Flexural behaviour of reinforced concrete members strengthened with prestressed CFRP laminate

Abstract

The use of fibre reinforced polymer (FRP) materials for the strengthening of reinforced concrete structures has many benefits in comparison to the traditional strengthening methods. Such reinforcements are lighter, easier to install, they will better meet aesthetical design and anticorrosion/durability requirements. The full potential of such high strength materials can be exploited by prestressing. This way, the deflection of the element will be reduced and the concrete cracking and steel yield loads will be increased. However, the use of FRP’s for strengthening may cause lots of issues for civil engineers regarding the appropriate evaluation of strengthened structures work, and technological issues during prestressing. While the concrete member is uncracked, both elements deform together and it can be considered as a full composite action. However, after concrete cracking, stress concentrations appear at the cracks and it reduces the contact stiffness of concrete and FRP. Afterwards, the slip occurs in the contact surface, what leads to a sudden and brittle FRP debonding failure mode. These are the most common failure modes. Therefore, a number of studies have been conducted on debonding like failure modes. However, the failure criterion or fracture energy values in the models found in the literature were derived from experimental narrow parametric scope pull-off / pull-out shear tests, while the bond behaviour and this criterion could be different for strengthened flexural reinforced concrete members with long bond lengths. It should also be noted that the effect of concrete-FRP partial shear connection is usually ignored in the assessment of element deformations. Considering the state of the art, a new analytical calculation model for the evaluation of concrete-FRP partial shear connection is proposed. Also, alternative new calculation techniques for the determination of the load carrying capacity, deflection, and cracking of the FRP strengthened RC members, taking into account FRP prestressing and concrete-FRP partial shear connection effects. Thus, the current thesis not only effectively evaluates the possible premature failure of the element, but also evaluates the full operation of concrete-FRP partial shear connection joint and its influence on the element deformations. Furthermore, a novel prestressing system for externally bonded FRP laminates is proposed in response to the technological challenges of prestressing.