## 2 Materials and Methods
### Macroscopic Properties during Sodium Chloride Action  
Figure 7 depicts the electrical resistivity of specimens during NaCl freeze-thaw cycles. According to Figure 7, the electrical resistivity decreases within the first 100 freeze-thaw cycles. The increase in NaCl concentration in pore solution in response to increasing freeze-thaw cycles may explain this improvement. However, the electrical resistivity increased as the number of freeze-thaw cycles increased from 100 to 300 cycles, owing to the formation of additional interior cracks following the NaCl freeze-thaw cycles [43]. Figure 7 shows that the longer the CO\({}_{2}\)-curing time, the lower the growing rate of electrical resistivity becomes. This is due to the fact that after CO\({}_{2}\) curing, the steel bars reinforced cement mortar's NaCl freeze-thaw corrosion resistance improves [44]. Figure 8 depicts the variation rate of ultrasonic velocity of specimens during NaCl freeze-thaw cycles. As shown in Figure 8, the ultrasonic velocity increases with the freeze-thaw cycles ranging from 0 to 50. It is believed that the cement's continuous hydration leads to the increase in compactness of cement mortar [45]. However, with the freeze-thaw cycles increasing from 50 to 300, the ultrasonic velocity decreases obviously. The further NaCl freeze-thaw cycles may induce the corrosion of steel bars and yield the inner cracks of cement mortar [46]. Besides, the frozen-thaw stress resultant from NaCl freeze-thaw cycles may also extend the propagation of internal cracks in cement mortar, hindering the transmission of ultrasound in concrete [47]. Therefore, the ultrasonic velocity of specimens decreases obviously with the freeze-thaw cycles increasing from 50 to 300. Figure 6 illustrates that when the CO\({}_{2}\)-curing age increases, the lowering rate of ultrasonic velocity decreases due to the better compactness of the higher CO\({}_{2}\)-curing age. Moreover, the increasing w/c increases the variation rate of ultrasonic velocity. This can be attributed to the increased compactness of cement mortar with the decreasing w/c. Figure 9 shows the mass-loss rate of specimens during NaCl freeze-thaw cycles. Figure 9 depicts how the mass grows as the number of freeze-thaw cycles. The continuous hydration of cement mortar during freeze-thaw cycles may be responsible for this increase. The continuous hydration of cement mortar increases the mass of hydration products. However, with freeze-thaw cycles increasing from 50 to 300, the mass of cement mortar decreases. The frozen-heave stress and reinforcement corrosion caused by NaCl freeze-thaw cycles may increase specimens' spalling and thus reduce the mass of cement mortar [48]. The mass-loss rate decreases at a longer CO\({}_{2}\)-curing age, due to the improved compaction of hydration products [49]. Moreover, specimens with a w/c of 0.3 display the lowest mass-loss rate. This is because a lower w/c may improve the compactness of cement mortar. However, the mass-loss rate of specimens with a w/c of 0.5 is lower than  
Figure 7: The electrical resistivity of specimens during NaCl freeze–thaw cycles. the specimens with a w/c of 0.4. This is because the higher carbonation degree of cement mortar with a w/c of 0.5 leads to higher compactness and less mass loss. The mass loss rate of steel bars through NaCl freeze-thaw cycles is shown in Figure 10. As illustrated in Figure 10, the mass-loss rate of steel bars increases with the freeze-thaw cycles. This is due to the fact that NaCl freeze-thaw cycles hasten the corrosion of steel bars, resulting in an increase in rust mass. However, the mass loss rate of steel bars increases with a higher w/c because the electrical conduction is improved with the increasing w/c [50]. Moreover, the mass-loss rate of steel bars decreases with the longer CO\({}_{2}\)-curing time due to the improved corrosion resistance.