While compacting concrete by a mechanical vibrator, the slump should not exceed ________ cm:
Correct Answer: B. 5.0 cm
📚 Detailed Explanation: Maximum Slump for Mechanical Vibration — 5.0 cm
Why B (5.0 cm) is correct: Mechanical vibrators are designed for relatively stiff to semi-plastic concrete mixes. IS 2505 (Internal vibrators) specifies that vibrators should not be used on mixes with slump exceeding 50 mm (5.0 cm). Above this slump, the concrete is too fluid and vibration energy causes differential settlement (segregation).
Slump Range and Vibrator Applicability
| Slump Range (mm) | Consistency | Vibrator Suitable? | Notes |
|---|---|---|---|
| 0–25 | Very stiff / earth moist | Yes — ideal | Hand compaction impractical; vibrator most effective |
| 25–50 | Stiff to semi-plastic | Yes — ideal range | Best use of internal vibrator; no segregation risk |
| 50–100 | Plastic (exceeds 5 cm limit) | Caution — segregation risk increases | Vibration should not be applied beyond 50 mm slump |
| >100 (flowing / SCC) | Highly fluid | No | Self-compacting concrete (SCC) needs no vibration |
What Happens with Excess Slump + Vibration
- Heavy coarse aggregates settle to the bottom of the pour.
- Water and cement paste (bleed water / laitance) rise to the top surface.
- Resulting concrete: strong dense bottom, weak porous top — non-homogeneous section.
- Rule: maximum slump for vibration = 5.0 cm (50 mm) per IS 2505.