Concrete workability for vertical members: The generally recommended slump for reinforced concrete columns is

Introduction / Context:
Slump is a simple field measure of concrete workability and consistency. Vertical elements like columns require enough workability for rebar congestion and proper compaction without segregation. Selecting an appropriate slump range is crucial for quality and speed of placement.

Given Data / Assumptions:

• Reinforced concrete (R.C.) columns with typical reinforcement congestion.
• Conventional placement without excessive pumping distances or high-range water reducers, unless noted.
• Standard ambient site conditions.

Concept / Approach:
Columns need a moderate to moderately high slump to flow around vertical bars and ties, especially with narrow forms. Too low a slump risks honeycombing; too high risks segregation and bleed. A commonly recommended range for columns is around 75–125 mm, balancing placability and cohesion. Specific codes or mix designs (using admixtures) may permit higher slumps while maintaining stability, but the traditional target range remains 75–125 mm for general work without special modifiers.

Step-by-Step Solution:
Review member type: vertical, congested reinforcement → needs higher workability than slabs.Avoid extremes: very low slump impedes compaction; very high slump may segregate.Adopt standard practice window: 75–125 mm is widely accepted for columns.Select the option matching this range.

Verification / Alternative check:
Site specifications and handbooks list typical slump ranges: slabs/beams ~25–75 mm; columns ~75–125 mm; tremie underwater concreting much higher with anti-washout admixtures.

Why Other Options Are Wrong:
25–50 or 25–75 mm: Often too stiff for congested column cages.50–100 mm: Borderline for many column details; may be insufficient.125–175 mm: Risk of segregation unless designed with special admixtures; not the general recommendation.

Common Pitfalls:
Chasing high slump by excess water instead of admixtures; ignoring that workability also depends on aggregate grading, shape, and admixture type.

Final Answer:
75 to 125 mm.