## 4 Microstructure Analysis of Asphalt Mixture under F-T Cycles
### Microstructure of Asphal Mixture after F-T Test
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & **F-T Cycles** & **TTN 1** & **LFN 2** & **MPN 3** & **SPN 4** & **LFR 5** (**x**) & **MPR 6** (**x**) & **SPR 7** (**x**) \\ \hline \multirow{3}{*}{Base matrix asphalt mixture} & 0 & 119 & 32 & 72 & 15 & 26.89 & 60.50 & 12.60 \\  & 5 & 112 & 29 & 65 & 18 & 25.89 & 58.04 & 16.07 \\  & 10 & 133 & 49 & 71 & 13 & 36.84 & 53.38 & 9.77 \\  & 15 & 135 & 51 & 75 & 9 & 37.78 & 55.56 & 6.67 \\ \hline \multirow{3}{*}{NTC modified asphalt mixture} & 0 & 153 & 46 & 88 & 19 & 30.06 & 57.52 & 12.42 \\  & 5 & 121 & 43 & 69 & 9 & 35.54 & 57.02 & 7.44 \\  & 10 & 168 & 74 & 82 & 12 & 44.05 & 48.81 & 7.14 \\  & 15 & 149 & 64 & 78 & 7 & 42.95 & 52.35 & 4.70 \\ \hline \multirow{3}{*}{BF modified asphalt mixture} & 0 & 187 & 65 & 97 & 25 & 34.76 & 51.87 & 13.37 \\  & 5 & 217 & 79 & 115 & 23 & 36.40 & 53.00 & 10.60 \\  & 10 & 223 & 84 & 109 & 30 & 37.67 & 48.88 & 13.45 \\  & 15 & 221 & 81 & 110 & 30 & 36.65 & 49.77 & 13.57 \\ \hline \multirow{3}{*}{NTC-BF composite modified asphalt mixture} & 0 & 141 & 51 & 70 & 20 & 36.17 & 49.64 & 14.18 \\  & 5 & 199 & 83 & 98 & 18 & 41.71 & 49.25 & 9.04 \\ \cline{1-1}  & 10 & 171 & 75 & 85 & 11 & 43.86 & 49.71 & 6.43 \\ \cline{1-1}  & 15 & 207 & 88 & 103 & 16 & 42.51 & 49.76 & 7.73 \\ \hline \hline \end{tabular}  
* Total Pore Number; 2 Large Pore Number; 3 Middle Pore Number; 4 Small Pore Number; 5 Large Pore Ratio;
* Middle Pore Ratio; 7 Small Pore Ratio.  
\end{table} 
Table 6: The void numbers of asphalt mixtures under F-T cycles. Figure 12: Variation of porosity after test F–T cycles of test asphalt mixture. Unlike the rapid increase of the number of small pores under F-T cycles, the variation regularity of the number of micro-pores and macro-pores is not obvious, which indirectly reflects the change of the internal pores of asphalt mixture under F-T cycles. There are two main reasons for the increase of the overall porosity of asphalt mixture. One is that a large number of micro-pores are produced in the sample due to F-T cycles, which makes the overall porosity increase. The other is that the freezing-thawing process makes the pores of the sample develop, which enlarges the pore area, and the transition from micro-pores to macro-pores leads to the increase in the number of macro-pores. At the same time, there are large pores forming though, resulting in the reduction of the number, and these two effects exist simultaneously. For example, matrix, nano and basalt fiber composite asphalt mixtures increase significantly with the number of small pores in the F-T cycle, resulting in a significant increase in porosity, while fiber-reinforced asphalt mixtures are mainly due to the development of internal pores in the F-T cycle resulting in an overall increase in porosity. Among the four kinds of asphalt mixtures, the internal structure of BF modified asphalt mixture rapidly produces macro-pores under F-T cycles, and the propagations lead to significant performance degradation. The internal structure of the four kinds of asphalt mixtures produce a large number of micro-pores in the early and middle stages of the F-T cycles, and the number of pores gradually develops with F-T cycles. When the number of F-T cycles was 15, the macro-pores were connected, and the pore characteristics also changed. According to the pore number and morphological characteristics under 0 to 15 F-T cycles, the change in amplitude of the pore characteristics of the base matrix asphalt mixture and NTC-BF composite modified asphalt mixture are the most stable.