Corrosion of Steel

Corrosion of Steel

Reinforcing steel corrosion is one of the most serious deterioration mechanisms in reinforced concrete structures and is also an important issue that needs be considered when evaluating and rehabilitating RC structures. Fortunately, there are two self-defense mechanisms that can be employed to protect reinforcing steel against corrosion ,physical protection provided by the dense and relatively impermeable structure of concrete and chemical protection provided by the high alkalinity of the pore solution. The first mechanism involves concrete of sufficient depth and good impermeable quality. The second one is a thin oxide covering that forms around reinforcing steel bars due to the high alkalinity of pore solutions, which contain high concentrations of soluble calcium, sodium and potassium oxides, in freshly mixed concrete. However, the random distribution of pore spaces suggests that aggressive substances such as chloride, carbon dioxide, oxygen, moisture, etc. may penetrate through weak points in the concrete cover trigger the corrosion of reinforcing steel bars in concrete and finally induce cracking of the concrete. The aggressiveness of the environment is a very important factor to consider when examining concrete that shows signs of possible distress Generally, corrosion attack is initiated either due to the carbonation of the concrete or due to diffusion of the chloride ions to the reinforcing steel bar surface or both. On account of different aggressive mechanisms, the corrosion due to concrete carbonation is much more uniform than that caused by chloride attack and, hence, it is much less susceptible to local attack .On the other hand, corrosion of reinforcing steel structural course in kerala and the consequent cracking of concrete due to the ingress of chloride ions to the reinforcing steel bar surface is more than that due to carbonation of concrete. Not only will corrosion affect the load-carrying capacity of the reinforcing steel bar, but it may also impair its ductility, which presents a serious problem for the safety of old and monumental constructions in seismically active areas. A lot of research has been done to the corrosion of reinforcement in RC, dealing with various issues related to the configuration of corroded reinforcing steel bars, the load– displacement relationship, residual strength, ductility, etc. The effect of reinforcement corrosion on the residual strength has been of great interest. The ductility of reinforcing steel is normally represented by two parameters, the ratio between the yield and the total strengths and the elongation ratio. The elongation ratio is the average strain of a corroded steel bar in its gauge length. The elongation ratio associated with a shorter gauge length is much greater than that of a greater gauge length and the gauge length was taken as five times the initial diameter of the steel bar . The elongation ratio decreases with increased corrosion level, although the rates of decrease are different for steel bars corroded in simulated solutions and those corroded in concrete. For the steel bars corroded in simulated solutions the decrease rate is about 0.2, which represents a moderate loss of ductility as corrosion increases; for the steel bars corroded in concrete , the decrease rate is about 0.8, which represents a drastic loss in ductility as corrosion increases. The contribution of the highly localised peak strain does not provide a correspondingly large contribution to overall elongation (Cairns et al., 2005); therefore, the steel bars corroded in concrete present a lower elongation ratio than those corroded in simulated solutions for the same cross-sectional loss. The reinforcing steel bars subjected to local or pitting attack may suffer a significant loss of ductility. As far as the fracture pattern is concerned, fracture of the reinforcing steel bar usually occurred at pitted sections and usually happened with a less ductile fracture when the notch was wide and deep.