Characterized Indices of Bond Thickness Variation on RC Beams Strengthened Externally by Bonded Carbon FRP

Abstract

Carbon fiber-reinforced polymer (CFRP) is a structurally sound and cost-effective material for strengthening and rehabilitation. The understanding of how to strengthen existing structural elements has progressed significantly, but the appropriate adhesive layer thickness for increased strengthening impact remains a gap in the literature. This study is designed to investigate and optimize bond layer thickness for enhanced strengthening of concrete structures. Nine (9) reinforced concrete beams were designed as prototypes and modelled using a 2.5 scale ratio in accordance with ACI 444. The dimensions of the modelled beam were 100 x 150, or 1100 mm. These beams were internally reinforced with 2Φ10 mm bars based on a 2.5 scale ratio as flexural reinforcement. 2Φ8 hanger rebars were provided at the compression zone. Φ6, bend in a rectangular shape and are spaced at 220 mm, which serves as shear reinforcement for the beams. The beam had a shear span to effective depth ratio (av/d) ratio of 2.5. The beam samples were externally strengthened by 200 g/m2 and 300 g/m2 of CFRP fabric, with bond thicknesses of 2 mm, 4 mm, 6 mm, and 8 mm. The experimental program was divided into phases in order to investigate the effects of adhesive layer thickness on bending strength, load carrying capacity, and ductility index. The beams were investigated as single-span on a 20-ton capacity loading frame with a one-third point load application over an effective length of 1100 mm. Results show the possibility of using an epoxy-bonded CFRP fabric to enhance the bending resistance of RC beams. The shear failure model was found dominant in the epoxy-bonded CFRP fabric in flexural strengthening. CFRP-strengthened fabric beams provide not only increased load resistance but also increased structural ductility. Results reveal that RC beams strengthened with CFRP fabrics with a bond thickness greater than 20 times the thickness of the CFRP fabric reduce the bending capacity and the ductility index. The authors recommend that a CFRP fabric to bond thickness ratio of not more than 0.05 for flexure strengthening could be structurally effective.