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Corrosion of Steel
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Corrosion  of reinforcements in concrete
could arise due to the following reasons :-
Environment, quality of concrete, depth of cover,
cracks and surface treatment of concrete

ENVIRONMENT - The site and location of a concrete structure such as an rural, coastal, marine or industrial zone will decide the extend of reinforcement corrosion in concrete. Like corrosion of steel structure,  corrosion of reinforcement steel in concrete is dependant on the degree of exposure to oxygen, water, carbon dioxide and chloride ions. For concrete in industrial zones, besides the exposure to oxygen and water "carbonation" process could reduce the alkalinity of the concrete. This reinforcement corrosion agent or process is particularly critical in areas of high humidity and vast temperature swing. In coastal and marine area the salt from sea splash would cause ingress of chloride ions thus causing rapid corrosion to unprotected reinforcements steel.     

QUALITY OF CONCRETE - The cement mix is an important parameter to good quality concrete.  Poorly mixed concrete would give rise to high permeability. The degree of permeability determines the rate of diffusion of chloride ions. A properly compacted dense concrete with low water cement ratio, small sized and well graded aggregate would give better bonding between aggregate and cement and this lead to low permeability. The concrete should then be properly cured to allow hydration of the cement.   

DEPTH OF COVER - Corrosion of concrete reinforcement was found to  occur frequently with depth of cover less than 20 mm. In cases of thin wall concrete where the 20 mm cover is hard to achieve hot dip galvanized reinforcement bars  

CRACKS - One main reason for the increased use of hot dip galvanized reinforcement steel now is the experience engineers faced with deterioration and even failure of concrete structures with uncoated steel reinforcements. Corrosion of uncoated reinforcement steel is likely when there are cracks in the concrete structure. The size of these cracks decides the durability of the concrete. Cracks can be caused either by shrinkage or thermal stress. Cracks that run parallel to the reinforcement bars are particularly harmful to the reinforcement in concrete. 

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