Aggregates for Concrete – Name of the Index Expressing Relative Sizes Across Coarse and Fine Fractions Which index number, obtained by summing cumulative percentage retained on a standard sieve series and dividing by 100, expresses the overall coarseness/fineness of combined aggregates?

Introduction:
Concrete mix design uses concise indices to summarize particle-size distribution. The fineness modulus (FM) condenses sieve analysis into a single figure that guides the balance between workability, cohesion, and economy for combined aggregates.

Given Data / Assumptions:

• Aggregates are tested on a prescribed sieve set.
• FM is computed from cumulative percentage retained values.
• A higher FM indicates a coarser aggregate blend.

Concept / Approach:

The FM helps compare and control grading from batch to batch. While detailed gradation curves are still needed, FM is a convenient control index that correlates with water demand and paste requirement in concrete.

Step-by-Step Solution:

Verification / Alternative check:

Concrete technology texts standardize FM computation and typical ranges for fine and coarse aggregates, ensuring mix consistency.

Why Other Options Are Wrong:

Proportioning ratio is not a defined index; grading zone classification groups sands but is not computed as FM. Uniformity coefficient and coefficient of curvature belong to soil mechanics, not concrete aggregate FM.

Common Pitfalls:

Confusing FM with grading zones; misusing FM outside its sieve set, leading to inconsistent control values.

Final Answer:

Fineness modulus (FM)