STRUCTURAL RETROFITTING OF CONCRETE BEAMS USING FRP - Debonding Issues

Sammanfattning: Popular Abstract in English Concrete is one of our most common building materials and is used both for buildings, bridges and other heavy structures. Typically, concrete structures are very durable, but sometimes they need to be strengthened. The reason may be cracking due to environmental effects, that a bridge is to be used for heavier traffic, new building codes, or damage resulting from earthquakes. Concrete is a material that can withstand compressive loads very well but is sensitive to tensile forces. Therefore, concrete structures are typically reinforced by casting in steel bars in areas where tension can arise. This cannot be done afterwards, and one strengthening method, is therefore to glue reinforcement on the exterior of the structure in the areas exposed to tension. Fibre composite can be used in reinforcing concrete structures externally. Fibre composite materials have low density, can be easily installed and are easy to cut to length on site. Therefore, fibre composite as external reinforcement for concrete structures has become very attractive and popular around the world. It is important to understand the behaviour of a strengthened structure well and realize what parameters affect the failure mode and load-bearing capacity. The aim of this thesis is therefore to investigate and improve the understanding of the behaviour of reinforced concrete beams strengthened with fibre composite. These structures have a critical problem implying that they may fail in a sudden manner. This failure involves separation between composite and concrete. Special attention is paid to this phenomenon, which is called debonding. One scope of this study was to develop computer modelling framework. Therefore, three dimensional computations were conducted considering the nonlinear behaviour of the materials. A new model for the concrete-fibre composite interface was included. The computations were verified against experiments. The results confirmed the ability of the computations to recreate the load-deflection behaviour, the crack distribution, and the failure modes. Simulations and experiments showed that application of fibre composite can increase the load capacity and the stiffness of the beams. The influence of several parameters such as length and width of fibre composite and properties of adhesive were investigated. Large width and length of fibre composite and soft adhesive would yield to reduce tendency of debonding and increase thus of the utilization of fibre composite and increase load capacity. The findings from this study yield a proposal for a modification of design code rules.