## 4 Microstructure Analysis of Asphalt Mixture under F-T Cycles  
### Microstructure of Asphalt Mixture before F-T Cycle  
#### 4.1.1 Pore Number Analysis  
Pore number analysis of the matrix asphalt mixture and the three modified asphalt mixtures under F-T cycles are shown in Table 4. This includes the number of total pores (NTP), large pores (NLP), medium pores (NMP), and small pores (NSP) and their corresponding ratios (i.e., RTP, RLP, RMP, RSP) for 10 sections per sample. Pores with an actual pore area of less than 1 mm\({}^{2}\) are defined as small pores, those with an actual pore area of 1-5 mm\({}^{2}\) are defined as medium pores, and those with an actual pore area \(>\) 5 mm\({}^{2}\) are defined as large pores.  
As listed in Table 4, the average number of pores is 119 for the matrix asphalt mixture. Among them, NSP is 32, NMP is 72 and NLP is 15. The corresponding porosity ratios are 26.89%, 60.50% and 12.60%, respectively, and the average porosity of the matrix asphalt samples is 5.67%. The volume porosity of asphalt mixture is changed by adding NTC and BF. The volume porosity of NTC-modified asphalt mixture is the lowest, followed by NTC-BF composite modified asphalt mixture, and BF-modified asphalt mixture is the largest.  
The total porosity of the three kinds of modified asphalt mixture is larger than that of the base matrix asphalt, especially BF-modified asphalt mixture. Compared with the base matrix asphalt mixture and NTC-modified asphalt mixture, the proportion of small pores in the asphalt mixture modified by BF is higher, while the proportion of small pores and large pores in the asphalt mixture is slightly increased by the addition of NTC, but the effect is not obvious. In addition, according to the porosity distribution of each section of asphalt mixture, the longitudinal porosity distribution of the sample can be roughly inferred, as shown in Figure 11.  
In Figure 11, the longitudinal pore distributions of NTC-modified asphalt mixture and BF-modified asphalt mixture show the characteristics of pore fluctuation at both ends and stability in  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & **NTP** & **NLP** & **NMP** & **NSP** & **RTP (\%)** & **RLP (\%)** & **RMP (\%)** & **RSP (\%)** \\ \hline Base matrix asphalt mixture & 119 & 32 & 72 & 15 & 5.67 & 26.89 & 60.50 & 12.60 \\ NTC modified asphalt mixture & 153 & 46 & 88 & 19 & 5.18 & 30.06 & 57.52 & 12.42 \\ BF modified asphalt mixture & 187 & 65 & 97 & 25 & 6.41 & 34.76 & 51.87 & 13.37 \\ NTC-BF composite modified asphalt mixture & 141 & 51 & 70 & 20 & 5.59 & 36.17 & 49.64 & 14.18 \\ \hline \hline \end{tabular}
\end{table}
Table 4: The porosity of test asphalt mixtures.  
Figure 11: Longitudinal pore distribution map of asphalt mixtures.  
the middle, while the pore distributions of the base matrix asphalt mixture and NTCA-BF composite modified asphalt mixture are more uniform. The non-uniform distribution of the end is closely related to the compaction mode, the non-uniform filler and the performance of the asphalt mixture. The results show that the pore distribution of the base matrix asphalt mixture and NTC-BF composite modified asphalt mixture are more uniform before the F-T cycle.