PSI - Issue 64

A. Cagnoni et al. / Procedia Structural Integrity 64 (2024) 944–950 2 Alessandro Cagnoni, Pierluigi Colombi, Marco A. Pisani, Tommaso D’Antino / Structural Integrity Procedia 00 (2019) 000 – 000 1. Introduction Prestress concrete is a construction technique that dates to the early decades of the 19 th century and has been increasingly adopted for the last fifty years. This technique is based on applying internal forces to the concrete that counteract the external loading, thus increasing the ultimate capacity of the concrete member. These internal forces are typically applied by tensioning steel tendons before pouring concrete (pretensioned members) or after pouring concrete (post-tensioned members). Nowadays, post-tensioning solutions are also used to strengthen existing reinforced concrete (RC) elements using external tendons, increasing their load carrying capacity and inducing concrete crack closure (Harajli (1993)). However, in specific applications (e.g., marine environments), tradition steel strands cannot be used and a possible solution is provided by fiber-reinforced polymer (FRP) tendons. Compared to the steel counterpart, FRP tendons generally have higher tensile and fatigue strength, lower creep strain, and better resistance to harsh environments. However, 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 FRP anisotropic behavior, which is characterized by a weak resistance in the radial direction. Currently, available anchoring systems may be classified into two main categories: mechanical anchors and bond anchors. The former utilize friction to transfer the load from the tendon to the concrete, while the latter rely on friction and chemical adhesion. Compared to mechanical anchors, bond anchors have a lower manufacturing cost, provide a better stress distribution along the composite tendon, and have higher efficiency (Zhang and Benmokrane (2004), Cai et al. (2015), Mei et al. (2020), Saeed et al. (2020), Jia et al. (2022)). On the other hand, mechanical anchors are more economical, easier to install, more compact, and do not require any curing time. Various authors have tried to reduce the incompatibility between the composite tendon and mechanical anchor. Among them, Al-Mayah et al. (2006), Schmidt et al. (2012), Al-Mayah et al. (2013), and Heydarinouri et al. (2021) studied metallic wedge-barrel anchor. Terrasi et al. (2011), Züst et al. (2022), and Shi et al. (2022) designed non-metallic wedge-barrel anchors, whereas Ye and Guo (2011) and Burningham et al. (2014) studied a clamp anchor system. In this paper, a novel mechanical system to anchor FRP tendons to concrete was designed and tested. The novel anchor system comprises three brass alloy wedges, a steel barrel, and a connecting ring. Preliminary quasi-static tensile tests were performed on carbon FRP (CFRP) bars with 8 mm nominal diameter. Three control specimens with bonded steel anchors were tested. Then, the newly developed wedge anchor system was used to anchor three GFRP bars and three CFRP bars. The experimental results are presented and discussed to provide a first insight into the effectiveness of the of the novel mechanical anchor system, thus providing fundamental information on its efficiency. 2. Materials and methods 2.1. FRP bars Two different types of fiber-reinforced polymer (FRP) composite bar were investigated in the experimental campaign, including glass FRP (GFRP) bars and carbon FRP (CFRP) bars. According to the information provided by the manufacturer, the GFRP rods were made of E-glass fibers embedded in a vinylester matrix, while the CFRP bars were made of high-strength carbon fibers embedded in a vinylester matrix. Both types of bar had a nominal diamater equal to 8 mm and were sand coated to improve their bond behavior with concrete. The GFRP bars were characterized by a nominal tensile strength of 1050 MPa, an elastic modulus of 46 GPa, and an ultimate strain of 2.0%. The CFRP bars were characterized by a nominal tensile strength of 2450 MPa, an elastic modulus of 130 GPa, and an ultimate strain of 1.8%. Since the carbon FRP bars with 8 mm nominal diameter were manufactured with a different method with respect to other bars from the same manufacturer, preliminary tensile tests were conducted to determine their mechanical properties. CFRP bar specimens had a total length of 750 mm. Two steel pipes 125 mm long were bonded to the ends of each bar. The tensile tests were conducted following the prescriptions of ASTM D7205/D7205M-21 (American society for testing and materials (ASTM) (2021)). Tensile tests were conducted in displacement control by monotonically increasing the displacement (stroke  ) of the machine at the rate of 0.050 mm/s. An axial extensometer with 100 mm gauge length was used to measure the bar axial strain. In Table 1, the tensile strength  max , corresponding ultimate strain  max , and elastic modulus E are reported for each specimen. The elastic modulus E was obtained by 945