Concentration Profile of Water-Soluble Chloride in 32 Year-Old Marine Concrete Piles

Abstract

The alkaline nature of concrete made from normal portland cements provides a natural protection against corrosion of steel bars embedded as reinforcement. Presence of chloride in small quantities, depending on the Cl/OH ratio tends to destroy this passivity of steel reinforcement even at a pH considerably above 11.5. The present paper compares numerical predictions, using Fick's second law of diffusion, against concentration profile of water-soluble chloride obtained from the atmospheric, tidal and submerged zones of six 32 year-old marine piles extracted from a wharf in the Port of Singapore. Concentrations of water-soluble chloride at the level of steel reinforcement were compared with the loss in mass of the reinforcing bars. The concrete was sampled using the drill and suction method at locations spaced approximately 1m along the entire length of the extracted piles. The results show that the numerical estimates were in good agreement with the actual values of water-soluble chloride concentrations obtained under Singapore conditions. Diffusion parameters calculated were within the expected range for the grade of concrete used, 30 MPa cube strength. Locations where reinforcement corrosion were found corresponded to those that showed presence of water-soluble chloride in concentrations as low as 0.025% by mass of concrete occurring mainly in the atmospheric zone.