Excess vibration during compacting of concrete can lead to:
Correct Answer: B. Segregation (Note: Commission revised answer key considered this question incorrect)
📚 Detailed Explanation: Over-Vibration Causes Segregation
⚠ Commission Note: The revised answer key issued by the examining commission considered this question incorrect/invalidated. Both options A (Bleeding) and B (Segregation) are consequences of over-vibration, creating ambiguity. The standard textbook primary answer is B — Segregation.
Primary effect of over-vibration: Segregation (B)
Secondary effect: Bleeding (A) — bleed water rises with migrating cement paste.
Secondary effect: Bleeding (A) — bleed water rises with migrating cement paste.
Stages of Vibration — Correct vs. Excessive
| Vibration Stage | Effect on Concrete | Result |
|---|---|---|
| Correct vibration | Air expelled; particles rearrange; paste fills voids | Dense, homogeneous, maximum strength |
| Under-vibration | Air remains trapped; honeycombing visible | Porous, weak, non-durable |
| Over-vibration | Gravity differential: dense aggregate sinks; lighter paste rises | Segregation + bleeding; weak top surface |
Over-Vibration Mechanism
| What Happens | Why |
|---|---|
| Coarse aggregate settles to bottom | Density ≈2650 kg/m³; settles under prolonged liquefaction |
| Cement paste rises to top | Density ≈1300 kg/m³; buoyed upward by denser sinking aggregate |
| Water (bleed water) rises to surface | Lowest density; migrates with cement paste to surface |
| Cement paste + water on surface dry | Forms laitance; cracks on drying; reduces surface strength |
How to Recognise Adequate Compaction
- Stop vibration when large air bubbles cease appearing at the surface.
- Cement paste (cream) just appears at the surface — this is the signal to stop.
- Never vibrate the same location continuously for more than 5–15 seconds.
- Primary answer = Segregation; bleeding is a secondary co-occurring effect.