External epoxy-bonded steel plates are sometimes used to strengthen existing concrete structures. Exposure tests have been carried out to determine the long term durability of the technique. Small unreinforced concrete beams were strengthened by bonding a mild steel plate centrally to one face using a structural epoxy adhesive. They were exposed at sites representing high rainfall, industrial and marine environments. Specimens were also kept under controlled laboratory conditions (20 degrees C and 65 per cent RH). During exposure half the specimens at each site were loaded and half were unloaded. To date, loading tests to failure have been carried out on specimens brought back to the laboratory after 1, 2 and 10 years. Light but sometimes extensive corrosion has occurred on all the plates removed from specimens exposed to natural environments. There was significantly more corrosion after 10 years compared with that observed after 1 and 2 years. Nevertheless there was, on average a small increase in strength with time although the naturally exposed specimens failed at marginally lower loads than those kept under controlled laboratory conditions because the control specimens had not corroded. Thus corrosion of the steel at the interface with the resin had not significantly affected the overall structural performance. The degree of corrosion was reduced significantly without loss in strength by coating the steel with an epoxy primer paint prior to bonding the concrete.

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