Thursday, November 24, 2011

Major Concrete Deterioration Mechanisms

Let us put sustainability away for some time and talk about the ways in which concrete gets deteriorated. The following will give you an idea about how does the deterioration mechanism take place which will make you cautious.

1.      Carbonation :
Carbonation is eventually inevitable, and is caused by the ingress of atmospheric carbon dioxide reacting with pore water to form carbonic acid. This neutralizes the alkalinity in the concrete. This occurs progressively, and a carbonation front moves through the concrete until it reaches the steel. The passive layer then breaks down as the pH falls from over 12 to around 8. Then it slowly corrodes the steel making it hazardous. The rate of carbonation is also affected by environmental conditions.

2.      Chloride Attack :
The second major cause of reinforcement corrosion is chloride attack. This is usually due to the following reasons:
·         De-icing salt ingress from roads and vehicles.
·         Sea-salt ingress from marine environments
·         Cast-in salt from contaminated mix components
·         Cast-in calcium chloride as a set accelerator.
Once the chloride level at the reinforcement exceeds 0.4% by weight of cement, there is a significant risk of corrosion, especially in presence of moisture.

3.      Design and construction defects:
The performance of reinforcement concrete can be severely reduced by poor design and construction techniques. These may cause reinforcement corrosion or the degradation of concrete itself, which in turn may lead to reinforcement corrosion. Insufficient concrete cover to the reinforcement is a major influence on the durability of reinforced concrete buildings and structures.
Problems which occur are:
·         Older codes do not specify adequate cover, especially in saline environments.
·         Details such as drips, groves of surfaces etc.
Problems arise during constructions:
·         Detailing that makes it difficult to achieve the specified cover
·         Incorrect reinforcement placing
·         Movement of reinforcement within shutters from the specified cover. 

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