Flakiness Limit – 75 mm passing and 60 mm retained fraction Aggregates passing a 75 mm sieve and retained on a 60 mm sieve are considered “flaky” if their least dimension (thickness) is less than 0.6 times the mean size of the fraction. What is that threshold (in mm)?

Introduction / Context:
Evaluating aggregate shape through flakiness index helps ensure reliable packing and mechanical performance. The threshold thickness for a size fraction is derived from a standard fraction of the mean sieve size.

Given Data / Assumptions:

• Sieve fraction: passing 75 mm and retained on 60 mm.
• Mean size = (upper + lower sieve) / 2.
• Flakiness threshold = 0.6 * mean size.

Concept / Approach:
For each fraction, the flakiness gauge slot equals 0.6 times the mean size. Particles passing that slot are classed as flaky. This geometric rule standardizes classification across all size bands.

Step-by-Step Solution:
Mean size = (75 + 60) / 2 = 67.5 mm.Threshold thickness = 0.6 * 67.5 = 40.5 mm.Select 40.5 mm from the options.

Verification / Alternative check:
Commercial flakiness gauges list a 40.5 mm slot for the 60–75 mm fraction, aligning with the calculation.

Why Other Options Are Wrong:

• 20.5, 30.5, 50.5 mm do not match the mandated 0.6 factor for the given fraction.
• “None of these” is invalid because the correct numeric value is present.

Common Pitfalls:
Using the lower sieve size directly instead of the mean size; the standard explicitly uses the mean.

Final Answer:
40.5 mm