Seashore gravels are of which of the following types?
Correct Answer: D. Rounded
📚 Detailed Explanation: Shape of Seashore Gravels
The shape of an aggregate is determined by how it was formed and the conditions it was subjected to during transport. Seashore (and river) gravels travel long distances, tumbling and rubbing against each other and the bed under the continuous action of waves and currents. This prolonged mechanical wear progressively removes corners and edges.
Why D (Rounded) is correct: The sustained attrition — the grinding and polishing of particles against each other and the seabed — wears down all sharp edges over geological time, producing smooth, rounded shapes. The same process occurs in river beds, which is why river gravels are also rounded. Angular aggregates, by contrast, are produced by fresh quarrying or blasting with minimal subsequent wear.
Aggregate Shape Classification
| Shape | Origin / Cause | Bond with Cement |
|---|---|---|
| Rounded | Seashore / river attrition | Weak (smooth surface) |
| Irregular | Partially worn natural deposits | Moderate |
| Angular | Fresh quarrying & crushing | Strong (rough surface) |
| Flaky | Laminated rock cleavage | Poor (high surface area, low thickness) |
Key Concepts for Students
- Rounded aggregates pack with the fewest voids (≈32–33%) but give the weakest cement bond — not ideal for high-strength concrete.
- Angular aggregates produce better interlocking, which is why crushed stone from quarries is preferred for structural concrete.
- Seashore gravels also carry chloride salts; they must be washed before use to prevent steel corrosion.