Keywords: strengthening, fiber-reinforced polymer, innovative materials, prestressing system, ultra-high tensile steel filament steel fibers.

Due to their light weight, high stiffness-to-weight and strength-to-weight ratios, and high resistance to environmental degradation, resulting in lower life-cycle costs, advanced composites materials, commonly known as fibre-reinforced polymers (FRP), are increasingly being considered for use in civil engineering applications. They range from the retrofit and rehabilitation of buildings and bridges to the restoration and strengthening of historical-monumental masonry structures. When composite materials started entering the civil engineering industry in the early 1990s, only three type of fibers were available: carbon, glass and later aramid impregnated with epoxy matrices. These were mostly available in the form of sheets to be installed by manual lay up. In later years FRP pulltruded bars and structural profiles, also started being used in retrofit applications in substitution for steel sections and profiles spreading further the use of FRP in civil engineering and architecture. Recently a new type of fibers have significantly entered the market of composite materials: ultra high tensile strength steel (UHTSS) fiber available in the form of cords made of twisted filaments. These are then installed at fixed spacing on a polyamide grid, creating different densities that have the advantage of being impregnated with either epoxies or cementitious matrices widening further the implementation of composites. These are especially for retrofitting masonry structures. Lastly natural fibers, basalt, flax and hemp started being investigated as an alternative to the aforementioned fibers for lower strength retrofit applications, especially once again for repairing and retrofitting low strength masonry, timber and other poor quality materials for which high strength fibers would not suit for retrofitting. This paper illustrates the different types of advanced composite materials available, differentiates between the types of fiber and the form in which they are available and compares their mechanical characteristics and main properties. Finally a rich list of case studies is presented in which these materials have been applied, differentiating between reinforced concrete and masonry structures and explaining with examples the different technologies implemented.

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