PSI - Issue 68

Aseel Salameh et al. / Procedia Structural Integrity 68 (2025) 166–172 A. Salameh et al. / Structural Integrity Procedia 00 (2025) 000–000

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the structure's load-bearing capacity. However, it is also the primary vulnerability of FRP strengthening systems. Debonding at the interface is known to be the most common mode of failure, often occurring unexpectedly and hindering the full potential of the FRP's high-performance qualities (Haddad et al., 2013; Tatar and Hamilton, 2016; Widanage et al., 2021). Inaccurate modelling of this bond behaviour can lead to significant underestimation or overestimation of the retrofitting efficiency. Thus, developing accurate models that capture the interfacial behavior is essential for understanding the mechanics of FRP retrofitting and predicting the performance of these strengthened structures. Researchers have reported different scenarios and models for a better understanding of this interface behaviour (Salameh et al., 2024). One key concept in these efforts is the bond-slip relationship, which quantifies the interaction between the FRP and concrete substrate. This relationship describes local shear stress distribution with respect to displacement (slip) and is used as a foundation for more advanced modelling techniques (Abdalla et al., 2017; Cho et al., 2011; Hawileh et al., 2014, 2024; Ko et al., 2014; Naser et al., 2012). To characterize the bond behavior between CFRP and concrete, bond-slip models are developed to define the relationship between bond stress and slip at the interface as mentioned previously. Three main approaches, numerical, analytical, and empirical are presented in the literature for predicting the bond behavior and debonding capability of FRP sheets affixed to concrete substrates. Empirical bond-slip models are directly derived from experimental testing including pull-off, shear, or beam-bending tests (Abdalla et al., 2017; Fathi et al., 2023; Guo et al., 2023; Naser et al., 2012; Pellegrino et al., 2008; Zhang et al., 2016). These models provide a straightforward relationship between bond stress and slip, represented by mathematical functions that fit the observed experimental data and this is expressed in mathematical forms as bilinear, trilinear, or polynomial relationships. The bond-slip curve produced from these tests generally exhibits an initial ascending branch (representing bond development), followed by a peak bond stress, and then a descending branch as debonding occurs (Abdalla et al., 2017; Nelson et al., 2020; Biscaia et al., 2015; Cho et al., 2011; Heydari Mofrad et al., 2019; Lu et al., 2005). Analytical models generate assumptions about interface properties and failure mechanisms based on theoretical equations derived from material behavior and mechanics, parameters such as bond strength, stiffness, and debonding capacity often incorporated. These models provide generalized formulas applicable to a wide range of conditions. However, their assumptions are less accurate compared to empirical models derived directly from experimental data (Nelson et al., 2020; Fathi et al., 2023; Liu, 2013). Many numerical models have been developed to simulate the bond slip behavior of FRP-concrete interfaces with the primary use of Finite Element Methods (FEM) (Assad et al., 2022a; Assad et al., 2022b; Assad et al., 2024). Making use of complex material properties and discretizing the interface into elements, these models could capture the nonlinear behavior, stress distribution, as well as debonding progression of the bond. It is because these numerical models can be approximated to any shape geometry, applied to all loading conditions and thereby boundary effects, so those tools have the potency in dealing with real-world structures. In most cases, numerical models are validated with experimental data to improve the accuracy and confidence of simulations (Lu et al., 2005; Müzel et al., 2020; Naser et al., 2012; Yuan et al., 2004). This research aims to integrate experimental bond-slip models into finite element analyses of CFRP-strengthened concrete members. This will provide more reliable predictions of bond behavior, structural capacity, and failure mechanisms by filling the gap between experimental observations and computational modeling. Nomenclature τ( ) Local bond/shear stress at different locations (MPa) Number of CFRP sheets ply Thickness of CFRP sheet (mm) Modulus of elasticity of CFRP sheet (MPa) Strain reading from ith strain gauge ( ) Location of the ith strain gauge (mm) along the sheet Concrete deformation FRP deformation ( ) Slip at distance x (mm) (0) Local slip at maximum bond/shear stress (mm) ( ) Slip at end of bonded length (mm) n Total number of strain gauges