Over the past decades a great deal of attention has been put on fibre reinforced polymer (FRP) bars, which moved rapidly from being a novel alternative to steel reinforcement to a well-established practice. FRP bars are characterized by a linear elastic mechanical response and, consequently, the structural behaviour of FRP RC is different than that of steel RC. Owing to their non-corrosive characteristics, FRP reinforcement is primarily used in application where durability is a concern and one of the most promising applications is in bridge construction. However, the lack of understanding of the performance of FRP RC elements subjected to complex combinations of actions limits the use of FRP reinforcement to deck slabs and does not extend to key structural elements such as transfer girders.
In particular, the shear behaviour of FRP RC elements is still poorly understood and very few studies have focused on size effect. Although shear in FRP RC beams is transferred through the development of the same resisting mechanisms as for steel RC, their individual contribution to overall shear resistance is different and needs to be carefully reassessed. In addition, most of FRP design codes are based on concepts developed for steel RC and do not account for the exact magnitude of these shear components.
The aim of this study is to better understand the shear mechanisms that develop in FRP RC beams and how these are affected by the size of the specimens (size effect). To achieve this, a comprehensive experimental study on FRP RC beams has been planned and is being conducted. The study investigates the different size of the beams as well as the presence of shear FRP reinforcement, while the other parameters are kept constant. A total of 6 FRP RC beams, similar in mechanical properties, but otherwise varying in overall depth from 260mm to 460mm were constructed. The first experimental series consisted of 3 FRP RC beams with no shear reinforcement, while the remaining beams had the minimum amount of FRP shear reinforcement (calculated according to ACI440). All of the beams were designed to fail in shear and were tested in 3-point bending.
Along with point wise transducers, a three dimensional digital image correlation system was employed in this study to obtain full-field measurements of deformations and gain additional insight into the development of shear transfer mechanisms.
The experimental results confirmed that the shear strength of FRP RC beams is size-dependent both for beams with and without web FRP reinforcement. The results showed a considerable size effect for the members without shear reinforcement, with a reduction in shear strength up to 60 %. The predictions based on current design provisions yielded in general conservative results providing not uniform margin of safety for the varying member depth. In addition, it was shown that DIC measurements yielded meaningful information on cracks initiation and their development, and helped to better understand the shear behaviour of large FRP RC beams.
Three point bending test: shear failure