Fineness Modulus (FM) classification of sand A natural sand sample has a fineness modulus (FM) of 2.5. According to common grading classifications used in concrete mix design, this sand is graded as:

Introduction / Context:
The fineness modulus (FM) is a single-number index representing the overall coarseness of an aggregate. For sand, FM helps select proportioning to achieve workability and reduce paste demand while avoiding segregation. Typical practice divides FM into bands such as very fine, fine, medium, and coarse.

Given Data / Assumptions:

• FM = 2.5 for the given sand.
• Standard sieve series and cumulative percentages used to compute FM.
• Typical classification bands used in mix design handbooks.

Concept / Approach:
For natural sand, an FM around 2.2–2.6 is generally referred to as “fine”. Medium sands tend to lie roughly from about 2.6 to 2.9, and coarse sands beyond that. The exact boundary values can vary slightly depending on guidance documents, but 2.5 comfortably falls in the “fine” category.

Step-by-Step Solution:
Interpret FM = 2.5 relative to standard bands.2.5 sits within the fine-sand range.Therefore, select “Fine sand”.

Verification / Alternative check:
Mix design charts often recommend adjusting sand FM to control workability. Finer sands (lower FM) increase water/paste demand; knowing the band helps choose suitable combined grading with coarse aggregates.

Why Other Options Are Wrong:

• Very fine suggests FM significantly below ~2.2.
• Medium or coarser categories correspond to FM values typically above 2.6.

Common Pitfalls:
Assuming FM alone guarantees performance; grading shape and percentage finer than 600 microns are also important to prevent bleeding or harshness.

Final Answer:
Fine sand