Monday, July 2, 2012

CAPpings - India - A constructor's perspective on Corrosion of RCC structures in Coastal Areas

Corrosion of reinforcing steel is one of the most important causes of deterioration of concrete structures in coastal environment

Steel and other ferrous metal along the coastline are susceptible to corrosion, which is mainly initiated by chloride ions present in the sea salts. It is further sustained by the oxygen present in the atmosphere, which diffuses through the moisture film to the surface of the metal.

Corrosion of reinforcing steel is one of the most important causes of deterioration of concrete structures in coastal environment. High permeability concrete, poor design detailing and construction defects such as inadequate depth of cover allow the ingress of salt and moisture into the concrete. High concentration of salt and moisture result in accelerated corrosion of reinforcing steel thereby significantly deteriorating the concrete structure.

Concrete structures in coastal areas can be divided into two categories based on their exposure: direct and indirect. Direct exposure includes structures that are fully or partially submerged and indirect exposure includes structures that do not come into direct contact with seawater. Buildings along the coastline are examples of indirect category of exposure.

Corrosion is commonly associated with deposition of sea salt in presence of moisture on steel and light metals. Chloride is the most significant corrosive species in the salt particles. Corrosion is influenced by the amount of salt on the metal surface and related to speed and direction of wind, distance of structure from the shore, elevation of the structures, degree of sheltering and frequency and amount of rain washing. Other factors that influence corrosion include time of wetness, relative humidity, metal surface temperature etc. Normal steel has poor resistance in coastal atmosphere and hence requires protection for durability.

When steel reinforcement corrodes, the corrosion product occupies more than three times the volume of the original steel, exerting great disruptive tensile stress on the surrounding concrete, leading to further cracking, more weather access and further corrosion. In mild cases, rust staining occurs whereas in more serious cases severe spalling of concrete may occur and ultimately the concrete members may fail completely.

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