PSI - Issue 64

Francesco Bencardino et al. / Procedia Structural Integrity 64 (2024) 932–943 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Italian Code (2018) for roofs not accessible except for normal maintenance and repair ( q k =0.50 kN/m 2 category H). For this reason, it is likely that the tension steel reinforcement in this section yielded, leading to concentrated, wide, and deep cracks. These results are likely attributed to some incorrect assessments during the design stage (e.g. raising the rebars before the null point in the bending moment diagram, see Figure 2 – Arrangement of the longitudinal steel bars). However, considering the minimum live load of 1.50 kN/m 2 as suggested by the Eurocode 1 (Part 1-1, 2002) or 2.00 kN/m 2 as proposed by the Italian Code (2018) for practicable roofs, at the Ultimate Limit State (ULS) even the M-M section is unable to withstand the minimum required loads. Based on what was reported above, it became clear that each beam needed strengthening in the overall clear span. 2.3. Externally Bonded FRP Systems for Strengthening Concrete Structures After the preliminary analysis, it was decided to develop an innovative solution using FRP composites for strengthening the degraded RC structural members. Within the last 20 years, many design guidelines were published to provide guidance for the selection, design, and installation of FRP systems for external strengthening of concrete structures. In Europe, the International Federation for Structural Concrete developed fib Bulletin 14 (2001) and fib Bulletin 90 (2019), which specifically addresses the use of externally bonded FRP reinforcement for RC structures. Additionally, the American Concrete Institute also contributed to the state of the art with its first publication and currently with ACI 440.2R-17 — Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. Furthermore, complementary guidelines have been provided by other organizations. The Concrete Society issued TR 55 (2012) — Design Guidance for Strengthening Concrete Structures Using Fibre Composite Materials, which provides additional insight and recommendations for FRP strengthening applications. Similarly, the Japan Society of Civil Engineer (JSCE, 2001) and the Canadian Standards Association (CSA S806:12, R2021) have contributed to the field with their own guidelines. Finally, in Italy, CND-DT 200 R1/2013 provides a comprehensive guide for the design and construction of externally bonded FRP systems for the strengthening of structures. As reported above, significant efforts have been made by the scientific community in recent years to promote a deeper understanding of this type of strengthening system; however, at the time of the intervention, consensus guidelines for the use of FRP materials in structural applications were not available. For this reason, the rehabilitation of the RC frames had to rely on the analysis of accurate experimental results. Below is a summary of the experimental results used to design the strengthening intervention. Subsequently, the same intervention was checked using the procedure described in CND-DT 200 R1/2013, in order to make a comparison and draw useful conclusions almost twenty years later. 3. C-FRP strengthening of the RC frames 3.1. Experimental tests and results The experimental campaign included four-point bending tests on C-FRP-strengthened RC beams with the aim of investigating the influence of the external strengthening system on the load-bearing capacity, deflection, and curvature of the specimens. The characteristics of the tested RC beams are summarized in Table 1. Specifically, one unreinforced beam was used as a control specimen; one beam was reinforced in flexure with a C-FRP plate, and the remaining beam was reinforced with a C-FRP plate and external anchorages (position and geometry detailed in Figure 4). This latter configuration aimed to provide additional information on the influence of using external anchorages on the strength, stiffness, and ductility of the beams. The terminal anchorages, Type A, were U-shaped (600 mm wide), designed to counteract the high peel and shear stresses at the ends of the plates and to control the slippage between the plate and the concrete. The intermediate anchorages (Type B), positioned along the span of the beam, were also U-shaped but of smaller in size (100 mm wide) and were designed to limit the movement of the bonded plate and control the slippage between the plate and the concrete substrate along the span of the beam. The geometry and cross-section of the beams are illustrated in Figure 4. In general, all beams had a rectangular cross-section with dimensions of 140×300 mm² and a total length of 5000 mm (with a clear span of 4800 mm). As depicted in Figure 4, all the beams had internal longitudinal reinforcements comprising four ϕ 16 mm steel rebars.