Selecting workability for shallow sections with vibration: For concreting shallow members that will be compacted using mechanical vibration, which workability level is most appropriate?

Introduction / Context:
Workability selection depends on member geometry and compaction method. When efficient vibration is available, mixes need not be very flowable; lower slumps reduce segregation and bleeding while still enabling full compaction.

Given Data / Assumptions:

• Shallow sections (short placement depth, relatively simple reinforcement).
• Mechanical vibration is used for compaction.
• Objective is adequate compaction with minimal segregation.

Concept / Approach:
With vibration, a low to moderate slump suffices. Very high workability is unnecessary and can trigger segregation, especially in shallow members where confinement is limited.

Step-by-Step Solution:
Step 1: Match workability to compaction method: vibration allows stiffer mixes.Step 2: Prefer lower slump ranges (often termed “low”) to control bleeding/segregation.Step 3: Select “Low” as most appropriate for shallow sections with vibration.

Verification / Alternative check:
Handbooks list slump ranges: for vibrated RC, low slump is commonly recommended for slabs and shallow beams, provided placement is not congested.

Why Other Options Are Wrong:
Very low: Risk of honeycombing if vibration/placement is not excellent.
Medium: Acceptable in some cases, but “low” better targets vibration efficiency and segregation control.
High/Very high: Unnecessary with vibration; increase segregation risk and water demand.

Common Pitfalls:

• Choosing high slump “for ease” and then suffering edge bleeding and coarse aggregate settlement.
• Ignoring reinforcement congestion; if very congested, reassess to medium slump or use plasticizers.

Final Answer:
Low (stiff but placeable with vibration)