## 5 Conclusions  
In this paper, a gradation optimization design for a dry-process CRTAM with a skeleton dense structure was carried out, and the relevant road performance was analyzed. The main conclusions are as follows:  
1. A skeletal intrusion compaction was used to optimize the grading of coarse aggregates and fine aggregates. Through the RCA test and a comparison of the standard grading, it was found that the gradation in the selected state was used as the synthetic gradation of the final skeleton compaction result;
2. When the volume by equal volume was replaced, the low-temperature performance was increasingly better with an increase in the dosage. Considering the economy and specifications, 2% CRT was selected. Marshall tests were carried out on six gradations of 2% CRT by unequal volume replacement and two common intermittent gradations. The results indicated that all of them met the requirement of \(VCA_{mix}\leq VCA_{DLC}\). It can be concluded that these eight gradations had a dense skeleton structure;
3. For high-temperature stability, as an overall trend, DS first increased and then decreased with an increase in the substitution ratio and reached its maximum when the substitution ratio was 1.5. The integrated relative deformation rate introduced the \(K\) value, and when the substitution ratio was 1.5 and 0.5, the performance was relatively good. The water stability of the specimens was significantly improved by adding the CRT, and the water stability under 50 compaction conditions was better than that of the 34 compacted specimens under the same conditions;
4. The low-temperature performance was characterized by the flexural strain energy density, which could simultaneously exhibit the bending strain and the bending strength. The value first increased and then decreased with an increase in the substitution ratio, and the low-temperature performance was best when the substitution value was 0.5;
5. In terms of a comprehensive consideration of the various performances and economic benefits, the best CRT content of the proposed CRTAM was 2%, and the best substitution ratio was 0.5.  
In previous research, crumb rubber activated by TOR has indeed been able to better improve the low-temperature performance of the mixture, and similar results were obtained in this study. Overall, this study successfully characterized an asphalt mixture with CRT and made a composition design of a CRTAM through the dry process. It is anticipated that this study will contribute to the further application of crumb rubber in asphalt pavement. In future research work, it is necessary to verify the durability of CRTAM. Fatigue properties of CRTAM could be further characterized to better understand the performance of the mixture under repeated loading. On the other hand, it can be verified whether the design method is universal for mixtures with other nominal maximum sizes of aggregates.  
Data curation, J.D., W.D., and Z.H.; formal analysis, J.D. and W.D.; methodology, F.M. and Z.F.; resources, F.M.; Writing--original draft, J.D.; Writing--review & editing, Z.F. and J.L. All authors have read and agreed to the published version of the manuscript.  
This project were supported by the National Key R&D Program of China (NO. 2018 YFB1600200) and the Special Fund for Basic Scientific Research of Central Colleges, Chang'an University (NO. 300102318208; 300102219513; 300102218509).  
The authors declare no conflicts of interest.