Aggregate grading curves – Visual signature of gap grading On a particle size distribution (grading) curve plotted as percentage passing versus sieve size (log scale), how is a gap-graded aggregate visually represented over the missing size range?

Introduction / Context:
Grading curves are essential tools for proportioning aggregates in concrete and asphalt. They reveal whether an aggregate is well-graded, uniformly graded, or gap-graded. Recognizing the visual cues helps diagnose workability, segregation, and void content issues quickly.

Given Data / Assumptions:

• Plot: percent passing on the vertical axis, sieve size (log scale) on the horizontal axis.
• Gap grading means a deliberate absence (or near absence) of particles in a particular size band.
• We are interpreting the shape of the curve, not numerical values.

Concept / Approach:
In a grading curve, a change in percent passing occurs only if some material exists within that size interval. If no particles lie in a band (true “gap”), percent passing remains unchanged as sieve size varies across that band. Graphically, this appears as a horizontal segment since the ordinate (percent passing) is constant.

Step-by-Step Solution:
Define gap grading: negligible content in a certain size range.Relate to the curve: across the gap, % passing does not change.A constant ordinate across changing abscissa yields a horizontal line segment.Therefore, the correct visual is a horizontal line through that interval.

Verification / Alternative check:
Check example blend curves in mix design texts: gap-graded blends commonly show “flat spots” (horizontal portions) where sizes are intentionally omitted.

Why Other Options Are Wrong:

• Vertical line: would imply an abrupt change at a single sieve, not an interval gap.
• N.E. or N.W. inclined lines: indicate increasing or decreasing % passing, i.e., material present across sizes.
• “None of these”: invalid since the horizontal segment is the standard representation.

Common Pitfalls:
Confusing a flat spot due to measurement scatter with a true gap; verify with mass retained data and repeat tests.

Final Answer:
By a horizontal line (constant percent passing over that size interval)