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 responsible. Many methods to repair and/or strengthen concrete structures exist, such as concrete overlays, shotcrete, external prestressing 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 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.

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