# Bonding Performance of Steel Rebar Coated with Ultra-High-Performance Concrete
## 3 Results and Discussion
### Load-Slip Relationship  
Figure 3 shows the load-slip curve for specimens 21_N containing a reinforcing bar without the UHPC coating and 21_C containing a reinforcing bar with the UHPC coating. When comparing the uncoated test specimen (21_N) with the UHPC-coated test specimen (21_C), noticeable changes (bond strength, toughness) in the coated test specimen can be confirmed. The load-slip curves of the two types of specimens were similar. A small slip change was shown in the initial loading section, and after reaching the maximum load the load gradually decreased, with the slip increasing. However, the bond strength of 21_C was 15.3% greater than that of 21_N and resulted in an increase in toughness performance. This was because the UHPC coating creates an additional interface and increases the bonding strength between the rebar and concrete, thereby delaying the progress of cracks. Figure 4 shows the load-slip curve of the uncoated rebars in concrete mixed with the PVA fiber. When the fibers were mixed, the slip resulted in the initial load section being smaller. As shown in Figure 4, the fiber increased the bond strength of the rebars, but not by much. The bond strength of specimens with 1% and 2% PVA fiber increased by about 6% and 13.1%, respectively, which were lower than the increases due to the UHPC coating. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Specimen** & \begin{tabular}{c} \(P_{max}\) \\ **(kN)** \\ \end{tabular} & \begin{tabular}{c} \(\tau_{max}\) \\ **(MPa)** \\ \end{tabular} & \begin{tabular}{c} \(\tau_{max-mean}\) \\ **(MPa)** \\ \end{tabular} & \begin{tabular}{c} \(S_{m}\) \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} \(S_{m-mean}\) \\ **(mm)** \\ \end{tabular} & \
\begin{tabular}{c} \(\tau^{*}_{max}\) \\ **(MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Failure** \\ **Mode** \\ \end{tabular} \\ \hline
21-C-0.2-CSF-1 & 49.77 & 19.65 & 19.78 & 1.43 & 1.44 & 4.08 & PO \\
21-C-0.2-CSF-2 & 50.40 & 19.90 & 19.78 & 1.45 & & 4.13 & PO \\ \hline
21-C-0.4-CSF-1 & 51.96 & 20.52 & 20.84 & 1.15 & 1.17 & 4.26 & PO \\
21-C-0.4-CSF-2 & 53.60 & 21.17 & 20.84 & 1.18 & & 4.40 & PO \\ \hline
21-C-0.6-CSF-1 & 58.54 & 23.12 & 22.89 & 1.75 & 1.80 & 4.80 & PO \\
21-C-0.6-CSF-2 & 57.40 & 22.67 & 1.84 & & 4.71 & PO \\ \hline
21-C-0.8-CSF-1 & 57.38 & 22.66 & 22.90 & 0.81 & 0.80 & 4.70 & SY \\
21-C-0.8-CSF-2 & 58.60 & 23.14 & 23.90 & 0.79 & & 4.80 & SY \\ \hline
21-C-1.0-CSF-1 & 62.39 & 24.64 & 24.86 & 0.87 & 0.88 & 5.12 & SY \\
21-C-1.0-CSF-2 & 63.50 & 25.08 & 24.86 & 0.90 & & 5.21 & SY \\ \hline
21-C-0.2-CSSF-1 & 46.70 & 18.44 & 18.66 & 0.53 & 0.52 & 3.83 & PO \\
21-C-0.2-CSSF-2 & 47.80 & 18.88 & 18.66 & 0.51 & & 3.92 & PO \\ \hline
21-C-0.4-CSSF-1 & 51.86 & 20.48 & 20.72 & 1.18 & 1.20 & 4.25 & PO \\
21-C-0.4-CSSF-2 & 53.10 & 20.97 & 20.72 & 1.23 & & 4.35 & PO \\ \hline
21-C-0.6-CSSF-1 & 54.46 & 21.51 & 21.77 & 1.83 & 1.88 & 4.47 & PO \\
21-C-0.6-CSSF-2 & 55.80 & 22.04 & 21.77 & 1.92 & & 4.58 & PO \\ \hline
21-C-0.8-CSSF-1 & 58.98 & 23.29 & 23.02 & 1.23 & 1.20 & 4.84 & SY \\
21-C-0.8-CSSF-2 & 57.60 & 22.75 & 23.02 & 1.18 & & 4.72 & SY \\ \hline
21-C-1.0-CSSF-1 & 58.27 & 23.01 & 23.19 & 0.70 & 0.72 & 4.78 & SY \\
21-C-1.0-CSSF-2 & 59.20 & 23.38 & 23.19 & 0.73 & & 4.85 & SY \\ \hline \end{tabular}  
* PO: pull out failure; SY: steel yield failure; \(\tau^{*}_{max}\) normalized bond strength; \({}^{**}\) reference [32] test results.  
\end{table} 
Table 5: _Cont._Figure 5 shows the load-slip curve for a pullout test specimen using concrete mixed with hooked steel fibers (HSF) in a reinforcing bar coated with UHPC. The load-slip curve of the test specimen in which 0.2-1.0% of HSF was mixed in the concrete matrix showed a  
Figure 4: Load–slip curves of uncoated rebars with PVA fiber [32]. Figure 3: Load–slip curves with and without UHPC coating. more evident resistance improvement on the adhesion surface when the UHPC was coated. With a small slip, the initial ascent section increased very steeply and showed a softening of the load reduction after reaching the maximum load.