Open issues on the use of carbon composite tendons as prestressed reinforcement of concrete members

In the last decades, fiber-reinforced polymer (FRP) composites gained popularity in the field of structural engineering thanks to their high mechanical properties and limited corrosion in harsh environments. These composites are used as reinforcement of new concrete members as well as for strengthening of existing structures. For existing reinforced concrete (RC) structures, different techniques are available to increase the strength and stiffness of the existing elements. Among them, a promising technology consists in inducing to the element a bending moment opposite to that induced by the applied load by using external prestressed FRP tendons. This technique increases the load carrying capacity of the beams and induces concrete crack closure, thus reducing the exposure of steel bars to the environment. In this paper, a literature review is carried out to investigate the open issues that hinder the diffusion of prestressing made with composite carbon fiber tendons. According to the literature, the main issue is represented by the anchor system that should guarantee high stress level into the composite material avoiding local ruptures. Another open issue is the definition of the maximum stress applied to the composite with respect to its tensile strength. In fact, high levels of stress may induce premature composite failure. Finally, the exposure of CFRP bars to some harsh environmental conditions and high temperatures may negatively affect the reinforcement effectiveness.