# Delaying the Occurrence of Bar Buckling in RC Columns Confined with SRG Jacketing
## 5 Conclusions  
The effectiveness of SRG jackets in delaying bar buckling in substandard RC columns under uniaxial compression was experimentally investigated. The study involved applying single- and double-layered SRG jackets with varying axial stiffness to 18 out of the 24 RC columns. Among the 18 specimens, 12 were intentionally designed to be prone to premature buckling, representing older construction practices, with a slenderness ratio (s/D\({}_{\rm b}\)) of 6.25 and 12.5. The remaining columns complied with current code requirements. The different jacket schemes were distinguished based on the fabric density and the number of plies used. These variations included jackets with 4 cords/in and one or two layers, as well as jackets with 12 cords/in and two layers. The following conclusions are drawn:  
(1) All the unconfined columns (control specimens) failed due to buckling of the longitudinal compression reinforcement developing similar axial strength (around 30 MPa, which coincides with the cylinder concrete compressive strength) regardless of the s/D\({}_{\rm b}\) values. Especially for the well-detailed specimens of group C with the dense stirrups, this implies that the stirrups' confining action does not offer any gain in the axial compressive strength. For those cases where s/D\({}_{\rm b}\) > 6 (groups A, B), the axial strain at the initiation of strength degradation was lower than the yielding threshold of the compression bars, implying elastic buckling of the internal reinforcement. Only the well-detailed specimens C developed a marginal ductility (double of the yielding threshold of the compression bars), implying inelastic buckling. (2) The majority of the SRG-jacketed columns experienced progressive failure until advanced state of axial deformation due to inelastic bar buckling. The cords ruptured predominantly at the middle height of the specimens, accompanied by progressive debonding of the textile material at the anchorage zone. When two-layered SRG jackets were applied to the specimens, cord rupture was observed in both layers. (3) Columns with poor detailing (s/D\({}_{\rm b}\) > 6, smooth stirrups with improper anchoring) were benefitted by a light, single-layered, 4 cords/in SRG jacket mainly as per the increase of the axial strain at the initiation of strength degradation over double the yielding threshold, thus approximating the response of the well-detailed unconfined C specimens. Moreover, this SRG configuration offered (a) a milder post-strength response in comparison to the unconfined C specimens and (b) a marginal increase of axial strength (10-15%). The also easily applicable double-layered 4 cords/in SRG jacketing had the potential to increase the concrete axial strength by 20-30% attaching a more ductile response in comparison to the lighter jacket (\(\varepsilon_{\rm cc}/\varepsilon_{\rm y}\) = 4 versus 2.5). Moreover, the denser (12 cords/in), two-layered SRG jacket provided an additional 10% increase in strength whereas the axial strain ductility reached a minimum of 6. A note of caution is that in the case of the high density and stiffness jacket (12 cords/in, two-layered jacket), the design engineer should assess whether its advantages outweigh the challenges posed by the application difficulties (mortar penetration, fitting at the rounded corners of the member). The evaluation of these findings denotes that the SRG jacketing is a promising method for enhancing the behaviour of old-type RC columns by delaying bar buckling and thus enabling the members to achieve greater strength and deformation capacity. (4) The analysis of experimental data, following the guidelines presented in Chapter 8 of the fib Bulletin 90 [23] for FRP jacketing and by also considering a proposed model that accounts for the impact of bar buckling on the deformation of the SRG jacket, demonstrates satisfactory predictions for columns with varying levels of detailing.