In reinforced cement concrete construction the corrosion of reinforcement takes place due to the presence of chlorides and sulphates beyond a critical limit and when sufficient alkalies is not obtained within the concrete to maintain steel in a positive condition.
The following factors are responsible for corrosion of steel reinforcement in concrete structure.
1. Quality of Concrete
Concrete consists of coarse aggregate, fine aggregate, cement and water. The right quality of materials with proper w/c ratio, correct mixing, adequate compaction by tamping or vibration and proper curing results in good quality concrete. if any of the above mentioned steps are not done in a specified manner, then that will result in a not so good concrete and there is a chance of corrosion of reinforcement.
High strength concrete, i.e. dense concrete is impervious to a large degree and generally resists the carrion of embedded steel.
2. Cover Thickness of Concrete Reinforcement
The reinforcement is protected by suitable concrete covering over it. The greater the cover thickness more is the degree of protection against the various climatic and other environmental conditions. For various structural members, the cover thickness should be different depending upon their importance and degree of exposure. Evenness of concrete cover over the reinforcement is also very important for its corrosion protection.
3. Condition of Reinforcement
The surface condition of the steel reinforcement, at the time of its placing in concrete, affects its corrosion rate. If the reinforcement is contaminated with salt or badly corroded, the corrosive action on reinforcement after placement in concrete is promoted rapidly.
4. Effect of Environmental and Other Chemicals
Chemical either from environmental or from within the concrete making materials are the main source of deterioration process. Due to attack of chemicals, the concrete develops cracks, which is the first sign of deterioration. The effect of chemicals is mainly due to presence of salt, carbonation, chloride attack and reaction of sulphates with tricalcium aluminate (C3A) present in cement.
Concrete is an intimate mixture of cement, aggregate and water which in the green state is highly alkaline. The hydration of cement develops calcium hydroxide which increases the pH value up to 12.5. In such alkaline conditions, the reinforcing steel is covered with a film of oxide which protects the steel.
5. Porosity of Concrete
The penetration of aggressive chemicals is possible due to permeability or porosity of concrete. the porosity of concrete depends on size, distribution and continuity of capillary pores. This depends upon the w/c ratio for given degree of hydration. The porosity also depends upon other factors, such as
Age of concrete
Degree of compactness
The size and grading of aggregate
Type of cement
6. Effect of High Thermal Stress
Normal concrete can withstand temperature upto 1000C. Beyond this temperature the deterioration of concrete starts. The concrete in industrial plants and power stations required special protective measures otherwise the concrete may develop thermal cracks. Cracked concrete structures are consequently affected by the environmental chemical and the process of corrosion starts.
7. Freezing and Thawing Condition
In cold regions, the moisture in the pores of concrete freezes. The ice formation gives rise to volumetric expansion which may excess bursting pressure of surrounding concrete mass. This results in development of cracks in concrete and can lead to corrosion of reinforcement.