The railroad between Gällivare and Luleå in the northern part of Sweden is called Malmbanan. Heavy loads of iron ore are transported on this railroad daily. During long time the weight of the load has been approximately 5 000 tons per train which correspond to 25 tons per axle. A wish from LKAB to increase the load up to 30 tons per axle or 6 000 tons per train exists. Calculations of the load bearing capacity of the bridges show that many of them will not be able to carry these higher loads without strengthening. There exist several methods for strengthening of a concrete structure. A strengthening method that have been more and more used during the last years is bonding of carbon fibre composite fabrics or laminates to the surface of the structure. The Swedish Rail Road authorities wanted to investigate the method more closely and of that reason a bridge were strengthened in Luleå during the summer of 1998. The bridge is built of concrete and has three spans. It is located at Kallkällan NW of the city centre. The objective of the test was not only to investigate the strengthening effect but also to study the work methods and the work environment during handling of the thermosetting plastic. Before the strengthening system was applied, the concrete surface was smoothened by sandblasting and grinding. Putty was used for bigger irregularities. The strengthening system consists of a hand lay up system with epoxy and unidirectional carbon fibre sheets. Later the concrete surface was coated by a polyurethane paint, with function to protect against UV-radiation and mechanical damage and also to give an increased aesthetic value. During the work the environmental and health aspects were taken into account and the epoxy, as well as the epoxy components were handled in a strict and careful manner, for example all the waste were placed in closed containers and sent to destruction. In the report, general equations, modes of action and design principles for strengthening by externally bonded fibre-reinforced polymers are described shortly. Some of the calculations that have been done for the bridge are also shown. For investigation of the strengthening system a comprehensive test program was undertaken on the bridge before and after the strengthening system was mounted. Both strains and deformations were measured when ore trains at different speeds passed on the bridge. The weight of each train was measured to be able to compare measurements of different trains with each other. Measured values from testing were relatively close to theoretical calculated levels, before and after the strengthening, both regarding strains and deformation. The measurement showed that the strengthening had increased the stiffness of the bridge by about 16%. Also measurements of the concrete quality were undertaken as well as an investigation of the bridge condition. The difficulties to decide the clamping of the slab in the load bearing beams and the chosen models for calculations have a big influence of the theoretical results. However, the measurements show that the method of strengthening concrete bridges by bonding carbon fibres to the surface works in an effective manner even when trains are frequenting the bridge during the strengthening work. In chapter 1 a short introduction and background to the project is given. In chapter 2 theoretical work are described and in chapter 3 the execution of the strengthening work is described. In chapter 4 the results and the performance from the measurements are recorded. In chapter 5 a comparison between theory and tests are made. In chapter 6 conclusions are presented and finally in chapter 7 proposal to further work is given.