## 13 Conclusions  
In this paper, the internal meso-structure images of AC-13, AC-20, AC-25 and PAC-13 asphalt mixtures were obtained by X-ray CT scanning technology, and the finite element simulation models considering heterogeneous meso-structure of mixtures were established by using digital image processing technology and mapped meshing method. According to the thermal properties of aggregate and asphalt, combined with the composition of asphalt mortar, the thermal parameters of asphalt mortar were calculated, and the temperature field variations and influencing factors of laboratory cylinder specimens during cooling and heating processes were investigated. The following conclusions can be obtained:  
1. There is little difference between temperatures of mortar, aggregate and disconnected voids in asphalt mixtures during both laboratory cooling and heating processes. Therefore, for dense-graded asphalt mixtures with mostly disconnected voids, the mixtures can be regarded as homogeneous materials with uniform thermal properties during thermal simulations, without distinguishing the specific components. However, for PAC mixtures with large contents of internally connected voids, the influence of the mixture meso-structure should be considered during thermal analysis.
2. In the processes of temperature variation, temperatures in different regions of a specimen are closer in AC mixtures than PAC mixture, whether near the center or near the edge of the specimen, because there is less void content in dense-graded asphalt mixture, and the heat transfer through the solid is fast and uniform; for PAC mixtures with gap-gradation, there are a lot of connected voids in the interior, which make heat resistances between the solids and results in uneven heat transfer inside the specimen. Therefore, in the same ambient environment, the temperature difference between the surface and center of the PAC specimen is relatively larger than that of the AC specimen, and the temperature and temperature accumulation value of each point in the interior is relatively more discrete.
3. By importance analysis of influencing factors, it can be found that the effect of mixture gradation is much greater than the mixture component and specimen regions. This is also mainly caused by the differences of void contents and void distributions between gapped-gradation and dense-graded mixtures.  
In summary, the large content of internally connected voids in porous asphalt concrete leads the great thermal property, which is different from dense-graded asphalt concrete. Thus, in thermal analysis, the heterogeneous meso-structure of porous asphalt concrete cannot be ignored; in contrast, it should be fully considered. Furthermore, when it comes to investigation of the rutting potential or aging levels of certain pavement structures with PAC, it is necessary to take the special thermal properties of the porous materials into account.  
**Author Contributions:** Conceptualization, J.J.; Data curation, Y.Z.; Formal analysis, Y.D. and L.Z.; Funding acquisition, Y.Z.; Investigation, Y.Z.; Methodology, J.J. and Y.D.; Resources, F.N.; Supervision, F.N.; Validation, Y.D. and L.Z.; Writing--original draft, Y.Z. and L.Z.; Writing--review and editing, J.J. and F.N. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Natural Science Foundation of China (NO. 51808115), the Cyan and Blue Talent Training Project of the Colleges and Universities in Jiangsu Province, the Science Foundation of Nanjing Vocational Institute of Transport Technology (Project No. JZ1802) and the 333 High-Level Talent Training Project of Jiangsu Province.  
**Conflicts of Interest:** The authors declare that there are no conflict of interest regarding the publication of this paper.