Which is the most important factor that affects the shrinkage of cement concrete?
Correct Answer: D. Amount of water added during mixing
📚 Detailed Explanation: Mixing Water Content = Primary Cause of Shrinkage
Why D (Amount of water) is correct: Shrinkage in concrete is primarily a phenomenon of the cement paste shrinking as water evaporates. The aggregate particles are essentially inert to drying shrinkage and in fact restrain paste shrinkage. Therefore, the amount of water in the paste is the dominant factor. More water → more evaporation → more shrinkage.
How Water Causes Shrinkage
| Water Type | Fate | Effect on Shrinkage |
|---|---|---|
| Chemically combined water (hydration) | Consumed by C3S, C2S reactions; becomes part of C-S-H gel | Causes autogenous shrinkage (small) |
| Adsorbed water (gel water) | Held in micro-pores; lost on low-humidity drying | Major contributor to drying shrinkage |
| Free (capillary) water | Fills capillary pores; evaporates on drying | Largest contributor to drying shrinkage |
Factors Affecting Shrinkage — Ranked by Importance
| Rank | Factor | Effect |
|---|---|---|
| 1 | Water content (w/c ratio) | Most important: higher w/c → more evaporable water → more shrinkage |
| 2 | Cement content | More cement = more paste = more shrinkage (paste shrinks; aggregate restrains) |
| 3 | Aggregate type and content | Higher aggregate content restrains shrinkage; stiff aggregates (granite) better than soft |
| 4 | Curing conditions | Longer/better curing reduces free water evaporation rate |
| 5 | Ambient humidity | Low humidity increases evaporation rate and shrinkage |
- Most important shrinkage factor: amount of water added during mixing (free evaporable water).
- IS 456:2000: total shrinkage strain taken as 0.0003 (3 × 10³ microstrain).
- Fine aggregate (sand) fills coarse aggregate voids, making concrete denser and reducing shrinkage somewhat.