The type of aggregates of same nominal size, which contains less voids when compacted are :
Correct Answer: A. rounded spherical
📚 Detailed Explanation: Aggregate Shape and Void Content
When aggregate particles are compacted into a given volume, the percentage of that volume occupied by air voids depends on the shape of the particles. Spheres pack most efficiently because they roll into each other's gaps; irregular or angular particles lock up in random orientations and leave more voids.
Why A (rounded spherical) is correct: Spherical (and near-spherical rounded) particles pack to approximately 26–33% void content in compacted state, compared to ≈35–45% for angular or irregular particles. The smooth curved surfaces of rounded particles allow them to slide into the most stable packing arrangement (closest packing). This is why rounded aggregates were historically preferred for mass concrete where minimising void content directly reduced cement usage — even though their smooth surface creates a weaker bond with paste.
Void Content by Aggregate Shape
| Aggregate Shape | Approximate Void Content (compacted) |
|---|---|
| Rounded / Spherical | ≈32–33% (minimum) |
| Irregular | ≈35–40% |
| Angular | ≈38–45% |
| Flaky | Highest (>45%) |
Key Concepts for Students
- Fewer voids = less cement paste needed to fill them and coat the aggregate — rounded aggregates are economical on cement.
- Despite lower void content, rounded aggregates are unsuitable for high-strength concrete because of their weak surface bond with cement paste.
- The angularity number in IS 2386 (Part I) quantifies this: it is the void percentage above 33% (reference for rounded aggregate). A higher angularity number means more voids and rougher aggregate.