# Experimental Study of Composite Hollow RC Column under Uniaxial Compressive Load
## 2 Test Program  
The steel-composite hollow RC columns with HSC were tested under compressive axial concentric loading. Figure 2 shows the design of the test columns in detail. The following variables were studied in the test program: (1) concrete strength (design normal-strength concrete: 20 MPa, design high-strength concrete: 50 MPa), (2) inner tube thickness, (3) hollow ratio (ratio between the inside diameter of the column and the outside diameter of the column), (4) column diameter, and (5) transverse reinforcement space.  
Figure 1: Cross-section of a steel-composite hollow reinforced concrete (RC) column.  
Figure 2: Dimensions of test specimens (red color: strain gauges).  
The specimens in groups 2-5 were made of concrete with a specified strength of 50 MPa. Normal-strength concrete was applied to the inner tube of group 1, which was subsequently compared to HSC specimens. To ensure that failure would occur in the region at the center of the column, both sides of the columns were reinforced with a bolted steel sheet made from a steel plate of 20 mm. Figure 2 shows the dimensions of the test specimens and strain gauges. The height of the specimens was 1200 mm and the experimental region at the column had a length of 800 mm. The diameter of the column was 350 mm. The strain gauges were attached at the transverse and longitudinal reinforcements in the inner tube.  
Figure 3 shows a diagram of the experimental setup. The test specimens were loaded on a rigid hydraulic press with load control capabilities at a maximum compressive load of 10,000 kN. The axial displacement was measured using four linear variable differential transformers located at the four corners of the columns and attached to the top, which is shown in Figure 3.  
Table 1 shows the characteristics of the test specimens. To study the concrete strength, two different concrete mixes designed with a 28-day strength of 50 MPa and 20 MPa were produced. The cured cylinder of concrete had an average strength of 43.5 MPa for HSC and 17.2 MPa for NSC. To measure the material properties of the reinforcement and steel tube, a tensile test was performed. The average tensile strength of steel reinforcement was 401 MPa, whereas that of the steel tube was 395 MPa.  
The test variables included the inner tube thickness (including normal-strength concrete), hollow ratio, column diameter, and transverse reinforcement space. Range of the inner tube thickness was from 0 to 6 mm. Furthermore, Specim Nos. 5 and 22 had transverse and longitudinal reinforcements instead of inner tube reinforcements in the inner side of the concrete. The hollow ratio, which is defined as the ratio between the outside and inside diameters of the column, ranged from 0.36 to 0.58. The diameter of the column ranged from 300 to 450 mm. The transverse reinforcement space increased from 80 to 150 mm, which is shown in Table 1.  
Figure 3: Diagram of experimental setup.  
Figure 4 shows the layout of the reinforcements and the setup of the specimens. The steel plates on the top and bottom of the specimens were reinforced to prevent any failure from ripping off due to compressive concentric loading. After the curing and placement of the concrete, the specimens were produced for testing, which is shown in Figure 4.