Concrete Placement – maximum drop height to avoid segregation To minimize segregation during placing, what is the recommended maximum free fall (vertical drop) height for concrete in normal practice?

Difficulty: Easy

Correct Answer: 150 cm

Explanation:


Introduction / Context:
Segregation occurs when coarse aggregate separates from mortar during handling or placing. Excessive free fall increases particle velocity and separation, leading to honeycombing, weak zones, and variable cover. Limiting drop height is a simple and effective control measure on site.


Given Data / Assumptions:

  • Conventional, non-flowing concrete without special anti-segregation admixtures.
  • Placement into formwork for typical beams, slabs, and columns.
  • Chutes, tremie pipes, or trunking used when higher drops are unavoidable.


Concept / Approach:

Many practical guides recommend limiting free fall to about 1.5 m for ordinary concretes. For taller forms, use inclined chutes or drop chutes to reduce velocity and maintain a vertical drop within the recommended height, thereby preserving uniformity and finish quality.


Step-by-Step Solution:

Identify typical limit → approximately 1.5 m (150 cm).Map to given options → select 150 cm.Note that for deep columns or tall walls, use tremie or trunk to keep fall within limit.


Verification / Alternative check:

Quality checklists for sites emphasize “free fall ≤ 1.5 m” unless special measures are adopted; this aligns with standard good practice.


Why Other Options Are Wrong:

100–125 cm is overly restrictive for most ordinary placements; 200–250 cm exceed common recommendations and can promote segregation and paste splashing.


Common Pitfalls:

Pouring from great heights into congested reinforcement; not using trunking; vibrating excessively to “fix” segregation rather than preventing it.


Final Answer:

150 cm

More Questions from Concrete Technology

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion