Increasing the moisture content (adding more water) in concrete at the time of mixing:
Correct Answer: A. Reduces the strength
📚 Detailed Explanation: More Mixing Water Reduces Concrete Strength
Why A (Reduces the strength) is correct: The compressive strength of concrete is inversely related to the water-cement (w/c) ratio — this is Abrams' Law. Adding excess moisture at mixing raises the w/c ratio. The extra water does not hydrate additional cement (there is a limit to how much water cement can combine with); instead it remains as free water in the paste and evaporates after hardening, creating capillary voids that reduce strength.
Abrams’ Law: Strength vs. w/c Ratio
Abrams' Law (approximate):
f = K1 / K2^(w/c)
f = K1 / K2^(w/c)
Where: f = 28-day compressive strength
K1 ≈ 97 MPa, K2 ≈ 4.0 (typical for OPC at standard conditions)
Effect of raising w/c by 0.1:
w/c 0.40 → f ≈ 42 MPa
w/c 0.50 → f ≈ 31 MPa (-26%)
w/c 0.60 → f ≈ 23 MPa (-45%)
w/c 0.70 → f ≈ 17 MPa (-60%)
What Excess Water Does in Concrete
| Fate of Excess Water | Effect on Concrete |
|---|---|
| Chemically combined (hydration water) | Required for C-S-H formation; contributes to strength (w/c ≈0.23–0.25 for full hydration) |
| Excess free water (above hydration requirement) | Remains in capillary pores; evaporates on drying → capillary voids → reduced strength and increased permeability |
Strength vs. w/c for M25 Concrete
| w/c Ratio | Approximate 28-day Strength (MPa) | Remarks |
|---|---|---|
| 0.40 | 38–42 | Low water; dense; strong; IS 456 severe exposure |
| 0.50 | 28–33 | Standard M25 design; IS 456 moderate exposure |
| 0.60 | 20–25 | Higher water; marginal for M25; not recommended |
| 0.70+ | <18 | Excess water; segregation likely; not structural |
- Increasing moisture content at mixing reduces strength (Abrams' Law: strength inversely proportional to w/c ratio).
- Excess water leaves capillary voids on evaporation → porous, weak concrete.
- IS 456:2000: maximum w/c for mild exposure = 0.55; severe = 0.45; very severe = 0.45; extreme = 0.40.