Drying shrinkage of concrete is directly proportional to:
Correct Answer: A. Cement content
📚 Detailed Explanation: Shrinkage Directly Proportional to Cement Content
Why A (Cement content) is correct: Drying shrinkage is a property of the cement paste, not the aggregate. Since aggregate particles do not shrink, the total shrinkage of concrete increases as the cement content (and hence paste volume) increases. More cement = more paste = more material that shrinks on drying. Aggregate acts as a rigid restraint that reduces total shrinkage relative to plain paste.
Why Cement Content Drives Shrinkage
| Factor | Effect on Shrinkage | Reason |
|---|---|---|
| Cement content (higher) | More shrinkage | More paste = more shrinkable material; paste shrinks, aggregate does not |
| Aggregate content (higher) | Less shrinkage | Aggregate restrains paste shrinkage; stiffer aggregates provide more restraint |
| w/c ratio (higher) | More shrinkage | More free water to evaporate; more capillary pores |
| Aggregate-cement ratio (higher) | Less shrinkage | Higher aggregate/paste ratio means less shrinkable paste per unit volume |
Understanding the Proportionality
| Scenario | Cement Content (kg/m³) | Relative Shrinkage |
|---|---|---|
| Lean mix (1:4:8) | ≈ 190 | Low (less paste volume; more aggregate restraint) |
| Standard M25 mix | ≈ 320 | Moderate |
| Rich structural mix | ≈ 400 | High (more paste; more shrinkage) |
| High-strength M60+ | ≈ 450–500 | Very high; shrinkage-reducing admixtures recommended |
Aggregate Restraint Factor (approximate):
Shrinkage of paste alone = εp (≈ 0.0008–0.001)
Shrinkage of concrete = εc (≈ 0.0003 per IS 456)
Ratio = εc / εp ≈ 0.3–0.4
Shrinkage of paste alone = εp (≈ 0.0008–0.001)
Shrinkage of concrete = εc (≈ 0.0003 per IS 456)
Ratio = εc / εp ≈ 0.3–0.4
Restraint provided by aggregate reduces paste shrinkage by 60–70%.
Higher aggregate content → higher restraint → less concrete shrinkage.
- Shrinkage is directly proportional to cement content (more paste = more shrinkage).
- Aggregate is inert to shrinkage and actually restrains paste shrinkage.
- To minimise shrinkage: reduce cement content (within durability limits), reduce w/c, increase aggregate content, use stiff aggregates.