Shrinkage in concrete is zero when relative humidity equals:
Correct Answer: A. 1 (100%)
📚 Detailed Explanation: No Shrinkage at RH = 1 (100% Relative Humidity)
Why A (1 i.e. 100% RH) is correct: Drying shrinkage is driven by the difference in vapour pressure between the pore water inside concrete and the ambient environment. This vapour pressure gradient drives evaporation of water from the concrete. At RH = 100%, the ambient air is fully saturated; the vapour pressure gradient is zero; no net evaporation occurs; and therefore drying shrinkage is also zero.
Relative Humidity and Shrinkage Relationship
| Relative Humidity (RH) | Vapour Pressure Gradient | Drying Shrinkage |
|---|---|---|
| 100% (RH = 1.0) | Zero (environment is saturated; no evaporation possible) | Zero — no shrinkage |
| 80% (RH = 0.80) | Small gradient; slow evaporation | Small; typical indoor condition |
| 60% (RH = 0.60) | Moderate gradient | Moderate; significant for thin members |
| 40% (RH = 0.40) | High gradient; rapid evaporation | Large; risk of plastic shrinkage cracking |
| 0% (RH = 0) | Maximum gradient; very rapid evaporation | Maximum shrinkage (theoretically) |
Practical Implications
| Application | RH Consideration |
|---|---|
| Moist curing | Maintains high RH (>95%) at concrete surface → minimises drying shrinkage during critical early period |
| Ponding curing | RH at surface = 100%; zero drying; most effective curing method for flat surfaces |
| Concrete in arid climates | Low ambient RH (30–50%) → rapid drying → high shrinkage; protect with wet hessian, polythene sheeting |
| Submerged concrete | RH = 100% underwater; swelling occurs (reverse of drying shrinkage) |
IS 456:2000 Design Practice:
Design shrinkage strain = 0.0003 (for RH of 50%)
This is the median exposure condition assumed for design.
Design shrinkage strain = 0.0003 (for RH of 50%)
This is the median exposure condition assumed for design.
At RH = 100%: shrinkage ≈ 0
At RH = 50%: shrinkage ≈ 0.0003 (design value)
At RH = 35%: shrinkage can exceed 0.0005
- No drying shrinkage occurs when RH = 1 (100%) — no vapour pressure gradient, no evaporation.
- Moist curing (RH near 100%) is the most effective way to minimise early shrinkage.
- IS 456 design shrinkage = 0.0003 assumes moderate humidity conditions (RH ≈ 50%).