Drying shrinkage of concrete is affected by which of the following?
Correct Answer: D. All options are correct
📚 Detailed Explanation: Three Factors Affecting Drying Shrinkage
Why D (All options) is correct: Drying shrinkage occurs as free water evaporates from the capillary pores of hardened concrete. All three listed factors directly influence either the amount of water available to evaporate or the rate at which it evaporates.
Mechanism of Each Factor
| Factor | Mechanism | Effect on Shrinkage |
|---|---|---|
| A. Relative humidity | At low RH, the vapour pressure gradient is high, driving rapid evaporation from capillary pores; at high RH, evaporation is suppressed | Lower RH → more/faster drying shrinkage; RH = 100% → zero drying shrinkage |
| B. Time | Drying is a time-dependent diffusion process; water evaporates progressively from the surface inward over months | Shrinkage increases with time, asymptoting to the ultimate value over years |
| C. Water-cement ratio | Higher w/c means more free (capillary) water in the mix; more evaporable water → more shrinkage on drying | Higher w/c → more porous paste → more drying shrinkage |
Drying Shrinkage Over Time (Typical Pattern)
| Time After Drying Begins | Approximate % of Ultimate Shrinkage Reached |
|---|---|
| 28 days | ≈ 14–34% (depends on w/c, humidity, member size) |
| 3 months | ≈ 40–70% |
| 1 year | ≈ 75–90% |
| Several years | Approaches 100% (ultimate) |
IS 456:2000 Design Assumption:
Total design shrinkage strain = 0.0003 (in absence of specific test data)
This is a conservative but practical value for structural design.
Total design shrinkage strain = 0.0003 (in absence of specific test data)
This is a conservative but practical value for structural design.
- Drying shrinkage is affected by relative humidity, time, and w/c ratio (all three).
- At RH = 100%, no drying occurs and drying shrinkage is zero.
- Autogenous shrinkage (due to chemical contraction during hydration) is separate from drying shrinkage.