## 7 Conclusions  
In this study, customized conditions for ultrasonic measurements were established for the stone block foundation of the Sungryemun Gate, a representative stone architectural heritage structure in the Republic of Korea, to enhance the reliability of the weathering evaluation. Furthermore, the extensively used method for evaluating the weathering was improved by proposing weathering standards that accounted for the material characteristics and provenance of the constituent stone materials.  
The physical damage on the stone block foundation of the Sungryemun Gate was mainly attributed to the blistering cracks (crack index 3.71), blistering (6.8%), scaling (7.5%), and disintegration (39.2%). Further, surface contamination and discoloration were primarily caused by the oxidation of the ferruginous mineral and moisture. The continuous attachment of air pollutants, salt crystals, clay minerals, and microorganisms may have increased the discoloration of the stone blocks.  
The average difference between the ultrasonic velocities of the original (average 2665 m/s) and new (average 3884 m/s) blocks belonging to the stone block foundation of the Sungryemun Gate was approximately 1219 m/s. Additionally, based on the assumption that fresh rock from the provenance site was used during the original construction, the calculation of the rate of weathering as a linear function exhibited that the ultrasonic velocity of pink feldspar granite and leucogranite of the original block decreased annually by approximately 1.8 and 2.4 m/s, respectively. This rate of weathering could be partly attributed to the intentional damage caused during the Korean War. Thus, it was estimated that the rate of weathering would be lower than these results under natural weathering conditions.  
In this study, the relative weathering grade based on the ultrasonic velocity of fresh rocks at the provenance site and absolute weathering grade based on the ultrasonic velocity of a universal hard rock were proposed. The evaluation of the weathering of the stone block foundation of the Sungryemun Gate exhibited that the original and new blocks exhibited a difference in weathering of approximately two stages. The absolute weathering grade underestimated the overall physical properties and strength compared with the relative weathering grade.  
This study makes a significant contribution to improve the reliability of non-destructive testing by proposing a customized model for performing ultrasonic measurements and evaluating the weathering of a stone architectural heritage. In particular, the weathering evaluation method based on the types and origin of the rock can be extensively used to diagnose and monitor similar stone architectural heritage structures.  
Conceptualization, Y.H.J.; methodology, Y.H.J. and C.H.L.; software, Y.H.J.; validation, C.H.L.; formal analysis, Y.H.J.; investigation, Y.H.J.; resources, Y.H.J.; data curation, C.H.L.; writing--original draft preparation, Y.H.J.; writing--review and editing, C.H.L.; visualization, Y.H.J.; supervision, C.H.L.; project administration, C.H.L.; funding acquisition, C.H.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.