During a concrete mix design process, what happens to the free maximum water cement ratio when grade of concrete is increased?
Correct Answer: B. Decreases
📚 Detailed Explanation: w/c Ratio Decreases as Grade Increases
Why B (Decreases) is correct: Concrete grade is defined by its 28-day characteristic compressive strength. Higher grade = higher required strength. Abrams' Law states: compressive strength is inversely proportional to w/c ratio. Therefore, to achieve higher strength, the w/c ratio must be lowered. IS 456 also specifies maximum permissible w/c ratios for durability, which decrease with increasing environmental exposure.
IS 456 — Maximum Free w/c Ratio by Grade and Exposure
| Exposure | Minimum Grade | Max w/c Ratio |
|---|---|---|
| Mild | M20 | 0.55 |
| Moderate | M25 | 0.50 |
| Severe | M30 | 0.45 |
| Very Severe | M35 | 0.45 |
| Extreme | M40 | 0.40 |
- Higher grade → lower maximum w/c ratio (to achieve higher strength).
- Abrams' Law: fc = A / Bw/c where A and B are constants; as w/c ↓, strength ↑.