Coarse aggregates are obtained from the residue left on sieves of size
Correct Answer: A. 10 mm and 4.75 mm
📚 Detailed Explanation: Coarse Aggregate Sieve Sizes
The definition of coarse aggregate centres on the 4.75 mm IS sieve as the boundary between coarse and fine aggregate. Within the coarse aggregate range, the 10 mm sieve is a significant demarcation used for the two most common single-size fractions.
Why A (10 mm and 4.75 mm) is correct: The 4.75 mm sieve is the defining boundary: material retained on it is coarse aggregate. The 10 mm sieve further divides the coarse aggregate into 4.75–10 mm (fine coarse aggregate / pea gravel / crushed stone chips) and >10 mm fractions. These two sieves together encompass the standard lower coarse aggregate size range referenced in IS 383. Option D (10 mm and 20 mm) are both within the coarse aggregate range but do not include the critical 4.75 mm boundary that defines “coarse aggregate” by IS 383.
Standard Coarse Aggregate Fractions (IS 383)
| Fraction | Sieve Range | Common Use |
|---|---|---|
| 10 mm single-size | Retained on 4.75 mm, passing 10 mm | Mix with 20 mm for combined grading |
| 20 mm single-size | Retained on 10 mm, passing 20 mm | Most common for structural concrete |
| 40 mm single-size | Retained on 20 mm, passing 40 mm | Mass concrete |
Key Concepts for Students
- 4.75 mm = the FA/CA boundary; material retained on 4.75 mm is always coarse aggregate, regardless of size.
- Most structural concrete uses 20 mm nominal maximum size aggregate, made by combining 10 mm and 20 mm single-size fractions.
- The combined CA must meet IS 383 grading envelope limits for the specified nominal maximum size.
