Air entraining agents are additives used in plastic concrete to increase its resistance to freeze-thaw cycles and improve its durability. These agents work by creating a large number of tiny air bubbles throughout the concrete mixture, which provide space for the water to expand when it freezes, reducing the potential for cracking.

The air entraining agents modify several properties of plastic concrete, including workability, segregation, and bleeding. Let's discuss each of these properties in detail:

1. Workability:
Workability refers to the ease with which concrete can be mixed, transported, placed, and finished. It is an important property as it affects the overall quality of the concrete and the efficiency of construction operations. Air entraining agents can significantly influence the workability of plastic concrete by:

- Increasing the water demand: Air entraining agents tend to increase the water demand of the concrete mixture, which can affect its workability. Adjustments in the water-cement ratio and superplasticizers may be required to maintain the desired workability.
- Reducing the slump: The addition of air entraining agents can reduce the slump of the concrete, making it less fluid. This can be advantageous in situations where high workability is not required, such as in vertical or overhead placements.

2. Segregation:
Segregation refers to the separation of coarse aggregates from the mortar in the concrete mixture. It can result in an uneven distribution of aggregates and reduce the overall strength and durability of the concrete. Air entraining agents help in reducing segregation by:

- Enhancing the cohesion: The tiny air bubbles created by the air entraining agents increase the cohesion of the concrete mixture, preventing the aggregates from settling down. This helps in maintaining a homogeneous distribution of aggregates throughout the mixture.

3. Bleeding:
Bleeding is the process in which water rises to the surface of freshly placed concrete due to the settlement of solid particles. It can lead to an excessive amount of water on the surface, which can weaken the concrete and adversely affect its durability. Air entraining agents can minimize bleeding by:

- Stabilizing the air void system: The air bubbles created by the air entraining agents form a stable network within the concrete, reducing the capillary pressure and preventing excessive bleeding.

In conclusion, air entraining agents modify the properties of plastic concrete by affecting its workability, segregation, and bleeding. They enhance the workability by increasing the water demand and reducing the slump. They also reduce the segregation by improving the cohesion of the mixture and minimize bleeding by stabilizing the air void system. Overall, the use of air entraining agents helps in producing more durable and resilient concrete, especially in areas prone to freeze-thaw cycles.