Chloride initiated reinforcement corrosion in marine concrete

University dissertation from Lund Institute of Technology, Divison of Building Materials, Box 118, S-221 00 Lund, Sweden

Abstract: Factors affecting the total chloride threshold for initiation of pitting corrosion in reinforced concrete were investigated by means of field and laboratory exposure tests. Factors affecting the chloride penetration into concrete were also investigated by means of field tests. The thesis consists of a summary (86 p.) and 11 papers attached. Measured total chloride thresholds for uncracked concrete based on sulfate resisting portland cement with w/c ratio 0.3-0.5 and exposed in the splash zone were in the range of 1.1-1.5 % total chloride by weight of binder. The effect of replacing the portland cement with 5 % silica fume on the chloride threshold was insignificant. Defects in the concrete micro structure and the steel surface at the steel-concrete interface were found to have a major negative effect on the chloride threshold. A linear relationship between free and total chlorides was found in concrete submerged in sea water. Concentration profiles measured for the free chloride and hydroxide ions indicated that these ions penetrate concrete in opposite directions but at approximately the same rate. It was suggested that hydroxide counter diffusion increases the chloride binding close to the exposed uncarbonated surface, which leads to an almost linear chloride binding. The effective chloride diffusivity calculated by fitting experimentally obtained total chloride concentrations to a solution to FickĀ“s second law of diffusion, decreases over time in field exposed concrete. The results after 5 years of field exposure indicated that the effective chloride diffusivity can be reduced by a factor of 8-10 by using 5 % silica fume in the binder and by reducing the w/c ratio from 0.40 to 0.30, as compared to a typical Swedish bridge concrete with w/c 0.40 and no pozzolan in the binder. The corresponding reduction of the required minimum cover would be 50 % for an initiation time of 100 years.

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