PSI - Issue 37

Ghusoon S. Alshami et al. / Procedia Structural Integrity 37 (2022) 367–374 Alshami et al./ Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction Fiber reinforced polymers (FRP) gained popularity in strengthening and retrofitting existing and new reinforced concrete (RC) structures. They have many advantages such as their high strength to weight ratios, durability, corrosion resistance, ease of instalment, and versatility. FRP sheets could enhance the capacity of different concrete members such as beams, columns, walls, and slabs by installing them on the the concrete external surfaces. Also, they could be applied on unique structures such as bridges, water tanks, and chimneys. The FRP sheets are usually bonded on concrete surfaces via epoxy resin by a trained staff that use special equipment during the installation process, such as aluminium rollers and squeegees. Such equipment releases entrapped air between the fibers to densify the FRP structure and prevent failures. There are different types of conventional FRPs with high strength fibers like carbon (CFRP), basalt (BFRP), glass (GFRP), aramid (AFRP), and high-strength steel (HSS). Many studies have proven that FRPs enhance concrete structures in terms of flexural capacity, shear capacity, durability, ductility, and fatigue life. For example, FRP laminates are installed on the tension side of a RC beam to enhance its flexural capacity and on its side face in the form of U-wraps, close wraps, or side bonded sheets to enhance its shear capacity (Siddika et al. (2019); Ali et al. (2014); Rasheed (2014); Zhuang et al. (2019); Hawileh at al. (2019); Hawileh at al. (2014); Helal at al. (2020); Mhanna et al. (2021); Abuodeh et al. (2019); Naser et al. (2019)). The main disadvantage of FPP strengthening systems is premature debonding that occurs when the FRP laminate debonds from the concrete surface due to its low strain levels and increased stress levels between the FRP and concrete. Therefore, the expected capacities will not be developed in strengthened members due to the occurrence of such brittle mode of failure. Flexural and/or flexural-shear cracks in the maximum moment region initiates debonding that is developed with high stress levels along the length of the bonded FRP laminate until the system fails. Delamination is another form of debonding where both the FRP laminates and concrete cover are detached from the concrete surface at the FRP end, where high stress concentrations are present. Debonding and delamination are two unfavorable failure modes that could be prevented by anchoring the FRP laminates to the concrete surface. Anchorage systems utilize the fiber which delays debonding failure and improves structural strength, ductility, and deformability. Anchors are compatible and could be used in different applications and structures like beams, column, slabs, and joints (Belarbi and Acun (2013); Sakar et al.(2014); Naser et al. (2019)). FRP composites consist of high strength fibers which are embedded in an epoxy matrix and bonded to the concrete. The quality of such bond depends on several factors and properties such as the material properties of the FRP and epoxy, concrete compressive strength, roughness, and cleanliness. Two main techniques are usually used to strengthen RC beams in flexure, which are externally bonded reinforcement (EBR) and near-surface mounted (NSM) systems. EBR is when the fibers are oriented along the longitudinal axis of the beam and its common failure mode is FRP debonding. The other technique is NSM, where a groove is cut in the concrete to insert the FRP bar or strip into it. (Ashour et al. (2004); Barris et al.(2020); Darain et al. (2016); Mostofinejad et al.(2013); Lorenzis and Teng (2007); Hawileh (2012)). Anchorage systems are used to prevent the two unfavorable failure modes, which are FRP debonding and delamination. These systems have become an interest in recent research studies due to their effectiveness and variety. Anchorage systems enhance the capacity of strengthened concrete structures and maintain their deformability, by providing a load transferring mechanism after debonding. Moreover, they are used to delay or prevent interfacial crack opening, increase the total interfacial stress, and to provide a stress transfer mechanism (Grelle and Sneed (2013)). Many types of anchorage systems are commonly used nowadays to anchor FRP sheets to RC members such as FRP spike anchors, U-anchors, transverse U-wrapping, FRP strips, and mechanically fastened anchors. The most common system is FRP spike anchors, where fibers with one end embedded in epoxy resin into the concrete and the other splayed on the FRP laminate. FRP anchors consist of three main components that play an important role in their strength, which are: (1) The dowel inserted in the concrete structure with an insertion angle β; 2) Fan po rtion set on the FRP sheet using epoxy; and (3) Key portion (transition segment) that includes the transition between the fan and the dowel. There are two types of FRP spike anchors which are straight anchors and bent anchors. Straight anchors are 180  anchor spikes installed in-line with the FRP laminates allowing tensile forces to be transferred from the FRP laminate to the FRP anchor embedment. The other type is bent anchors which include 90o and other angles anchor spikes where one end is installed in a predrilled hole at a specified angle to the plane of the concrete and the other end splayed on the FRP laminate (Grelle (2011); Aljaafreh et al. (2018); Del Rey Castillo (2017)).