Experimental Investigation On Hybrid Bonded FRP For Strengthening Reinforced Concrete Beams

Strengthening and repairing of reinforced concrete (RC) members and structures using fiber-reinforced polymer (FRP) composites have become popular in last decades. This has been due to the advantages of FRP materials such as their high tensile strength-to-weight ratio, good corrosion resistance and the moderate cost. In the strengthening of RC members or structures, FRP materials are normally externally bonded to their surface using epoxy resin. Such as externally bonding FRP (EB-FRP) materials to the underside of RC beams is a traditional method to enhance the flexural capacity of RC beams. However, the poor bonding strength between the FRP and the beams surface normally induces the premature failure which is interfacial debonding which reduces the utilizing rates of the tensile capacity of FRP. To solve the problem, various techniques have been developed such as installing a large anchor or U-jacket at the end of FRP plate, mechanically fastening system (MF-FRP) and the application of the near surface mounted FRP (NSM). These techniques show good advantage in terms of the bonding strength of FRP to concrete. However, they would not prevent premature failure completely. So other new retrofit techniques are needed to improve the bond strength between FRP and concrete.A new hybrid bonding technique (HB-FRP) that combines EB-FRP and a new type of mechanical fastening had been proposed by the Department of Building and Construction in City University of HK last year. This new technique used a new mechanical fastener which is simple and many of these fasteners can be placed along the length of beams to produce a significant resistance to the separation of the FRP from the concrete substrate. Previous study showed that the interfacial bond strength in HB-FRP strengthened slab is about 7.5 times greater than which in EB-FRP strengthened slab, indicating clearly this method is simple and efficient. Meanwhile, more comprehensive investigation is needed to understand the basic principles of HB-FRP system to develop design models and guidelines. Also a new type of concrete anchor or screw with easy installability, better strength reliability and stability will be needed to replace the small concrete nails previously studied for experimental research or practical application. This thesis presents an experimental study on bond strength and failure modes of two EB-FRP strengthened RC beams and seven HB-FRP strengthened RC beams which divided into two groups, which was conducted to develop a better understanding of the behaviour and failure mechanisms of HB-FRP system. Tests show that the failure modes of HB-FRP beams are different with EB-FRP reference beams, the maximum bond strength and was 5.4 times that of conventional EB-FRP system. Furthermore, design guidelines and formula are given based on data analysis.