## 2 Experimental Program
### Test and Measuring Devices  
The schematic diagram of the test device is illustrated in Figure 3a, and the corresponding field layout of the test device is shown in Figure 3b. In Figure 3a, No.1, No.2 and No.3 represent the reaction wall, vertical reaction steel column and reaction steel beam respectively, in which the steel beam and steel column are connected by high-strength bolts. No.4 represents the electro-hydraulic servo actuator that can push and pull back and forth, which is connected to the specimen by high-strength bolts through a special loading end. In addition, No.5, No.6, No.7 and No.8 refer to the hydraulic jack, specimen, pressure beam made of steel and roller device, respectively. It should be noted that a roller device is placed between the jack and the reaction steel beam to facilitate the free horizontal movement of the jack with the specimen. After the specimen was assembled according to the predetermined test axial pressure ratio, two identical hydraulic jacks were used to synchronously apply a constant vertical load to the top of the column, in order to simulate the actual situation in frame structures. After each of the vertical loads reached a stable value, the specimen was tested under a low-frequency cyclic lateral load. According to the Chinese standard of seismic test methods of buildings [26], the loading process included two main steps, namely a load-control step and a displacement-control step. In addition, in the load-control \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Member** & \begin{tabular}{c} **Yield Strength** \\ **(N/mm2)** \\ \end{tabular} & \begin{tabular}{c} **Ultimate Strength** \\ **(N/mm2)** \\ \end{tabular} & \
\begin{tabular}{c} **Modulus of Elasticity** \\ **(N/mm2)** \\ \end{tabular} &
\begin{tabular}{c} **Yield Strain** \\ **(\(\mu\)\(\varepsilon\))** \\ \end{tabular} \\ \hline Steel tube & 406.5 & 478.3 & \(2.18\times 10^{5}\) & 1865 \\ Longitudinal reinforcement & 470.5 & 672.8 & \(1.99\times 10^{5}\) & 2364 \\ Stirrup & 419.9 & 548.5 & \(2.16\times 10^{5}\) & 1944 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The measured mechanical properties of steel. Figure 3: The test set-up arrangement of the (**a**) test device and (**b**) loading scene. \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Coarse Aggregate** & \begin{tabular}{c} **Grading** \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} **Apparent Density** \\ **(kg/m3)** \\ \end{tabular} & \begin{tabular}{c} **Bulk Density** \\ **(kg/m3)** \\ \end{tabular} & \
\begin{tabular}{c} **Water Absorption** \\ **(\%)** \\ \end{tabular} &
\begin{tabular}{c} **Water Content** \\ **(\%)** \\ \end{tabular} \\ \hline Natural & 5–20 & 2722 & 1435 & 0.05 & 0.00 \\ \hline Recycled & 5–20 & 2655 & 1270 & 3.16 & 1.82 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The physical properties of the coarse aggregate. step, each step was cycled once, and in the displacement-control step, each step was cycled three times, as depicted in Figure 4. The specific loading steps were as follows. First of all, the load-control was adopted for graded loading, with each 5 kN being one level, until the specimen reached the yield load \(\mathbf{P}_{\text{y}}\). After that, the displacement-control was used, taking the multiple of the yield displacement \(\mathbf{\Delta}_{\text{y}}\) as the grade difference to control the loading progress. The yield displacement was the horizontal displacement of the column end when the steel tube or longitudinal bar in the frame first reached the yield strain, which in this test was 6 mm. Finally, the test was finished when the load of the specimen dropped to about 85% of the peak load \(\mathbf{P}_{\text{u}}\). During the test, the horizontal load and displacement were measured by the load and displacement sensor of the electro-hydraulic servo loading system. The strain of steel tube, longitudinal reinforcement, stirrups and beam RAC was measured by strain gauges.