Keywords: fibre reinforced polymer, carbon fibre, debonding, finite element analysis, reinforced concrete beam.

Fibre reinforced polymers (FRPs) are used for strengthening structural members such as beams, slabs and columns as a result of their wide range of application and high strength. For instance, the flexural and shear capacity of beams can be increased by attaching FRPs to the flexural and side surfaces of beams. For concrete beams strengthened with FRPs, debonding in FRPs is critical because the beams often show sudden failure as a result of debonding in FRPs, and the capacity of the FRPs is not fully achieved at the time of beam failure.

This paper presents analytical studies to evaluate debonding performance of reinforced concrete (RC) beams with FRPs of different sizes attached externally. The analytical modelling approach is developed to simulate the structural behaviour of RC beams having different lengths, widths and numbers of FRP layers. The commercial finite element (FE) software, ABAQUS version 6.10-3, is used for the modelling. Simulations are planned such that reinforced concrete beams (200x250x3200mm) are subjected to four point-bending with simply supported conditions. The beams are strengthened with two, three, or four layers of CFRPs with different FRP widths and lengths. Type A models have same CFRP length of 1800mm, while B and C types are designed such that the length of each CFRP layer is different by 300mm. The CFRP widths are varied as 200mm, 300mm, and 400mm. Also, the 2A200U model is strengthened with a U-strap for shear strengthening as well as two layers of carbon FRPs (CFPRs) on the tension side.

Based on the results, following conclusions can be drawn:

1. CFRP can significantly improve the load capacity of a RC beam. The stiffness of the beams strengthened with CFRPs increases with the increase of CFRP thickness, however, beams strengthened with four or more layers of CFRPs show earlier failures as a result of debonding of the FRPs.
2. The effect of the CFRP length on the load-displacement relationship is not significant if the length of the first layer of CFRP reaches 60% of beam span length. When multiple layers of CFRPs are used for strengthening, beams with the tapered layers of CFRP aarly failure caused by debonding is avoided.
3. When the width of the CFRP is wider than the width of the RC beam, concrete cover separation occurs along the location of reinforcing steel bars as a result of high interfacial stresses between concrete and FRPs. Compared to wide CFRPs, U shaped straps are more effective to increase the load capacity of the RC beam by preventing early failure of RC beams strengthened with FRPs resulting from debonding.

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