Quantity of water used per cubic meter of concrete
Correct Answer: C. 5 percent of aggregate weight plus 30 percent of cement weight
📚 Detailed Explanation: Estimating Water Quantity in Concrete
Water in concrete serves two distinct functions: (1) it hydrates the cement to form the binding C-S-H gel, and (2) it coats and lubricates the aggregate particles to give workability. The empirical formula for estimating total water content combines these two requirements.
Why C is correct: The approximate water requirement is:
• 30% of cement weight: The theoretical water needed for complete hydration of cement (w/c for complete hydration ≈ 0.25–0.30). For a typical cement content of ~300–400 kg/m³, this adds 90–120 litres.
• 5% of aggregate weight: Water needed to wet the large aggregate surface area and make the mix workable. With ~1700 kg/m³ total aggregate, this adds ~85 litres.
Total: ~175–205 litres/m³, consistent with typical concrete mix designs.
Option D (5% cement + 30% aggregate) would give ~15 + 510 = 525 litres — far too much water for any structural concrete.
• 30% of cement weight: The theoretical water needed for complete hydration of cement (w/c for complete hydration ≈ 0.25–0.30). For a typical cement content of ~300–400 kg/m³, this adds 90–120 litres.
• 5% of aggregate weight: Water needed to wet the large aggregate surface area and make the mix workable. With ~1700 kg/m³ total aggregate, this adds ~85 litres.
Total: ~175–205 litres/m³, consistent with typical concrete mix designs.
Option D (5% cement + 30% aggregate) would give ~15 + 510 = 525 litres — far too much water for any structural concrete.
Water Content Calculation Example
| Component | Typical Qty (per m³) | Water Contribution |
|---|---|---|
| Cement | 350 kg | 30% × 350 = 105 litres |
| Aggregate (FA+CA) | 1700 kg | 5% × 1700 = 85 litres |
| Total water | ~190 litres/m³ |
Key Concepts for Students
- This is an empirical estimate; actual water content varies with aggregate size, workability, and admixtures.
- The minimum water for complete hydration is w/c = 0.23 — any less and hydration is incomplete; any more adds porosity.
- In IS 10262 mix design, actual water content is determined from tables based on maximum aggregate size and slump requirements.