Nonlinear constitutive model for time-dependent behavior of FRP-concrete interface

The constitutive behavior of the FRP-concrete interface is a prime issue when evaluating the strengthening effects of concrete structures strengthened with externally bonded fiber-reinforced polymer (FRP) sheets or plates. This paper is devoted to developing a new nonlinear viscoelastic model for the study of the long-term behavior of the FRP-concrete interface. The model has the ability to describe the creep of the FRP-concrete adhesive layer and the creep fracture propagation along the FRP-concrete interface. The linear viscoelastic behavior of the FRP-concrete interface is taken into account by using Maxwell’s generalized rheological model through a step-by-step time increment procedure. The nonlinear time-dependent behavior of the adhesive layer is considered through the micro slip criterion model (MSCM), which depends on a displacement failure criterion and the interfacial fracture energy. The proposed model is implemented with a finite element model and it has been calibrated by using the results of time-dependent double-lap shear test specimens. The results demonstrate the reliability of the proposed model and its capability to predict the time-dependent debonding propagation along the FRP-concrete interface. It is also shown that the model can be used to predict a wide range of creep fractures not only under low sustained loading but also under high sustained loading.