An Accurate and Efficient Finite Element for Reinforced Concrete Beams Flexurally Retrofitted with FRP

This paper presents an efficient frame finite element (FE) able to accurately model reinforced concrete (RC) beams strengthened in flexure with externally bonded fiber reinforced polymer (FRP) strips and plates. The proposed element employs a force-based formulation and considers distributed plasticity with fiber-discretization of the cross-sections. The FRP strip/plates are modeled as an additional layer with linear elastic-brittle behavior in tension and zero-strength in compression. The failure strain in tension is the lower value between the rupture strain of the FRP material and the strain at debonding. The cross-section stress resultants are obtained under the Euler-Bernoulli kinematic assumption. Realistic nonlinear hysteretic constitutive models are used for both concrete and steel materials. The presented FE is used to predict, based on very coarse FE meshes, the ultimate load-carrying capacity of beams subjected to three- and four-point bending loading, for which experimental results are available in the literature. Comparison of numerical and experimental results are presented in terms of the ratio between the numerical prediction and the experimental value of the maximum shear force for several sets of tests from different authors. The experimental configurations considered cover a wide range of different geometry, material properties, steel reinforcements and FRP reinforcements. The obtained numerical results compare remarkably well with the experimental results. The proposed FE is able to model flexural collapse (steel yielding, concrete crushing), collapse due to FRP rupture, and FRP debonding. The major features of this frame FE are its simplicity and its efficiency in terms of mesh refinement. In addition, the proposed frame element can be used without modification for FE analyses based on cyclic and dynamic loadings. Thus, this FE is suitable for modeling and analyzing flexural rehabilitation of FRP-strengthened RC frame structures. FRP-strengthening is particularly useful for structures subjected to demanding environmental conditions.