Size Effect in Shear Critical FRP RC Beams

Owing to its non-corrosive characteristics, Fibre-Reinforced Polymer (FRP) reinforcement can be a promising alternative to conventional steel reinforcement in structural elements exposed to severe environments, such as in bridge girders. However, the use of FRPs in such elements is limited, as their shear behaviour is still very little understood. In fact, only limited number of studies have focused on size effect, which is associated with a reduction of shear strength in larger elements, and how this affects the relative shear contributions of concrete and FRP shear reinforcement. Many design recommendations adopted size effect provisions developed for steel RC beams without investigating the validity of these models in great depth or accounting for the unique mechanical properties of FRP and their effect on overall structural behaviour (e.g. wider cracks and larger strain).
This study aims to advance the understanding of the influence of beam’s depth on the shear behaviour and development of shear resisting mechanisms in FRP RC beams, so as to develop more refined and reliable shear predicting models, especially for larger elements.
Based on detailed experimental measurements obtained from three point bending tests carried out on fifteen FRP RC beams with and without external FRP shear links, a decomposition of the basic shear resisting mechanisms is performed to estimate the individual contributions of concrete and shear reinforcement.
The results confirm a considerable size effect for members without shear reinforcement (up to 40 % reduction for larger elements), and indicate that this is strongly related to the maximum strain that can be attained in the flexural reinforcement. The analysis shows that both contributions of concrete and shear reinforcement are variable, and change with increasing strain in the flexural and shear reinforcement. However, even when relatively large strain values are mobilised in the reinforcement, the additive nature of these shear resisting components can be effectively maintained, as long as the individual contributions offered by concrete and shear reinforcement are sufficiently accounted for.
Based on the experimental observations, a unified design model for FRP and steel RC beams is proposed. The model accounts for size effect, increases the safety margins for larger RC beams and reduces the COV in shear predictions by up to 25%.