Corrosion of reinforcing steel in concrete structures submerged in seawater

For an under-sea tunnel project, laboratory and field tests have been carried out to study the influence of permanent submersion in re-circulating seawater on the corrosion of steel in reinforced concrete. For each concrete mixture, cube specimens of plain concrete and beams of reinforced concrete were made. Sound as well as pre-cracked beams with a crack width of about 0.03, 0.3 and 1.0 mm were produced. The following parameters were measured as a function of time: chloride penetration as well as electrical resistivity in plain concrete specimens, and the corrosion rate of steel in reinforced concrete beams by visual and microscopic examinations. The results show that chloride diffusion rate in concrete was reduced by decreasing the water-cement ratio (w/c) and by using fly ash or silica fume. In polymer impregnated concrete (PIC) specimens chloride penetration was negligible until about three months of permanent submersion in seawater: after this period, the penetration rate surprisingly increased according to a capillary suction mechanism. The electrical resistivity results were in agreement with those on chloride diffusion. The corrosion rate, in terms of corroded surface area and pit depth, was not detectable on sound uncracked beams, within a 2 year period of permanent submersion in seawater, regardless of the w/c (0.65-0.35). It is expected that, even at longer ages, the corrosion rate will be negligible due to lack of oxygen which is needed for the corrosion process. In pre-cracked concrete beams, with crack widths greater than 0.2 mm, a pitting corrosion process was observed on the steel reinforcement close to the crack tip. No technical advantage in reducing the corrosion of steel was recorded when PIC, instead of regular concrete, was used.