## 1 Introduction  
A weir installed across a river must contain the river and secure water resources under normal circumstances, enabling sufficient water to pass over the cross-sectional area in times of flooding [1; 2; 3; 4]. A fixed weir often causes overflow in the upper stream in times of flooding, which causes problems such as degraded river water flux due to soil accumulation. As a result, many existing concrete fixed weirs are no longer functional and are being demolished. The current trend is to re-install moveable weirs [1; 5; 6; 7]. Representative forms of moveable weir installation technology include rubber weirs, hydraulic flip gates, open/close-type water gates, and hinge-type automatic water gates [1; 8; 9; 10; 11]. A moveable weir is valuable not only for the purpose of using and controlling a river, but also in other areas of water resource management [1; 3; 5; 10]. A rubber weir is an air supply/discharge-type pneumatic moveable weir, but, while it is environmentally friendly with no sources of river contamination, it is weaker than steel gates [1; 3; 5; 6; 7; 10]. Hydraulic steel flip gates have sufficient durability but the problem of possible river contamination from the hydraulic operating oil [1; 3; 5; 6; 7; 10]. Improved moveable weirs boast the advantages of rubber weirs and flipping gates and have been in use since the 1980s [1; 2; 5; 6; 7; 11; 12; 13]. Figure 1 shows an improved moveable weir [1; 7]. It uses steel panels for all parts except the air bag, so there is concern regarding corrosion in a river environment as well as various other problems such as decreased durability with long-term use and possible river ecosystem damage due to the leakage of corrosive materials [1; 5; 6; 7]. Figure 2 shows the corrosion of the steel panels [1; 7]. In a previous study, a hybrid panel with glass fiber-reinforced polymer (GFRP) panel layers on both sides of the steel panel was developed to replace the solely steel panel, thereby avoiding corrosion concerns [1; 7]. In addition, the surface shape of the steel panel was changed to accommodate the characteristics of the GFRP panel [7]. The surface deformation of the steel improved the bond properties between the steel and the GFRP panels [1; 7]. In addition, hybrid panels with outer GFRP layers or panels prevent the exposure of the steel to the ambient environment [1]. However, GFRP composite materials may exhibit performance degradation when exposed to various environments [1; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. Especially in the case of a hybrid panel made by attaching steel and GFRP panels, there is the possibility of fracturing at the bond surface [1; 24; 25; 26; 27]. Previous studies have evaluated the performance of hybrid panels when exposed to chemically harsh environments, such as those containing high levels of sulfate and sea water [1]. However, these conditions are rarely encountered by the hybrid panels since they are most commonly used in rivers. Therefore, it is necessary to evaluate the performance of hybrid panels under relevant exposure conditions. Such conditions include exposure to winter and long-term freezing, repeated freeze-thaw cycles, summer heat and repeated dry-wet cycles, and long-term exposure to high temperatures. With repeated dry-wet cycles and long-term exposure to high temperatures, there is a high possibility of fracture at the bond interface because of the thermal expansion coefficient differences between the GFRP material and the steel panel. Furthermore, as moisture penetrates through the weir environment, there is the possibility of fracture  
Figure 1: Improved moveable weir installed in a river. Figure 2: Corrosion of steel panels. via the corrosion of the steel panel. In the winter, with repeated freeze-thaw cycles and long-term freezing conditions, there is also a possibility of fracturing at the interface between the GFRP material and the steel panel with subsequent performance degradation of the GFRP composite.