The authors study the effect of water migration and carbonation progress on the corrosion rate of reinforcing bars, focusing on the spalling of cover concrete of railway viaducts. As a result, when un-carbonation depth (=cover depth – carbonation depth) becomes less than 10 mm with supplied water, the risk that the spalling of cover concrete occurs is high. On the other hand, even if un-carbonation depth is less than 10 mm, when the water is not supplied, the risk that the spalling of cover concrete occurs is low. In addition, when the water is supplied, the corrosion rate of the reinforcing bars is 0.2 to 9.3*10-3 mm/year. On the other hand, even if un-carbonation depth is less than 10 mm, when the water is not supplied, the corrosion rate is 0.2 to 2.7*10-3 mm/year.
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