## 4 Results
### Test Results under Accelerated Environmental Conditions  
The results of long-term freezing tests are shown in Figure 9. The flexural strengths of the control specimens were 267.46, 265.25, and 240.80 MPa as the panel was thickness increased from 9.2 to 12.2 to 15.2 mm, respectively. The flexural strengths of the sand-blasted specimens were 355.57, 438.25, and 461.31 MPa, respectively. All of the evaluated specimens satisfied the residual strength goal of \(\geq\)65% (Figure 9b). As panel thickness increased from 9.2 to 12.2 to 15.2 mm, the residual strengths of the control and sand-blasted specimens were 78.34%, 82.57%, and 86.58%, respectively, and 89.54%, 93.20%, and 93.93%. The sand-blasted specimens showed higher residual strengths. In addition, interfacial delamination was suppressed at the surface, indicative of higher bond strength. Moisture penetration into the bond interface between the steel and GFRP panels accelerated fracturing, due to the expansion of water as it freezes. The sand-blasted specimens exhibited higher bond strength with a smaller degree of interfacial delamination via moisture penetration and subsequent freezing and expansion. As a result, both the initial and residual flexural strengths were enhanced. The results of the repeated dry-wet environment exposure are shown in Figure 10. As panel thickness increased from 9.2 to 12.2 to 15.2 mm, the control specimens showed flexural strengths of 279.35, 285.81, and 270.29 MPa, respectively. The sand-blasted specimens exhibited the significantly greater values of 316.69, 383.60, and 428.22 MPa, respectively. The flexural strength of the control specimens increased for panel thicknesses, up to 12.2 mm, but then decreased at 15.2 mm. This result shows that the flexural strength of the control specimens was limited by the bond strength between the steel and GFRP panels. After testing, fractures were evident at the bond interface before fracturing of the GFRP panel occurred. In contrast, the bond strength of the sand-blasted specimens was higher such that interfacial failure was determined primarily by the strength of the GFRP panel. In terms of residual strength, all of the panels satisfied the goal of this study (\(\geq\)65%; Figure 10b). As the thickness of the hybrid panel increased from 9.2 to 12.2 to 15.2 mm, the control specimens showed residual strengths of 81.84%, 88.96%, and 97.19%, respectively, while those of the sand-blasted specimens were 79.74%, 81.58%, and 87.19%. Figure 9: _Cont._  
The results of the repeated freeze-thaw cycling are shown in Figure 11. As the thickness of the hybrid panel increased from 9.2 to 12.2 to 15.2 mm, the control specimens showed flexural strengths of 279.30, 265.21, and 228.23 MPa, respectively, while those of sand-blasted specimens were 320.819, 394.24, and 415.18 MPa. Generally, repeated freeze-thaw cycles allow moisture to penetrate GFRP composite materials, resulting in fracture of fibers and resin by subsequent expansion during freezing. In a similar fashion, moisture penetration into the bond interface between the steel and GFRP panels may cause fractures in the bond interface. In such cases, the degree of fracture is determined by the strength of the bond interface. In this study, sand-blasted specimens exhibited higher bond strengths and a greater resistance to moisture penetration in repeated freeze-thaw cycles. As a result, the residual flexural strength remained high after testing. In terms of residual strength, all of our panels satisfied the goal of this study (\(\geq\) 65%; Figure 10(b)). As the thickness of the hybrid panel increased from 9.2 to 12.2 to 15.2 mm, the control specimens exhibited residual strengths of 80.65%, 81.93%, and 82.06%, respectively, while those of sand-blasted specimens were 80.78%, 83.84%, and 84.54%. Figure 9: Flexural strength of hybrid panels following long-term freezing tests. Figure 10: Flexural strength of hybrid panels after repeated wetting and drying cycles.