Excess vibration during compacting of concrete can lead to:
Correct Answer: B. Segregation (Note: Commission invalidated this question in the revised answer key)
📚 Detailed Explanation: Over-Vibration Causes Segregation (and Bleeding)
⚠ Note: This question was invalidated by the examining commission in the revised answer key. The answer key originally intended option B (Segregation) as the primary answer; however, over-vibration also causes bleeding (option A), creating ambiguity.
Primary consequence: Segregation (B)
Secondary consequence: Bleeding (A)
Secondary consequence: Bleeding (A)
What Happens During Over-Vibration
| Stage | Effect | Consequence |
|---|---|---|
| 1. Vibration begins | Concrete temporarily liquefies; particles become mobile | Air expelled — desired effect |
| 2. Continued vibration | Gravity differential: heavy aggregates (density ~2650 kg/m³) settle faster than paste (density ~1200–1400 kg/m³) | Aggregates sink → segregation |
| 3. Excess vibration | Free water driven upward with migrating paste | Bleeding — water and cement paste rise to surface |
| 4. Final result | Dense, aggregate-rich bottom; weak, paste-rich top | Non-homogeneous section with weak top layer |
Correct vs. Excessive Vibration
| Vibration Type | Effect |
|---|---|
| Correct vibration | Expels air; uniform density; concrete surface shows cream/paste appearance |
| Under-vibration | Air remains; honeycombing; low density; low strength |
| Over-vibration | Segregation + bleeding; non-homogeneous; weak top surface (laitance) |
- Over-vibration → segregation (primary) and bleeding (secondary).
- Stop vibration when bubbling ceases and paste appears at surface — do not continue beyond this point.
- Commission invalidated due to both A and B being partially correct; textbook primary answer is B (Segregation).