: The strengthening or retrofitting of existing concrete structures to resist higher design loads, correct strength loss due to deterioration, correct design or construction deficiencies or increase ductility has traditionally been accomplished using conventional materials and construction techniques. During the last decade, fiber reinforced polymers (FRPs) have emerged as an alternative to traditional materials for repair and rehabilitation. The most common technique for strengthening concrete structures utilizing FRP materials is externally bonded reinforcement (EBR) technique. The bond between the concrete and the FRP composite becomes an important factor that controls the efficacy of the repair. However, it has been shown that the total strength capacity of the beam is often not attained due to premature debonding of the FRP composite from the concrete substrate. In this case, ultimate load cannot always increase with an increase in the bond length, and the ultimate tensile strength of FRP composite may never be reached, however long the bond length is. This leads to important concept of effective bond length, beyond which any increase in the bond length cannot increase the ultimate load. A large number of parameters affecting the ultimate load. Compressive and tensile strength of concrete, types, elasticity modulus and thickness of the FRPs were investigated in the study.The main intention of the current study is to experimentally evaluate the effective bond length of externally bonded FRP composite by means of single shear tests. To do so, 34 concrete prisms with dimensions of 150×150×350 mm were strengthened in 6 groups. Carbon fiber fabric, glass fiber fabric, carbon fiber laminate and steel plate were used for strengthening concrete prisms. The concrete prisms were cast in three series (i.e. series I-III). Series I consisted of specimens with compressive strength less than 30 MPa, series II included specimens with compressive strength in a range from 30 to 40 MPa and finally in series III, the compressive strength of specimens were more than 40 MPa. FRP composites and steel plates were adhered to all specimens using conventional EBR technique and bond length remained constant and equal to L=200 mm. Afterwards, all strengthened specimens were subjected to single shear test using a 300 kN hydraulic jack. In order to obtain the displacement field an image-based full field measurement technique, i.e. particle image velocimetry (PIV) was used to investigate displacement field during loading process. To do so, successive digital images were taken from the field undergoing deformation. Comparison of experimental results of the current study with the existing models of effective bond length shows the incapability of the existing guideline models in accurate predictions of FRP-to-concrete effective bond length. Consequently, with the support of experimental results of this study appropriate calibration factors were introduced to effective bond length models in well-known existing guidelines i.e. fib Bulletin 14 and ACI 440.2R. Keywords: FRP composites, bond strength, effective bond length, single-shear test, externally bonded reinforcement (EBR), particle image velocimetry (PIV).