In this study, to develop a growth prediction method of rust on weathering steel, the rust progress characteristics under constant temperature and humidity have been conducted. In order to achieve a long life of weathering steel bridges, determination of several problems are required. Especially, rust evaluation technology, rust prediction method and repair technology of corroded weathering steel are significant. Different concentration (0.3%, 1.0%, 3.0%) of salt water sprayed on steel test pieces and,- were placed in a constant temperature and humidity test vessel, to observe the progress of rust in in-plane direction. Four patterns of exposure environment were tested. Exposure environment were combination of temperature (40°C, 20°C) and relative humidity (50%, 95%). To calculate the increase of rust area, specimen surface was confirmed. Higher salt water concentration (3%) brought rapid rust development. Furthermore, the increase of rust area was highly dependent on temperature than humidity. From time observations, it becomes clear that increase of rust area can be approximated to a logarithmic curve.
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