For a given degree of hydration, the effect of increasing the water-cement ratio in concrete is:
Correct Answer: A. To increase permeability
📚 Detailed Explanation: Higher w/c Ratio Increases Permeability
Why A (Increase permeability) is correct: Permeability of hardened concrete is governed by the capillary pore system in the cement paste. For a given degree of hydration, a higher w/c ratio means more water was in the original mix relative to cement. Since hydration can only consume a limited amount of water (approximately w/c = 0.23–0.42 depending on degree of hydration), excess water remains in the paste as capillary pores. More excess water = larger, more interconnected pore network = higher permeability.
Water-Cement Ratio and Capillary Porosity
| w/c Ratio | Capillary Pore Volume | Permeability | Durability |
|---|---|---|---|
| 0.35–0.40 | Very low (<5% at full hydration) | Very low (dense paste) | Excellent |
| 0.45–0.50 | Low to moderate | Low | Good |
| 0.55–0.60 | Moderate | Moderate | Acceptable for mild exposure |
| >0.65 | High (interconnected capillaries) | High | Poor; not durable in aggressive environments |
Mechanism: How w/c Controls Permeability
| Step | What Happens |
|---|---|
| 1. Mixing | Water fills space between cement particles; w/c determines initial porosity |
| 2. Hydration | C-S-H gel fills some pore space; chemically combines some water |
| 3. Residual capillary water | Water in excess of what hydration consumes remains as capillary pores |
| 4. Evaporation | Capillary water evaporates; pores remain empty; interconnected network |
| 5. Higher w/c = more residual pores | Larger, more numerous, more interconnected pores = higher permeability |
Powers (1958) relationship (approximate):
Capillary porosity (p) = (w/c – 0.36*α) / (w/c + 0.32)
Where α = degree of hydration (0–1)
Capillary porosity (p) = (w/c – 0.36*α) / (w/c + 0.32)
Where α = degree of hydration (0–1)
At full hydration (α = 1.0):
w/c = 0.40: p ≈ 0% (no capillary pores; essentially impermeable)
w/c = 0.50: p ≈ 9% (some capillary pores; moderate permeability)
w/c = 0.60: p ≈ 18% (significant porosity; high permeability)
- Higher w/c ratio → more excess water → more capillary pores → increased permeability.
- At w/c ≤0.40 with full hydration, capillary porosity approaches zero (essentially impermeable paste).
- IS 456 limits w/c to 0.40 for extreme exposure to achieve maximum impermeability and durability.