Article Repair and strengthening of reinforced concrete columns with CFRPs Okan Ozcan 1 , Baris Binici 2 , Erdem Canbay 2 and Guney Ozcebe 2 Abstract In this study, seismic retrofitting of undamaged and moderately damaged RC columns with CFRP was investigated. The experimental program was conducted on five RC columns with inadequate tie spacing, 90-degree hooks at tie ends, and plain (undeformed) reinforcing bars. Columns were tested under a constant axial load of 27% of axial capacity and under reversed lateral cyclic loading. The primary objective of this study was to investigate the influence of sustained axial load during repairing, damage, and corner-rounding radius of CFRP wraps on the seismic performance of RC columns. Improved performance in terms of ductility, dissipated energy, and secant-stiffness degradation was observed for the CFRP strengthened columns. Reducing the corner-rounding radius deteriorated the seismic behavior of the columns, while the stiffness degradation response remained unaffected. The sustained axial load during repairing had only minor effect on column behavior. After CFRP repairing, the increase in yield drift ratio and curvature was observed to be in proportion with the corresponding consumed ductility values during damage cycles. A drift-based design equation for FRP repaired columns was proposed that considers the damage amount and axial load level as the major parameters as consistent with the experimental results. Keywords Reinforced concrete column, repair, strengthening, FRP Introduction Poor performance of reinforced concrete (RC) columns during recent earthquakes in Turkey (Kocaeli 1999, Du¨zce 1999, and Bingo¨l 2003) demonstrated the lack of deformability in building columns due to inadequate seismic detailing of confining steel reinforcement. Convenient strengthening and repairing techniques can be utilized for upgrading such columns. The tradi- tional method of repairing or strengthening RC col- umns by means of concrete 1–5 or steel jacketing 6,7 involves complicated and cumbersome implementation procedures. There are several studies in the literature on seismic upgrading of RC columns by using carbon fiber reinforced polymer (CFRP) sheets. 8–23 It was shown that CFRP wrapping of columns enhances the seismic behavior by confining the column section and preventing rebar buckling. Throughout these studies, repairing with concrete 1–3 and CFRP jacketing 10–13,18 was investigated only for columns that had deformed rebars for longitudinal reinforcement. In Turkey, the use of plain bars was quite common until the 1999 Kocaeli and Du¨zce earthquakes. Considering inadequate bond characteristics of plain bars, seismic behavior of such columns is inferior due to the need of longer development length and signifi- cant fixed-end rotations. The literature review con- ducted reveals that there was no experimental data on CFRP repair of RC columns with plain bars to enhance the deformability of the section. This study examines the performance of both strengthened and repaired 1 Department of Civil Engineering, Akdeniz University, Antalya, Turkey. 2 Department of Civil Engineering, Middle East Technical University, Ankara, Turkey. Corresponding author: Erdem Canbay, Department of Civil Engineering, Middle East Technical University, Ankara 06531, Turkey Email: ecanbay@metu.edu.tr Journal of Reinforced Plastics and Composites 29(22) 3411–3424 ! The Author(s) 2010 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684410376332 jrp.sagepub.com