Variation in the water demand for concrete is caused due to variation in
Try it first: pick an option, then read the explanation.
Correct answer: D. All option are correct
Detailed Explanation: Factors Causing Variation in Water Demand
The water demand of a concrete mix is not fixed; it changes when the properties of the fine aggregate change. Understanding which sand properties affect water demand is essential for maintaining consistent workability across different batches.
Why D (All option are correct) is correct:Option A — Surface texture: Rough, angular particles (crushed sand) need more water to lubricate their surfaces than smooth, rounded particles (river sand). A rough texture means more surface area in contact with paste per unit mass.
Option B — Grading: A fine-graded sand (high surface area per unit volume) needs more water for the same workability than a coarsely-graded sand. Zone IV sand (finest) requires significantly more water than Zone I sand (coarsest).
Option C — Fine particles (silt/clay <75 µm): These have extremely high surface area and absorb water very readily, greatly increasing water demand even when present in small percentages (1–2% silt can increase water demand by 10–15 litres/m³).
Effect of Sand Properties on Water Demand
| Sand Property | Effect on Water Demand |
|---|---|
| Rough surface texture | Increases water demand |
| Finer grading (lower FM) | Increases water demand |
| High silt/clay content (<75 µm) | Significantly increases water demand |
Key Concepts for Students
- An increase in water demand without a corresponding increase in cement (to maintain w/c ratio) directly reduces concrete strength.
- IS 456 requires that silt content (material <75 µm) in fine aggregate not exceed 3% by mass for concrete, specifically to control water demand.
- Plasticiser admixtures (water reducers) can compensate for increased water demand from fine or rough-textured sands.