A large number of concrete structures are exposed to sea water either directly or indirectly. The coastal and offshore structures are exposed to the simultaneous action of a number of physical and chemical deterioration process. The concrete in sea water is subjected to chloride-induced corrosion of steel, freezing and thawing, salt weathering, abrasion by sand held in water and other floating bodies.
- Sea water generally contains 3.5% of salt by weight. The ionic concentration of Na+ and Cl- ions are highest, typically 11000 and 20000 mg/l respectively.
- The pH of seawater varies between 7.5 and 8.4.
- Sea water also contains some amount of Carbon-di-oxide.
Deterioration of Concrete in Seawater:
- The deterioration of concrete in sea water is characterized by the form of erosion or loss of constituents from the parent mass.
- The most severe attack of sea water on concrete occurs just above the level of high water.The portion between low and high water marks is less affected and the parts below the water level which are continuously remain immersed are least affected.
- The crystallization of salt in the portion of concrete above high water level is responsible for disruption of concrete.
- In place of the cold climatic region, the freezing of water in pores at the spray level of concrete is responsible for causing lack of durability in concrete.
Corrosion of Steel:Since concrete in not 100% impervious. The water that permeates into the concrete causes corrosion of steel. The product of corrosion being of higher volume than the material they replace, exert pressure which results in a lack of durability to reinforced concrete.
From the foregoing discussion, it will be easy to formulate steps to improve the durability of concrete in sea water.
- Apart from the right type of cement, the other factors to be considered is the use of rich concrete with a low water-cement ratio. It will make the concrete impervious to attack of sea water, and also having very little capillary pores does not hold water to cause expansion either by freezing or crystallization of salt.
- Provision of an adequate cover is another desirable step for increasing the durability of concrete.
- A good compaction, well-made construction joints etc. are other points helping the durability of concrete in sea water.