Numerical analysis of concrete elements strengthened with carbon fiber reinforced polymers

Sammanfattning: Due to the growing number of ageing reinforced concrete structures such as buildings and bridges, there is an increasing interest in condition assessment and strengthening methods. Deterioration processes and demands for higher loads contribute to this interest. If the service life of a structure can be extended by repair and/or strengthening instead of building a new structure, much money can be saved. A strengthening method that has gained increasing acceptance and popularity in recent years is strengthening with fiber reinforced polymer (FRP) composites. One common type of FRP material is carbon FRP (CFRP). The FRP strengthening system consists of the strengthening material, a FRP, and the bonding material, usually an epoxy adhesive. Strengthening with FRP can generally be divided in two types of techniques: externally bonded sheets/plates and near surface mounted reinforcement (NSMR). The technique of bonding FRP sheets/plates externally encompass of bonding the FRP onto a prepared surface on a structural member. In the NSMR technique, a longitudinal groove is cut in the surface of a structural element, followed by applying the bonding material into the groove and inserting a FRP bar. The most important characteristic of a FRP strengthening system is the ability to transfer loads acting on a deficient structural member to the strengthening material. A detrimental event for a strengthened member is failure in the bond region, denoted as debonding. This means failure is occurring prior to the designed or predicted capacity of the strengthened member and must be avoided. The behavior of the complete composite system with concrete, adhesive, FRP, and internal reinforcement is quite complex. A sophisticated alternative when studying the performance of concrete structural members strengthened with FRP is the finite element (FE) method. To properly model the problem at hand, several considerations must be made in a FE analysis: solution procedures, material models, boundary conditions, etc. This thesis aims to contribute to the understanding of the behavior of FRP strengthened structures in general and NSMR CFRP strengthened concrete members in particular. Nonlinear 3D FE analysis is utilized to investigate the behavior of FRP strengthened slabs, with and without openings, and two different kinds of bond testing methods: a beam bending test and a NSMR anchorage test. The results from the slab tests show that slabs with openings can be strengthened with externally bonded CFRP sheets and the performance is even better than for traditionally steel reinforced slabs. The numerical evaluations show good agreement with the experimental results. The results from the beam bending tests indicate that the externally bonded sheet and plate have an effective bond length and that NSMR have an anchorage length larger than tested bond lengths. The numerical results are sensitive to the values of the fracture energy, the tensile strength, and the shape of the softening response for the concrete. The results in the NSMR anchorage test show that the failure behavior could be captured and explained only by combining the experimental observations and the FE analysis. The failure mode is a combined failure in the concrete and the adhesive; however, the maximum transferable load is obtained when a major crack at the very end of the bond length is developing in the concrete.