מודול הדקוּת (Fineness Modulus)

Fineness modulus is calculated by summing the cumulative percentage retained on each sieve and dividing the total by 100. First, record the weight retained on each sieve, then calculate cumulative weights and express them as percentages of total sample weight. Finally, add all cumulative percentages and divide by 100 to obtain the fineness modulus value.

A higher fineness modulus value indicates coarser aggregates, while lower values indicate finer particles. For fine aggregates, fineness modulus typically ranges from 2.3 to 3.1. This numerical index effectively reflects variations in aggregate gradation and provides more meaningful differentiation for fine aggregates than for coarse aggregates.

When aggregates from the same source are used, slight variations in fine aggregate gradation are less visually obvious than with coarse aggregates. The fineness modulus value captures these subtle differences more insightfully for fine aggregates, making it a more sensitive indicator of particle size distribution in finer materials.

Sand typically has a fineness modulus ranging from 2.3 to 3.1. Lower values within this range indicate finer sand with smaller particles, while higher values indicate coarser sand with larger particles. This range helps classify sand quality for construction and concrete applications.

To express aggregate retained as a percentage, divide the cumulative weight retained on each sieve by the total sample weight, then multiply by 100. For example, if 60 grams are cumulatively retained on a sieve from a 500-gram sample, the cumulative percentage retained is 12 percent.

When all particles are retained on or above a particular sieve and nothing passes through it, the cumulative percentage retained for that sieve and all sieves stacked underneath it is entered as 100 percent. This indicates that no material finer than that sieve size is present in the sample.

Fineness modulus is a numerical index that represents the grading of aggregates by measuring the coarseness or fineness of particles. It provides a single quantitative value that characterizes the particle size distribution of an aggregate sample, enabling standardized comparison and quality control in construction materials.