Compaction of concrete by a mechanical vibrator is unsuitable if the concrete mix is:
Correct Answer: D. Plastic
📚 Detailed Explanation: Vibrators Unsuitable for Plastic (Wet) Concrete
Why D (Plastic) is correct: A mechanical vibrator works best on stiff, low-slump mixes where hand compaction is ineffective. When used on plastic (highly workable, high-slump) concrete, the vibration is too intense for the already-fluid mix — it causes the heavy aggregate to settle to the bottom while lighter cement paste and water migrate upward, resulting in severe segregation.
Concrete Consistency vs. Vibrator Suitability
| Mix Type | Slump (mm) | Vibrator Suitable? | Reason |
|---|---|---|---|
| Very dry (no slump) | 0 | Surface vibrator / vibrating roller | Too stiff for internal vibrator alone; needs high energy |
| Earth moist | 0–25 | Yes (internal vibrator) | Vibration fluidises the dry, stiff mix effectively |
| Semi-plastic | 25–50 | Yes (ideal range) | Vibration most effective; no segregation risk |
| Plastic | >50 mm (high slump) | Unsuitable — causes segregation | Mix too fluid; vibration causes aggregate to sink |
What IS 2505 Specifies
IS 2505 (Internal Vibrators) recommends mechanical vibration for mixes with a maximum slump of 50 mm (5 cm). Above this, the mix is considered too workable for safe vibration without segregation risk.
Consequences of Over-vibration / Vibrating Plastic Mix
- Heavy aggregates settle to the bottom (aggregate sinking).
- Cement paste and bleed water rise to the top (laitance formation).
- Non-homogeneous concrete: weak top, dense bottom — opposite of what is desired.
- Rule: vibrators are for stiff mixes; plastic (wet) mixes do not need vibration — they can be hand-worked or rodded.