Using a too-wet (high w/c) concrete mix may cause:
Correct Answer: D. All of these
📚 Detailed Explanation: Effects of Excessively Wet Concrete Mix
Why D (All of these) is correct: Excess water in a concrete mix creates a cascading series of problems. Lower paste viscosity reduces cohesion (segregation risk), excess evaporation leaves large capillary voids (lower density and strength), and a higher w/c ratio directly reduces compressive strength per Abrams' Law.
Three Consequences of a Too-Wet Mix
| Consequence | Mechanism | Quantified Impact |
|---|---|---|
| A. Segregation | Excess water lowers paste viscosity and yield strength; cannot suspend dense aggregate particles which sink under gravity | 5% voids from segregation → >30% strength loss |
| B. Lower density | Excess water occupies space in the mix; evaporates after hardening leaving capillary pores and voids; concrete becomes more porous and lighter | Density can drop from ~2400 kg/m³ to 2250 or less |
| C. Lower strength | Abrams' Law: compressive strength is inversely proportional to w/c ratio; more water directly dilutes the paste quality; more voids form on evaporation | Doubling w/c from 0.4 to 0.8 can reduce strength by 50%+ |
Abrams' Law (approximate):
f = K1 / K2^(w/c)
where:
f = compressive strength
K1, K2 = empirical constants (≈ 97 MPa and 4.0 for typical conditions)
w/c = water-cement ratio
f = K1 / K2^(w/c)
where:
f = compressive strength
K1, K2 = empirical constants (≈ 97 MPa and 4.0 for typical conditions)
w/c = water-cement ratio
Result: Every 0.1 increase in w/c reduces strength by ≈ 15–20%
Water-Cement Ratio vs. 28-day Strength (M25 concrete)
| w/c Ratio | 28-day Strength (MPa) | Remarks |
|---|---|---|
| 0.40 | 38–42 | Rich, low-water mix; excellent durability |
| 0.50 | 30–35 | Standard M25 design w/c |
| 0.60 | 22–28 | Adequate for M20 if other factors controlled |
| 0.70 | 16–20 | Lean, wet mix; not recommended for structural concrete |
| 0.80 | 10–14 | Very wet; high segregation; not suitable |
- Too-wet concrete causes: segregation, lower density, and lower compressive strength (all simultaneously).
- IS 456:2000 maximum w/c ratios: 0.45 for moderate exposure; 0.40 for severe exposure.
- Never add water at the site to “improve workability” — this is a code violation and causes all three defects.