Flakiness Limit – 25 mm passing and 20 mm retained fraction An aggregate that passes the 25 mm I.S. sieve and is retained on the 20 mm sieve is termed “flaky” if its least dimension (thickness) is less than 0.6 times the mean size. What is this threshold value (in mm) for the stated size fraction?

Introduction / Context:
Aggregate shape affects workability, strength, and durability of concrete and bituminous mixes. The flakiness index quantifies the proportion of particles whose least dimension (thickness) is below a prescribed fraction of their mean size. Understanding how to compute the threshold for a given sieve fraction is essential for quality control.

Given Data / Assumptions:

• Sieve fraction: passing 25 mm and retained on 20 mm.
• Mean size = average of the two sieve sizes.
• Flaky definition: least dimension < 0.6 * mean size.

Concept / Approach:
For each sieve fraction, standards specify a slot width equal to 0.6 times the mean size. Particles that pass through this slot in the flakiness gauge are counted as flaky. Therefore, the numeric threshold is simply 0.6 multiplied by the mean of bounding sieve sizes.

Step-by-Step Solution:
Mean size = (25 + 20) / 2 = 22.5 mm.Threshold thickness = 0.6 * 22.5 = 13.5 mm.Compare with options and select 13.5 mm.

Verification / Alternative check:
Reference flakiness gauges are manufactured with slots sized at 0.6 times the mean sieve for each fraction; for the 20–25 mm fraction, the slot is 13.5 mm, confirming the calculation.

Why Other Options Are Wrong:

• 22.5 mm is the mean size, not the threshold.
• 18.5 mm, 16.5 mm, 15.5 mm do not match the 0.6 factor requirement.

Common Pitfalls:
Confusing “least dimension” (thickness) with length or breadth; flakiness pertains to thinness, not elongation.

Final Answer:
13.5 mm