The maximum size of aggregate in concrete depends upon :
Correct Answer: D. All option are correct
📚 Detailed Explanation: Factors Governing Maximum Aggregate Size
The maximum nominal size of coarse aggregate used in concrete is not arbitrary — it is limited by the physical geometry of the structural element being cast. Aggregate particles that are too large cannot pass through confined spaces, causing honeycombing (voids in hardened concrete) and poor compaction. IS 456:2000 Clause 5.3 specifies these geometric constraints.
Why D (All option are correct) is correct: Three geometric criteria govern the maximum aggregate size: (1) Section thickness — maximum size should not exceed 1/4 of the minimum cross-sectional dimension. (2) Spacing of reinforcement — aggregate must pass freely between bars; typically size ≤ (bar spacing − 5 mm). (3) Clear cover — aggregate must not be larger than the cover to reinforcement so it doesn’t get wedged at the surface and create a weak cover zone.
IS 456:2000 Guidelines for Maximum Aggregate Size
| Constraint | Rule (IS 456) |
|---|---|
| Section thickness | ≤ 1/4 of minimum member dimension |
| Reinforcement spacing | ≤ (clear spacing between bars − 5 mm) |
| Clear cover | ≤ clear cover to reinforcement |
| General max for beams/slabs | 20 mm (common practical limit) |
Key Concepts for Students
- Larger aggregate = less cement required (less surface area to coat), but it is limited by geometry and workability requirements.
- For mass concrete (dams), up to 150 mm aggregate is used; for concrete floors, it is limited to 10 mm.
- Always check all three geometric constraints and use the smallest limiting value as the maximum permissible size.