Concrete is unsuitable for compaction by vibration if it is Plastic.

Explanation:

Introduction:
Compaction is a process used in construction to increase the density of soil or concrete. It is achieved by applying mechanical force to the material, usually through vibration or pressure. However, not all types of concrete are suitable for compaction by vibration. One such type is plastic concrete.

Plastic Concrete:
Plastic concrete refers to a state of concrete where it is still workable and has a high water content. It is often used in applications where the concrete needs to be molded into complex shapes or structures. Plastic concrete is easy to handle and manipulate due to its high fluidity.

Unsuitability of Plastic Concrete for Vibration Compaction:
Plastic concrete is unsuitable for compaction by vibration due to the following reasons:

1. Lack of Cohesion: Plastic concrete has a high water content, which reduces its cohesion and stability. When subjected to vibration, the excessive water causes the particles to separate and settle, resulting in a loss of strength and density. The vibration forces the water to rise to the surface, leaving voids and reducing the overall compactness of the concrete.

2. Segregation of Aggregates: The excessive water content in plastic concrete can cause the aggregates (such as sand and gravel) to segregate. During vibration, the heavier aggregates tend to settle at the bottom, while the lighter particles rise to the top. This leads to an uneven distribution of aggregates, resulting in a non-uniform and weaker concrete structure.

3. Loss of Workability: Vibration causes the excess water in plastic concrete to separate and rise to the surface, leaving the concrete with a reduced workability. This makes it difficult to achieve a uniform compaction and may result in the formation of voids and honeycombs within the concrete.

4. Reduced Strength: The excessive water content in plastic concrete, coupled with the loss of cohesion and segregation of aggregates, compromises the strength and durability of the final hardened concrete. It may lead to the formation of cracks, reduced load-bearing capacity, and increased permeability.

Conclusion:
In conclusion, plastic concrete is unsuitable for compaction by vibration due to its high water content, lack of cohesion, segregation of aggregates, and loss of workability. It is important to use the appropriate type of concrete for compaction to ensure a strong, durable, and well-compacted structure.