Effect of Aggregate Size on Workability of Concrete

Introduction:
The workability of concrete refers to its ability to be easily mixed, placed, and finished without segregation or excessive bleeding. It is an important property of concrete as it determines the ease of construction, the homogeneity of the mixture, and the final strength and durability of the hardened concrete. Aggregate size plays a crucial role in determining the workability of concrete.

Effect of Aggregate Size:
The size of aggregates used in concrete can vary from fine sand particles to large rocks. Under certain limits, the size of the aggregate can have a significant effect on the workability of concrete. The following points explain the effect of aggregate size on workability:

1. Interparticle Void Space:
- Larger aggregates occupy more space and leave fewer voids between them.
- This reduces the amount of water required to fill the voids and coat the aggregates, resulting in a lower water-cement ratio.
- A lower water-cement ratio leads to a stiffer and less workable concrete mix.

2. Surface Area and Friction:
- Smaller aggregates have a larger surface area compared to larger aggregates.
- The increased surface area causes more friction between particles, which hampers the flowability of the concrete mix.
- Consequently, smaller aggregates tend to reduce the workability of concrete.

3. Segregation and Bleeding:
- If the aggregate size is too large, it may cause segregation and bleeding in the concrete mix.
- Segregation refers to the separation of coarse aggregates from the rest of the mix, leading to an uneven distribution of aggregates.
- Bleeding occurs when water rises to the surface due to the settlement of fine particles, causing a non-uniform mix.
- Both segregation and bleeding negatively impact the workability of concrete.

Conclusion:
Under certain limits, the larger size of aggregates tends to increase the workability of concrete. This is because larger aggregates decrease the interparticle void space, reducing the water requirement and resulting in a stiffer mix. On the other hand, smaller aggregates have a larger surface area and higher friction, which reduces the flowability of the mix and lowers workability. It's important to choose the appropriate aggregate size to achieve the desired workability and ensure the proper placement and finishing of concrete structures.