The aggregate which is obtained from the seashore or rivers and produces minimum voids in the concrete is known as ……..
Correct Answer: D. Rounded aggregates
📚 Detailed Explanation: Rounded Aggregates and Minimum Voids
The void content in a compacted aggregate mass depends on the shape of the particles. Spherical and rounded particles pack closest to an ideal arrangement because their smooth, curved surfaces allow them to settle into each other's gaps without angular interference.
Why D (Rounded aggregates) is correct: River and seashore gravels are subjected to sustained attrition (water action) that wears down all edges and corners, producing smooth, roughly spherical particles. When compacted, these rounded particles achieve the minimum void content of approximately 32–33%. Angular aggregate, by contrast, creates voids of 38–45% due to bridging and random orientation. Flaky aggregate has the highest void content due to its plate-like shape.
Void Content by Aggregate Shape (Compacted)
| Shape | Approx. Void Content | Cement Required |
|---|---|---|
| Rounded (river/seashore) | 32–33% (minimum) | Least |
| Irregular | 35–40% | Moderate |
| Angular (crushed) | 38–45% | More |
| Flaky | >45% | Most |
Key Concepts for Students
- Minimum voids means less cement paste is needed to fill them, making rounded aggregate economical.
- Despite minimum voids, rounded aggregates are unsuitable for high-strength concrete because their smooth surface gives a weak bond with cement paste.
- The angularity number for rounded aggregate = 0 (reference standard); angular crushed stone typically has angularity number 7–11.