In recent years, the premature deterioration of reinforced concrete (RC) structures seen in falling concrete blocks has become a problem; improving the durability of RC structures poses a new challenge. One of the main causes of RC structure deterioration is the corrosion of reinforcing bars due to chloride ion penetration. The standard specification of concrete by the Japan Society of Civil Engineers shows a method for checking durability in terms of chloride ion content. However, since investigation data on the scale of chloride ion penetration are limited, the method is not necessarily rational in quantative terms. As a result, the Railway Technical Research Institute (RTRI) has defined a method for determining the rational concrete covering (i.e. the shortest distance from the concrete surface to the reinforcing bar surface) so that bars are not affected by the chloride ion quantity. This method is based on investigations of actual chloride ion quantities found in railway structures near coastlines nationwide.
The investigations were conducted by measuring the depth direction distribution of the chloride ion quantity in concrete on approximately 120 railway bridge piers located within a distance of about 1 km from the coastline (Fig. 1 and 2). The density of chloride ions at the concrete surface was estimated by considering the movement of chloride ions in the concrete as a one-dimensional linear diffusion phenomenon. Based on this estimation, the value one hundred years after construction was estimated. The investigations showed that the density of surface chloride ions varies from region to region, and the density value is below that specified by the Japan Society of Civil Engineers (Fig. 3).
The concrete covering required for RC structures was calculated based on the above investigation results. The calculations showed that the covering required to keep the quantity of chloride ions below the corrosion occurrence limit can be reduced on the Sea of Japan side to 50 mm+ lower than the value obtained by the Japan Society of Civil Engineers. In Pacific or Inland Sea regions, investigation of chloride ions is no longer needed since the required concrete covering for structures within 250m of the coastline is determined by a neutralization check in such regions where conditions are not harsh (Fig. 4). These results will be reflected in the design standard for railway structures, which is currently under revision.