FRP shear strengthening of RC beams and walls

Abstract: The shear failure of Fibre Reinforced Polymers (FRP) strengthened reinforced concrete (RC) beams has not been studied to the same extent as the bending failure mechanism in the past decade. The complex nature of the shear failure mechanism just for reinforced concrete beams is still under debate among scientists and not solved yet. If we add the FRP strengthening to the already existing unknown issues, it is quite clear why attention was not focused on the shear failure of strengthened beam. In other words an extra uncertainty to the already existing ones is complicating more the problem of shear in concrete. It is of utmost importance to understand the shear failure mechanism of reinforced concrete beams and for this all the known theories for designing reinforced concrete beams subjected to shear are presented: truss analogy, theory of plasticity for concrete and modified compression field theory. The use of these theories in two of the most used standards is also exemplified. Further on, a design model forthe shear strengthening of concrete beams by using fibre-reinforced polymers (FRP) is presented in Appendix I, and the limitations of the truss model analogy are highlighted. The fracture mechanics approach is used in analyzing the bond behaviour between the FRP composites and concrete. The fracture energy of concrete and the axial rigidity of the FRP are considered to be the most important parameters. The effective strain in the FRP when the debonding occurs is determined. The limitations of the anchorage length over the cross section are analyzed. A simple iterative design method for the shear debonding is finally proposed. Since the model's predictions are found satisfactory but not really precise, a deep literature review has been performed (Paper I). All the significant theoretical models for predicting the shear capacity of FRP strengthened RC beams developed during the years are analyzed, commented and compared with an extensive experimental database. The database contains the results from more than 200 tests performed in different research institutions across the world. The results of the comparison are not very promising and the use of the additional principle in the actual shear design equations should be questioned. The large scatter between the predicted values of different models and experimental results is of real concern bearing in mind that some of the models are used in present design codes. Further on, the influence of the FRP composites on the openings of shear (squat) walls is analyzed. In the same manner as for RC beams the current design methods existing in ACI (2005) and Eurocode (2004 a, b) are presented. Since the strengthening of shear walls was studied even less than FRP strengthened beams RC beam an up to date literature review of the experimental and theoretical work is presented. A concept, developed in collaboration with the Civil Engineering Department from "Politehnica" University of Timisoara, is presented for testing RC walls with openings subjected to lateral and gravitationalloads (Appendix III). From a matrix of 50 different practical configurations of openings are selected 12 walls which are the subject of an ongoing experimental program. 8 walls with different opening configurations are subjected to cyclic lateral loading under constant gravitational load to simulate the seismic behaviour of FRP strengthened walls with openings. 4 walls with different opening configurations are tested to monotonic gravitational loading up to failure.