Retrofitting of reinforced concrete and steel structures due to reasons such as drawbacks in old codes, changing the usage of structure, destructive environmental factors and deficiencies in construction process is from important issues in structural engineering. Strengthening of structures with FRP composites is one of the methods that have been welcomed by researchers due to the unique properties of FRPs including their high tensile strength, light weight, simple application and resistance to corrosive environment. Although the main purpose of retrofitting is to increase the overall load carrying capacity of the structures, however, the failure mechanism of the strengthened member is also very important. In a reinforced concrete beam, four modes of flexural failure mechanism is possible: a) concrete crushing after steel yields, b) concrete crushing before steel yields, c) FRP rupture after steel yields, and d) FRP rupture before steel yields. To achieve any of these modes, premature surface deboning should be controlled and prevented. Debonding of FRP sheets from concrete substrate is directly affected by the method of connecting FRP laminates to the concrete; i.e. the conventional surface preparation, and the new method of grooving in the form of externally bonded reinforcement on grooves, EBROG, has been recently introduced in the literature. To examine the effects of the aforementioned methods, 36 reinforced concrete beam specimens with the dimensions of 120×140× 1000 mm were cast and strengthened with FRP laminates and subjected to four-point flexural loading. Then, the flexural failure mechanisms of the beams were investigated using both EBROG and conventional EBR methods. 6 concrete beam specimens with the dimensions of 100×100× 500 mm with 4 reinforced concrete beam with the dimensions of 120×140× 1000 mm were tested to control the effect of delaying between different steps in EBROG method. Numerical investigation on experimental samples from this paper and other scientific sources has been done. The results indicate EBROG is more effective than EBR to control debonding and increasing load carrying capacity of beams in all four flexural failure mechanisms. Putting delay between steps of EBROG method reduces the efficiency of it. ACI 440.2R-08 code does not have sufficient accuracy in predicting load carrying capacity and failure mechanism of beams strengthened with FRP. Key Words: FRP, EBR, EBROG, Debonding, Flexural Failure Mechanism
