# Cyclic Behaviors of Geopolymeric Recycled Brick Aggregate Concrete-Filled Steel Tubular Column
## 3 Test Results and Discussion
### Skeleton Curves  
Skeleton curves obtained from the successive peak point from hysteresis curve are shown in Figure 10. Initially the displacement linearly changed with an increase of the load, then the growth of load slowed down once yielded. Similar to the discussion in Section 3.2, no evident difference was noticed in the development of load-displacement curve before the yielding of steel tube for different specimens. The bearing capacity and the degradation behavior was significantly influenced by the variation parameters. The mechanical behavior index at the feature point in skeleton curves was analyzed and the corresponding characteristic value is outlined in Table 5. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Specimen NO.**} & \multirow{2}{*}{**Direction**} & \multicolumn{2}{c}{**Yield Point**} & \multicolumn{2}{c}{**Peak Point**} & \multicolumn{2}{c}{**Failure Pint**} & \multirow{2}{*}{\(\mu=\Delta_{\mu}/\Delta_{\gamma}\)} \\  & & \(P_{\gamma}/\)**kN** & \(\Delta_{\gamma}/\)**mm** & \(P_{\gamma}/\)**kN** & \(\Delta_{\gamma}/\)**mm** & \(P_{\gamma}/\)**kN** & \(\Delta_{\gamma}/\)**mm** & \(\mu=\Delta_{\mu}/\Delta_{\gamma}\) \\ \hline \multirow{3}{*}{STG4-0-0.25} & Pos. & 47.88 & 11.09 & 59.68 & 18.05 & 50.73 & 44.34 & 4.0 \\  & Neg. & 46.84 & 10.69 & 57.7 & 22.9 & 49.05 & 35.28 & 3.3 \\  & Ave. & 47.36 & 10.89 & 58.69 & 20.48 & 49.89 & 39.81 & 3.65 \\  & Pos. & 31.44 & 7.73 & 50.5 & 19.5 & 42.93 & 29.76 & 3.85 \\ \multirow{3}{*}{STG4-0.3-0.25} & Neg. & 46.95 & 12.54 & 57.7 & 25.3 & 49.05 & 38.83 & 3.1 \\  & Ave. & 39.2 & 10.14 & 54.1 & 22.4 & 45.99 & 34.3 & 3.48 \\ \multirow{3}{*}{STG4-0.5-0.25} & Pos. & 37.63 & 9.23 & 51.7 & 18.8 & 43.95 & 34.8 & 3.77 \\  & Neg. & 42.56 & 10.23 & 53.2 & 18.9 & 45.22 & 31.32 & 3.06 \\ \multirow{3}{*}{STG4-0.5-0.25} & Ave. & 40.1 & 9.73 & 52.45 & 18.85 & 44.59 & 33.06 & 3.42 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Measured load and displacement at feature points. Figure 10: (**a**) Skeleton curves for the replacement ratio of RBA; (**b**) Skeleton curves for the thickness of the steel tube; (**c**) Skeleton curves for the axial compression ratio; (**d**) Skeleton curves for cementitious material. From Figure 10a and Table 5, it was clearly found that the peak load reduced with a higher replacement ratio of RBA, and compared with STG4-0-0.25, the bearing capacity of STG4-0.3-0.25, STG-0.5-0.25, STG4-0.7-0.25 and STG4-1-0.25 decreased by 9.48%, 11.25%, 16.92% and 17.68%, respectively. It was analyzed that due to the inherent flaw of RBA, the initial damage accumulated with the increase of RBA replacement ratio, and the negative effect caused by the inherent flaw became influential, leading to a more negative effect on the bearing capacity. Moreover, the difference in the failure mode was more evident since the confinement effect was weakened once yielding of the steel tube occurred. In Figure 10b, the increase of the steel tube thickness was beneficial for the initial stiffness and the peak load, while it had little influence on the degradation in behavior. The reason for this was that in the degrading stage, the confinement from steel tube was no longer effective due to the bulge of the steel tube. The bearing capacity of STG6-0.5-0.25 was increased by 23.98% compared with STG4-0.5-0.25, since a thicker steel tube provided better confinement to inner concrete, which in return provided favorable lateral support for the steel tube to prevent premature buckling under a small load. In Figure 10c, the influence on the bearing capacity was insignificant when the axial compression ratio was less than 0.25. The degradation in resistance of GRBACFST column was gentler under a small axial compression ratio. The applied axial load would result in a more serious P-\(\Delta\) effect with a higher axial compression, leading to premature yielding or bulging of the steel tube; moreover, the inner concrete was also more prone to crack and crash.