PSI - Issue 64

Paula Folino et al. / Procedia Structural Integrity 64 (2024) 1452–1459 Folino et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The need to increase the load-bearing capacity of existing reinforced concrete structures may be due to different situations such as, among others, a change in the construction destination, the incorporation of additional constructions to be supported by the existing structure, an error in the structural design or in the construction stage, a degradation due to environmental actions, or an exposure to a fire or an impact. In what respects to shear capacity, which is the focus of this paper, numerous techniques have been developed to improve shear strength of reinforced concrete beams, including concrete jacketing, shotcrete, fiber-reinforced polymer (FRP) sheets or plates, and steel plates or profiles, among many others (Delatte, 2009; Helene & Pereira, 2003). Concrete jacketing involves adding an extra layer of concrete to the sides and often the lower part of the beam. The thickness of this additional layer is determined by factors such as the desired strength and the quality of the concrete, usually falling within a range of 40-100 mm according to research, although practical constraints may require thicknesses beyond 80mm (Mitropoulou et al., 2022). Adequate surface preparation, concrete fluidity, and proper casting are vital for ensuring strong adhesion and long-lasting effectiveness of the reinforcement. In the case of shotcrete reinforcement, the method entails building up the section with a layer of concrete pneumatically projected onto the existing surface, allowing for the adaptation to complex geometries. However, this process can be noisy and produce significant waste, making it less suitable for scenarios where minimizing disturbance to occupants or neighboring areas is essential. It should also be noted that the resulting concrete layer, unless carefully proportioned and applied, tends to have more porosity compared to conventionally cast and compacted concrete, reducing its resistance and durability (Bamonte et al., 2016). FRP sheets or plates, typically made of materials such as carbon fiber or glass, have also been extensively studied as a method for shear strengthening of reinforced concrete beams, and are applied in one or multiple layers to the beam's surface using epoxy adhesives. These strengthening techniques present high strength-to-weight ratios, are corrosion-resistant, and manage to achieve thicknesses as low as 3 to 4mm (Raza et al., 2019; Naser et al., 2019). Nonetheless, the epoxy adhesive is sensitive to UV radiation, and their thin profile makes them susceptible to fire unless provided with additional protection (Tetta and Bournas, 2016). Similarly, steel plates or profiles can be applied to the beam's lateral surface, either bonded with epoxy adhesives or anchored with bolts. This method offers geometric versatility, allowing for reduced thicknesses, but fire protection is often required, and corrosion susceptibility may demand additional protection depending on the environment (Raza et al., 2019; Liu et al., 2018). One technique that has shown promising results in shear strengthening of reinforced concrete beams is the use of Steel Fiber Reinforced Concrete jacketing, leading to improvements in durability and adhesion between new and existing concrete (Ruano et al., 2014; Martinola et al., 2007), and since no reinforcement is used, the thickness is limited by fiber length and volume content (Folino et al., 2020), with minimum values reaching up to 30 mm (Gholampour et al., 2019). Additionally, fiber-reinforced concrete exhibits good corrosion resistance and higher surface crack mitigation compared to other methods (Xargay et al., 2018). However, there are still open questions regarding minimum thicknesses, applicability ranges, impact of surface preparation on performance, degradation under the action of high temperatures and structural design methodology for determining jacket thickness. This paper presents part of the results of an extensive experimental study comparing different alternatives for strengthen reinforced concrete beams using Fiber Reinforced Self-Compacting High Strength Concrete jacketing. 2. Experimental Campaign 2.1. Materials The experimental campaign firstly involved the preparation of reference concrete beams reinforced with steel rebars to be strengthened. For this purpose, elaborated Normal Strength Concrete (NSC) with a target compressive strength of 40MPa was provided by a local company and transported to the site by a mixer truck and then poured into the formworks. Compaction of fresh concrete was carried out using a three-phase vibrator with a 25mm diameter. Reference beams were structurally designed for failing in shear. Ribbed steel bars used in this campaign are classified as ADN 420 according to local standard IRAM-IAS-U-500-528, with a prescribed yielding stress of 420 MPa, a tensile