Maximum shrinkage in concrete occurs after drying for:
Correct Answer: A. 28 days
📚 Detailed Explanation: Maximum (Rate of) Shrinkage Occurs by 28 Days
Why A (28 days) is correct: The initial rapid phase of drying shrinkage occurs in the first 28 days of drying. During this period, most of the free capillary water evaporates from the concrete, causing the largest volume reduction. After 28 days, the remaining adsorbed and gel water evaporates much more slowly, and further shrinkage is minor.
Drying Shrinkage Progression
| Drying Duration | Approximate Cumulative Shrinkage | What's Happening |
|---|---|---|
| 3 days | ≈ 5–10% of ultimate | Surface water evaporating; thin surface layer drying |
| 7 days | ≈ 10–20% of ultimate | Drying penetrating deeper; rapid evaporation continues |
| 28 days | ≈ 14–34% of ultimate | Most free capillary water evaporated; maximum rate phase ending |
| 3 months | ≈ 40–70% of ultimate | Adsorbed water evaporating; slower rate |
| 1 year | ≈ 75–90% of ultimate | Gel water loss; very slow; hygrothermal cycling |
| Several years | ≈ 100% (ultimate reached) | Equilibrium with ambient conditions |
Factors That Affect How Quickly 28-day Shrinkage Completes
| Factor | Faster Shrinkage | Slower Shrinkage |
|---|---|---|
| Ambient humidity | Low RH (<50%) | High RH (>80%) |
| Temperature | High temperature → faster evaporation | Low temperature → slower evaporation |
| Member thickness | Thin members dry faster (high surface/volume ratio) | Thick members dry slowly (water diffuses from core) |
| w/c ratio | High w/c → more water to evaporate sooner | Low w/c → less free water; more is bound |
- The maximum rate of shrinkage occurs in the first 28 days of drying when free capillary water evaporates.
- IS 456:2000 design shrinkage strain = 0.0003 (total, not just 28-day).
- Shrinkage movement joints in large concrete structures are essential to accommodate this deformation.