The risk of segregation in concrete is higher for which of the following conditions?
Correct Answer: D. All options are correct
📚 Detailed Explanation: Three Factors That Increase Segregation Risk
Why D (All options) is correct: Segregation occurs when inter-particle cohesion in the concrete mix is insufficient. All three listed factors reduce cohesion or increase the tendency for differential settlement of coarse aggregate.
Analysis of Each Factor
| Factor | Mechanism | Result |
|---|---|---|
| A. Wetter mix (high w/c) | Thinner, less viscous paste has lower cohesion and yield stress → cannot hold heavy aggregates in suspension | Aggregates settle; mortar separates |
| B. Larger maximum aggregate size | Settling velocity increases with particle size (Stokes' Law: V ∝ d²). A 40 mm aggregate settles 4× faster than a 20 mm aggregate | Faster differential settlement; higher segregation risk |
| C. Coarser cement grinding | Coarser particles have lower specific surface area → less C-S-H gel per unit time → less cohesive paste in early stages | Paste cannot hold aggregate in suspension |
Void Content vs. Strength Loss (from Segregation)
| Voids in Concrete (%) | Approximate Strength Reduction |
|---|---|
| 0 (fully homogeneous) | 0% (full design strength) |
| 2% | ~10% |
| 5% | >30% |
| 10% | ≈50% |
Additional Causes of Segregation
- Dropping concrete from height >1.5 m (IS 456 limit).
- Over-vibration: vibrating plastic/wet mixes exceeding 50 mm slump.
- Poor aggregate gradation (gap-graded or single-sized).
- Adding more water than required by the mix design.
- Large density difference between aggregate particles and mortar.