Seismic strengthening of sub-standard rc columns through a novel hybrid thin jacketing method

In this study, a novel hybrid thin jacketing method, in which fiber-reinforced polymer (FRP) sheets are used together with conventional steel reinforcing bars and cement-based repair mortar for strengthening of sub-standard full-scale columns, is proposed. The application of the method is comprised of three parts, i) cover concrete is removed and additional longitudinal steel reinforcing bars are installed with anchors to the foundation, ii) cover concrete is re-formed using repair mortar and iii) columns are wrapped externally in transverse direction with carbon FRP (CFRP) sheets. The major advantages of this strengthening method are unaltered dimensions of columns after strengthening and ease in application. Within the scope of the study, two full-scale sub-standard reinforced concrete (RC) columns were tested under reversed cyclic lateral displacements simulating seismic actions. One of the columns was tested prior to strengthening while the other was tested after strengthening with the specified hybrid thin jacketing method. Three special features of the specimens tested in this study are i) insufficient transverse steel reinforcement with large spacing and ii) high axial load to capacity ratio 0.75 and iii) high shear force corresponding to moment capacity to shear capacity ratio of 0.62 and 0.80 according to ACI 318-19 (2019) and Turkish Building Earthquake Code (TBEC, 2018), respectively. Based on TBEC (2018), columns had a V_e/(f_ctmbd) ratio of 1.12 indicating a high shear demand, where, V_e, f_ctm, and d are the shear force corresponding to moment capacity, direct tensile strength of concrete, and effective depth of the section. In addition, the ratio of transverse reinforcement area to the minimum required transverse reinforcement area according to ACI 318-19 (2019) and TBEC (2018) was 0.19 and 0.77, respectively. The experimental findings showed that the proposed easy-to-apply hybrid thin jacketing method is effective in terms of enhancement of seismic performance of sub-standard RC columns.