For the construction of cement concrete floor, the maximum permissible size of aggregates is
Correct Answer: D. 10 mm
📚 Detailed Explanation: Maximum Aggregate Size for Cement Concrete Floors
Cement concrete floors are relatively thin sections (typically 75–150 mm thick) that require a smooth, dense finish. Using large aggregate in a thin floor slab would create bridging effects, voids, and difficulty in achieving a smooth surface. The maximum aggregate size is therefore restricted more tightly than in beams or columns.
Why D (10 mm) is correct: For floor construction, the maximum nominal aggregate size is limited to 10 mm. This ensures that: (a) aggregate can pass freely through the thin cross-section without bridging; (b) a smooth top surface can be achieved by finishing; (c) the concrete can be properly compacted with a screed or vibrating beam. The 1/4 rule (max size = 1/4 of section depth) for a 75 mm slab gives ≤18.75 mm, but 10 mm is the practical code-recommended limit for floors.
Maximum Aggregate Size for Common Applications
| Application | Maximum Aggregate Size |
|---|---|
| Mass concrete (dams) | Up to 150 mm |
| General structural concrete | 20 mm or 40 mm |
| RCC slabs and beams | 20 mm |
| Cement concrete floors | 10 mm |
| Plaster / mortar | Fine aggregate only (<4.75 mm) |
Key Concepts for Students
- 10 mm is the standard answer for floors and thin sections in Indian examinations — memorise this figure.
- Using aggregate larger than 10 mm in floors causes surface irregularities and voids beneath large stones, weakening the wearing surface.
- The IS 456 general rule (max size = 1/4 of minimum section dimension) supports 10 mm for 40 mm thick sections and is the limiting case for very thin slabs.