Reading a grading curve — horizontal segment between 20 mm and 4.75 mm If a grading (percent passing) curve is horizontal between the 20 mm IS sieve and the 4.75 mm IS sieve, what does it indicate about the aggregate fractions present?

Introduction / Context:
A grading curve plots cumulative percent passing versus sieve size. The shape of this curve directly reveals the presence or absence of material in particular size ranges. Interpreting these shapes is critical to diagnosing workability issues, segregation, and paste demand in concrete mixes or asphalt blends.

Given Data / Assumptions:

• Standard IS sieves: 20 mm down to 4.75 mm considered.
• “Horizontal” means percent passing is constant across the range.
• No measurement errors assumed.

Concept / Approach:
If the percent passing does not change between two sieve sizes, it implies that no additional material passes the smaller sieve compared with the larger one. In other words, there are no particles in that intermediate size band. Thus, a horizontal segment from 20 mm to 4.75 mm signifies an absence of particles larger than 4.75 mm and smaller than 20 mm—i.e., missing the entire intermediate fraction, including nominal 10 mm sizes and the exact boundary sizes, effectively indicating a gap-graded condition in that zone.

Step-by-Step Solution:

Verification / Alternative check:
Sieve analysis results would show the same mass retained on both sieves, confirming no intermediate fraction present.

Why Other Options Are Wrong:

• Choosing only a single missing size underestimates the complete gap indicated by the horizontal segment.

Common Pitfalls:
Misreading semi-log plots where equal spacing does not mean equal size ratios; ignoring that “horizontal” signifies constant passing percentage.

Final Answer:
All of the above