IntroductionThe concrete structures built in marine conditions are always exposed to seawater either directly or indirectly. The coastal and offshore structures are always in contact with seawater and there are number of physical and chemical deterioration processes takes place. So, Concrete structures effected by seawater requires special attention.
Composition of SeawaterThe 71% of earth’s surface is covered by water bodies and in which nearly 96.5% is covered by seawater only. So, large number of concrete structures effected by concrete either with direct contact or indirectly by the winds carrying seawater sprays.
Fig 1: Bridge constructed on sea sideGenerally seawater contains 3.5 per cent of soluble slats by weight. The ionic concentration of Na+ and Cl- are maximum in seawater, normally 11,000 and 20,000 mg/lit respectively. Seawater also contains Mg2+ and SO42- about 1400 and 2700 mg/li respectively. The pH of seawater fluctuates between 7.5 and 8.4. The average pH is taken about 8.2. Seawater also contains some amount of CO2. If higher concentration of CO2 dissolved in seawater then the pH may fall below 7.5. The following table gives you the major ions concentration in some of the famous world seas.
|World seas/Major Ions||Concentration of ions (mg/lit)|
Effect of Seawater on Concrete StructuresThe constituents of seawater reacts chemically with constituents of cement concrete which results damage to the concrete structure in several ways. The magnesium sulfate present in seawater reacts with calcium hydroxide of cement and forms calcium sulfate as well as magnesium hydroxide precipitation. Magnesium sulfate also reacts with hydrated calcium aluminate and forms calcium sulpho aluminate. These final formations are the primary reasons for chemical attack on concrete structures. The deterioration of concrete structures by seawater is more due to leaching rather than expansion of concrete. Leaching more effects the small concrete structures than expansion while large concrete structures are effected by leaching as well as expansion. Sulfates attack the concrete and cause expansion but due to the presence chlorides in seawater the swelling of concrete retards. Hence, erosion and loss of concrete takes place without showing much Expansion.
Fig 2 : Deterioration of Concrete in SeawaterThe lime content present in the concrete also lost due to leaching. Both calcium hydroxide and calcium sulfate are soluble in seawater this will result in increased leaching action. The temperature is also a factor chemical attack, higher the temperature more will be the attack. Concrete is not 100% impervious. When seawater enters into the pores of concrete and reaches the reinforcement then corrosion will occur. It will affect the durability of structure.
Fig 3: Corrosion of Reinforcement due to salt waterAnother case is that concrete damaged by abrasion. Seawater may carry sand and silt especially at the shallow end of the sea. When it forcibly contacts the concrete surface abrasion occurs. Abrasion also occurs due to mechanical force buy wave action.
Theoretical AspectsIf concrete structure is built in seawater, then the most affected area of the structure is well above the high water mark. The area between low water level and high water level is less affected while the area which is continuously submerged under the seawater is least effected. The reason behind this is, when the seawater forcibly contact the area above the high water mark due to wave action, some salty water gets deposited in the concrete pores. When this area dried the water will crystallized into salt particles and disruption of concrete takes place. Similarly, when the water in concrete pores is allowed to freeze in cold climates, the concrete will expand and lost its durability.
Fig 4: Diagrammatic Representation of Concrete Exposed to Seawater
How to Improve Durability of Concrete in Seawater?To improve durability of concrete structure which are exposed to marine conditions,
- Cement with low C3A content should be preferable to make concrete.
- Prepare rich concrete with low water cement ratio which makes the concrete impervious. Then the pores in concrete are very small and they cannot hold seawater results in the prevention of expansion by freezing of water and crystallization of salt in the pores.
- The concrete is of low water cement ratio. To make it workable for construction, Water reducing admixtures can be added to the concrete which is recommended by ACI 318 and ACI 357.
- The admixtures should not contain chloride in any form otherwise corrosion of reinforcement takes place.
- Adequate cover should be provided for reinforcement in concrete structure to enhance durability. ACI 357 recommended cover for reinforcement bars which is shown in below table.
Zone Cover over reinforcing steel, inches Cover over post tensioning ducts, inches Atmospheric zone not subjected to slat spray 2 3 Atmospheric zone subjected to salt spray 2.5 3.5 Submerged zone 2 3 Cover of stirrups 0.5 inch less than above case 0.5 inch less than above case
- Good compaction and well-made construction joints in the structure helps the concrete structure to withstand against expansion caused by seawater.
- Use of pozzolanic material in the preparation of concrete is good against salt water.
- For better durability, High pressure steam cured concrete elements can be used for construction of structure in marine conditions.
- Both ACI 318 and ACI 357 recommended that suitable air entraining agents can be used to prevent the effect of seawater on concrete.
- Aggregates used for making concrete should be thoroughly washed with fresh water to reduce the chloride ion concentration in it.
Fig 5: Sea Waves Collision With Concrete Structure