Explanation:

To understand why rounded aggregates require less cement paste for a given water content, we need to consider the properties of aggregates and their effect on the workability of concrete.

1. Workability of Concrete:
Workability refers to the ease with which concrete can be mixed, transported, placed, and compacted without segregation or excessive bleeding. It is an important property as it affects the durability, strength, and overall performance of concrete.

2. Shape of Aggregates:
The shape of aggregates has a significant impact on the workability of concrete. Aggregates can be classified into two main types based on their shape: rounded and angular.

3. Rounded Aggregates:
Rounded aggregates have a smooth and rounded shape, similar to pebbles or river stones. They are naturally occurring aggregates that have undergone erosion and weathering processes. These aggregates have several characteristics that contribute to their workability advantages:

a) Reduced Surface Friction:
The smooth surface of rounded aggregates reduces the friction between particles, resulting in lower internal resistance during mixing and handling. This characteristic allows concrete to flow more easily, enhancing its workability.

b) Improved Lubrication:
Rounded aggregates provide better lubrication between particles due to their smooth surfaces. This lubrication reduces the amount of water required to achieve a desired level of workability. Consequently, less cement paste is needed to maintain the proper water-cement ratio, resulting in a more economical mix.

c) Enhanced Particle Packing:
The rounded shape of aggregates allows for better particle packing. When combined with angular aggregates, the rounded particles fill the voids between angular particles, resulting in a denser and more compact concrete mix. This improved particle packing further contributes to the workability of the concrete.

4. Angular Aggregates:
In contrast to rounded aggregates, angular aggregates have sharp edges and corners. These aggregates tend to interlock with each other, increasing the internal friction and reducing the workability of concrete. More cement paste is required to lubricate and separate the angular particles, resulting in a less economical mix.

Conclusion:
In summary, rounded aggregates require less cement paste for a given water content due to their reduced surface friction, improved lubrication, and enhanced particle packing. These characteristics contribute to the improved workability of concrete, making rounded aggregates a favorable choice in many construction applications.