In reinforced concrete slab detailing, if D denotes the overall slab thickness, what is the maximum allowable diameter of reinforcing bars to be used within the slab section?

Introduction / Context:
Bar diameter limits in slabs help ensure proper concrete flow, compaction, cover, and crack control. Oversized bars in thin slabs can cause congestion, inadequate cover, or honeycombing, which compromise durability and strength. Hence, standard detailing rules cap the maximum bar size relative to the slab thickness D.

Given Data / Assumptions:

• D is the overall slab thickness (measured in mm).
• We seek the limiting diameter for reinforcement used in the slab.
• Assume conventional RC slab practice with normal cover and bar spacing requirements.

Concept / Approach:

A widely adopted detailing guideline for slabs is: maximum bar diameter ≤ D / 8. This ensures a balanced relationship between bar size and slab thickness, allowing proper concrete placement and adequate distribution of steel for crack control.

Step-by-Step Solution:

Verification / Alternative check:

Design offices often also limit spacing by min(3*d, 300 mm) where d is effective depth; the D / 8 bar-size limit works compatibly with such spacing caps for typical slab thicknesses.

Why Other Options Are Wrong:

• D/6: Too large; risks poor compaction and cover violation.
• D/10 or D/12: Conservative but not the standard maximum limit widely cited for slabs.
• 0.25 * D: Excessively large; not a recognized detailing limit.

Common Pitfalls:

• Using one or two very large bars instead of multiple smaller bars, which worsens crack control.
• Ignoring minimum clear spacing to allow aggregate passage.

Final Answer:

D / 8