The density of concrete:
Correct Answer: C. Increases with increase in size of aggregates
📚 Detailed Explanation: Concrete Density Increases with Larger Aggregate Size
Why C (Increases with increase in aggregate size) is correct: Aggregate particles (density ≈ 2650 kg/m³) are denser than hardened cement paste (density ≈ 1800–2200 kg/m³). Larger aggregate particles have a smaller surface area-to-volume ratio, requiring less paste to coat them. This means more aggregate volume per unit of concrete volume, raising the overall density. Additionally, larger particles pack with fewer micro-voids between them.
Why Larger Aggregates Increase Density
| Aggregate Size | Surface Area/Volume Ratio | Paste Required | Concrete Density |
|---|---|---|---|
| 10 mm (small) | High (more surfaces to coat) | More paste needed | Lower (paste dilutes density) |
| 20 mm (medium) | Moderate | Moderate paste | Moderate |
| 40 mm (large) | Low (fewer surfaces to coat) | Less paste needed | Higher density |
Typical Concrete Densities
| Concrete Type | Density (kg/m³) | Key Factor |
|---|---|---|
| Plain cement concrete (PCC) | 2300–2400 | Standard aggregate (granite, basalt) |
| Reinforced concrete (RCC) | 2400–2500 | Steel + concrete |
| Heavyweight concrete | 3000–6000 | Baryte or iron ore aggregate; radiation shielding |
| Lightweight concrete | 300–2000 | Lightweight aggregate (LYTAG, pumice) or foamed |
- Concrete density increases with increasing aggregate size.
- Larger aggregate = less surface area/volume = less paste needed = more dense aggregate material per unit volume.
- IS 456 takes concrete density as 2400 kg/m³ (plain concrete) and 24 kN/m³ for structural calculations.