On Edge Stresses Control in Strengthened RC Beams with FRP Strips: Adhesive Layer Profile Effect

A high-order analytical model for the analysis of reinforced concrete (RC) beams strengthened with fiber-reinforced plastic (FRP) strips bonded with adhesive layer of variable thickness is presented. The model is based on the closed-form high-order approach, and it provides the means for the analysis of beams retrofitted with generally curved FRP strips and adhesive layers of arbitrary profile. The analysis is comprehensive and includes the local and overall response of the structure. An emphasis is put on the stress concentration that occurs at the edge of the FRP strip and in many cases leads to brittle and sudden failure of the strengthened member. The field equations and the boundary and continuity conditions are derived using the variational principle of virtual work along with the kinematic relations of small deformations. The governing equations of the generally curved FRP strip include large curvatures and are introduced via coordinate transformation from its local curvilinear system into the global Cartesian one. The derived model is used for the investigation of various adhesive profiles and their influence on the shear and vertical normal stresses at the edges of the FRP strip. The results focus on the stress concentrations involved and reveal that proper design and application of the adhesive profile can significantly reduce the edge stresses, thus preventing the brittle mode of failure. The paper is concluded with a summary and recommendations for the analysis, design, and application of the strengthening process.