Magazine of Concrete Research, 2011, 63(1), 1–11 doi: http://dx.doi.org/10.1680/macr.2011.63.1.1 Paper 1000157 Received 01/09/2010; revised 07/03/2011; accepted 15/03/2011 Published online ahead of print xx/yy/zzzz Thomas Telford Ltd & 2011 Magazine of Concrete Research Volume 63 Issue 1 Retrofitting of reinforced concrete beams in shear using external prestressing technique El-Shafiey and Atta PROOFS Retrofitting of reinforced concrete beams in shear using external prestressing technique Tarek El-Shafiey Professor of Concrete Structures, Faculty of Engineering, Tanta University, Tanta, Egypt Ahmed Atta Assistant Professor, Faculty of Engineering, Tanta University, Tanta, Egypt Retrofitting of existing concrete structures has become an important issue nowadays in the construction industry. There are many causes for repair and/or strengthening, but normally change of use, increased demands on the structure, errors in the design or/and construction phase or accidents are governing. Many methods to repair and/or strengthen concrete structures exist such as concrete overlays, shotcrete, external prestressed using cables and so on. This research presents laboratory tests of nine reinforced concrete beams with 2 m span strengthened in shear with external prestressing technique. In addition to a non-strengthened reference beam. the present work studied in three groups to examine beams strengthening using different configuration of prestressing details in shear domain, the effect of repairing beams cracked in shear with different damage levels, and the strengthening of beams using carbon fibre reinforced polymer plates in flexure and prestressing in shear. The tests results showed that prestressing is a very effective strengthening method to increase the shear load-carrying capacity of existing concrete beams. The external post-tensioning method can reduce the effect of the existing crack and increase the member capacity up to 95% of the original capacity. The failure of beam strengthened with external carbon fibre reinforced polymer laminate in flexure and using external post-tensioning technique in shear failed owing to peeling of the carbon fibre reinforced polymer laminate. In addition, a numerical model was presented and verified with the experimental result and showed good agreement. Introduction Generally reinforced concrete members are designed to fail in flexure under ultimate loading conditions. Therefore, most of the studies have focused on the flexural strengthening of reinforced concrete members such as girders. However, some structural members need to be strengthened for shear only due to the type of loading or error in design. External post-tensioning is one of the widely used strengthening techniques, owing to its advan- tages. Some of the advantages include economical construction, easy monitoring and maintenance, and easy tendon placement and concreting. External post-tensioning refers to a prestressing technique where the tendons are placed outside the concrete section. Strengthening or repairing of existing concrete members using external post-tensioning is quite different to the construc- tion of new concrete members using external post-tensioning. Many factors influence the efficiency of the strengthening of existing concrete members, the main one being the effect of existing cracks. These effects need to be investigated to achieve an optimum strengthening technique using external post-tension- ing. The flexural behaviour of concrete members by external post- tensioning has been studied in detail by many researchers (Atta et al., 2009; Ghallab and Beeby, 2005; Miyamoto et al., 2002; Ng, 2003; Ng and Tan, 2006; Tan and Tjandra, 2007). However, to the authors’ best knowledge, there have been relatively limited investigations on the shear strengthening and the effect of shear in externally post-tensioned members (Aravinthan, 2006; Recup- ero et al., 2005; Suntharavadivel, 2006; Tan and Ng, 1998; Wolf and Frosch, 2007). All previous researches were focused on prestressing of members in the horizontal direction only. Even most of the current design codes do not have provision for the existing shear cracks (except some guidelines recommend these cracks should be repaired prior to strengthening). Therefore, the aim of this study was to gain more understanding about the efficiency of using external prestressing technology in vertical direction for repairing or strengthening reinforced concrete beams in shear. The main objective was to investigate and establish experimentally the response of beams failed in shear under the new configuration of the prestressing technique. To achieve those objectives, nine specimens were tested. The effect of strengthen- ing of beams using different prestressing details in shear domain, the effect of repairing of beams cracked in shear with different damage levels and combined strengthening of beams using carbon fibre reinforced polymer (CFRP) plates in flexure and prestressing in shear were studied. The obtained results are presented and analysed. An analytical model was presented and examined against experimental work. Article Number = 1000157 1 1