PSI - Issue 64

Tom Molkens et al. / Procedia Structural Integrity 64 (2024) 1484–1491 Tom Molkens & Mona El-Hallak/ Structural Integrity Procedia 00 (2024) 000 – 000

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1. Introduction In the past few years, fiber-reinforced polymer (FRP) reinforcement has been widely used in concrete structures, especially for applications where high strength, low deadweight, and high corrosion resistance or electromagnetic transparency are desired. Its construction lifecycle and maintenance process led to a reduction in greenhouse gas emissions, hence a lower environmental impact than conventional materials.(Sbahieh, et al., 2022). A parameter study (Preinstorfer, et al., 2022) shows that in the design of FRP-reinforced structures, the serviceability limit state often becomes normative. As a result, higher reinforcement ratios are used, on the other hand, recently developed thermoplastic GFRP rebars facilitate the recycling process (Benmokrane, et al., 2021). However, the advantages of Glass Fiber Reinforced Plastics (GFRP) over Carbon Fiber Reinforced Plastics (CFRP) i.e., six times lower environmental impact and cost difference of about a factor of 12, limits the underlying research to a study of the redistribution capacity of GFRP-reinforced structures through experimental tests on double-span beams. What is innovative about this study is that, unlike other studies, the top reinforcement was not applied continuously, but only above the support point, and slip was measured during the experiment. Due to the modified detailing, a clear redistribution of bending moments could take place. From the deformations measurements, strains and reactions carried out, force-deformation relationships, moment-curvature, and redistribution ratio can be determined. GFRP-reinforced concrete is characterized by the combination of the two brittle materials, so the required ductility is expected to be neglected as reported in past studies (Grace, et al., 1998; Santos, et al., 2013). It was highlighted that ductility had to be achieved by using either the plastic behavior of concrete or concrete failure as the failure criterion for beams. In addition, the redistribution capacity could be an issue due to the anisotropic behavior of GFRP reinforcement and larger crack widths/deformations (fib Bulletin 40, 2017). Nevertheless, the redistribution capacity has been observed in experiments for internal reinforcement using strain gauges and measurement of reaction forces (Rahnam & El-Salakawy, 2016), and for hybrid reinforcement (Almahmood, et al., 2020). Furthermore, the interaction surface properties between the GFRP bars and concrete matrix may still cause some plasticity of the composite reinforced structure. This behavior has been reported elsewhere with external reinforcement (de Castro & Keller, 2009), simple beam tests (Lin & Zhang, 2013), and based on pull-out tests (Fava, et al., 2016). Accordingly, later published tests either implicitly (Basa, et al., 2018) or explicitly (Basa, et al., 2020) address the influence of the bond between rebar and concrete matrix. Previous in-house research (Molkens & Lauwens, 2023), in which GFRP reinforced concrete slabs were subjected to accidental loading, also demonstrated the beneficial effect of slip of the GFRP reinforcement. However, without slip at the interface between concrete and the FRP-rebars, there is no redistribution (Peng, et al., 2023), highlighting the need for appropriate detailing. Recent research (Mistretta, et al., 2023) on shear capacity also points to the importance of slip behavior in analytical models for its evaluation. 2. Design rules In the design of reinforced concrete structures, ductile behavior and rotational capacity are often used implicitly. This allows several effects such as shrinkage, creep, temperature, and differential settlement to be neglected (EN 1992 1-1, 2005). The possible redistribution capacity of the structure is used if the occurrence of these effects does not lead to (excessive) damage. In fact, according to the same standard, the structural calculation should take into account the influence of possible moment redistribution in all aspects of the design. However, the application of "limited" redistribution is subjected to a number of conditions, including i) only applicable to continuous slabs and beams that are predominantly subjected to bending, ii) with a ratio between the lengths of adjacent spans between 0.5 and 2, and iii) if sufficient rotational capacity is also available. The redistribution ratio δ is defined as the ratio between the redistributed moment ( M RED ) and the linear elastic moment ( M LEA ), as shown in Eq. (1). It should be noted that several authors also use the degree of moment redistribution, MR = (1- δ) in %. = ≥ + / (1) To invoke this, the current European regulations (EN 1992-1-1, 2005) have requirements that depend on a minimum value k i , the deformation properties of the concrete via the ε cu2 value or the ultimate strain (included in the k j factor, depending on the concrete quality), and the ratio between the compressed zone (height x u ) and the effective depth ( d )