## 3 Results and Analysis
### Bond Strength  
The experimental results of pull-out tests are shown in Table 5.  
To precisely understand the effect of \(V_{f}\) and high temperature on the bond strength, the steel fiber reinforcement factor (\(\beta_{V}\)) and temperature reduction factor (\(\beta_{T}\)) were introduced, as shown in Equations (2) and (3). \[\beta_{V}=\frac{\tau_{u}^{V}}{\tau_{u}^{0}} \tag{2}\]  
\[\beta_{T}=\frac{\tau_{u}^{T}}{\tau_{u}^{0}} \tag{3}\]where \(\tau_{u}{}^{V}\) is the ultimate bond stress when steel fiber is mixed with \(V_{f}\) (\(V_{f}\) = 0.5%, 1% and 1.5%); \(\tau_{u}{}^{0}\) is the ultimate bond stress of the NC specimen. \(\tau_{u}{}^{T}\) is the ultimate bond stress when the temperature of the bond zone is \(T\) (\(T\) = 200, 400 and 600 \({}^{\circ}\)C); \(\tau_{u}{}^{20}\) is the ultimate bond stress at room temperature (20 \({}^{\circ}\)C). From Figure 10 and Table 5, it can be seen that both ultimate bond stress and peak slip increase as \(V_{f}\) increases. As the temperature rises, the ultimate bond stress decreases and the peak slip increases. The effect of \(V_{f}\) on bond strength is shown in Figure 11. With the increase in \(V_{f}\), \(\beta_{V}\) generally shows a linear growth trend. \(\beta_{V}\) is small at 20 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, i.e., less than 1.5 in both cases. The slope of the \(\beta_{V}\)-\(V_{f}\) curve is steeper between 400 and 600 \({}^{\circ}\)C, which indicates that the effect of \(V_{f}\) on the improvement in bond strength is more obvious between 400 and 600 \({}^{\circ}\)C.  
\(\beta_{V}\) of the same \(V_{f}\) increases as the temperature rises. The main reason for this is that high temperatures significantly reduce the bond strength of NC. The effect of temperature on bond strength is shown in Figure 12. It can be seen that there is less \(\beta_{T}\) decay of NC and SFRC between 20 and 200 \({}^{\circ}\)C. \(\beta_{T}\) of NC shows large decay between 200 and 400 \({}^{\circ}\)C. However, the degree of \(\beta_{T}\) decay of SFRC is consistent with that for temperature between 20 and 200 \({}^{\circ}\)C. The slope of the \(\beta_{T}\)-\(T\) curve for SFRC is greater  
Figure 10: Ultimate bond stress at different high temperatures. between 400 and 600 \({}^{\circ}\)C than between 200 and 400 \({}^{\circ}\)C. This indicates that the bond strength between rebar and SFRC experiences greater decay between 400 and 600 \({}^{\circ}\)C.  
At the same temperature, \(\beta_{\rm T}\) increases as \(V_{f}\) is increased. This indicates that the increase in \(V_{f}\) improves the bond strength between rebar and SFRC. The reasons for the effect of \(V_{f}\) and temperature on bond strength can be explained as follows. 1. The interaction of the steel fibers and the aggregate creates a rigid skeleton for bridging cracks [17; 21; 22]. As a result, the bridging action not only retards the expansion of concrete cracks but also moderates the degree of stress concentration at crack tips. This allows the scale and number of crack sources within the concrete matrix to be effectively controlled. 2. The difference in the thermal properties of rebar and SFRC at high temperatures destroys the bond behavior of rebar and SFRC. Firstly, there was a significant difference in the thermal properties of rebar and SFRC. During the heating process, the thermal conductivity of rebar was higher than that of SFRC. Therefore, the stress concentration generated by the temperature difference exists in the bond zone between rebar and SFRC. Secondly, while the thermal expansion of rebar and SFRC were of the same order of magnitude under temperature influence, the temperature expansion of rebar and SFRC above 400 \({}^{\circ}\)C varied considerably [24]. The differences in the expansion and deformation of the rebar and SFRC aggravated the extension of cracks in the concrete between the ribs in the radial direction of the rebar. This led to a decrease in the bond strength between rebar and SFRC as the temperature rose. 3.