## 4 Results and Discussion
### Effect of Freeze-Thaw Cycles on Aggregate-Bitumen Interface Shear Strength  
In the seasonally frozen regions, freeze-thaw (F-T) damage is the main pavement disease, causing a variety of poor conditions in bitumen pavement, such as cracks, pits, potholes, and slush. In order to evaluate the effect of freeze-thaw cycles on aggregate-bitumen interface shear strength of NHSSMA, damaged BA and NHSSMA after 10, 20, and 30 F-T cycles were collected for an aggregate-bitumen interface shear strength test. The details of a freeze-thaw cycle test is described below. First, neat and modified asphalt binders were heated to a fluid state and poured into a fixed-size plate to ensure the dimensions of asphalt binder samples was approximately \(6\times 250\times 250\) mm. The purpose of this was to make the thickness of the prepared asphalt binder samples as close as possible to the thickness of the asphalt film in the actual pavement. Then, the samples were submerged in a container containing water, then the container with specimens were placed in the precision temp-enclosure at \(-15\)\({}^{\circ}\)C and frozen for 10 h.  
Finally, the base asphalt and modified asphalt samples were soaked in water at 15 \({}^{\circ}\)C for 16 h through adjusting the temperature controller. As described above, a complete freeze-thaw aging cycle was completed. Then, after 10, 20, and 30 freeze-thaw aging cycles, damaged samples were collected for aggregate-bitumen interface shear strength test to evaluate the effect of freeze-thaw cycles on aggregate-bitumen interface shear strength of NHSSMA. Moreover, another set of experiments was designed to consider the effect of snow-melting agent on asphalt in the spring-thaw season. A 20% concentration of CaCl\({}_{2}\) solution was selected for the F-T cycle. The aggregate-bitumen interface shear strength of modified and unmodified asphalt with freeze-thaw cycles under different conditions are given in Figure 9.  
As shown in Figure 9a, 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of BA increases with the increase of F-T cycles, and then decreases slightly. After 30 F-T cycles, 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of BA decreased by 18.9% to 0.4 MPa. The 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of NHSSMA decreased with the increase of F-T cycles. After 30 F-T cycles, 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of NHSSMA decreased by 24.6% to 0.546 MPa. It can be seen that 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength decay rate of NHSSMA is greater than BA under the F-T cycles, but 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of NHSSMA is still higher than that of BA at different F-T cycles. This indicates that the incorporation of NHSS increases the 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of asphalt under the F-T cycles. It can be concluded from Figure 9b that the snow-melting agent has a greater influence on 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength, and the decay rate under a snow-melting agent is higher than that under clear water. After 30 F-T cycles, 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of BA and NHSSMA decreased by 20.3% and 43.4%, respectively. However, whether it is a clear water or CaCl\({}_{2}\) solution, 10 \({}^{\circ}\)C aggregate-bitumen interface shear strength of NHSSMA is greater than that of BA. This shows that NHSS can improve the aggregate-bitumen interface shear strength of asphalt in the spring-thaw season. As shown in Figure 9c,d, after 30 F-T cycles, \(-10\)\({}^{\circ}\)C aggregate-bitumen interface shear strength of BA and NHSSMA decreased by 4.8% and 21.8%, respectively. After 30 F-T cycles, \(-10\)\({}^{\circ}\)C aggregate-bitumen interface shear strength of BA and NHSSMA with snow-melting agent decreased by 10% and 31.3%, respectively.