## 4 Test Results and Analyses
### Bond Properties of Single Steel Fiber  
#### 4.3.1 Bond Strength  
Figure 11 presents the bond strength \(\tau_{\rm d}\), \(\tau_{\rm max}\), and \(\tau_{\rm res}\) of steel fibers. The hooked-end steel fiber HF3 has the maximum \(\tau_{\rm d}\), \(\tau_{\rm max}\), and \(\tau_{\rm res}\). HF2, HF4, and HF5 have a similar \(\tau_{\rm d}\) of about 53-55% of HF3, and their \(\tau_{\rm max}\) are 61%, 53%, and 70% of HF3, successively. HF2 has the \(\tau_{\rm res}\) of 55% of HF3. HF4 and HF5 have no \(\tau_{\rm res}\) due to their broken fibers. HFb has the smaller \(\tau_{\rm d}\), \(\tau_{\rm max}\), and \(\tau_{\rm res}\) of about 39%, 39%, and 36% of HF3, successively. HF6 has the smallest \(\tau_{\rm d}\), \(\tau_{\rm max}\), and \(\tau_{\rm res}\) of 22%, 24%, and 22% of HF3, successively. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**NO.**} & \multicolumn{2}{c}{**Key Point P**} & \multicolumn{2}{c}{**Key Point D**} & \multicolumn{2}{c}{**Key Point R**} \\ \cline{2-7}  & \(P_{\rm max}\) (N) & \(s_{\rm p}\) (mm) & \(P_{\rm d}\) (N) & \(s_{\rm d}\) (mm) & \(P_{\rm r}\) (N) & \(s_{\rm r}\) (mm) \\ \hline HFb & 637.2 & 0.888 & 388.6 & 0.258 & 399.7 & 2.50 \\ HF2 & 1481.6 & 1.332 & 800.0 & 0.256 & 953.6 & 2.61 \\ HF3 & 2788.4 & 1.276 & 1728.8 & 0.26 & 1974.6 & 2.68 \\ HF4 & 1320.4 & 0.864 & 831.9 & 0.252 & – & – \\ HF5 & 2908.8 & 0.76 & 1326.1 & 0.322 & – & – \\ HF6 & 1439.8 & 1.246 & 792 & 0.335 & 1018.5 & 2.74 \\ CF & 1587.2 & 1.457 & 682.5 & 0.241 & 607.5 & 2.5 \\ CF2 & 1929.6 & 0.944 & 1177.1 & 0.333 & 641.5 & 2.46 \\ IF1 & 2064.4 & 0.439 & 1465.7 & 0.243 & – & – \\ IF2 & 2636.0 & 0.59 & 2003.4 & 0.404 & 1258.2 & 1.01 \\ IF3 & 2297.4 & 0.52 & 1585.2 & 0.297 & – & – \\ IF4 & 1066.8 & 0.156 & 1066.8 & 0.156 & – & – \\ MF & 1652.4 & 0.334 & 1057.5 & 0.22 & – & – \\ LF & 601.3 & 0.314 & 511.1 & 0.22 & – & – \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Key points of pull-out load-slip curves. The crimped steel fiber CF2 has the maximum \(\tau_{\rm d}\), \(\tau_{\rm max}\), and \(\tau_{\rm res}\). CF has the \(\tau_{\rm d}\), \(\tau_{\rm max}\), and \(\tau_{\rm res}\) about 31%, 47%, and 51% of CF2, successively. The indentation steel fiber IF3 has the maximum \(\tau_{\rm max}\). Next is IF4 and IF2. IF1 has the smallest \(\tau_{\rm d}\) and \(\tau_{\rm max}\) of about 61% and 59% of IF3. IF4 has the same \(\tau_{\rm d}\) and \(\tau_{\rm max}\) due to no slope changing observed in the ascending portion of the pull-out load-slip curves of IF4. Compared with other types of steel fibers, the milling steel fiber MF has a good \(\tau_{\rm d}\) and \(\tau_{\rm max}\), and the large-end steel fiber LF has the smallest \(\tau_{\rm d}\) and \(\tau_{\rm max}\). These differences are also caused by their different mechanical anchoring mechanisms. Figure 12 presents the strength use efficiency \(u_{\rm sf}\) of different deformed steel fibers. Considering the safety coefficient of 1.40 for the tensile strength of steel fiber higher than 1100 MPa as that of a high-strength steel bar [38], the \(u_{\rm sf}\) of 71.4% would keep sufficient reliability to avoid the fracture of steel fiber. At the same time, ensuring the full use of fiber strength, HFb, CF2, and IF3 are suitable in the mortar of this test. The \(u_{\rm sf}\) of HF6 is only 37%. This indicates that the fiber strength has not fully played due to the weak mechanical anchoring in the mortar. Therefore, the hooked-end shape of HF6 does not match its fiber strength. The \(u_{\rm sf}\) of IF2 is 44.6%, due to the mortar cracking  
Figure 11: The bond strengths of characteristic points of different deformed steel fibers: (**a**) \(\tau_{\rm d}\); (**b**) \(\tau_{\rm max}\); (**c**) \(\tau_{\rm res}\). Figure 12: The strength uses efficiency \(u_{\rm sf}\) of different deformed steel fibers. in the test. This indicates that the designed embedded length of IF2 is too long, and IF2 would be suitable for concrete higher than 60 MPa.