For heavily reinforced concrete members, the nominal maximum size of coarse aggregates should be restricted based on bar spacing and cover. Which criterion should govern the maximum aggregate size?

Introduction / Context:
Aggregate size directly affects workability, segregation risk, and the ability of concrete to pass through congested reinforcement. In heavily reinforced sections, limiting the nominal maximum size helps ensure proper compaction and minimizes voids near bars and cover regions, thereby improving durability and bond.

Given Data / Assumptions:

• Member is heavily reinforced (close bar spacing and smaller covers).
• We compare two limits: bar clear spacing and concrete cover.
• Aggregate must pass between bars and avoid cover conflicts.

Concept / Approach:

To guarantee placement, maximum aggregate size should be strictly less than both the minimum clear spacing between bars and the minimum cover dimension. A practical rule sets it as 5 mm less than each of these dimensions, with the governing limit being the smaller of the two.

Step-by-Step Solution:

Verification / Alternative check:

Field experience confirms that obeying the tighter constraint avoids blocking and honeycombing in congested cages, especially with larger aggregates.

Why Other Options Are Wrong:

• (a) or (b) alone: Ignores the other geometric constraint, risking placement issues.
• Greater of (a) and (b): Non-conservative; could permit aggregates that cannot pass one of the restrictions.
• Equal to largest bar diameter: Unrelated to flow between bars or near cover.

Common Pitfalls:

• Ignoring tie/secondary reinforcement when judging clear spacing.
• Not accounting for construction tolerances; a 5 mm margin is intentional.

Final Answer:

smaller of (a) and (b)