Effective bond length of FRP-to-concrete adhesively-bonded joints: Experimental evaluation of existing models

Bond behavior of adhesively-bonded fiber reinforced polymers (FRPs) to concrete substrate has been investigated by many researchers worldwide. An interesting aspect of 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 capacity of the joint. Effective bond length of FRP composites, in fact, is an important part of all strengthening calculations, and conservative design guideline predictions can lead to waste of composite materials in strengthening projects. Consequently, the main intention of the current study is to evaluate the accuracy of existing guideline models of effective bond length by means of single-shear bond tests. To do so, carbon FRP (CFRP) sheets with a wide range of bond length from 20 to 250 mm, were adhered to 22 concrete prisms using externally bonded reinforcement (EBR) technique. The specimens were then subjected to single-shear test and debonding loads as well as the effective bond length of the CFRP sheets were determined. Moreover, an image based technique, i.e. particle image velocimetry (PIV) was used to verify the estimated effective bond length by analyzing strain distribution along the CFRP strips during loading process. Experimental results of the current study show that fib Bulletin 14 model overestimates debonding loads and effective bond length. The model adopted by ACI 440.2R-08 also overestimates effective bond length while accurately predicts debonding loads. Appropriate calibration factors were introduced to modify the existing models for CFRP sheets.