Combating Chloride

The backbone of construction, concrete structures that are durable and can sustain seismic activity are made by reinforcing steel in concrete. For dynamic structures such as flyovers and bridges, reinforcement is a must. Today’s multi-storeyed buildings and malls with straight or curved shapes also require reinforced concrete structures (RCC). However, a key negative is the corrosion of reinforcement.

Steel can be imbedded in concrete in any shape. The steel frame of a structure is first erected and then covered with concrete. However, the presence of this steel can result in corrosion of an RCC structure. So, how do chlorides reach steel in the embedded structure? 

The fact is, concrete does not seal the RCC structure because it is porous so it always retains some moisture. Second, the outer layer of concrete, which is called ‘cover’, is always in contact with moisture and pollutants whose concentration varies from place to place and season to season. For instance, a relatively dry environment like Delhi or Rajasthan will have lower moisture and pollutants compared to a wet environment as in Mumbai and Chennai, which have higher humidity. Further, a highly populated city and industrial city may have a high concentration of pollutants such as carbon dioxide and sulphur dioxide and coastal cities also have chlorides in the atmosphere. All these pollutants enter the porous concrete structure by hitting the cover and then making their way into the steel through the concrete pores. As soon as a minimum concentration of these pollutants reaches the steel-concrete interface, the corrosion begins as in Figure 1.

 About the author: Professor AS Khanna is retired from IIT Bombay and is Chairman, SSPC India.

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