The water-cement ratio is given by
Correct Answer: B. Weight of water/Weight of cement
📚 Detailed Explanation: Definition of Water-Cement Ratio
The water-cement ratio (w/c ratio) is one of the most fundamental parameters in concrete technology. It directly controls the strength, workability, and durability of hardened concrete and is the basis of Abrams' Law.
Why B (Weight of water / Weight of cement) is correct: The w/c ratio is always expressed as a mass ratio (weight/weight), not a volume ratio. This is because water has a density of 1.0 g/cm³ but cement has a specific gravity of ~3.15 g/cm³. If volumes were used, the ratio would give a completely different number. The mass-based definition gives a consistent, reproducible value independent of the physical form of the materials. Options A (cement/water) is the inverse — the cement-water ratio, sometimes used in strength calculations. Options C and D mix mass and volume units.
W/C Ratio Formula and Typical Values
| Parameter | Value/Range |
|---|---|
| Formula | w/c = Mass of water / Mass of cement |
| Units | Dimensionless (kg/kg or g/g) |
| Min for complete hydration | ~0.23 (theoretical) |
| Practical min for workability | 0.35–0.40 |
| Typical structural concrete | 0.40–0.55 |
| IS 456 max (mild exposure) | 0.55 |
Key Concepts for Students
- w/c = mass of water / mass of cement — never volume/weight or weight/volume.
- The higher the w/c ratio, the more water relative to cement — giving more workability but lower strength.
- Abrams' Law (1919) established the inverse relationship: compressive strength ∝ 1 / (w/c ratio).