The American Standard Society for Testing and Materials requires a number between 2.3 and 3.1 for fine aggregate on the fineness modulus scale. Fineness modulus estimates the proportions of aggregate in construction materials. Aggregate is inert granular material such as sand, gravel or crushed stone used to form concrete. The fineness modulus number affects the workability and finish of concrete.
Before calculating fineness modulus, lab technicians pour aggregate onto sieves. Fineness modulus is the sum of the total percentages of the aggregate retained on each sieve divided by 100. Natural sand or crushed stone is used to achieve a fine rating, with most particles passing through a 3/8-inch sieve. The sand and stone comes from a pit, river, lake or seabed. Aggregate is described as coarse when the number is higher than 3.1. Coarse aggregate comes from quarry rock, boulders, cobbles, large gravel or crushed concrete. When choosing aggregate materials, grading, durability, particle shape, surface texture, abrasion and skid resistance, unit weights and voids, absorption, and surface moisture are factors to consider. In 1925, Duff Abrams introduced the concept of fineness modulus. The American Standard Society for Testing and Materials members deliver the test methods, specifications, guides and practices that support industries and governments worldwide.