Workability of concrete for a given water content is good if the aggregates are:
Correct Answer: A. Rounded aggregate
📚 Detailed Explanation: Rounded Aggregates Give Best Workability
Why A (Rounded aggregate) is correct: The shape of aggregate critically affects workability:
Rounded: Smooth curved surfaces → minimum inter-particle friction → easiest flow → maximum workability for given water. River gravel is the classic example. However, lower surface area = slightly reduced bond with cement paste = marginal reduction in compressive strength.
Angular/crushed: Rough, sharp edges → high friction, interlocking → needs more water for the same workability → but higher surface area = better bond = higher strength.
Flaky/elongated: Worst for both workability and durability.
Rounded: Smooth curved surfaces → minimum inter-particle friction → easiest flow → maximum workability for given water. River gravel is the classic example. However, lower surface area = slightly reduced bond with cement paste = marginal reduction in compressive strength.
Angular/crushed: Rough, sharp edges → high friction, interlocking → needs more water for the same workability → but higher surface area = better bond = higher strength.
Flaky/elongated: Worst for both workability and durability.
Aggregate Shape vs. Workability and Strength
| Shape | Workability | Strength |
|---|---|---|
| Rounded | Best (least friction) | Slightly lower |
| Irregular | Good | Moderate |
| Angular (crushed) | Lower | Higher (better bond) |
| Flaky/elongated | Worst | Low |
- Rounded aggregates = best workability for a given water content due to minimum inter-particle friction.
- The trade-off: rounded aggregates have slightly less paste bond area, so angular crushed aggregate is preferred for high-strength concrete despite lower workability.