Reinforced concrete ribbed slabs (narrow ribs): What is the maximum permissible nominal size of coarse aggregate to be used when casting the ribs of a slab, considering the narrow rib width and the need for proper compaction and cover?

Introduction / Context:
In reinforced concrete ribbed (joist) slabs, the ribs are relatively narrow. Selecting an appropriate nominal maximum size of coarse aggregate is essential to ensure good workability, proper compaction, and adequate cover around reinforcement without honeycombing.

Given Data / Assumptions:

• Narrow rib width typical of ribbed slabs.
• Standard cover and bar spacing must be maintained.
• Concrete must flow and compact properly within the rib.

Concept / Approach:
Code-based and good-practice limits state that the nominal maximum aggregate size should be less than the minimum clear distance between bars, less than three-fourths of the clear cover, and usually not more than about one-third of the section depth in very thin members. For narrow ribs, a 10 mm aggregate is widely adopted to avoid blocking and to improve consolidation quality.

Step-by-Step Solution:
Check rib width and reinforcement layout to identify clear spacing.Apply the rule: max size ≤ min(clear spacing, 3/4 cover, practical thin-section limit).For narrow ribs, a 10 mm nominal size satisfies these constraints and improves placement quality.

Verification / Alternative check:
Site trials and slump tests often show that larger aggregates (15–20 mm) increase the risk of voids and segregation in tight ribs. Reducing to 10 mm improves passing ability around bars and corners and reduces vibration effort.

Why Other Options Are Wrong:
5 mm and 7.5 mm: These are closer to fine aggregate/very small coarse aggregate; unnecessarily small for economy and strength in most ribs.15 mm and 20 mm: Too large for narrow ribs; higher risk of honeycombing and cover violations.

Common Pitfalls:

• Ignoring the bar spacing/cover check and choosing aggregate solely by availability.
• Using high slump to compensate for large aggregate instead of selecting the right size.

Final Answer:
10 mm