FRP-RC Beam in Shear: Mechanical Model and Assessment Procedure for Pseudo-Ductile Behavior

Abstract

This work deals with the development of a mechanics-based shear model for reinforced concrete (RC) elements strengthened in shear with fiber-reinforced polymer (FRP) and a design/assessment procedure capable of predicting the failure sequence of resisting elements: the yielding of existing transverse steel ties and the debonding of FRP sheets/strips, while checking the corresponding compressive stress in concrete. The research aims at the definition of an accurate capacity equation, consistent with the requirement of the pseudo-ductile shear behavior of structural elements, that is, transverse steel ties yield before FRP debonding and concrete crushing. For the purpose of validating the proposed model, an extended parametric study and a comparison against experimental results have been conducted: it is proven that the common accepted rule of assuming the shear capacity of RC members strengthened in shear with FRP as the sum of the maximum contribution of both FRP and stirrups can lead to an unsafe overestimation of the shear capacity. This issue has been pointed out by some authors, when comparing experimental shear capacity values with the theoretical ones, but without giving a convincing explanation of that. In this sense, the proposed model represents also a valid instrument to better understand the mechanical behavior of FRP-RC beams in shear and to calculate their actual shear capacity.

Shear Capacity of Reinforced Concrete T-Beams Retrofit with Externally Bonded CFRP Fabric: A New Perspective

“…In order to understand this historic inability to accurately predict the FRP contribution to shear resistance, it is necessary to consider the way in which this contribution is determined experimentally. As noted by Petrone and Monti (2014), most research efforts have concentrated on accurately determining an FRP contribution, rather than describing a comprehensive mechanical model for retrofit beam behaviour. This has led to an experimental approach that focuses on identifying a distinct component of shear resistance V frp that represents the FRP 'contribution'.…”

“…Despite considerable research effort and widespread use of this strengthening approach in practice, questions about the accuracy (Lima andBarros 2011, Chen andCheng 2019) and appropriateness (Petrone and Monti 2014) of the models underpinning existing guidance have been raised. In particular, the assumption underlying most current guidance -that the concrete, steel and FRP contributions to resistance are independent -is drawn into question by experimental studies (Bousselham et al 2006, Belarbi et al 2012, Mofidi et al 2014).…”

“…An apparent reduction in the enhancement available from the externally bonded FRP associated with increasing transverse reinforcement provision as a result of premature separation of the FRP material from the concrete substrate has been termed an "adverse shear interaction" (Chen et al 2013, Chen et al 2017. Models have addressed this interaction using modification factors considering the ratios of steel and FRP reinforcements (Mofidi and Chaallal 2011) or through mechanical models enforcing weak (Pellegrino and Modena 2008) or strong (Petrone and Monti 2014) compatibility conditions on the steel and FRP strains. The role of bond between the concrete substrate and the CFRP in relation to the effective capacity of the externally bonded system has been extensively investigated (Khalifa et al 1998, Triantafillou and Antonopoulos 2000, Denton et al 2004, Mofidi and Challal 2011, Chen et al 2013, Chen et al 2017.…”

Comparisons of Design Methods of Shear Capacity and Flexural Capacity Using Fiber Reinforced Polymer in Concrete Beam

“…concrete, FRP, stirrups. According to (Monti, 2014), ACI overestimates the shear strength of reinforced concrete strengthened with FRP.…”

Assessment of the existing models to evaluate the shear strength contribution of externally bonded frp shear reinforcements

“…This might be due to possible changes in the strut orientation or additional cracking that may change the contribution of the concrete or existing transverse reinforcement to the shear strength. The interaction of the FRP shear reinforcement with the transversal steel or the concrete is only considered in a few number of the existing formulations (Modifi and Chaallal [12], Monti and Liotta [13], Kotynia [14]; Colotti [15]; Ali et al [16]; Petrone and Monti [17]). Bousselham and Chaallal in [8] concluded that the contribution of concrete remains more or less unchanged after the formation of diagonal cracking for small and medium size beams.…”