Compaction suitability — when is vibration not effective for concrete? Concrete is generally unsuitable for compaction using internal or surface vibrators if its consistency is:

Introduction / Context:
Vibration compacts concrete by mobilizing the mix so particles can re-arrange and entrapped air can escape. If the mix is too stiff, vibrators cannot create sufficient particle mobility, leading to honeycombing and poor consolidation.

Given Data / Assumptions:

• Normal concrete (not roller-compacted or zero-slump special mixes).
• Internal needle or surface vibrators are available.
• Workability is assessed by slump or feel.

Concept / Approach:

Dry, very low-slump concrete lacks the paste fluidity required for vibratory energy to rearrange aggregates effectively. Earth-moist material behaves like damp soil and is also problematic, but “dry” clearly identifies the unsuitable condition. Semi-plastic to plastic mixes respond well to vibration and are standard for reinforced concrete.

Step-by-Step Solution:

Verification / Alternative check:

Guides recommend target slumps (e.g., 50–100 mm) for normal vibration, avoiding zero-slump mixes unless specialized methods are used.

Why Other Options Are Wrong:

• (b) is also difficult but the definitive unsuitable class is “dry”.
• (c) and (d) are routinely vibrated.

Common Pitfalls:

• Over-vibration causing segregation in highly workable mixes; adjust time and insertion spacing.

Final Answer:

Dry (very low slump).