## 3 Test Results and Discussion  
### Failure Modes  
Table 3 summarizes the test results such as peak loads, displacements at peak loads, and loads corresponding to several crack propagation phenomena occurring in the concrete block. All of the test specimens basically showed very similar crack propagation patterns, which are illustrated in Figure 6. Cracking first occurred on top of the concrete block in the direction perpendicular to the lateral load (Stage 1) and then propagated into its side surfaces (Stage 2). As the load further increased, cracks were developed in the direction parallel to the lateral load appearing on top of the concrete block (Stage 3), and concrete crushing occurred on its compression side (Stage 4). In all of the test specimens, where the proposed perfobond shear connectors were installed, the crack pattern of a half circle shape was observed on top of the concrete block, as shown in Figure 6a. This phenomenon was not observed in the reference specimen SB_D10_NL, where the steel pile was strengthened with conventional steel rebars.  
Figure 5: Test setup.  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Cracking Load (kN)**} & \multicolumn{4}{c}{**Peak Load**} & **Displacement** \\ \cline{2-7}
**Specimen** & **Stage 1** & **Stage 2** & **Stage 3** & **Stage 4** & **Lateral Load** & **Bending Moment** & **at Peak Load** \\  & & & & & **(kN)** & **(kN\(\cdot\)m)** & **(mm)** \\ \hline SB_D10\_NL & 99 & 99 & 192 & 277 & 342 & 308 & 50.0 \\ PC\_D05\_NL & 82 & 108 & 164 & 239 & 279 & 251 & 32.2 \\ PC\_D05\_L3 & 58 & 79 & 150 & 227 & 262 & 236 & 33.2 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of the test results.  
Figure 7 shows the cracking pattern of the reference specimen SB_D10_NL on its top and vertical faces. Cracks started to develop on the top and side surfaces of the concrete block in the direction perpendicular to the applied load at the load level (\(P\)) of 99 kN. As the load further increased, additional cracks were propagated on the top surface in the direction parallel to the applied load at \(P\) = 192 kN. Finally, concrete crushing occurred in the compression zone of the top surface at \(P\) = 277 kN, and the developed cracks were further propagated until the test specimen reached its complete failure stage.  
Figure 8 shows the cracking pattern of Specim PO_D10_L6 on its top and side faces. This type of crack patterns was observed in all of the test specimens, where the proposed perfobond shear connectors were installed, although their peak load values were slightly different from each other. On the top surface of the concrete block, cracks started to develop in the direction perpendicular to the applied load at \(P\) = 69 kN and then were propagated into the side surface of the concrete cube at \(P\) = 78 kN. Top surface cracks were developed in the parallel direction to the applied load at \(P\) = 187 kN. Concrete crushing happened on top of the concrete cube at \(P\) = 235 kN. The test was over  
Figure 6: Typical cracking patterns of test specimens. (**a**) Top surface of the concrete block; (**b**) Side surface of the concrete block.  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Cracking Load (kN)**} & \multicolumn{4}{c}{**Peak Load**} & **Displacement** \\ \cline{2-9}
**Specimen** & **Stage 1** & **Stage 2** & **Stage 3** & **Stage 4** & **Lateral Load** & **Bending Moment** & **at Peak Load** \\  & **Stage 2** & **Stage 3** & **Stage 4** & **(kN)** & **(kN:m)** & **(mm)** \\ \hline PC\_D10\_L3 & 72 & 86 & 193 & 265 & 286 & 257 & 68.6 \\ PC\_D10\_L6 & 69 & 78 & 187 & 235 & 303 & 273 & 48.0 \\ PO\_D05\_L3 & 62 & 84 & 217 & 273 & 319 & 287 & 39.2 \\ PO\_D10\_L3 & 74 & 153 & 262 & 287 & 326 & 293 & 43.6 \\ \hline \hline \end{tabular}
\end{table}
Table 3: _Cont._  
Figure 7: Failure mode of Specim SB_D10_NL.  
at \(P=303\) kN, when half-circle shaped cracks were intensively formed on the tension side of the top surface as shown in the figure.