Water cement ratio is
Correct Answer: B. weight of water to that of cement
📚 Detailed Explanation: W/C Ratio is Weight-Based
Although water is often measured in litres on site, the w/c ratio is always defined on a weight (mass) basis. This ensures the ratio is consistent across different measurement systems and independent of temperature effects on volume.
Why B (weight of water / weight of cement) is correct: Water has a density of 1 g/cm³ (1 kg/litre), so its mass and volume are numerically equal in SI units. Cement, however, has a specific gravity of ~3.15, so 1 litre of cement weighs 3.15 kg. If you measured volumes, 100 litres of water + 100 litres of cement would give a misleading w/c of 1.0, when the actual mass ratio is only 100/315 = 0.317. Option C is wrong because volume and weight ratios are NOT equivalent for this purpose (only numerically similar for water because ρ=1 kg/L).
Why Weight, Not Volume?
| Property | Water | Cement |
|---|---|---|
| Density (g/cm³) | 1.00 | 3.15 |
| 100 litres weighs (kg) | 100 kg | 315 kg |
| Mass basis w/c | 100/315 = 0.317 ✓ | |
| Volume basis w/c (wrong) | 100/100 = 1.0 ✗ | |
Key Concepts for Students
- In practice: w/c = litres of water per kg of cement (since 1 litre water = 1 kg).
- For a 50 kg cement bag: w/c = 0.45 means 0.45 × 50 = 22.5 litres of water.
- Volume ratios are used in mix design only for aggregate proportioning (absolute volume method), not for w/c ratio.