Aggregate Nominal Size for RCC – Commonly Accepted Choice Considering cover, spacing of bars, and workability in typical reinforced concrete members, which nominal maximum size of coarse aggregate is generally acceptable for RCC?

Introduction / Context:
Choosing an appropriate nominal maximum size of aggregate affects workability, segregation resistance, and the ability to place concrete around reinforcement. For general RCC work, a balanced choice is preferred.

Given Data / Assumptions:

• Typical RCC elements such as slabs, beams, and columns.
• Normal reinforcement congestion and standard covers.
• Conventional vibration during placing.

Concept / Approach:

The aggregate size must be smaller than clear cover and the minimum clear spacing between bars to avoid blocking. A too-large size impedes flow around bars, while too small a size may increase water demand for a given workability.

Step-by-Step Solution:

Verification / Alternative check:

Many standard specifications and SOR schedules list 20 mm nominal size as default for RCC unless special conditions call for 10 mm or 12.5 mm for congested zones, or 25 mm for massive pours.

Why Other Options Are Wrong:

• 10 mm, 15 mm: Useful in thin sections or heavy congestion but not the general default.
• 25 mm: May be acceptable in lightly reinforced, thicker members but not universally adopted.
• 40 mm: Typically for mass concrete, not for regular RCC with close bar spacing.

Common Pitfalls:

Forgetting to coordinate aggregate size with pump line, cover, and spacing near laps or anchorage regions where congestion increases locally.

Final Answer:

20 mm