For a given aggregate ratio, increasing the water cement ratio:
Correct Answer: A. increases shrinkage
📚 Detailed Explanation: W/C Ratio and Drying Shrinkage
Drying shrinkage is the volumetric contraction of concrete as evaporable water leaves the capillary pores during drying. The water-cement ratio is the primary factor controlling the amount of evaporable water available.
Why A (increases shrinkage) is correct: At higher w/c: more mixing water is present per unit of cement → more water evaporates during drying → more capillary pore space is created → greater volumetric contraction = greater drying shrinkage. Option B (no change) is incorrect — shrinkage is strongly linked to w/c. Option C (increases strength) is the opposite of Abrams' Law. Option D (decreases shrinkage) is also opposite to the actual effect.
Effect of W/C on Shrinkage
| W/C Ratio | Evaporable Water | Drying Shrinkage |
|---|---|---|
| 0.35 (low) | Less | Lower |
| 0.45 (medium) | Medium | Medium |
| 0.55 (high) | More | Higher |
| 0.65 (very high) | Most | Highest |
Key Concepts for Students
- Higher w/c = more drying shrinkage — another reason to minimise w/c in structural concrete.
- Drying shrinkage causes cracking if the concrete is restrained — controlling w/c is the primary prevention strategy.
- Lower w/c simultaneously gives: higher strength + lower shrinkage + lower creep + lower permeability.