Shear Strengthening of RC Beams with Externally Bonded FRP Composites: Effect of Strip-Width-to-Strip-Spacing Ratio Amir Mofidi 1 and Omar Chaallal, M.ASCE 2 Abstract: The results of an experimental and analytical investigation of shear strengthening of reinforced concrete (RC) beams with ex- ternally bonded (EB) fiber-reinforced polymer (FRP) strips and sheets are presented, with emphasis on the effect of the strip-width-to-strip- spacing ratio on the contribution of FRP (V f ). In all, 14 tests were performed on 4,520-mm-long T -beams. RC beams strengthened in shear using carbon FRP (CFRP) strips with different width-to-spacing ratios were considered, and their performance was investigated. In addition, these results are compared with those obtained for RC beams strengthened with various numbers of layers of continuous CFRP sheet. More- over, various existing equations that express the effect of FRP strip width and concrete-member width and that have been proposed based on single or double FRP-to-concrete direct pullout tests are checked for RC beams strengthened in shear with CFRP strips. The objectives of this study are to investigate the following: (1) the effectiveness of EB discontinuous FRP sheets (FRP strips) compared with that of EB continuous FRP sheets; (2) the optimum strip-width-to-strip-spacing ratio for FRP (i.e., the optimum FRP rigidity); (3) the effect of FRP strip location with respect to internal transverse-steel location; (4) the effect of FRP strip width; and (5) the effect of internal transverse-steel reinforcement on the CFRP shear contribution. DOI: 10.1061/(ASCE)CC.1943-5614.0000219. © 2011 American Society of Civil Engineers. CE Database subject headings: Fiber reinforced polymer; Concrete beams; Reinforced concrete; Sheets; Shear strength; Bonding. Author keywords: Carbon-fiber-reinforced polymers; Concrete beams; Continuous sheet; Discontinuous sheet; Shear strengthening; Strip-width-to-strip-spacing ratio. Introduction and Background Significant interest has been shown in the use of externally bonded (EB) fiber-reinforced polymer (FRP) sheets and laminates for strengthening and repair of existing reinforced concrete (RC) beams and slabs, especially for bridges and buildings. Because of its intricacy, shear strengthening of RC beams with FRP is some- what less well documented than FRP strengthening of beams in flexure and of columns (Bousselham and Chaallal 2004). Research studies carried out during the past decade have provided valuable outcomes, especially concerning the effect of FRP axial rigidity on the shear-strength enhancement of RC beams (Triantafillou 1998; Khalifa et al. 1998). On the basis of these research studies, many codes and design guidelines (hereafter called “the guidelines”) for RC structures strengthened with EB FRP have been published worldwide (e.g., ACI 440.2R-08, fib-TG 9.3-01, CAN/CSA S806-02, CAN/CSA S6-06, CNR-DT200-04, and HB 305-08). However, in a statistical comparison with available experimental results, Lima and Barros (2007) have revealed that some of the guideline equations (ACI 440 and CNR-DT200) lead to unduly conservative designs that use quantities of FRP materials well in excess of what is required. Efficient and accurate FRP strengthen- ing design equations will greatly benefit the economic aspects of FRP strengthening projects. Based on available experimental data, Triantafillou (1998) dis- covered that FRP effective strain decreases as FRP rigidity in- creases. Khalifa and Nanni (2000) realized that increasing the amount of carbon FRP (CFRP) may not always result in a propor- tional increase in shear resistance. They noted the existence of an FRP quantity threshold beyond which further strengthening effec- tiveness is questionable. In addition, their experimental tests showed that CFRP strips can outperform continuous CFRP sheets. Other experimental tests by Khalifa and Nanni (2002) and Zhang and Hsu (2005) confirmed these observations. Unlike shear-strengthening methods that use FRP continuous sheets, with discontinuous FRP sheets, the quantity of FRP present varies with the strip-width-to-strip-spacing ratio (w f =s f ). The FRP rigidity parameter (R f ) can be used to quantify the amount of FRP used with respect to the FRP-strip-width-to-FRP-strip-spacing ra- tio. The FRP rigidity parameter can be expressed as a function of (w f =s f ) as follows: R f ¼ ρ f × E f ð1a Þ where ρ f ¼ 2t f b w × w f s f ð1b Þ In the calculation of FRP rigidity, the only variable is the FRP-strip-width-to-FRP-strip-spacing ratio (assuming that the FRP sheet characteristics and the RC beam cross-sectional dimen- sions are constants). Based on the writers’ updated database, which 1 Ph.D. Candidate, Dept. of Construction Engineering, Univ. of Quebec, École de Technologie Supérieure, Montreal QC, Canada H3C 1K3. E-mail: amir.mofidi.1@ens.etsmtl.ca 2 Professor of Construction Engineering, Univ. of Quebec, École de Technologie Supérieure, 1100 Notre-Dame St. West, Montreal QC, Canada H3C 1K3 (corresponding author). E-mail: omar.chaallal@etsmtl.ca Note. This manuscript was submitted on July 13, 2010; approved on March 24, 2011; published online on March 26, 2011. Discussion period open until March 1, 2012; separate discussions must be submitted for in- dividual papers. This paper is part of the Journal of Composites for Con- struction, Vol. 15, No. 5, October 1, 2011. ©ASCE, ISSN 1090-0268/ 2011/5-732–742/$25.00. 732 / JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / SEPTEMBER/OCTOBER 2011 Downloaded 24 Nov 2011 to 142.137.229.172. Redistribution subject to ASCE license or copyright. Visit http://www.ascelibrary.org