Trrl Lr 839

Current codes, if rigidly enforced, prelude highway bridge painting on site throughout the winter and much of the spring and autumn in an average year. The temperature must be not less than 4øC and the relative humidity not above 90 per cent. As the real criterion is to avoid painting on wet surfaces a change of emphasis away from relative humidity measurements is suggested. A simple wetness detector has been developed using the principle of a resistance change between two electrodes in the presence of moisture. The complete system enabled readings of surface wetness air and steel surface temperatures to be collected over monthly intervals on a battery operated chart recorder. The results of 14 months measurements on four structures is described. steelwork sheltered by concrete desks on tall compositely constructed viaducts over land was dry for a very large proportion of the year. It is implied that the painting season for some structures could be extended