Reinforcement steel is an important building material which makes concrete to resist the tensile forces which means it provides the bending strength to concrete. Buildings are planned for the long life, but during the service life reinforcement steel gets corroded due to many factors which reduce the stability and sometimes building may collapse.
Here we discuss the reasons of building failure due to corrosion of steel bars which helps to understand the process of corrosion of steel bar and its effect on the structure.
The main factors which promote the corrosion of reinforcement steel are the presence of moisture and least or improper concrete cover. Other factors which also promote corrosion in concrete are the process of carbonation, electrolysis, alkali-aggregate reaction, the presence of impurities and chemical in water, etc. We have already discussed the factors cause the steel corrosion in the concrete structures. Most of the R.C.C Building in recent times undergone repairs or fails or collapse due to corrosion. The massive repairing and strengthening happening to the building are due to the cause of corrosion.
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Courtesy – Exponent
When the human body is affected by the disease like cancer, they die after some period, if they are not attained in the initial phase itself it spread fast and can be controlled if detected initially. Similarly, when concrete is affected by corrosion, the building may collapse with time, if not prevented or cured properly and at the beginning itself. Different conditions of the building affect the steel corrosion.
Reasons of Corrosion of the Steel Bar:
- Alternate wetting and drying of RCC element.
- When the concrete members have poor concrete cover.
- When water leakage occurs in the building.
- When the building has RCC member with patterned cracks due to other reasons.
- When the concrete member bulges or buckles in the building.
- When the building has defective concrete, spalling or loose plaster in the ceilings.
- When the reinforcement steel or concrete is exposed to corrosive environments.
- Inadequate waterproofing on terrace, toilets, balconies, chhajjas, joints etc.
Above situations in the buildings increases the rate of corrosion and causes spalling, cracks or deterioration during the service life which may ultimately leads to building failure.
What Happens when the Buildings Face above Situation?
All the situations which are discussed above lead to the penetration and reaction of chloride or carbon dioxide with concrete.
Corrosion of steel bar is the process in which the rust film is formed on reinforcement bar due to the chemical or electrochemical reaction in concrete. The chemical reaction of the iron i.e. Fe get oxidized to iron oxide FeO or Fe2O3 to form a film of scale around the steel bar. The film of scale causes expansion of bar, which ultimately creates cracks in concrete. Due to cracks the concrete absorbs more CO2 from the cracks and ultimately increase the process of carbonation of concrete. It is also the alkalinity of concrete which protects the steel from corrosion. The pH of concrete is around 12 to 13. Hence CO2 enters, it loose alkalinity and its capacity to protect steel gets on reducing. Hence vicious circle begins and ultimately results loss of area of steel bars, spalling of concrete, dislodging cover of reinforcement bars from the concrete. If corrosion of reinforcement bars is not repaired at an early stage, the building may lead to serious repair or the building may collapse partially or totally over a period of time, particularly when columns are affected.
Stages of Corrosion of Steel Bar:
- The first stage is the penetration of chloride or carbon dioxide which is not visible to eyes.
- The second stage is the reaction of chloride or carbon dioxide with concrete which is also not visible, but the corrosion testing machine can detect it.
- The third and last stage is the damage like spalling, cracking, etc. in concrete which is visible to eyes.
In conclusion, the deterioration, spalling, and cracks of concrete decreases the stability during the service life of the building and it may collapse due to the corrosion of the steel bars over a period of time. Corrosion in RCC structure is like cancer in human body, if not attended in time. It can cause serious problems. You therefore need to take all preventive measures at the time of design and construction of building. And once it appears in the building, you need to take immediate curative as well as preventive actions.
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