The fineness modulus is an empirical figure that provides an index of the average particle size in an aggregate sample. It is determined by summing the cumulative percentages retained on a specified series of sieves and dividing the sum by 100. For example, if a sample has cumulative percentages retained of 10, 30, 60, 80, and 90 on the sieves used, the sum would be 270, and the resultant modulus would be 2.7.
This calculation offers a simplified method for characterizing the overall gradation of an aggregate. It is valuable in concrete mix design, allowing engineers to estimate the workability and water demand of the mix. A higher modulus indicates a coarser aggregate, which typically requires less water for a given workability. Historically, this value has been used as a quick quality control check, ensuring consistency in aggregate supplies and facilitating predictable concrete performance.
