## 5 Conclusions  
In this study, the moisture susceptibility and low-temperature stability of lignin fiber (and/or) glass fiber modified asphalt mixtures were experimentally evaluated. The results  
Figure 7: Results of cracking resistance in the low-temperature test. (**a**) The tensile strength of asphalt mixtures. (**b**) The tensile strain of asphalt mixtures.  
indicated that adding either lignin or glass fiber or a composite mixture of both to asphalt mixes can improve resistance to water damage and low-temperature cracking, hence providing better pavement properties as compared to ordinary pavement. Both lignin fiber and glass fiber were added at a percentage of 0.3% to the AC-16 mixture. Based on the results from the Marshall immersion test, freeze-thaw splitting, and low-temperature bending tests, which were applied to assess the behavior of the studied asphalt mixtures before and after adding fibers, the main conclusions are as follows:  
* The optimum bitumen content of the mixtures obtained from the Marshall stability test increased by adding lignin and glass fibers because the fiber has a high absorption of bitumen due to its large specific surface area.
* The \(MSR\), \(TSR\), maximum tensile strength, and strain of asphalt mixtures were enhanced by the addition of lignin and glass fibers. Lignin and glass fibers significantly improved the moisture susceptibility and low-temperature resistance.
* Lignin fiber had a greater effect on the bending strength and ultimate bending strain compared to glass fiber due to its large specific area and absorption effect; however, the influence of glass fiber on improving the water stability performance was more significant than that of lignin fiber.
* The asphalt mixture reinforced with composite admixture showed significant improvements with respect to moisture susceptibility and low-temperature stability as compared to the other mixtures.  
In summary, pavement construction using composite technology will have a good impact on enhancing the service life and the overall performance of asphalt mixes as compared to the usage of single additives.  
Conceptualization and experimental design: A.K. and M.A.; experiments, validation, and formal analysis: A.K. and M.A.; project administration: D.L.; writing--original draft preparation: A.K. and M.A.; writing--review and editing: A.K., M.A., and M.G.; supervision: D.L., Y.Y., M.G., and Y.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Post-Doctoral Science Foundation (No. 2019M653645), the National Natural Science Foundation of China (52078418), Project of strategic planning department of ministry of science and technology (HYJC2019 FG/072 HZ), Science and technology project of Yulin (CityCXY-2020-046), Sinohydro Bureau 11 Co., Ltd. (20201225), the Central University's Special Research Fund Interdisciplinary Project (vjj2017175), and the Research Fund Project of Xi'an JiaoTong University (1191320036).  
A financial support from CSC (Chinese Scholarship Council) is gratefully acknowledged.  
The authors declare that they have no conflict of interest.  
The following abbreviations were used in this manuscript:  
C Control asphalt mix  
L Lignin fiber modified asphalt mix  
G Glass fiber modified asphalt mix  
LG Composite of lignin fiber and glass fiber modified asphalt mix  
TSR Tensile strength ratio  
SMA Stone matrix asphalt  
FRAC Fiber-reinforced asphalt concrete  
PI Penetration index  
TTOT Thin film oven test  
O.A.C Optimum asphalt content  
\(MSR\) Ratio of residual stability  
\(MS_{2}\) Marshall stability after 48 h of water immersion