Internal friction between the ingredients of concrete, is decreased by using
Correct Answer: D. more water and coarse aggregates
📚 Detailed Explanation: Reducing Internal Friction in Fresh Concrete
Internal friction refers to the resistance between solid particles (cement, aggregate) as they move past each other during mixing and placement. Reducing it improves workability.
Why D (more water and coarse aggregates) is correct:
More water: Water fills inter-particle voids and acts as a lubricating film between particles, directly reducing friction.
Coarse aggregates: Compared to fine aggregates, coarse particles have much lower specific surface area (surface area per unit volume). Less surface area = less particle contact area = less friction per unit volume.
Option A (less water) is wrong — less water increases friction. Option B (fine aggregates) is wrong — fine aggregates increase friction due to very high specific surface area. Option C (rich mix) is wrong — while rich mixes have more paste, richness alone does not reduce friction as directly as water does.
More water: Water fills inter-particle voids and acts as a lubricating film between particles, directly reducing friction.
Coarse aggregates: Compared to fine aggregates, coarse particles have much lower specific surface area (surface area per unit volume). Less surface area = less particle contact area = less friction per unit volume.
Option A (less water) is wrong — less water increases friction. Option B (fine aggregates) is wrong — fine aggregates increase friction due to very high specific surface area. Option C (rich mix) is wrong — while rich mixes have more paste, richness alone does not reduce friction as directly as water does.
Factors Affecting Internal Friction
| Factor | Effect on Friction |
|---|---|
| More water | Decreases (lubricates particles) |
| Less water | Increases |
| Coarse aggregate | Decreases (low specific surface) |
| Fine aggregate (Zone IV) | Increases (high specific surface) |
| Angular aggregate | Increases (interlocking) |
| Rounded aggregate | Decreases (ball-bearing effect) |
Key Concepts for Students
- Workability = reducing internal friction: any factor that reduces friction improves workability.
- The combined use of more water AND coarser aggregate is the most effective approach — each acts by a different mechanism.
- Chemical admixtures (plasticisers) reduce friction at the molecular level without adding water — the modern preferred approach.