The 28-day cube strength of mass concrete using aggregates of maximum size 5 cm for gravity dams should be:
Correct Answer: D. Below 200 kg/sq.cm
📚 Detailed Explanation: Concrete Strength for Gravity Dams
Why D (Below 200 kg/cm²) is correct: Gravity dams resist hydrostatic force through their own weight. The concrete does not need to be high-strength — it needs to be:
1. Mass-efficient (high density to provide weight)
2. Low heat of hydration (large pour volume → thermal cracking risk)
3. Durable (water-retaining structure)
These requirements lead to mixes with low cement content, large aggregates (≥ 50 mm up to 150 mm), and relatively low 28-day strength — typically below 200 kg/cm² (≈ 20 N/mm²).
1. Mass-efficient (high density to provide weight)
2. Low heat of hydration (large pour volume → thermal cracking risk)
3. Durable (water-retaining structure)
These requirements lead to mixes with low cement content, large aggregates (≥ 50 mm up to 150 mm), and relatively low 28-day strength — typically below 200 kg/cm² (≈ 20 N/mm²).
Mass Concrete for Gravity Dams vs. Structural Concrete
| Property | Mass Concrete (Gravity Dam) | Structural Concrete (RCC) |
|---|---|---|
| 28-day strength | Below 200 kg/cm² (<20 N/mm²) | M20–M40 (20–40 N/mm²) |
| Aggregate size | Up to 150 mm | 20–40 mm |
| Cement content | Low (150–200 kg/m³) | Higher (300–450 kg/m³) |
| Heat of hydration | Minimised (critical concern) | Less critical |
| Design philosophy | Gravity (weight-dependent) | Structural (strength-dependent) |
- Gravity dam mass concrete: 28-day cube strength = below 200 kg/cm².
- Primary focus is on controlling heat of hydration, not maximising strength.