PSI - Issue 62

Francesco Bencardino et al. / Procedia Structural Integrity 62 (2024) 972–982 Francesco Bencardino/ Structural Integrity Procedia 00 (2019) 000 – 000

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4.1. Material properties of reinforcement – FRPs FRP materials to be used for the bending and shear reinforcement interventions of the structural elements being analyzed are the following: • Uni-directional carbon fiber fabric (CFRP) of 620 g/m 2 , 500 mm wide and 0.348 mm thick, specific for structural consolidations, to be applied with the fibers in a perpendicular direction (β=90°) or appropriately inclined (β=45°; β=135°) with respect to the longitudinal axis of the beam element, to achieve an increase in the shear resistance of the element. This CFRP had a tensile breaking stress of 4800 MPa and a tensile elastic modulus of 230 GPa. • 1.4 mm thick carbon fiber plate, pre-impregnated with epoxy resin produced by pultrusion, 120 mm wide, specific for the structural consolidation of predominantly bent RC-elements having a tensile breaking stress >3000 MPa and a tensile elastic modulus of 170 GPa. Furthermore, the partial factors for the materials, the resistance models, and environmental conversion factors were (according to CNR DT 200/2004): • γ f = 1.10 failure of the composite in “ edges detachment ” application; • γ f,d = 1.20 collapse due to delamination in “ edges detachment ” application; • γ Rd = 1.00 bending resistance model; • γ Rd = 1.20 shear resistance model; • η a = 0.85 external exposure, aggressive environment, carbon/epoxy FRP system; • α fE = 0.90 stiffness reduction coefficient for in situ impregnated systems; • α ff = 0.90 resistance reduction coefficient for in situ impregnated systems. In this study, special attention was given to debonding considerations during the design phase. The calculation process took into account the specific debonding strain values that were different between CFRP plates (about 0.22%) and CFRP fabrics (about 0.14%). Specifically, 0.00304 and 0.00215 for 1 ply and 2 plies of CFRP plate, respectively; 0.001413 and 0.000981 for 1 ply and 2 plies of CFRP fabric, respectively. This tailored approach ensures the effective adhesion of the FRP-strengthening materials to the concrete substrate, optimizing the overall structural performance and durability of the rehabilitated bridges. Effective bond length, often referred to as “ effective anchorage length ” , is a critical concept in structural engineering and construction. It pertains to the length over which FRP-strengthening bonded on the concrete substrate to ensure that it provides the intended structural strength. It should be remarked that, when strengthening a RC-beam using FRPs, it is often recommended to apply bending reinforcement before the shear reinforcement. This sequence is chosen for several reasons. The primary purpose of FRP-strengthening is often to enhance the flexural (bending) capacity of the beam. This is important because flexural forces are typically the main cause of structural failure in beams. By applying FRP sheets or wraps to the tension face (bottom) of the beam, the section ’ s flexural capacity can be significantly increased. Moreover, sequential application of bending and shear reinforcement simplifies the construction process. When FRPs are applied for bending, the material can be applied uniformly along the entire length of the beam. This provides better control over the installation process and minimizes the risk of defects. Lastly, when bending FRP strengthening is applied first, it provides a more solid anchor for the subsequent shear FRP-application. This anchorage reduces the likelihood of edge debonding in the shear strengthening process because the shear FRP-strengthening can be bonded to the pre-installed bending reinforcement, creating a more robust connection. Vice versa, the shear strengthening improves the edge bond of the bending strengthening previously installed preventing premature detachments. The design was guided by the principle of minimum intervention according to ISCARSAH ’ s Recommendations (2003). With reference to the two central beams, a minimum capacity-resistant bending moment was found to be equal to 1908 kNm, corresponding to a safety factor of 1.04. To achieve this, a first CFRP layer (made up of 3 plates) needed to be applied for 7.5 m and a second layer for 5.0 m symmetrically installed with respect to the midspan (Fig. 7). Shear reinforcement was necessary for all four beams of the deck, the two internal ones (Fig. 8a) and the two of the edge (Fig. 8b) of each span, as the design shear stress is greater than corresponding resistance. The shear reinforcement was achieved by applying strips of fabric, on one or more layers, adhering to the external surface of the element to be strengthened. Distinctive elements of the reinforcement were the thickness, width and pitch of the attached fabric strips to the lateral faces of the beam and the inclination angle (β) of the fibers with respect to the