Durability and strength are two most important criteria for the design of reinforced concrete structures. These are the two requirements for the long term performance of concrete structures.
Any deficiency in any of the two i.e. durability and strength, could make the structure unfit for the intended purpose. If the structure is not durable, but it has sufficient strength, then the strength of structure reduces with the age due to deterioration of concrete and reinforcement due to surrounding environment.
Standard codes in limit state design method have made the durability check as an integral part of the design process. When checking for durability of the reinforced concrete structure, durability of concrete as well as steel reinforcement is considered. Any lack of the required durability reduces the designed life of the structure.
Environment plays an important role while selecting durable materials for reinforced concrete structures. For environments such as coastal areas where rate of corrosion is very high, special care is taken for the corrosion allowance of reinforcement, epoxy painting of reinforcement, required cover to reinforcement, grade of concrete to be used, water-cement ratio, quality of water for construction, proper placement and compaction of concrete.
- Durability of Reinforced Concrete in Different Environmental Conditions
- Table – Classification of Exposure Conditions (According to IS 456- Table-19)
- Table – Nominal Cover for Durability (BS 8110)
- Table – Increased cover for special conditions for concrete below M25 Grade (IS456-Cl.25.4)
- Table – Minimum cement content and water-cement ratio for Durability (IS 456, Table-19)
Durability of Reinforced Concrete in Different Environmental Conditions
Some precautions to be taken care while constructions of reinforced concrete structure for durability are:
- Proper amount of minimum cover specified by the code should be provided.
- Minimum cement content in concrete mix and maximum water-cement ratio guidelines based on type of environment provided by the code should be followed.
- Using good quality lab tested coarse and fine aggregates suitable for construction and free from (or within permissible limits) impurities such as dust, alkalies, chlorides, sulfates etc. should be used.
- Based on environment attack on structure, suitable type of cement, concrete admixtures and water-cement ratio should be used.
- Good placement and compaction of concrete.
- Following formwork removal schedule as per type of construction as per guidelines given by standard codes.
- Proper curing of concrete for the required period of time.
Some of the major durability problems caused by environment are:
1. Corrosion of steel reinforcement:
This corrosion of steel reinforcement can be controlled by:
- Cover to reinforcement (IS 456, Cl.25.4)
- Minimum cement content (IS 456, Table-19)
- Proper water-cement ratio
- Proper compaction and curing
- Using epoxy coated reinforcement.
2. Deterioration of Concrete:
Due to chemical attacks such as sulfates present near chemical industries atmosphere, soil and ground water, the concrete even without reinforcement steel deteriorates.
Concrete in plant drains or sewers are also exposed to such environment and deteriorates due to sulfate action. To overcome the problem of such deterioration of concrete, following points should be followed:
- Using proper cement types such as sulphate resistant cement or cement with low C3A content
- Using minimum cement content as specified by the code
- Using proper water-cement ratio for the concrete
- Using protective coating to concrete surface buried under the soil.
Table – Classification of Exposure Conditions (According to IS 456- Table-19)
|Type of Exposure||Environment Description|
|Mild||Protected concrete surface|
|Moderate||Sheltered from rain or permanently under water or in contact with non-aggressive soils|
|Severe||Alternate wetting and drying or exposed to sea water|
|Very severe||Exposed to sea water spray or corrosive fumes|
|Extreme||Exposed to abrasive action like sea water carrying solids|
Table – Nominal Cover for Durability (BS 8110)
|Exposure Condition||Concrete Grade (Size of aggregate – 20mm)|
- Minimum grade of concrete for Reinforced concrete work is 25 (According to BS 8110).
- Grades of concrete shown in Table include a relaxation of 5 MPa.
- The cover specified is the nominal cover to reinforcement, including links.
- Cover may be reduced to 15mm in places marked by * when the aggregate size does not exceed 15mm.
Table – Increased cover for special conditions for concrete below M25 Grade (IS456-Cl.25.4)
|Condition||Additional Cover (mm)|
|Members totally immersed in sea water||40|
|Members periodically immersed in sea water or subjected to sea water spray||50|
|Members exposed to harmful chemicals and in contact with earth faces contaminated with chemicals||15 to 50|
(Average value =35)
Notes: For concrete grade M25 and above, the additional cover specified may be reduced to half. In all cases, the cover to reinforcement should not exceed 75mm.
Table – Minimum cement content and water-cement ratio for Durability (IS 456, Table-19)
|Exposure Condition||Minimum Cement Content|
|Maximum water cement ratio|