S1: Rounded aggregate → high workability, low strength. S2: Angular aggregate → high workability, low strength. S3: Rounded aggregate → low workability, high strength. S4: Angular aggregate → low workability, high strength. Which are correct?
📚 Detailed Explanation: Aggregate Shape — The Workability-Strength Trade-Off
Aggregate shape creates an inherent trade-off: the same roughness that reduces workability (friction) also improves concrete strength (mechanical bond). This is a frequently examined concept.
S1 TRUE — Rounded: high workability, low strength. Smooth surfaces mean low friction in fresh concrete (high workability). But smooth surfaces also provide weak mechanical bond with cement paste (lower strength). River gravel is the classic example.
S4 TRUE — Angular: low workability, high strength. Rough, jagged surfaces create high friction in fresh concrete (low workability). But these same surfaces provide excellent mechanical interlock with cement paste (higher strength). Crushed stone is the classic example.
S2 (angular → high workability) and S3 (rounded → high strength) are both wrong — they swap the properties.
Summary Table
| Shape | Workability | Concrete Strength |
|---|---|---|
| Rounded (river gravel) | HIGH | LOWER |
| Angular (crushed stone) | LOW | HIGHER |
- S1 and S4 state the correct combination: Rounded = high workability + low strength; Angular = low workability + high strength.
- For high-strength concrete (w/c <0.40), angular crushed aggregate is mandatory despite lower workability.
- Admixtures bridge the gap: plasticisers restore workability of angular-aggregate mixes without raising w/c.