# Delaying the Occurrence of Bar Buckling in RC Columns Confined with SRG Jacketing
## 3 Test Results and Discussion
### Response under Monotonic Axial Load  
The axial stress versus axial/lateral strain response curves of the tested columns per group are presented in Figure 6, aiming to demonstrate the influence of the increased jacket stiffness (from lighter to darker colour in each diagram) to a specific bar slenderness ratio \(\mathrm{s/D_{b}}\) (green-coloured curves correspond to \(\mathrm{s/D_{b}}=12.5\); blue-coloured curves correspond to \(\mathrm{s/D_{b}}=6.25\); and orange coloured curves correspond to \(\mathrm{s/D_{b}}=4.17\)). In general, it is observed that as the SRG stiffness increases, both axial strength and axial deformation capacity (here considered the deformation beyond which an abrupt reduction of strength occurs) increase for all three stirrup arrangements. Additionally, the lateral deformation capacity, which may quantify the confining pressure provided by the SRG jacket and accounts for concrete core dilation and stretching due to outwards bending of the bars, increases as the bar slenderness ratio (\(\mathrm{s/D_{b}}\)) decreases (Figure 6). All control columns developed similar strength (around 30 MPa, which coincides with the cylinder concrete compressive strength) regardless of the \(\mathrm{s/D_{b}}\) value. However, their axial deformation capacity increased as the \(\mathrm{s/D_{b}}\) decreased. The vertical dashed line in Figure 6 denotes the upper strain limit of an SRG system in direct tension (i.e., 0.025 according to [24; 25; 26]). This limit is compared with the experimental lateral strain values at the initiation of strength drop for each specimen (see values \(\varepsilon_{\mathrm{lat,cc}}\) in column (4) of Table 2). Since the recorded values often exceed the established limit value (i.e., 0.025 [24; 25; 26]) by a significant margin, it has been decided not to use these data in the analysis. However, for the sake of completeness, they are presented here. Figure 5: Control (X = A, B, C) and SRG-jacketed columns (DiX) at failure. Figure 6: Axial stress–axial/lateral strain curves of the tested columns of (**a**) group A (s/D\({}_{\rm b}\) = 12.5); (**b**) group B (s/D\({}_{\rm b}\) = 6.25); (**c**) group C (s/D\({}_{\rm b}\) = 4.17). Figure 7a-c depict how effective a particular jacket configuration is in increasing the strength and deformation capacity with progressive improvement of the longitudinal bars support conditions, i.e., by lowering the \(\rm{s/D_{b}}\) ratio (the same colour code is used as in in Figure 6). Note that all unconfined columns developed the same strength regardless of the \(\rm{s/D_{b}}\) ratio. The single-layered 4 cords/in density SRG jackets slightly improved the strength (up to 15% in comparison the control specimens which was close to 30 MPa) but kept the axial deformation capacity at the same level with decreasing \(\rm{s/D_{b}}\) (Figure 7a). The two-layered 4 cords/in density SRG jackets increased the strength by the same level (around 38 MPa, corresponding to 25% compared to the value of the control specimen) but of higher axial deformation in case of \(\rm{s/D_{b}}=4.17\) (Figure 7b). The two layered 12 cords/in SRG jacket improved the strength (increase of 25% to 40% in comparison to 30 MPa in the control specimens) by sustaining the axial deformation to similar levels as \(\rm{s/D_{b}}\) decreased (Figure 7c).