# Delaying the Occurrence of Bar Buckling in RC Columns Confined with SRG Jacketing
## 1 Introduction  
Reinforced concrete (RC) critical structural elements (columns/beams) that do not conform to modern codes for seismic resistance introduced in the early 1980s are characterized by inferior material quality (concrete compressive strength less than 20 MPa, steel yielding stress lower than 400 MPa) and inadequate transverse reinforcement detailing. Especially, columns' areas of high moment and axial load demands due to seismic excitations (plastic hinge regions) suffer large inelastic deformations, which with the absence of dense and well anchored stirrups undergo quick deterioration of strength [1]. Especially for the transverse reinforcement used in construction practices before the 1980s in South Europe, it typically consists of smooth bars as small as 5 mm in diameter, with their ends simply overlapping at the corners. These stirrups are sparsely placed, resulting in a maximum spacing s\({}_{\text{max}}\) = min[min[b, h], 12D\({}_{\text{b}}\)], where b and h are the dimensions of the cross section, and D\({}_{\text{b}}\) is the diameter of the longitudinal reinforcement bars. For example, for b = h = 300 mm and D\({}_{\text{b}}\) = 14 mm, then s\({}_{\text{max}}\) equals 168 mm (refer to Chapter 4 in [1]) which means that only a single stirrup's legs cross a potential diagonal crack with nil contribution to shear resistance of a column under seismic excitation. Moreover, this lack of appropriate size and arrangement of stirrups leads to a significant unsupported length of the compressive longitudinal reinforcing bars, making them susceptible to buckling when reversed seismic sway induces increased axial loads to columns. As a consequence, there is an increased risk of brittle failure in the structural element because it possesses inadequate resistance to reversed cyclic load. The safe performance of the entire building can be compromised by localized damage in the plastic hinge regions of the columns, where sideways buckling of the compression reinforcement is expected due to the lateral shear distortion in that area. Mainly, the slenderness ratio s/D\({}_{\text{b}}\)[2, 3, 4, 5, 6] along with the stiffness and the anchorage detailing of the stirrups play a key role in the stability of compression reinforcing bars. Theslenderness ratio \(\rm s/D_{b}\) could range between 6 and 8 in case of high to moderate ductility RC members [7], whereas for \(\rm s/D_{b}\) lower than 5 (\(\rm s/D_{b}<5\)), the bar in compression is able to develop its full strain-strain response as in the case of tension. For values higher than eight (\(\rm s/D_{b}>8\)), the bars could reach buckling state when their compressive stress reaches yielding point [5; 6]. According to the practice detailing of the pre-1970s era, \(\rm s/D_{b}\) could receive any value between 10 and 40 [7]. The beneficial impact of confinement on the compressive behaviour of substandard RC columns has been consistently observed in previous studies [6; 7; 8; 9; 10; 11; 12; 13]. External wrapping using either FRP (fibre-reinforced polymers) or TRM (textile-reinforced mortars) has been found to increase the axial compressive strength and associated strain ductility. This improvement is attributed to the prevention of premature buckling of the reinforcing bars, delaying it to occur at higher levels of strain ductility [8; 12]. When the longitudinal reinforcement approaches a state of instability at the critical axial strain, the bar undergoes lateral bending to maintain compatibility with the increasing axial strain of the supporting concrete core [4]. By wrapping the column with composite fabrics, the confined concrete experiences enhanced strength and strain capacity. As long as the increased axial strength of concrete due to confinement is able to undertake the overload released by the buckled reinforcement, the member continues to deform until strain concentrations limit the effectiveness of the composite jacket as lateral support for the longitudinal reinforcement [12].