The risk of segregation in concrete is higher for:
Correct Answer: D. All options are correct
📚 Detailed Explanation: Factors That Increase Segregation Risk
Why D (All options) is correct: Segregation occurs when the cohesion of the concrete mix is insufficient to hold all ingredients together. All three factors independently reduce cohesion or increase the tendency of coarse aggregate to separate, so all three increase segregation risk.
Analysis of Each Factor
| Factor | How It Increases Segregation Risk |
|---|---|
| A. Wetter mix (high w/c) | More water = thinner, less viscous paste = weaker matrix to hold aggregates in suspension. High slump concrete (low cohesion) cannot resist differential settlement of heavy aggregates |
| B. Larger max. aggregate size | Larger aggregates are heavier per particle; settling velocity increases with particle size (Stokes' Law). A mix with 40 mm aggregate segregates more easily than a 20 mm mix under similar conditions |
| C. Coarser grinding of cement | Coarsely ground cement particles have lower specific surface area → less hydration product per unit time → less cohesive paste early on → reduced ability to hold aggregates in suspension |
Additional Causes of Segregation
| Cause | Mechanism |
|---|---|
| Dropping concrete >1.5 m | Impact separates heavy aggregate from mortar |
| Over-vibration | Prolonged vibration causes differential settlement |
| Pumping too fast | Aggregate impacts pipe bends and separates from paste |
| Poor aggregate gradation | Gap-graded mixes (missing intermediate sizes) more prone to segregation |
How to Prevent Segregation
- Use minimum water content consistent with required workability (low w/c).
- Use well-graded aggregates with appropriate maximum size for the member.
- Use properly ground cement (correct fineness per IS specification).
- Do not drop concrete more than 1.5 m; transport carefully; do not over-vibrate.
- Use admixtures (viscosity-modifying agents) for high-slump/SCC mixes to maintain cohesion.