The aim of this research was to identify the benefits of strengthening metallic structures with carbon fibre reinforced polymer (CFRP) plates whilst the structures are subjected to preload and imposed loading. When a carbon fibre plate is bonded to a metallic substrate there is a curing period during which the adhesive develops its full strength. When it is not economic to take a structure out of service, the adhesive will be subjected to imposed loading during curing and uncertainty remains with regard to the effectiveness of the bond, which needed to be investigated. For the beams, it was found from laboratory testing that reinforcement using CFRP plates had increased the stiffness by approximately 35%. This increase was found to be greatly affected by exposure to imposed loading at an early stage of curing, where adhesive can sustain damage (debonding). Finite element analysis was carried out to investigate the effect of cyclic loading on the performance of the adhesive. Another focus of this research dealt with strengthening of cast iron struts with existing preload and geometrical imperfection. Studies [Moy & Lillistone, 1999] have shown that the intensity of the preload affects the amount of benefit gained, as tensile strength of cast iron determines when failure occurs, thus preventing the CFRP composite from developing its full capability. Therefore, the effects of geometrical imperfections and different levels of preload prior to reinforcement on the stiffness and load bearing capacity of cast iron struts was investigated. At present, the guidance for engineers/designers with regard to the use of CFRP plates in reinforcing metallic structure is not fully developed and only deals with fully cured adhesive. However, with the aid of FE analysis it is possible to obtain the material damage in the adhesive due to imposed loading and recalculating the structures stiffness.