Which of the following methods reduces the drying shrinkage of concrete?
Correct Answer: D. All options are correct
📚 Detailed Explanation: Three Methods to Reduce Drying Shrinkage
Why D (All options) is correct: Drying shrinkage is primarily a cement paste phenomenon. Strategies to reduce it focus on reducing the amount and porosity of paste, reducing free water, and increasing aggregate restraint.
Methods to Reduce Shrinkage
| Method | Mechanism | Effect on Shrinkage |
|---|---|---|
| A. Low w/c ratio | Less free water in the mix → less evaporable water → fewer capillary pores formed | Directly reduces drying shrinkage; also improves strength and durability |
| B. Less cement content | Paste = cement + water; less cement = less paste volume that shrinks. Aggregate does not shrink and restrains paste shrinkage | Reduces total shrinkage, but must maintain minimum cement content for durability (IS 456) |
| C. Proper mix design | Correct aggregate gradation minimises void space requiring paste; well-graded aggregate reduces paste demand for a given workability; stiffer aggregate (higher modulus) provides more restraint | Optimised aggregate/paste ratio minimises shrinkage |
Additional Strategies to Reduce Shrinkage
| Strategy | Why It Helps |
|---|---|
| Extended moist curing | Delays drying onset; allows more hydration products to fill pores → lower porosity → less evaporation |
| Shrinkage-reducing admixtures (SRAs) | Reduce capillary tension that drives shrinkage by lowering surface tension of pore water |
| Expansive cements | Chemically expand during hydration to offset later shrinkage |
| Minimize member thickness (slabs) | Thinner members dry faster but have less moisture gradient; use expansion joints for large areas |
- Drying shrinkage reduced by: low w/c ratio, minimal cement content, and proper mix proportioning (all three together).
- Aggregate restrains paste shrinkage — maximising aggregate volume (within design limits) reduces shrinkage.
- Harder, stiffer aggregates (granite, quartzite) provide more restraint than softer aggregates.