Which of the following statements is true?
Correct Answer: D. All option are correct
📚 Detailed Explanation: Role of Water in Concrete Strength
Water in concrete serves two roles: it activates cement hydration (chemical role) and provides workability (physical role). Both the quality and quantity of water are critical. All three statements in this question capture important and distinct truths about how water governs concrete performance.
Why D (All option are correct) is correct: Statement A is correct — impure water introduces chlorides, sulphates, or organic matter that disrupt hydration and reduce strength (IS 456 sets permissible impurity limits). Statement B is correct — well-graded aggregates need less water to achieve the same workability; mechanical compaction also reduces the water needed. Statement C is correct — concrete strength is inversely related to the water-cement ratio (Abrams’ law); adding 10% extra water raises the w/c ratio enough to reduce compressive strength by ≈15%.
Water–Cement Ratio and Strength Relationship
| Factor | Effect on Concrete |
|---|---|
| Poor quality water (high solids) | Disrupts hydration; reduces strength and durability |
| Poorly graded aggregate | More water needed for workability; higher w/c ratio |
| 10% excess water | Raises w/c; ≈15% strength reduction |
| Good compaction | Achieves same density with less water |
Key Concepts for Students
- Abrams’ Law: Compressive strength = A / Bw/c — strength decreases as w/c ratio increases, for fully compacted concrete.
- IS 456:2000 specifies maximum w/c ratios (e.g., 0.45 for M25 in moderate exposure) precisely to control this effect.
- Quality checks per IS 456: pH ≥6, TDS ≤15,000 ppm, sulphates ≤400 mg/litre, chlorides ≤2000 mg/litre for RCC.