PSI - Issue 64

Amir Mofidi et al. / Procedia Structural Integrity 64 (2024) 999– 1008 Mofidi et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Rehabilitation of reinforced concrete (RC) beams in shear is possible using different techniques. One of the progressive and effective strengthening techniques is the near-surface mounted (NSM) fibre-reinforced polymer (FRP) technique. The NSM FRP material used in this technique is in the shape of bars or laminates. In order to use this method for shear-strengthening, grooves are cut in the concrete side covers of RC beams before the NSM FRP is installed in the grooves and bonded with epoxy adhesive perpendicular to the longitudinal axis of the shear-deficient RC beams. The first practical use of the NSM technique dates back to 1949 when NSM steel rods were grouted into the longitudinal cut grooves to strengthen a bridge slab in flexure (Asplund 1949). Later, Nanni et al. (1999) strengthened a highway bridge in flexure using NSM FRP rods. In 2001, De Lorenzis and Nanni (2001) used the NSM FRP method to strengthen RC beams in shear for the first time. Six beams were constructed and tested in this study. De Lorenzis and Nanni (2001) proposed a model to predict shear contribution of NSM FRP circular bars. Later, Parretti and Nanni (2004) modified the earlier model to predict the shear contribution of NSM FRP circular and rectangular bars. Barros and Dias (2006) investigated twenty specimens with different test parameters such as different steel and FRP shear reinforcement ratios; and compared the NSM technique results in shear-strengthening with those of beams strengthened with externally bonded (EB) FRP sheet technique. In this study, a previously proposed model was used with fixed bond stress and CFRP effective strain values obtained in pullout bending tests with NSM CFRP laminate system. In 2009, Rizzo and De Lorenzis (2009a), tested a number of beams with NSM round bars and strips reinforcement. They proposed two separate models with a simplified and a more sophisticated approach, to predict the FRP contribution to the shear capacity (Rizzo and De Lorenzis 2009b). In the former model, an ideally plastic bond-slip behavior of the NSM reinforcement was used while in the latter one detailed bond – slip modeling of the NSM reinforcement was utilized. A series of tests were also conducted by Dias and Barros from 2008 to 2013 using NSM FRP laminates to strengthen RC beams in shear (Dias and Barros 2008; 2010; 2011; 2012; and 2013). A predictive model was proposed by them to predict the NSM FRP laminates contribution in shear (Dias and Barros 2013). Perera et al. (2014) proposed two approaches for NSM FRP bars including the use of neural networks as a means of predicting shear strength and solving a multi-objective optimization problem with genetic algorithms. The performance of both methodologies was compared with the experimental data in the literature. Bianco et al. (2009; 2010; and 2014) proposed a three-dimensional mechanical model for simulating the NSM FRP strips by fulfilling equilibrium, kinematic compatibility, and constitutive law of the adhered materials and the bond between them. Mofidi et al. (2016 and 2018) tested six full-scale RC T-beams to study the effects of important parameters on NSM FRP technique. The experimental results of Mofidi et al. (2016) and other researchers were used to develop a model to predict the shear contribution of NSM FRP rods and laminates in RC beams strengthened in shear. A comparison with other existing models showed that the model achieved a better correlation with the experimental data than the other existing equations. In 2023, Mofidi et al. (2023) proposed a new design model using a different bond model than that of Mofidi et al. (2016 and 2018) to improve the accuracy and ease of use of the equations. In this research study, the existing design procedures in the literature regarding the shear contribution of NSM FRP in shear-strengthened RC beams were evaluated. A multi-metric comparative study was conducted to show the accuracy of each model. 2. Contribution of NSM FRP to shear strength In order to calculate the nominal shear resistance of a beam strengthened with NSM FRP in shear, V n , a simple equation has been proposed which has three parts for the contribution of concrete, V c , transverse steel reinforcement, V s , and the contribution of NSM FRP reinforcement, V f (Eq. 1). (1) Various investigations have been conducted on the contribution of NSM FRP bars or laminates to the shear resistance. In this study, state-of-the-art existing design models have been studied and compared. n s f V V V V = + + c