Reinforced concrete slab detailing (IS practice): for a slab thickness of 75 mm, what is the maximum bar diameter permitted for reinforcement in the slab?

Introduction / Context:
Slab reinforcement bar diameters are limited to ensure adequate concrete cover, crack control, and proper distribution across thin sections. Indian practice (e.g., IS 456) limits the diameter of bars in slabs relative to slab thickness.

Given Data / Assumptions:

• Slab thickness t = 75 mm.
• Rule of thumb/IS guidance: maximum bar diameter in slabs should not exceed t / 8 for main bars.
• Adequate cover and spacing are achievable.

Concept / Approach:
Compute the limit using the slab-thickness criterion. The chosen bar should not exceed this limit, ensuring proper anchorage, crack distribution, and workability during placement and compaction.

Step-by-Step Solution:
Compute t / 8 = 75 / 8 = 9.375 mm.Select a standard bar ≤ 9.375 mm → 8 mm qualifies; 10 mm exceeds the cap.Therefore, the maximum permissible diameter is 8 mm.

Verification / Alternative check:
Checking cover requirement and spacing confirms 8 mm bars are routinely used in thin slabs and toppings.

Why Other Options Are Wrong:

• 10 mm and 12 mm exceed the t/8 limit for 75 mm slabs.
• 6 mm is permitted but not the maximum; the question asks for the maximum allowable.

Common Pitfalls:
Using column/beam bar limits; forgetting that slabs have stricter limits due to thin sections.

Final Answer:
8 mm diameter