The objective of this study is to predict the initiation of the spalling of the cover concrete of RC bridges by estimating the rebar corrosion rate. The rebar corrosion rate was studied based on the developed estimation method by means of the visual observation of such phenomena as the spalling of the cover concrete of the columns and barriers of railway RC bridges in service. The result indicated that the rebar corrosion rate in wet condition was 2.2 times faster than that in dry condition,

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... the effect of dry-wet condition to the rebar corrosion rate was larger than that of cover depth and uncarbonated depth. In addition, based on the existing equation of the rebar corrosion rate in consideration of the dry-wet condition of concrete, the corrosion rate of these RC bridges in service was estimated on the assumption that the water content is 5% at the concrete surface in case of wet condition which is recognized by the existence of the water-bearing mark, and on the assumption that the water content is 2-4% at the concrete surface in case of dry or dry-wet condition. By means of the proposed method which is based on the estimation of the rebar corrosion rate, it is possible to predict the initiation of the spalling of the cover concrete of RC bridges in service.