Which of the following decreases the workability of concrete?
Correct Answer: A. Angular aggregates
📚 Detailed Explanation: Angular Aggregates Reduce Workability
The shape of aggregate particles is one of the most significant factors affecting the workability of fresh concrete. Angular surfaces interlock mechanically and create friction, while smooth surfaces allow particles to slide past each other freely.
Why A (Angular aggregates) is correct: Angular aggregates (crushed stone) have: rough, irregular surfaces; sharp corners that interlock; high internal friction angle in the fresh mix. All of these resist flow → lower workability. Option B (well-graded aggregates) improve packing efficiency and fill voids, reducing overall paste demand and improving workability. Option C (air-entraining admixture) introduces tiny air bubbles that act as ball-bearings, improving workability. Option D (rounded aggregates, e.g. river gravel) have smooth surfaces and low friction → high workability.
Aggregate Shape vs. Workability and Strength
| Shape | Workability | Strength |
|---|---|---|
| Rounded (river gravel) | Highest | Lower (weak bond) |
| Sub-rounded | High | Medium |
| Sub-angular | Medium | Medium |
| Angular (crushed) | Lowest | Highest (mechanical bond) |
- Angular aggregates reduce workability but improve strength — the shape trade-off in concrete.
- Air-entraining agents improve workability because microscopic air bubbles roll like ball-bearings between particles.