STUDY OF CHLORIDE DIFFUSION IN CONCRETE AND CORROSION OF STEEL REINFORCEMENT

Abstract

This project investigates the ingress of chloride and the consequent corrosion of bare and galvanized steels in concrete. Special attention was focused on the application of the diffusion model to the study of chloride ingress. The variables included exposure condition, grade of concrete, depth of cover and length of exposure. Corrosion monitoring of embedded steel by electrochemical potential measurement was also studied. The investigation involved 80 weeks' field exposure tests and field survey of a 24-year old concrete structure in the marine environment. For the exposure tests, concrete prisms with bare and galvanized steel specimens embedded at different depths of cover were exposed at the atmospheric, tidal and submerged zones. The field survey covered the mean-tidal, upper-tidal and splash zones. Analysis was mainly based on the results of chloride content, rust covering ratio and electrochemical potential measurements. Examination by itereo-microscope and energy dispersive spectrometric analysis were also carried out. To study the application of the diffusion model, an iterative computer program based on the model was developed. It was found that the ingress of chloride was highly dependent on the grade of concrete, the depth of cover and the exposure conditions. The chloride content in low grade concrete were typically 11 / 2 to 3 times that of standard grade concrete while chloride content decreased dramatically with increased depth of cover. The level of chloride ingress was found to be highest at the submerged zone followed by the tidal and atmospheric zones. Generally, chloride ingress in concrete specimens was found to increase with exposure time. The field survey results also showed that chloride contents tend to increase with height above the mean tide level. In general, the diffusion model appears to fit the ingress of chloride at the tidal, splash and submerged zone reasonably well (taking note the possibility of other ingress mechanisms). Diffusion coefficient values of the 24-year old jetty ranqed from 2 .13 x 10-8 cm2 /s at the mean tidal zone to 5.50 x 10-8 cm2/s at the splash zone. The importance of an adequate steel-concrete interfacial film for the protection of bare steel in concrete was indicated. Galvanizing was found to be effective in protecting the steel substrate to a certain extent. The activity of corrosion which is dependent on the level of chloride ingress was influenced by the same parameters of grade of concrete, depth of cover and length of exposure. For exposure condition, the saturation state of the concrete and the availability of oxygen also have a significant influence. Corrosion was most active in concrete specimens at the tidal zone. In the field survey of the 24-year old structure, the upper-tidal and splash zones areas were found to be most susceptible to corrosion. Electrochemical potential measurement proved to be a fairly consistent and reliable method for monitoring and assessing the corrosion of steels embedded in concrete.