## 5 Conclusions  
In this paper, CT was used to scan four kinds of asphalt mixtures before and after freeze-thaw cycles. Based on image processing technology, the number of pores, pore distribution characteristics and pore morphology of test asphalt mixtures were studied and analyzed. The results provide the basis for understanding the performance attenuation mechanism of asphalt mixture and establishing the respective attenuation models after F-T cycles. The main conclusions are as follows:  
* The volume porosity of asphalt mixture is influenced by adding the modifiers NTC and BF to the base asphalt. The volume porosity of BF-modified asphalt mixture is the largest, followed by base matrix asphalt, NTC-BF composite modified asphalt mixture, and NTC- modified asphalt mixture. * According to the longitudinal pore distribution law of asphalt mixtures, the longitudinal pore distribution of NTC-modified asphalt mixture and BF-modified asphalt mixture show the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample Type** & **ALA 1** & **ASA 2** & **AD 3** & **AC 4** & **TASAO 5** & **MAAVIA 6** & **Classification** \\  & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(degree)** & **Coefficient** \\ \hline  & 3.01 & 1.45 & 2.58 & 9.57 & 2.08 & 88.73 & 0.73 \\ Base matrix asphalt mixture & 2.98 & 1.43 & 2.61 & 9.87 & 2.08 & 94.61 & 0.8 \\  & 2.51 & 1.37 & 2.01 & 7.13 & 1.83 & 91.93 & 0.92 \\  & 2.57 & 1.39 & 2.07 & 7.41 & 1.85 & 92.1 & 0.97 \\ \hline  & 2.96 & 1.21 & 1.92 & 6.69 & 2.45 & 90.81 & 0.86 \\ NTC modified asphalt mixture & 2.75 & 1.47 & 2.41 & 9.37 & 1.87 & 94.12 & 0.75 \\  & 2.57 & 1.24 & 2.07 & 7.49 & 2.07 & 93.8 & 0.82 \\  & 2.34 & 1.37 & 2.04 & 7.46 & 1.71 & 90.22 & 0.92 \\ \hline  & 2.43 & 1.18 & 1.68 & 5.81 & 2.06 & 91.17 & 0.97 \\ BF modified asphalt mixture & 2.59 & 1.21 & 1.79 & 6.77 & 2.14 & 89.7 & 0.88 \\  & 2.54 & 1.21 & 1.79 & 7.09 & 2.10 & 89.58 & 0.85 \\  & 2.69 & 1.27 & 1.84 & 7.57 & 2.12 & 92.55 & 0.83 \\ \hline  & 2.78 & 1.21 & 1.81 & 6.66 & 2.30 & 91.07 & 0.90 \\ NTC-BF modified asphalt mixture & 2.51 & 1.16 & 1.87 & 7.19 & 2.16 & 90.91 & 0.85 \\  & 2.37 & 1.19 & 1.78 & 6.53 & 1.99 & 91.33 & 0.88 \\  & 2.51 & 1.14 & 1.74 & 6.91 & 2.20 & 90.66 & 0.83 \\ \hline \hline \end{tabular}  
* Average Long Axis; 2 Average Short Axis; 3 Average Diameter; 4 Average Circumference; 5 Long Axis to Short Axis Ratio; 6 Main Axis and Vertical Inclusion Angle.  
\end{table} 
Table 7: The pore morphology of the test asphalt mixture under F–T cycles. characteristics of pore fluctuation at both ends and stability in the middle, while the pore distributions of the base matrix asphalt mixture and NTC-BF composite modified asphalt mixture are more uniform. * After F-T cycles, the internal structure of BF-modified asphalt mixture rapidly produces macro-pores, which spread and cause significant performance degradation. As for the base matrix asphalt mixture, NTC-modified asphalt mixture and NTC-BF composite modified asphalt mixture, a large number of pores produce in the early and middle stage of F-T, then the number of pores develops slowly as the number of F-T cycles increases. Among the four kinds of asphalt mixtures, the pore characteristics of the base matrix asphalt mixture and NTC-BF composite modified asphalt mixture retain the best stability after F-T cycles. * With increasing F-T cycles, a series of changes take place in the pore number and morphology of the asphalt mixture: the porosity increases, and the moisture entering into the specimen with F-T cycles continuously increases. Furthermore, high temperature water bath melting increases the contact area between the specimen and water which aggravates the loss phenomenon. All of these destroy the stability of the asphalt mixture skeleton, decrease the adhesive force between asphalt and aggregates, and weaken the performance of the asphalt mixtures. In this study, the influence of F-T cycles on the microstructure variation of test asphalt mixtures was intuitively analyzed by CT scanning technology.