Corrosion Repairs and Strengthening of the Reinforced Concrete
Corrosion of reinforcement in concrete is one of the most important factors determining the economic life of marine structures. Under normal conditions, the reinforcement inside the concrete does not corrode. This is due to the high alkalinity level of the concrete, such as pH 13. In the high pH environment, a protective passive film layer forms around the reinforcements and this passive layer perfectly protects the reinforcement against corrosion. This protection is even better than an epoxy coating, and it repairs itself even when scraped. Technically,corrosion of the reinforcement in the concrete can only start with the deterioration of this passive film layer.
There are two basic reactions that damage the protective passive film layer. The first of these is a chemical reaction called carbonation or carbonization. The carbon dioxide gas in the atmosphere enters into the concrete by diffusion and reacts with the active calcium ions released as a result of the hydration of the cement inside, forming calcium carbonates and lowers the pH of the concrete by showing an acidic effect. When the pH drops below 10, the protective passive film layer around the reinforcement breaks down and corrosion starts. The volume of rusted reinforcement expands and cracks occur in the concrete. As a result of these cracks, the reinforcements exposed to direct atmospheric effects continue to corrode rapidly and begin to lose their cross-sectional area. Carbonization reactions can be stopped easily by preventing gas diffusion into reinforced concrete. The speed of this corrosion depends entirely on the quality and thickness of the concrete forming the rust around the reinforcement.
A second and more dangerous reaction is chloride attacks. When chloride ions penetrate into concrete, they act as a catalyst in a reaction that breaks down the protective passive film layer around the reinforcement. The deterioration of the said protective passive film layer starts when the weight ratio of chloride ions to cement exceeds 0.2%, but corrosion progresses very rapidly, especially in marine structures where 1% chloride ion by weight is present to cement, especially in marine structures that are constantly wet and dry. This means approximately 0.15 wt% chloride concentration in concrete.