To prevent segregation, the maximum permissible height for placing concrete is:
Correct Answer: C. 150 cm
📚 Detailed Explanation: Maximum Placement Height 150 cm (IS 456:2000)
Why C (150 cm) is correct: IS 456:2000 Clause 13.2 specifies that concrete shall not be dropped from a height exceeding 1.5 m (150 cm) to prevent segregation. Beyond this height, the impact energy separates heavier coarse aggregates from the mortar, destroying the homogeneity of the mix.
Why Height Causes Segregation
| Drop Height | What Happens | Outcome |
|---|---|---|
| ≤150 cm | Concrete falls with moderate energy; stays together | No segregation; concrete remains homogeneous |
| >150 cm | High kinetic energy on impact; aggregate bounces and separates | Coarse aggregate separated from mortar; segregation |
Methods to Place Concrete Without Exceeding 150 cm
| Situation | Solution |
|---|---|
| Deep columns or walls | Use a tremie pipe, elephant trunk, or chute to guide concrete to the bottom |
| Deep foundations | Tremie pipe method; concrete placed at the bottom and displaces water/soil upward |
| High-rise slabs | Pump concrete directly to the placement point at each level |
| General placing | Use elephant trunk hose; lower the drop height at all times |
- Maximum placement height: 150 cm (1.5 m) per IS 456:2000 to prevent segregation.
- Use tremie pipes or elephant trunks when placement depth exceeds 1.5 m.
- Concrete placed from too great a height also loses entrained air, affecting durability.