## 5 Conclusions  
In this study, the change in adhesion strength according to the reduction of the adhesion area of the stubborn mortar in relation to the detachment defects of the indoor wall tiles was analyzed, and the following conclusions were drawn:  
1. Through the ISO standard adhesion strength evaluation method it was confirmed that the spot-bonding method is also able to secure stable adhesion, but only in sections where the spot-bonding mortar was securely installed, leading to the hypothesis that, as long as a higher surface area application of the cementitious adhesive is achieved, a stable installation of the tile by spot-bonding can also be achieved.
2. By using a new evaluation method consisting of sectioning the tile into individual pieces across the entire tile surface, an adhesion strength evaluation in the spot-bonding method using large tiles was made possible by schematic mapping of the tile with high and concentrated adhesion strength. Through this schematic analysis of adhesion strength throughout the tile surface, the results confirmed that the adhesion strength of the tile cementitious adhesive surface ratio should be 100% in order to achieve adhesion strength of the tile above the standard regulation for approximately 80% of the tile area. Further investigation is required to assess different material types, different load conditions during tile pressing, and ambient conditions need to be conducted in the near future to establish a reliable data set on the tile spot-bonding installation method, but it is hoped, nonetheless, that the evaluation methodology proposed in this study is viable for continuous studies on this topic.  
Based on the results of this study, the following conclusions can be offered; the spot-bonding installation method for tiles is still effective, but only when having secured an AR of 100% (by which only about 75% of the tile surface would achieve stable adhesion). It is evident that the practice of spot-bonding will not be completely discontinued any time soon, as the economic advantages of spot-bonding for contractors are too high. If such is the case, what should at the very least happen is, the results of this study should be referenced to understand the exact risks of poorly conducted spot-bonding tile installation. The results of the evaluation method demonstration have experimentally proven that if spot-bonding is to be used, then few glob applications (AR 60-80%) should not be conducted. When evaluating the adhesion stability to large-sized tiles in the future, a test method of splitting and measuring the entire surface of the tile can be applied, and a study on the standard test method for this is required.  
Conceptualization, J.-h.L., K.-h.O. and S.-k.O.; methodology, X.H. and B.J.; software, experimental plan, B.-s.K., B.-i.K. and S.-k.O.; formal analysis, resources, J.-h.L.; investigation, J.-h.L., B.J., X.H. and B.-i.K.; data curation, J.-h.L.; technical information, B.J. and X.H.; writing--original draft preparation, J.-h.L. and K.-h.O.; writing--review and editing, B.-i.K. and S.-k.O.; supervision, X.H. and S.-k.O. All authors have read and agreed to the published version of the manuscript.  
This research was carried out by the research fund support (21 CTAP-C151778-03) of the Ministry of the Land, Infrastructure, and Transport's Ministry of the Land, Infrastructure, and Transport Technology Promotion Research Project.  
Not applicable.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the decision to publish the results.