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: Shrinkage is a phenomenon of the cement paste, not the aggregate. Since aggregate does not shrink, increasing cement content (and hence paste volume) directly increases total concrete shrinkage. This relationship is direct and linear within normal mix design ranges.
Why Cement Content Governs Shrinkage
| Constituent | Shrinkage Behaviour | Role in Mix |
|---|---|---|
| Cement paste | Shrinks on drying (loses 0.08–0.10% of dimension) | Phase that shrinks; more cement = more shrinkable paste |
| Coarse aggregate | Does NOT shrink; dimensionally stable | Restrains paste shrinkage; rigid filler |
| Fine aggregate (sand) | Does NOT shrink; dimensionally stable | Fills voids; also restrains paste shrinkage |
| Water (excess) | Evaporates; leaves capillary voids | Higher w/c → more evaporable water → more shrinkage |
Methods to Minimise Shrinkage
| Method | Why It Reduces Shrinkage |
|---|---|
| Lower cement content (within durability limits) | Less paste to shrink |
| Saturated aggregates | Pre-wetted aggregates do not absorb mix water; reduces effective w/c; less evaporation |
| Lower w/c ratio | Less free water to evaporate |
| Working at low temperatures | Slower evaporation rate; more complete hydration; less free water |
| Shrinkage-reducing admixtures (SRAs) | Reduce surface tension of pore water; decrease capillary stress driving shrinkage |
IS 456:2000 Design Shrinkage Strain = 0.0003 (for RH ≈ 50%)
Aggregate restraint factor:
Paste shrinkage alone ≈ 0.0008–0.001
Concrete shrinkage ≈ 0.0003 (IS 456 design value)
Restraint factor = 0.0003 / 0.0009 ≈ 0.33
Aggregate restrains ≈67% of what the paste would shrink alone.
- Shrinkage is directly proportional to cement content (more paste = more shrinkage).
- Aggregate (coarse and fine) is inert to shrinkage and restrains paste contraction.
- Reduce shrinkage with: lower cement, saturated aggregates, low w/c, cool working conditions.