## 4 Test Results and Discussion
### Characteristic Loads
#### 4.3.2 Impact of Curved Section Thickness  
Figure 12 illustrates the cracking stresses of all panel specimens. As shown in Figure 12, the cracking stresses of UHPC panel specimens decreased from 9.35 MPa to 7.42 MPa as the curved section thickness increased from 25 mm to 40 mm. This phenomenon is attributed to the wall effect of steel fiber distribution in UHPC; namely, the fibers near the concrete surface were basically distributed two-dimensionally, while they were distributed three-dimensionally within the thicker cross-section [29]. The parallel orientation of fibers with the tensile stress can significantly improve the tensile resistance of UHPC, increasing the utilization efficiency of steel fiber's tensile strength. Additionally, the reinforcing effect of the fibers on concrete strongly depends on the orientation and distribution of fibers [30]. Therefore, as the curved section thickness decreases, more steel fibers are aligned with the direction of tensile stress in the curved section, leading to an increase in the cracking stresses of the UHPC panel specimens. However, the glass fiber had a length of 30 mm, which was like the thickness of the curved section. It resulted in no reorientation of glass fibers and, thus, consistent cracking stress in GRC panel specimens.  
Figure 12: Cracking stress of UHPC and GRC panel specimens.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Cracking Load (N)** & **Ultimate Load (N)** & **\(L\) (mm)** & **Cracking Stress (MPa)** & **Mean (MPa)** & **\(\mu_{cr}\)/\%** & **\(\mu_{a}\)/\%** \\ \hline U25 & 1334.1 & 1863.9 & 361 & 9.35 & & 126.8 & 81.0 \\ U28 & 1491.1 & 2256.3 & 371 & 8.57 & & 87.7 & 115.0 \\ U31 & 1677.5 & 2472.1 & 380 & 8.07 & & 71.2 & 86.7 \\ U34 & 1952.1 & 2687.9 & 374 & 7.69 & & 70.4 & 39.1 \\ U37 & 2256.3 & 3276.5 & 378 & 7.60 & & 64.3 & 39.2 \\ U40 & 2550.6 & 3168.3 & 381 & 7.42 & & 68.1 & 29.2 \\ \hline G25 & 588.6 & 1030.0 & 367 & 4.19 & & - & - \\ G28 & 794.6 & 1049.6 & 380 & 4.68 & & - & - \\ G31 & 981.0 & 1324.3 & 368 & 4.57 & & - & - \\ G34 & 1147.7 & 1932.5 & 374 & 4.52 & & - & - \\ G37 & 1373.3 & 2354.4 & 378 & 4.62 & & - & - \\ G40 & 1520.5 & 2452.5 & 385 & 4.47 & & - & - \\ \hline \hline \end{tabular} Note: \(\mu_{cr}\) and \(\mu_{a}\) are the increased ratios of the cracking stress and ultimate load of UHPC panels to those of GRC panels with the same curved section thicknesses.  
\end{table}
Table 8: Characteristic loads and stress on the panel specimens.  
Figure 13 shows the increased ratio of panel specimens with varying curved section thicknesses. As shown in Figure 13a, the cracking loads of UHPC and GRC panel specimens significantly increased as the curved section thickness increased. Every 3 mm increase in the curved section thickness resulted in approximately 18.2% and 31.7% increases in cracking loads of UHPC and GRC panel specimens, respectively. The increases in cracking loads of UHPC and GRC panel specimens were stable due to the crack stress.  
As illustrated by Figure 13b, before the curved section thickness reached 40 mm, the ultimate load of UHPC and GRC panel specimens, respectively, increased by 14.0% and 27.6% with every 3 mm increase in the curved section thickness. For the UHPC panel specimens, there was a small decrease in ultimate load after the thickness reached 40 mm. It might be attributed to the fact that more steel fibers were aligned in the non-parallel direction of the panel surface, leading to insufficient utilization of the tensile strength of the steel fibers and, therefore, the lower ultimate load of U40. Moreover, the most efficient curved section thickness for both UHPC and GRC panel specimens was 37 mm in this study, as U37 had the greatest ultimate load among UHPC panels, and the ultimate load of G37 was slightly smaller than that of G40 by 7%. Therefore, it can be concluded that there was an upper limit in the thickness of the curved section, and the influence of fiber distribution and wall effects need to be carefully considered in the design.