Conflicts in design for strengthening of concrete structures using fibre-reinforced polymer composites

The use of fibre-reinforced Polymer (FRP) composites as an externally-bonded tension reinforcement is an effective technique for the strengthening of under-performing concrete structures. The FRP-based strengthening design processes are significantly more conservative than the conventional norm in structural design. For example, the safety factors prescribed on FRP material properties could be up to 3 times as high than those typically prescribed on structural concrete. It is believed that such an approach will account for the considerably greater extent of uncertainties comprising of those inherent to the existing structure as well as those arising from the lack of knowledge and time-testimony of using FRP composites for structural strengthening. However, FRP composites involve substantially peculiar characteristics compared to steel when used as a tension reinforcement. Therefore, the design processes for strength (for new RC structures) and additional strength (for strengthening the existing RC structures) have conflicting design requirements and objectives. Thus, an approach of prescribing significantly higher factors of safety will not only be against the economic and environmental sustainability requirements, but can also fall short in providing the required safety margins and could instigate negative implications and undesirable side-effects under some design scenarios. This paper summarises important design conflicts arising from differential treatments in uncertainty, mechanics, failure mode, detailing and reliability management within the safety formats prescribed by various strengthening design standards. The implications of these conflicts, on the course and quality of strengthening design solutions, and that on their resultant safety-contents are discussed.