If a grading curve is horizontal between the portions of 20 mm I.S. Sieve and 4.75 mm I.S. sieve, the graded aggregates do not contain
Correct Answer: D. All option are correct
📚 Detailed Explanation: Gap Grading and Horizontal Grading Curves
A particle size distribution (grading) curve plots the cumulative percentage passing (y-axis) against sieve size (x-axis on log scale). The slope of the curve at any point indicates whether that size range is present. A horizontal segment means the cumulative percentage does not increase — no particles are present in that size range.
Why D (All option are correct) is the official answer: If the grading curve is perfectly horizontal between the 20 mm and 4.75 mm sieves, all particle sizes within that band — including 20 mm, 10 mm, and 4.75 mm — are absent. This is the definition of gap grading: a deliberate exclusion of intermediate sizes. However, note that many educators interpret “between” to mean only the intermediate sizes (10 mm etc.) are missing, not the boundary sizes themselves. The official answer key accepts D as correct.
Reading a Grading Curve
| Curve Feature | Meaning |
|---|---|
| Steep slope | Many particles of that size range present |
| Horizontal (flat) | No particles in that size range — gap grading |
| Smooth S-curve | Well-graded aggregate — ideal for concrete |
Key Concepts for Students
- Gap grading can produce workable concrete with high strength if the void content of the coarser fraction is exactly filled by the finer fraction — but it is sensitive to segregation.
- Well-graded (continuous) aggregates are preferred in general structural concrete to minimise void content and cement paste requirement.
- On grading curves, always note that the steepest segment identifies the dominant particle size in the mix.