Workability limits under vibration: when compacting concrete using a mechanical vibrator, the slump value should not exceed which limit for quality placement?

Introduction / Context:
Vibration greatly assists compaction; therefore, very high slump is unnecessary and may cause defects. Selecting an appropriate slump prevents segregation and bleeding.

Given Data / Assumptions:

• Normal reinforced concrete with internal or surface vibrators.
• Self-compacting concrete is excluded.

Concept / Approach:
Guides recommend a slump range of about 25–75 mm when vibration is applied. The upper bound 75 mm (7.5 cm) limits the risk of loss of cohesion under vibratory action.

Step-by-Step Solution:
Acceptable slump band under vibration ≈ 25–75 mm.Therefore maximum recommended slump ≈ 7.5 cm.

Verification / Alternative check:
IS 456 and site best practices align with the 25–75 mm recommendation for vibrated concrete.

Why Other Options Are Wrong:

• 10–15 cm exceed safe limits and risk segregation; 2.5–5 cm are within range but are not the upper limit.

Common Pitfalls:

• Adding water to raise slump when vibrators are available, reducing strength and durability.

Final Answer:
7.5 cm