# Experimental Study of Composite Hollow RC Column under Uniaxial Compressive Load
## 3 Test Results  
### Investigation of Test Results  
Figure 5 shows Specim HSC-HS_130 at different loading stages. During the ascending part of loading, cracks appeared in the concrete cover after the load exceeded the maximum strength of the column. The cover concrete became suddenly separated at the initial load level. After the crack occurred in the cover concrete on the loading side with an increase in compressive concentric loading, the crack propagated at an angle of 45\({}^{\circ}\). Figure 5(a) shows the axial load-displacement interactions. The maximum strength of this specimen was 3700 kN. After the maximum strength of the column was reached, the strength reduced rapidly due to the buckling of longitudinal reinforcement or yielding of the transverse reinforcement, which is shown in Figure 5(b),c. We confirmed that the strains of longitudinal reinforcements changed from the compressed state to the tensile state, which is shown in Figure 5(c). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Variables** & **No.** & **Name** & **Concrete** & **Column** & **Outside Diameter** & **Inner Tube** & **Transverse** & **Number of Longitudinal** \\  & & & **Strong** & **Diameter** & **of Inner Tube** & **Tickness** & **Reinforcement Space** & **Reinforcements** \\ \hline  & 1 & HSC-h\({}_{\pm 0.0}\) & 43.5 & 350 & 0 & 0 & 100 & 6 \\ Inner tube thickness & 2 & HSC-h\({}_{\pm 1.0}\) & 43.5 & 350 & 165.2 & 1 & 100 & 6 \\ Inner tube thickness & 3 & HSC-h\({}_{\pm 0.0}\) & 45.5 & 350 & 165.2 & 3 & 100 & 6 \\  & 4 & HSC-h\({}_{\pm 0.0}\) & 43.5 & 350 & 173.2 & 6 (nulable loop) & 100 & 6 \\  & 6 & HSC-h\({}_{\pm 0.0}\) & 43.5 & 350 & 139.8 & 6 & 100 & 7 \\ Hollow Ratio & 7 & HSC-h\({}_{\pm 0.0}\) & 44.5 & 350 & 165.2 & 6 & 100 & 7 \\  & 8 & HSC-h\({}_{\pm 0.0}\) & 43.5 & 350 & 190.7 & 6 & 100 & 6 \\  & 9 & HSC-h\({}_{\pm 0.0}\) & 43.5 & 350 & 263.6 & 6 & 100 & 6 \\  & 10 & HSC-h\({}_{\pm 0.0}\) & 43.5 & 350 & 165.2 & 6 & 100 & 4 \\
**Column diameter** & 11 & HSC-h\({}_{\pm 0.0}\) & 45.5 & 350 & 180.7 & 6 & 100 & 6 \\  & 12 & HSC-h\({}_{\pm 0.0}\) & 43.5 & 400 & 216.3 & 6 & 100 & 8 \\  & 13 & HSC-h\({}_{\pm 0.0}\) & 43.5 & 400 & 287.4 & 6 & 100 & 9 \\  & 14 & HSC-h\({}_{\pm 0.0}\) & 45.0 & 350 & 168.2 & 6 & 80 & 6 \\ Space of hoop rebar & 15 & HSC-h\({}_{\pm 0.0}\) & 43.5 & 350 & 168.2 & 6 & 100 & 6 \\  & 16 & HSC-h\({}_{\pm 0.0}\) & 45.0 & 350 & 168.2 & 6 & 130 & 6 \\  & 17 & HSC-h\({}_{\pm 0.0}\) & 45.5 & 350 & 168.2 & 6 & 130 & 6 \\  & 18 & NSC-h\({}_{\pm 0.0}\) & 72.0 & 350 & 0 & 0 & 100 & 6 \\ Inner tube thickness & 19 & NSC-h\({}_{\pm 1.0}\) & 17.2 & 350 & 165.2 & 1 & 100 & 6 \\ normal-strength & 20 & NSC-h\({}_{\pm 0.0}\) & 17.2 & 350 & 168.2 & 3 & 100 & 6 \\ concrete & 21 & NSC-h\({}_{\pm 1.0}\) & 17.2 & 350 & 165.2 & 6 & 100 & 6 \\  & 22 & NSC-h\({}_{\pm 0.0}\) & 17.2 & 350 & 173.2 & 6 (nulable loop) & 100 & 6 \\ \hline \hline \end{tabular}  
* Denotation: *HSC-In_t, 0.0—First term: compressive strength of concrete, Second term: parameters, and Third term: dimensions. *NSC: normal-strength concrete, HSC: high-strength concrete, In: inner tube thickness, In:Hoop: diameter of inner hoop rebar, H: hollow ratio, D: diameter of column, and HS: space of hoop rebar.  
\end{table} 
Table 1: Details of test specimens (unit MPa, mm). Figure 4: Layout of reinforcements (**a**) and setup of specimens (**b**). Figure 5: Appearance of test specimens at different loading stages (HSC-HS_130). Figure 6: _Cont._  
In general, the strength of the RC column with HSC decreased after it reached its maximum strength. On the contrary, the steel-composite hollow RC column had a bifurcation point (3), as shown in Figure 5(a). This phenomenon occurred due to the resistance after the buckling of inner tube, as shown in Figure 5(d). This column had 65% of the maximum strength after the inner steel tube collapsed. Figure 7 and Table 2 show the images of specimens after testing. These specimens had two trends depending on the splicing of the transverse reinforcements.