Workability requirement — select the case demanding higher workability In which of the following structural situations is higher workability of concrete especially necessary to ensure full compaction and proper cover?

Introduction / Context:
Workability must be matched to the placing conditions: congestion of reinforcement, section thickness, and vibration capability. Insufficient workability in congested sections leads to voids, honeycombing, and inadequate cover, compromising durability and strength.

Given Data / Assumptions:

• Normal vibration equipment available.
• Choice among varying section thickness and reinforcement densities.
• Target is adequate flow around bars and into tight corners.

Concept / Approach:
Thin, heavily reinforced members have small clear cover and limited bar spacing; the concrete must flow readily to fill gaps and encapsulate reinforcement. Therefore, higher workability (achieved by appropriate water–cement ratio with admixtures, optimized grading, and air entrainment where appropriate) is required. Thick sections or lightly reinforced members are less demanding, as there is more space for consolidation and bar spacing is larger.

Step-by-Step Solution:

Verification / Alternative check:
Placement guidelines assign higher target slumps or use plasticizing admixtures for congested reinforcement details such as beams with multiple layers of bars or heavily reinforced shear walls.

Why Other Options Are Wrong:

• (a) Plain concrete is easier to place; lower workability suffices.
• (b) Thick and lightly reinforced: ample space for consolidation.
• (d) Thick and heavily reinforced can need higher workability, but not as critical as thin sections with tight spacing.

Common Pitfalls:
Attempting to improve workability only by adding water; instead, use admixtures and grading optimization to maintain low w/c.

Final Answer:
Member that is thin and heavily reinforced