## 4 Results and Discussion
### Effect of Moisture on Aggregate-Bitumen Interface Shear Strength  
In the spring-thaw season, after the asphalt film breaks, the aggregate-bitumen interface will be exposed to the moisture environment and the penetration speed of the moisture to the aggregate-bitumen interface will increase exponentially compared with the diffusion speed through the asphalt film. In order to evaluate the variation of the shear strength of the aggregate-bitumen interface that is directly exposed to the moisture environment, the dry test shear sample was completely immersed in a water bath at 10 \({}^{\circ}\)C for 4, 8, 24, and 48 h and then taken out for testing. The test results include the average aggregate-bitumen interface shear strength and coefficient of variation (CV). The formula for CV is as follows.  
\[\text{CV}=\frac{\sigma}{\mu},\]  
Figure 9: The changes of aggregate-bitumen interface shear strength of BA and NHSSMA with freeze-thaw cycles under different conditions. (**a**) The 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of BA and NHSSMA under different freeze-thaw cycles. (**b**) The 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of BA and NHSSMA with snow-melting agent under different freeze-thaw cycles. (**c**) The \(-\)10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of BA and NHSSMA under different freeze-thaw cycles. (**d**) The \(-\)10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of BA and NHSSMA with snow-melting agent under different freeze-thaw cycles.  
where \(\sigma\) is data standard deviation and \(\mu\) is data average. The test results are given in Figure 10. It can be seen from Figure 10 that 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of BA and NHSSMA gradually decrease with increasing immersion time, and the shear strength decreased significantly in the early stage of water immersion. After 4 h of immersion, 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of NHSSMA and BA are greatly reduced, with a decrease of 43.1% and 48.3%, respectively. After that, 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of modified and unmodified asphalt decreases slowly as the immersion time increases. After 48 h of immersion, 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of NHSSMA and BA are reduced by 51% and 55.4%, respectively. This indicates that when the asphalt film has a large range of rupture, the moisture can quickly penetrate into the aggregate-bitumen interface, which has a significant effect on the aggregate-bitumen interface shear strength. Moreover, whether it is strength value or the descend range, NHSSMA is better than BA at different immersion times, which indicates that the addition of nano hydrophobic silane silica improves the ability of asphalt binder to resist moisture damage.  
As seen in Figure 10, CV of test results for modified and unmodified asphalt gradually increase with the immersion time, and CV of test results for the two types of asphalt binders is smallest when the immersion time is zero. This indicates that the aggregate-bitumen interface shear strength test can obtain more accurate data in the dry environment, and the longer the immersion time, the greater the discreteness of the test data. CV of all test results measured in this paper is within 8.4%, which proves that the aggregate-bitumen interface shear strength test has good repeatability.