is 2 when the main skeleton has exerted its maximum capacity to resist the load and has approached the critical value. The Gauss regression model can well describe the relationship between the coordination number and the corresponding amount of coarse aggregates.  
(3) Gradation is the major determinant of the mesoscopic evaluation indices, \(\overline{n}_{c}\) and C value, for the asphalt mixture main skeleton. The performance of asphalt has an insignificant influence on the mesoscopic evaluation indices; it mainly affects the development trend of macroscopic rutting. In the design process of DSAM, it is necessary to optimise the gradation with the aim of increasing the coordination number and reducing the C value so as to enhance the load resistance capacity of the main skeleton. In the preparation of asphalt mixture specimens using the wheel rolling method, the design criteria of mesoscopic evaluation indices for the main skeleton are \(\overline{n}_{c}\)\(\geq\)1.5 and C \(\leq\) 15%, which can be used as reference for the design of DSAM with a nominal maximum particle size of 13.2 mm.  
(4) The conclusions of this study, particularly the design criteria of mesoscopic evaluation indices for the main skeleton, are obtained from limited laboratory tests. Consequently, the conclusions have limitations; nevertheless, better reference values have been achieved for the research concepts. More of the factors that affect the main skeleton structure should be considered, and various asphalt mixtures should be analysed in a follow-up study. The mesoscopic evaluation indices, test criteria, and design criteria of the main skeleton should be demonstrated further and optimised based on large amounts of indoor and outdoor test data to better guide the gradation design of DSAM.  
Conceptualization: [PERSON], [PERSON] and D.Y.W.; data curation: [PERSON], [PERSON], X.Q., and X.X.; funding acquisition: [PERSON] and [PERSON]; investigation: [PERSON], X.Q., X.X., and M.K.J.; methodology: D.Y.W. and L.W.S.; project administration: [PERSON] and [PERSON]; resources: [PERSON]; writing--original draft: [PERSON]; writing--review and editing: [PERSON] and [PERSON].  
The research was funded by the National Natural Science Foundation of China, grant number 51278203.  
The authors declare no conflict of interest.