## 2 Experimental Program  
### Test Specimens  
For this study, a total of seven specimens were manufactured and tested. Among the seven specimens, six were strengthened with the newly proposed perfobond shear connectors, and the remaining specimen with conventional steel reinforcing bars. Figure 3 illustrates the details of the two types of test specimens, including their geometrical configurations and infilled concrete depths. The perfobond shear connectors were connected to the outer surface of the steel pile cap by bolting. In addition, a number of L-shaped steel plates were connected to the inner surface of the steel pile cap by bolting. They play the role of shear keys and provide additional shear resistance. The center of the steel pile cap coincided with that of the concrete block in all specimens.  
Figure 3: Details of test specimens. (**a**) Specim strengthened with conventional steel rebars; (**b**) Specim strengthened with perfobond shear connectors (unit: mm).  
The details of the seven test specimens are summarized in Table 1. In all of the specimens, the diameter of the steel pile (D) is 400 mm. The main test parameters include the type of perfobond shear connectors, infilled concrete depths, and the number of L-shaped steel plates. Two types of perfobond shear connectors were used in the test specimens. The first type (PO) has open holes, while the other (PC) closed holes, shown in Figure 4. They all have sectional dimensions of 100 mm by 8 mm and a length of 620 mm. The reference specimen strengthened with conventional steel rebars (SB) had the infilled concrete depth of 700 mm, which is greater than the diameter of the steel pile and satisfies the requirement of the development length for the rebars suggested in [10]. In contrast, the specimens strengthened with the perfobond shear connectors had two different infilled concrete depths corresponding to the diameter (D10) and half diameter (D05) of the steel pile, respectively. Three different numbers of L-shaped steel plates, which are 0 (NL), 3 (L3), and 6 (L6), were installed in the specimens strengthened with the perfobond connectors. The effects of the test parameters on the bending resistance capacity of the strengthened steel pile cap are discussed in Section 3 in detail.  
The compressive strength of concrete was measured in accordance with the standards of ASTM C39 [11], and the measured average strength was found to be 31.9 (MPa). Five different sizes of rebars such as H10, H13, H16, H19, and H22 were used in the test specimens. The steel types used for steel piles and steel plates are SPS490 and SS400, respectively. Their yield strength, ultimate strength, and maximum strain at fracture were measured per ASTM A370 [12]. The summary of the measured properties is provided in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Properties**} & \multirow{2}{*}{**Concrete**} & \multicolumn{4}{c}{**Rebars**} & \multicolumn{2}{c}{**Steel Pile**} & \multicolumn{1}{c}{**Steel Plate**} \\ \cline{3-8}  & & **H10** & **H13** & **H16** & **H19** & **H22** & **SPS400** & **SS400** \\ \hline compressive strength & 31.9 & - & - & - & - & - & - \\ yield strength & - & 607.5 & 524.4 & 490.1 & 475.7 & 579.6 & 242.1 & 321.4 \\ ultimate strength & - & 707.1 & 650.9 & 584.9 & 581.8 & 569.2 & 418.8 & 441.5 \\ maximum strain at fracture & - & 17.2 & 18.1 & 18.1 & 18.3 & 17.3 & - & 33.7 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Material properties of test specimens (unit: MPa (strength), % (strain)).  
Figure 4: Types of perfobond shear connectors. (**a**) Perfobond shear connector with closed holes; (**b**) Perfobond shear connectors with open holes (unit: mm).