PSI - Issue 64

952 A. Cagnoni et al. / Procedia Structural Integrity 64 (2024) 951–958 2 Alessandro Cagnoni, Pierluigi Colombi, Marco A. Pisani, Tommaso D’Antino / Structural Integrity Procedia 00 (2019) 000–000 1. Introduction Among sources of pollution, those related to the construction industry are critical due to the high water consumption, resources extraction, and gases release Garner (2024). Thus, strengthening existing structures instead of demolition and new construction is a preferable solution to reduce the environmental impact of the construction industry. Among strengthening techniques for reinforced concrete (RC) members, applying an external force using external steel tendons is a possible solution. External tendons allow to increase the strength and stiffness of the member (Harajli (1993), Harajli et al. (1999), Pisani (1999), Tan and Tjandra (2007)). Moreover, this technique may induce concrete crack closure, thus reducing the exposure of steel bars to the environment (Harajli (1993)). In specific conditions, steel strands are non-compatible with the environment and different type of tendons may be used as an alternative. Fiber reinforced polymer (FRP) tendons represent a viable option due to their high resistance to harsh environments (e.g., marine environment), high strength, and high strength-to-weight ratio. Among the various polymeric composite products, those made with carbon fibers (CFRP) are the most promising thanks to their higher strength and creep resistance compared to those made with glass (D’Antino and Pisani (2019)) and aramid fibers. Tendons with carbon fibers include CFRP rods but also carbon fiber composite cables (CFCC). A limited number of studies investigated the reliability of external CFRP tendons for strengthening RC members (Elrefai et al. (2012), Burningham et al. (2015)) and of internal prestressed CFRP tendons for new concrete members. Several open issues are limiting the diffusion of these innovative tendons. In this paper, open issues on the use of carbon composite tendons are reported and discussed, including the anchorage system, stress level at jacking, and exposure to various aggressive environments. 2. Anchor systems One of the major obstacles in the development and spread of prestressed concrete with FRP tendons is related to the anchor system. The difficulties of anchoring composite tendons come from the mechanical properties of FRP composites, which have an anisotropic behavior with lower properties in the radial than in the longitudinal (i.e., fiber aligned) direction. An efficient anchor should be capable of limiting stress concentration along the gripped portion, thereby ensuring full exploitation of the composite properties. Not all anchors are able to develop the full tensile strength of the tendon. However, an acceptable criteria is provided by the Canadian standard CAN/CSA-S806-02 (Canadian Standards Association (CSA) (2002)), which requires the anchor to be capable of developing at least 90% of the nominal tensile strength of FRP tendon. The various types of anchors for composite tendons can be grouped in two main categories: mechanical and bond anchors. 2.1. Mechanical anchors Mechanical anchors are based on friction to transfer stresses from the tendon to the anchor system and vice-versa. Compared to bonded systems, mechanical anchors are generally more economical, easier to install, do not need curing time, and are more compact. Different types of mechanical system are used, including wedge-barrel and clamp anchors. Typically, mechanical anchors do not work well with composite tendons due to the anisotropic characteristics, brittleness, and weak transverse properties of FRPs. Various studies were conducted to reduce the incompatibility between composites and mechanical anchors by optimizing the geometries and materials of the mechanical parts. Al-Mayah et al. (2006) designed a barrel-wedge anchor system with the addition of a sleeve material. The geometries and materials were optimized to reduce stress concentrations. The authors observed that the initial presetting force and the size of the rods had no effect on the performance of the anchor system. Al-Mayah et al. (2013) eliminated the sleeve layer and, to compensate for its effect, changed the material of wedges. The authors discovered that the hardness plays a key role in this type of anchor. Schmidt et al. (2012) developed a wedge anchor system using an integrated aluminum sleeve. They confirmed that the hardness is crucial to avoid local stress concentrations. Heydarinouri et al. (2021) combined the technologies presented in Al-Mayah et al. (2013) and Schmidt et al. (2012), and the system obtained was tested under monotonic and cyclic tensile tests. The results showed that the developed anchorage system was not sensitive to the loading frequency. Other authors designed wedges using materials different from metal alloys. Terrasi et al. (2011) tested wedges made of a thermoplastic polymer, Züst et al. (2022) used polymeric wedges, and