In many cases, existing structures require reinforcement for strengthening purposes because the standards to which they were designed are no longer considered to be adequate, use of the structure has exceeded that for which it was originally designed, the structure is damaged due to fatigue, corrosion or impact or simply because the structure has reached the end of its expected life.
Conventionally, such structures are strengthened by the addition of new material of the same type as the existing structure, for example reinforcement of steel or aluminium structures by welding or mechanical fastening techniques, reinforcement of cast iron, timber or fibre-reinforced plastics structures by mechanical fastening techniques, and reinforcement of fibre-reinforced plastics structures by bonding. Where new material of a different type is to be added to a structure, a combination of bonding and mechanical fastening may be used, for example in the attachment of steel plates to a concrete structure. However, these methods have problems associated with either the practicality of their implementation at the worksite or with the structural efficiency of the materials used. Additionally, methods involving welding or mechanical fastening can often result in zones of increased stress which may cause weakening or failure of previously sound areas of the original structure.
Advanced fibre reinforced polymer composites have properties which make them very well suited to the reinforcement of existing structures. Typical fibres include the whole range of carbon fibres, some aramid fibres and some of the high performance glass fibres. The advantages of these materials compared with the common engineering materials from which most structures are manufactured are high specific properties, leading to low weight and compactness, creating a low burden on existing structures and providing easy handling. Additionally, the ability to use adhesive bonding to attach the reinforcing materials reduces stress concentrations in the original structure and the freedom from maintenance of the reinforcing materials reduces through life costs. Advanced fibre reinforced polymer composites are therefore considered to be improved reinforcement materials.
Advanced fibre reinforced polymer composites are usually manufactured in highly controlled workshop environments sometimes called "clean rooms". Most manufacturing techniques for advanced fibre reinforced polymer composites are based on working in such environments.
EP-A-0378232 describes one use of advanced fibre reinforced polymer composites to reinforce existing concrete structures. Because the method described results in a composite material having a low fibre volume fraction (and hence comparatively low stiffness and strength) and a low strength bondline, the method is limited in its application to concrete. The materials used in this process are a combination of a pre-preg, and a general purpose room temperature-setting epoxy adhesive.