# Performance Evaluation of Reinforced Recycled Aggregate Concrete Columns under Cyclic Loadings
## 3 Parametric Investigation
### Influences of \(f_{y}\) and \(\rho_{\mathrm{s}}\)  
Four classes of steel reinforcement in accordance with the Chinese concrete design code (GB 50010-2010 [78]) were employed to conduct the parametric study. The values of steel yield strength \(f_{y}\) corresponding to the four classes were 300, 335, 400, and 500 MPa. The elastic modulus of the steel reinforcement \(E_{\mathrm{s}}\) was taken as 200 GPa. The numerical models of RRC columns in this section were established based on the specimens reported in Yang [43] via varying the values of \(f_{y}\) described above. Figure 10 illustrates the calculated hysteresis loops for the RRC columns, and Figure 11 shows the seismic performance indexes (i.e., \(P_{\mathrm{u}}\) and \(\mu\)) of those columns. From Figures 10 and 11 it can be concluded that improving \(f_{y}\) definitely resulted in a marked increase in \(P_{\mathrm{u}}\). But increasing \(f_{y}\) also reduced the ductility of RRC columns. This reduction can be explained by the inconsistent increase in the yield displacement (i.e., \(\Delta_{y}\)) and the ultimate displacement (i.e., \(\Delta_{0.85}\)) when higher-strength steel was used. The former (\(\Delta_{y}\)) was increased more significantly than the latter (\(\Delta_{0.85}\)), thus reducing the ductility ratio, as defined in Equation (5). From Figure 10a, it was also noticed that the value of \(P_{\mathrm{u}}\) at a specific yield strength (\(f_{y}\)) generally decreased with an increase in \(r\) because the AWM was adopted as discussed in Section 3.1. It should be highlighted that a similar study on the axial load capacity of RRC columns manufactured using the ETWM have been reported in the previous investigation conducted by authors (i.e., [64]), showing that the ETWM generally leads to a slight increase in the load capacity as a function of \(r\). Figure 10: Effect of steel yield strength on hysteresis loops of RRC columns (\(n=0.3\)). (**a**) \(r=0\%\), (**b**) \(r=50\%\), and (**c**) \(r=100\%\). The influence of area ratio of steel reinforcement (\(\rho_{\rm s}=A_{\rm s}/A\)) on the seismic performance of RRC columns was also discussed. The values of \(\rho_{\rm s}=1.57\%\), \(2.79\%\), \(4.36\%\), or \(6.28\%\) were considered by varying the diameter of the longitudinal steel reinforcement (i.e., \(d=12\), \(16\), \(20\), and \(24\) mm). Figure 12 shows the hysteresis loops of RRC columns with different \(\rho_{\rm s}\). Figure 13 shows the influence of \(\rho_{\rm s}\) on the seismic performance indexes. As expected, an increase in \(\rho_{\rm s}\) resulted in a substantial increase in \(P_{\rm u}\). This was because the steel reinforcement ratio contributed significantly in the flexural strength of the members. On the other hand, an increase in \(\rho_{\rm s}\) resulted in a reduction in \(\mu\) of RRC columns. In fact, the ductility characteristic was not only affected by concrete but also related to the stress level in the longitudinal bars caused by the axial compressive load; the following derivation clearly shows this. Figure 11: Effect of steel yield strength on seismic performance of RRC columns. (**a**) Capacity. (**b**) Ductility. Figure 12: Effect of steel ratio on hysteresis loops of RRC columns (\(n=0.3\)). (**a**) \(r=0\%\), (**b**) \(r=50\%\), and (**c**) \(r=100\%\).