Water-cement ratio is measured ……………….. of water and cement used per cubic metre of concrete.
Correct Answer: B. weight by weight
📚 Detailed Explanation: W/C is Weight by Weight
Revisiting the fundamental definition: the water-cement ratio must be expressed on a weight (mass) basis because cement and water have very different densities.
Why B (weight by weight) is correct: w/c = (mass of water per m³) / (mass of cement per m³). For example, in a mix with 180 kg/m³ water and 400 kg/m³ cement: w/c = 180/400 = 0.45 (weight by weight). Using volume: volume of water = 180 L; volume of cement = 400/3.15 = 127 L; volumetric ratio = 180/127 = 1.42 — a completely different and useless number. The mass ratio of 0.45 is the physically meaningful and universally accepted definition.
Comparison of Measurement Bases
| Basis | Formula | Example (water=180, cement=400 kg/m³) | Use? |
|---|---|---|---|
| Weight/Weight | 180/400 | 0.45 | Correct |
| Volume/Volume | 180 L / (400/3.15) L | 1.42 | Incorrect |
| Weight/Volume | 180 kg / 127 L | 1.42 kg/L | Incorrect |
Key Concepts for Students
- w/c is always weight (mass) by weight (mass) — never volume by volume.
- Since water density = 1 kg/L, the w/c numerically equals litres of water per kg of cement — convenient for site use.