Optimizing the Performance Characteristics of Beams Strengthened with Bonded CFRP Laminates

Ductility is of fundamental importance in the design of concrete structures. With structures using conventional materials such as concrete and steel, ductility of a member as a whole can be satisfactorily defined in terms of deflection, curvature or energy absorption capacity as exemplified by the area under the load-deflection curve. However, when structural members strengthened with externally bonded fibre reinforced polymer (FRP) laminates are considered, conventional definitions of ductility become less precise because of the brittle behaviour of FRP materials, and their resultant effect on the performance of the strengthened beam. Furthermore, such beams when strengthened without the provision of external anchorages will fail very suddenly, with abrupt debonding of the laminate and substantial loss of load capacity. This paper intends to propose a new criterion to evaluate the structural performance of such strengthened composite beams, and the efficiency of the external anchorage system. The design criterion termed, Performance Factor, incorporates both the deformability and strength of composite beams. Unlike the concept of toughness as applied to materials, the Performance Factor incorporates the effect of numerous parameters which influence structural design. To examine the reliability of this parameter a series of eleven reinforced concrete beams were tested to evaluate the structural performance of beams strengthened with and without externally bonded carbon fibre reinforced polymer (CFRP) laminates, and with different types of internal reinforcement and external anchorage systems. The structural behaviour of these beams was then evaluated using the Performance Factor. The results show that the criterion of Performance Factor call realistically represent the overall structural behaviour of the strengthened composite beam both in terms of strength enhancement and deformability. It is also shown that the reinforcement details of the beam to be strengthened have a strong influence on the effectiveness of the arrangement and disposition of the external anchorages designed to optimize the structural behaviour of the strengthened composite beam.