## 3 Experimental Variables and Results
### Experimental Results and Analysis
The ultimate load of the normal N24-H20-R1 member was 244.1 kN, and that of the S120-H12-R1 panel with the same rebar content was similar at 247.7 kN. For the normal N24-H20-R1 panel, the resistance capacity of the rebar decreased after the ultimate load was reached, leading to an expected cutting failure, which prompted termination of the test. In contrast, the UHPFRC-F series exhibited a gradual decrease in load after the ultimate load was reached because of the cross-linking effect of the steel fibers. Notably, the N24-H20-R1 member and S120-H12-R1 maintained similar performances despite the reduction in the panel thickness of approximately 49% (from 200 mm to 120 mm). The load-deflection relationships of the NC N24-H20-R1 member and the UHPFRC-I series were analyzed, resulting in the following findings. As shown in Figure 12, the initial cracking loads of SH120-H20-R1, SH120-H20-R2, and SH120-H20-R3 were 99.1, 95.2, and 70.9 kN, respectively, with deflections of 1.2, 1.3, and 1.0 mm, respectively. The N24-H20-R1 panel exhibited nonlinear behavior after a linear increase in load, with a yield load of 201.0 kN and a deflection of 3.0 mm. In contrast, SH120-H20-R1 and SH120-H20-R2 showed yield loads of 334.5 and 363.0 kN, respectively, with deflections of 4.8 and 4.0 mm, respectively. For panel SH120-H20-R3, the yield load could not be measured as the load rapidly increased. Unlike the normal N24-H20-R1 member, the UHPCR-I series  
Figure 10: Load–deflection curve (UHPFRC-F series). demonstrated a gradual decrease in the load-carrying capacity after the ultimate load was reached due to increased deflection and widening of the main crack. The ductility of a concrete structure refers to its ability to undergo deformation until it fails outside the elastic range and can be used as a measure of its ability to absorb energy [55]. Generally, the ductility of concrete structures can be quantified by a ductility index. Unlike regular concrete structures, the UHPFRC structures exhibited ductility due to strain hardening from the initial crack stage. In this study, the ductility index of the test members was calculated based on the initial and ultimate states of deflection using Equation (1). \[\mu=\frac{\Delta_{cr}}{\Delta_{u}} \tag{1}\]  
Figure 11: Comparison of load–deflection curves of N24-H20-R1 and UHPFRC-F series specimens. Figure 12: Comparison of load–deflection curves obtained from N24-H20-R1 and UHPC-I series specimens. where \(\mu\) represents the ductility index of the member, \(\Delta_{\sigma}\) denotes the deflection at the initial cracking load, and \(\Delta_{u}\) denotes the deflection at the ultimate load. The ductility index of the conventional concrete specimen N24-H20-R1 was 6.44, whereas for the UHPFRC-T series, the ductility indices for S120-H12-R1, S120-H12-R2, and S120-H12-R3 were 7.08, 4.93, and 7.81, respectively. Except for the SH-H12-R2 specimen, the ductility index increased by 10% to 37% compared to that of the conventional concrete specimen. Additionally, for the UHPFRC-I series, the ductility indices for SH120-H20-R1, SH120-H20-R2, and SH120-H20-R3 were 5.00, 3.92, and 5.30, respectively, demonstrating lower values than that of the conventional concrete specimen. As shown in Figure 6, considering the cross-section of the specimen with the insulation material, the neutral axis, which plays a significant role in ductility, was formed in the insulation material. In other words, while in the regular UHPFRC specimen, the resistance against bending due to material deformation was demonstrated by the fiber-reinforced concrete, in the specimen with the insulation material, the insulation became the primary contributor. Therefore, the ductility effect could be inferred to be diminished in the specimen with the insulation material. Using UHPFRC, the flexural performance was evaluated by selecting the steel fiber content, panel thickness, and thermal insulation as the primary variables.