# Bonding Performance of Steel Rebar Coated with Ultra-High-Performance Concrete
## 3 Results and Discussion  
The compressive strength of the UHPC presented in Table 1 was evaluated from a total of 5 specimens. Among them, the average values of the three specimens were calculated after excluding two specimens with a standard deviation of 5% or more of the compressive strength. After curing, the average compressive strength on the 28 th day was 145.3 MPa. The ultra-high-strength UHPC was coated on the surface of the reinforcement to a certain thickness, and in addition a new type of interface was formed with the general concrete mixed with three types of steel fibers. Composite pullout resistance capability was expected at the newly formed interface 1 and interface 2. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Mix Type** & **Fiber Volume Fraction, \(V_{f}\) (\%)** & **Specimen Name for Pullout Test** \\ \hline
21-N (OPC) & None & 21-N \\ Steel-1 (PVA0) & None & PVA 0\% \\ Steel-2 (PVA1) & 1\% & PVA 1\% \\ Steel-3 (PVA2) & 2\% & PVA 2\% \\
21-C(UHPC coating) & None & 21-C \\
21-C-SF-A & 0, 0.2, 0.4, 0.6, 0.8, 1.0 & 21-C-\(V_{f}\)-HSF \\
21-C-SF-B & 0, 0.2, 0.4, 0.6, 0.8, 1.0 & 21-C-\(V_{f}\)-CSF \\
21-C-SF-C & 0, 0.2, 0.4, 0.6, 0.8, 1.0 & 21-C-\(V_{f}\)-CSSF \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Specimens in the pullout test. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Mix Type**} & **Cement** & **Water** & **Sand** & **Coarse** & **Blast Furnace** & **Fly Ash** & **Fiber** & **Super-** \\  & **(kg/m3)** & **(kg/m3)** & **Aggregate** & **Slag** & **(kg/m3)** & **(kg/m3)** & **Content** & **Plasticizer** \\  & & & & **(kg/m3)** & **(kg/m3)** & **(kg/m3)** & **(kg/m3)** \\ \hline
21-N (OPC) & 239 & 164 & 922 & 928 & 30 & 30 & - & 1.79 \\ Steel-1 (PVA0) & 580 & 255 & 580 & - & - & 580 & 0 & 6.96 \\ Steel-2 (PVA1) & 580 & 255 & 580 & - & - & 580 & 13 & 6.96 \\ Steel-3 (PVA2) & 580 & 255 & 580 & - & - & 580 & 26 & 11.6 \\
21-C-0.2-SF & 239 & 164 & 922 & 928 & 30 & 30 & 0.31 & 1.79 \\
21-C-0.4-SF & 239 & 164 & 922 & 928 & 30 & 30 & 0.63 & 1.79 \\
21-C-0.6-SF & 239 & 164 & 922 & 928 & 30 & 30 & 0.94 & 1.79 \\
21-C-0.8-SF & 239 & 164 & 922 & 928 & 30 & 30 & 1.26 & 1.79 \\
21-C-1.0-SF & 239 & 164 & 922 & 928 & 30 & 30 & 1.57 & 1.79 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Concrete mix proportions. Except for the specimen with high CSF and CSSF ratios, all specimens showed pullout failure before the yielding of rebars, without splitting cracks appearing on the surfaces of the specimens. The specimens with 0.8% and 1% of CSF or CSSF showed pullout failure after the yielding of rebars. The interfacial debonding behaviors of the pullout test specimen consisting of UHPC-coated steel rebar and different structural fibers on the interfacial bonding zone are discussed in this section. The pullout failure of the rebars was defined when the applied load reached the maximum point. The corresponding values of the maximum nominal bond stress and slip were then determined. Since the interfacial stress distribution between the rebar and concrete matrix was not constant along the embedded part, the average bond strength was calculated as follows:  
\[\tau_{max}\ =\ \frac{P_{max}}{\pi d_{b}l_{b}} \tag{1}\]  
where \(\tau_{max}\) is the bond strength, \(P_{max}\) is the maximum tensile load, \(d_{\text{b}}\) is the rebar diameter, and \(l_{\text{b}}\) is the embedment length. The various test specimens' performances as a result for the pullout test are summarized in Table 5. In this table, \(f_{c}\) is the concrete strength used for making pullout specimens, \(\tau_{max}\) is the bond strength, and \(S_{m}\) is the slip value at the maximum load. The mean values of the bond strength and the slips are summarized in the table. The normalized bond strength(\(\tau^{{}^{\prime}}_{max}\)) [32; 33; 34; 35] describing the effect of concrete strength was calculated as follows:  
\[\tau^{{}^{\ast}}_{max}\ =\ \frac{\tau_{max}}{\sqrt{f_{c}}} \tag{2}\]