The minimum grade of plain concrete in sea-water or exposed directly along the sea coast shall be at least M 20.

Explanation:

Introduction:
The grade of concrete refers to the strength of the concrete mix. It is denoted by the symbol 'M' followed by a number, which represents the compressive strength of the concrete in megapascals (MPa). The higher the grade, the stronger the concrete.

Concrete in Sea-Water or Exposed along the Sea Coast:
When concrete is exposed to sea-water or is directly exposed along the sea coast, it is subjected to harsh environmental conditions such as the presence of saltwater and salt-laden air. These conditions can lead to corrosion of the reinforcement and deterioration of the concrete.

Importance of Minimum Grade:
To ensure the durability and longevity of concrete structures in such harsh environments, it is important to use a minimum grade of concrete that can withstand the corrosive effects of sea-water and salt-laden air. The minimum grade is determined based on the required strength and durability of the structure.

Why M 20 Grade:
The minimum grade of plain concrete in sea-water or exposed directly along the sea coast is specified as M 20. This means that the concrete mix should have a minimum compressive strength of 20 MPa.

Reasons for Choosing M 20 Grade:
1. Strength: M 20 grade concrete has a minimum compressive strength of 20 MPa, which provides sufficient strength to withstand the load and environmental conditions in sea-water or along the sea coast.

2. Durability: The higher strength of M 20 grade concrete helps in resisting the corrosive effects of sea-water and salt-laden air, thereby enhancing the durability of the structure.

3. Workability: M 20 grade concrete has a good workability, which makes it easier to place and compact in the formwork.

4. Availability: M 20 grade concrete is readily available and commonly used in construction projects, making it an economical choice for sea-side structures.

5. Code Requirements: The minimum grade of concrete for sea-side structures is specified in the relevant building codes and standards. M 20 grade is the minimum requirement based on the recommendations of these codes.

Conclusion:
In conclusion, the minimum grade of plain concrete in sea-water or exposed directly along the sea coast should be at least M 20. This grade provides the necessary strength, durability, workability, and availability for constructing durable and long-lasting structures in such harsh environments.