Strengthening or retrofitting of existing reinforced concrete (RC) structures may be performed to provide higher design load resistance, increase stiffness or correct strength loss due to deterioration. During the last decade, an increase in the application of fiber reinforced polymers (FRPs) has been observed in all engineering branches due to certain advantages of the material including high tensile strength, light weight, corrosion resistance and ease of application. The aforementioned benefits of FRP composites have led engineers and researchers all over the world to utilize FRPs as alternative to conventional strengthening material, i.e. steel; for strengthening of RC members. The most common technique for flexural/shear strengthening of RC beams and slabs utilizing FRP materials is externally bonded reinforcement (EBR) technique. Despite certain benefits of EBR technique such as simple and rapid installation, the main problem which has greatly hampered the use of EBR technique is premature debonding of FRP composite from the beam substrate. In this case, FRP-to-concrete bond behavior is a key factor controlling debonding failures in FRP-strengthened members. On the other hand, a very important aspect of the FRP-to-concrete bond behavior is that there exists an effective bond length beyond which an extension of the bond length cannot increase the ultimate load. Due to importance of the bond behavior of FRP-to-concrete bonded joints, especially FRP-to-concrete effective bond length, the main intention of the current study is to experimentally evaluate the effective bond length of externally bonded FRP reinforcements by means of single-shear bond tests. To do so, 30 concrete prisms with dimensions of 150 × 150 × 350 mm were cast and CFRP sheets having different bond length were adhered to 22 concrete specimens using conventional EBR technique, and the other 8 specimens were strengthened using grooving method (GM). Afterwards, all strengthened specimens were subjected to single-shear bond test using a 300 kN hydraulic jack specially designed at Isfahan University of Technology (IUT). In order to benefit from full field deformation measurements, a non-contact image based technique, i.e. particle image velocimetry (PIV), was used to investigate the bond behavior of tested specimens. Moreover, numerous validation experiments were conducted to verify the ability of PIV technique in precise deformation measurements by means of four-point flexural tests on steel and RC beam specimens. 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. ACI 440.2R and fib 14 overestimate the effective bond length by a mean error of 100 percent. In the other hand, due to high stiffness bond of FRP to concrete when using GM, all GM specimens failed due to CFRP rupture regardless of grooves length. Consequently, experimental results of the current study strongly verified the capability of GM for eliminating premature debonding failure. Keywords: FRP composites, debonding, single-shear test, FRP-to-concrete effective bond length, externally bonded reinforcement (EBR), grooving method (GM), particle image velocimetry (PIV).