Synopsis:Ten reinforced concrete blocks (1.5 x 1.5 x 0.5 m) were cast during the autumn of 1982 and exposed in the tidal zone in the Trondheim fjord in March 1983. The blocks were made of concrete with two different strength grades and with 0, 10 and 20 % silica fume addition. The blocks have been subject to several investigations during the years, especially with respect to chloride intrusion. The last examination was executed at approximately 21 years of exposure. The results indicate that the activity factor of silica fume regarding resistance against chloride intrusion is in the same order as for compressive strength (approximately 3). The chloride diffusion coefficients have been calculated for several exposure periods. This makes a calculation of the aging effect, and long term development of the effective chloride diffusion coefficient, possible. The concrete was composed according to the common practice in Norway in the early 1980s, and the concrete proportions are therefore hardly relevant to day. The effect from addition of silica fume with respect to durability and chloride intrusion for well documented, long term field exposed concrete is however considered very valuable.