## 5 Conclusions  
By simulating the freeze-thaw weathering of yellow sandstone in different environments, the variation law of sandstone indexes was analyzed, the influence of freeze-thaw modes on the damaging effect of freeze-thaw was studied, and the quantitative relationship between physical property indexes and mechanical indexes was discussed. According to the rock samples and weathering conditions used in this experiment, the following conclusions were obtained:  
1. The environment of rocks is an important prerequisite for weathering protection. The damage characteristics of rock samples caused by different freeze-thaw modes are quite different on the macro level. The damage to sandstone is the most serious under the action of immersion freeze-thaw, and the overall damage of some rock samples occurs within 80 cycles. The damage caused by periodic saturated freeze-thaw is  
Figure 19: Schematic diagram of weathering in part of Qingyang North Grottoes. (**a**) Moderate weathering; (**b**) heavy weathering; (**c**) strong weathering.  
the next most serious. Capillary freeze-thaw damage is the least significant, and it is significant only in the range of 2 mm at the bottom. (2) The change in the testing environment affects the water content of the rock, leading to a certain difference in the test results of wave velocity. As the curve change rates of wave velocity and the number of freeze-thaw cycles are different in different testing environments, attention should be paid to this difference in on-site nondestructive testing. (3) As the number of freeze-thaw cycles increased, the UCS of sandstone showed a decreasing trend of first fast and then slow, while the strength loss rate shows a rising trend of first fast and then slow, both of which show a good exponential relationship with the number of freeze-thaw cycles. In the early stage, the strength loss increases rapidly. After 20 cycles, the strength attenuation accounts for about 60% of the total attenuation value of 80 cycles, and then the strength loss gradually slows down. (4) There are different correlations between different non-destructive weathering indexes and UCS. Due to the differences in damage uniformity and severity, the fitting parameters are different in different freeze-thaw modes. Therefore, the range of fitting parameters should be determined according to the weathering environment of rock when the nondestructive testing indexes are used to judge or evaluate the mechanical parameters of rock. (5) Damage variable evaluation is an important analysis to measure the irreversible deterioration degree of rock materials. The weathering degree of rock samples undergoing the same cycle times is different, among which the damage degree of immersion freeze-thaw is the highest, followed by periodic saturated freeze-thaw, and the damaging effect of capillary freeze-thaw is the least. This is consistent with the overall results of the experiment.  
Conceptualization, N.P. and J.H. ([PERSON]); methodology, N.P. and [PERSON]. ([PERSON]); software, Y.Z.; validation, Y.D.; formal analysis, [PERSON] ([PERSON]), Y.Z. and B.S.; investigation, B.S.; writing--original draft preparation, N.P. and J.H. ([PERSON]); writing--review and editing, Y.Z. and J.H. ([PERSON]); funding acquisition, [PERSON] ([PERSON]) and N.P. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Key Research and Development Program of China (No. 2019 YFC1520500), and the National Natural Science Foundation of China (No. 51808246). Not applicable. Not applicable. Not applicable. Not applicable. Data Availability Statement: The data used to support the findings of this study are available from the corresponding author upon request. The authors declare no conflict of interest.