## 3 Effects of Freeze-Thaw Cycles on Degradation of Performance
### Stability  
The Marshall stability tests were carried out after 0, 5, 10, 15, and 20 freeze-thaw cycles. The test results are shown in Figure 8.  
From Figure 8, it can be seen that the stability strength of each type of asphalt mixture was continuously reduced and the loss ratio caused by damage was continuously increased under freeze-thaw cycles. The stiffness modulus decreased slightly at the early stage of freeze-thaw cycles. After 10 freeze-thaw cycles, the performance degradation rate decreased slightly, but the loss ratio caused by damage development was relatively uniform generally. After 20 freeze-thaw cycles, the loss ratio caused by damage of NBAM group was significantly smaller than AM group. The loss ratio of stability for AM caused by damage under 20 freeze-thaw cycles is 63%, and for NBAM is 46%. The high-temperature performance of AM decreased more significantly under freeze-thaw cycles, when added nano-TiO\({}_{2}\)/CaCO\({}_{3}\) and basalt fiber, the stability was obviously improved, and the stability loss rate was significantly reduced. The NBAM behaved an initial stability improvement of 17% over AM and NBAM increased the stability of 67% than the AM after 20 freeze-thaw cycles. The freeze-thaw resistance of NBAM was significantly better than that of AM. From the results of mesoscopic voidage in Figure 8, as far as the whole trend was concerned, the voidage gradually increased and it increased faster at the early stage of freeze-thaw cycles. The stability gradually decreased and it decreased faster at the early stage of freeze-thaw cycles. The variation of pores was obviously related to the decline of pavement performance. Asphal mixture freeze-thaw damage could be divided into two stages:  
Figure 8: Stability changes: (**a**) Relationship between stability and freeze-thaw cycles; (**b**) relationship between stability damage and freeze-thaw cycles. Figure 7: Variation of porosity of asphalt mixtures under freeze-thaw cycles. In the initial stages of freeze-thaw cycles (i.e., 0-5 cycles), the voidage increased gradually. This is because open pore water froze and expanded, which resulted in cracks in asphalt mixture. With the increase of freeze-thaw cycles, the mesoscopic cracks gradually expanded and changed into macro cracks, the voidage increased rapidly. At this stage, due to the internal cracks in asphalt mixture, the strength of asphalt mixture decreased rapidly, but the decline degree was more related to asphalt mixture types. Since the integrity of the structure was mainly provided by the bonding of the binder between aggregates, the strength of asphalt mixture decreased rapidly in the first stage after asphalt mixture cracked at the mesoscopic level. The reduction at this stage has a great relationship with asphalt mixture types. After adding nano-TiO\({}_{2}\)/CaCO\({}_{3}\), the viscosity of the asphalt binder increased, which slowed down the cracking under the force bring by the expansion of water when it freezes. When basalt fiber was added, basalt fiber was disordered in asphalt mixture. When the crack direction was perpendicular or oblique to the fiber direction, basalt fiber played as anchor, which was similar to the reinforcement that steel provided for concrete, preventing further expansion of the crack. The uniaxial static and dynamic creep results showed that the high-temperature stability of nano-TiO\({}_{2}\)/CaCO\({}_{3}\) and basalt fiber composite modified asphalt mixtures after freeze-thaw was significantly improved compared with other test groups. 2. In the second stage of freeze-thaw cycles (i.e., 5-15 cycles), the internal pores gradually became connected when the opening voidage increased to a certain degree, which made it possible to effectively disperse part of the frost heaving force during the freezing process. Therefore, the opening pore growth slowed down and the volume change caused by water decreased. At this stage, the water gradually began to erode the interface between asphalt and aggregate.