What cause corrosion in concrete reinforcement
This article is about What cause corrosion in concrete reinforcement. Sometimes we find corrosion in concrete reinforcement bars. Although reinforcement bars are inside the concrete, they are still affected by corrosion. what happen actually to these concrete reinforcement ?
What cause corrosion in concrete reinforcement? The following factors are responsible for corrosion of steel reinforcement in concrete structure :
1. Concrete Quality.
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. Concrete Reinforcement Cover Thickness.
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. Reinforcement Condition.
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. Chemical and Environmental Effects of the Surroundings.
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. Concrete Porosity.
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. High Thermal Stress.
Normal concrete can withstand temperature up to 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.
The article about What cause corrosion in concrete reinforcement above is written by Sanjay Kumar, Virendra Kumar, M. Prasad and it is originally published on NSCP – 2001. For more updates about Civil Engineering articles, please subscribe.