On a grading curve, the gap grading is represented by :
Try it first: pick an option, then read the explanation.
Correct answer: A. a horizontal line
Detailed Explanation: Reading a Grading Curve — Gap Grading
A particle size distribution (grading) curve is a graph of cumulative percentage passing (y-axis) versus sieve size (x-axis, usually on a log scale). The shape of this curve tells you the nature of the aggregate gradation.
Why A (horizontal line) is correct: On the grading curve, a rise in the cumulative percentage passing means particles of that size are present. A horizontal segment means the cumulative passing does not increase — no particles exist in that size range. This is the graphical signature of gap grading: a deliberate (or natural) absence of intermediate particle sizes. A vertical line would be impossible (cumulative % cannot decrease with increasing sieve size for a single sample).Grading Curve Shapes and Their Meanings
| Curve Feature | Type of Grading | Interpretation |
|---|---|---|
| Smooth S-curve (steep middle, gentle ends) | Well-graded | Particles spread across all sizes — ideal for concrete |
| Horizontal segment | Gap-graded | Missing sizes in that range |
| Very steep, nearly vertical | Uniform / single-size | Nearly all particles the same size |
Key Concepts for Students
- Well-graded aggregate has the smoothest S-curve and the fewest voids, requiring less cement paste — preferred for structural concrete.
- Gap grading can be intentional (to improve packing of two specific sizes) but risks segregation of the fresh concrete.
- Single-size aggregate produces a near-vertical line and has the highest void content of all — used in no-fines concrete and porous pavements.