What is the impact of increasing the fineness modulus of aggregates on the workability of concrete?

Increasing the fineness modulus (FM) of aggregates generally reduces the workability of concrete. The fineness modulus is an empirical index that indicates the overall fineness or coarseness of an aggregate sample. It is calculated by summing the cumulative percentages retained on a specified series of sieves (typically #100, #50, #30, #16, #8, #4, 3/8 in., 3/4 in., 1 1/2 in., 3 in., and 6 in.) and dividing the sum by 100. A higher FM indicates a coarser aggregate gradation, meaning the aggregate contains a larger proportion of larger particles. When the FM of aggregates increases, the surface area of the aggregate blend decreases, meaning there is less fine material to fill the voids between the larger particles. This results in a harsher mix with less paste available to coat the aggregate surfaces, leading to reduced workability. The concrete becomes stiffer, more difficult to place and consolidate, and may be more prone to segregation. To maintain workability with a higher FM aggregate, the water content or paste content (cement and supplementary cementitious materials) typically needs to be increased. For instance, if changing from an aggregate with an FM of 2.5 to one with an FM of 3.0, the concrete mix will likely become less workable, requiring adjustments to the mix design to achieve the desired slump and consistency. It's important to note that while a higher FM generally reduces workability, extremely low FM values (very fine aggregates) can also reduce workability due to excessive surface area and water demand.