Finite element modelling of beams strengthened with FRP sheets during short and long-term loads

FRP (Fiber Reinforced Polymer) technique was often diffused to repair damaged reinforced concrete bridges. In this work it is proposed to investigate the complex mechanism of stress-strain evolution at the FRP interface, during short or long-time loadings, until complete debonding. This study has been per- formed by means of a fully three-dimensional approach within the context of damage mechanics, to appropri- ately catch transversal effects as well as normal stresses, developing a realistic and comprehensive study of the delamination process. The adhesion properties have been studied through a contact model incorporating an elastic-damage constitutive law, relating inter-laminar stresses acting in the sliding direction. A F.E. re- search code (FRPCON) has been developed, incorporating a numerical procedure accounting for Mazars damage law inside the contact algorithm. The code is able to describe the delamination process considering the various surface preparations of the concrete part. Long-time behaviour of this composite structures has been studied by means of two visco-elastic formulations: i) Bazant B3 law has been considered in the con- crete component, where creep effect is composed by three different terms, i.e. the elastic part, basic creep and drying creep; ii) different long-time behaviour for fibres and matrix have been implemented for FRP compo- nent, using a micromechanical approach. The experimental results of long-time bending tests have been used to calibrate and validate the numerical models.