Finite element prediction of brittle failure in RC beams strengthened with externally bonded FRP

The use of fiber reinforced polymer (FRP) composites for repairing and strengthening of reinforced and prestressed concrete, as well as steel and masonry structures has proved to be an effective way of upgrading the infrastructure because FRP products offer an excellent corrosion resistance, high strength and stiffness to weight ratio, ease of transportation, handling, and application. When strengthening RC members with FRP material, a typical strength increase is desired. A conventional failure by either, concrete crushing in the compressive region, or rupture of FRP composite in the tensile soffit, can provide the desired strength enhancement. However, in most cases, the desired strengthening capacity will not be achieved due to premature failures which take place before the ultimate capacity is reached. Concrete cover separation (CCS) has been frequently observed pre-mature failure in RC beams strengthened or retrofitted with FRP plates. This study investigates, using three-dimensional finite element (FE) models, the behavior of RC beams strengthened in flexure with externally bonded Carbon FRP (CFRP) material, when CCS is the mode of failure. Four FRP strengthened RC beams, previously tested in four-point bending and experienced CCS failure, were simulated at several loading stages, cracking, yielding, and failure. A section of the FE model, at the cut-off location of CFRP, was examined to determine the failure load. A stress failure criterion, based on the concrete strength, is implemented. At each load step in the model, normal stresses at the section were observed and compared to the concrete tensile strength, and when the maximum tensile stress exceeds the concrete's tensile strength, CCS failure is assumed to initiate and the accompanying load step is considered to be the ultimate load. Comparisons with the experiment, have suggested a good correlation of results between FE and the experiment, and validated the post-processing analysis procedure.