## 1 Introduction  
Concrete structures may be highly prone to early degradation and damage, especially in their most vulnerable parts, if they have been improperly designed and constructed or built with inappropriate materials or when the environmental conditions are severe. This leads to a significant degradation in their safety, durability, and functionality, thereby increasing the frequency and scale of natural disasters and safety accidents. In particular, safety accidents are rapidly increasing in concrete structures, and the resultant damage is also increasingly severe in scale and scope. The aging and degradation of structures and their structural performance is accelerated by the degradation of the performance and functionality of their concrete parts. This degradation process is considered to be caused by a variety of factors, as mentioned above, such as the natural aging of structures, environmental changes including climate change, quality errors in design and construction, and changes in the load condition due to extension or design change. Concrete structures can be effectively used and fully implement their functionality over the required or intended period of time only when their safety is thoroughly monitored and reviewed at all times. Further, when damage occurs or may occur, maintenance and retrofitting measures must be immediately taken to ensure safety. The world has recently seen large-scale earthquakes caused by environmental changes, including climate change, increasingly causing significant damage to various facilities, and especially structures. In Japan, China, and Taiwan, the neighboring countries of the Korean Peninsula, earthquake-induced damage is dramatically increasing. In some ways, this implies that the Republic of Korea is not free from the danger of earthquakes, whether directly or indirectly. Notably, the 2005 Fukuoka Earthquake in Japan [1], the 2008 Sichuan Earthquake in China [2], and the 2016 Kuzamoto Earthquake in Japan [3] all occurred within the Eurasian Plate to which the Korean Peninsula belongs. This is explicit evidence that large-scale earthquakes may occur in Korea. As is well known, the 2016 Gyeongiu Earthquake and the 2017 Pohang Earthquake in the country already revealed the vulnerability of facilities and structures in the region, in terms of seismic safety, to a significant extent. These accidents were a huge wake-up call to the possibility of nationwide disasters. In the 2016 Gyeongiu Earthquake, not many buildings or structures were subject to severe damage, except for some column damage in buildings located near the epicenter, including schools and residential buildings. In the 2017 Pohang Earthquake, however, newly built piloti structures and multi-unit dwellings (apartment buildings), including school facilities with non-seismic details, underwent severe damage [4]. In particular, reinforced concrete (RC) columns without sufficient shear reinforcement were found to be highly vulnerable to shear failure, as shown in Figure 1, which then emerged as an urgent and important issue to be addressed in the country's seismic policy development for years to come. As demonstrated in the earthquakes in Gyeongju and Pohang, earthquakes are increasing in Korea in both frequency and intensity. Thus, to prevent structures from collapsing in the case of a large-scale earthquake, and to minimize resultant human and property  
Figure 1: Damaged buildings after the 2017 Pohang Earthquake: (**a**) RC school building; (**b**) RC public building; (**c**) piloti building. damage, it is necessary to develop an economical and effective seismic retrofitting method to improve the seismic performance of structures that are likely to be vulnerable to earthquakes, and especially RC structures with non-seismic details, which are highly likely to undergo shear failure in their columns. Also, these seismic retrofitting measures must be conducted in an efficient and economical manner based on the expected earthquake magnitude and resultant damage.