Fineness Modulus (FM) of concrete aggregates is an empirical figure representing the mean particle size of an aggregate sample. The number is required in the selection of proportions for the concrete mix design process, but its real influence is not always understood by users. Fineness modulus is determined using size fractions from the sieve analysis of aggregates and can be calculated for any designated series of nominal particle/sieve sizes. When designing concrete mixes, the FM of the fine aggregate alone is required for effective proportioning of mixes since changes to the grading of the coarse aggregate portion have less effect on overall concrete properties. Aggregate proportioning can also be controlled using separate determinations of fine and coarse aggregates and blending techniques to optimize the FM for a concrete mix.
Duff Abrams, an American materials researcher in the early part of the 20th century, was influential in studying concrete properties and characteristics. Along with defining the importance of water/cement ratio and developing the slump test, he defined fineness modulus as a way of characterizing concrete aggregates and estimating the correct proportions to use in mix designs. In explaining the premise in 1925, he said, “Aggregate of the same fineness modulus will require the same quantity of water to produce a mix of the same consistency and give a concrete of the same strength.”
How to Calculate Fineness Modulus
The simple calculation for FM is detailed in ASTM C136 Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates. After the sieve analysis is performed, the sum of the cumulative percentages retained on the following sieves are divided by 100: 150-μm (No. 100), 300-μm (No. 50), 600-μm (No. 30), 1.18mm (No. 16), 2.36mm (No. 8), 4.75mm (No. 4), 9.5mm (3⁄8in.), 19.0mm (3⁄4in), 37.5mm (11⁄2in). Larger sieves can also be included by increasing the ratio of 2:1.No additional equipment is required to make the determination, just the same ASTM E11 test sieves used in the aggregate tests for particle size analysis.
In a nutshell, fineness modulus describes the gradation curve as well as the texture and uniformity of the material. FM of the fine aggregate fraction is also an important predictor of other characteristics of both plastic and hardened concrete like workability, finishability, shrinkage, porosity, permeability, strength and the tendency to crack. A higher FM index indicates coarser aggregate that will produce a “harsh” mix, prone to segregation and that is difficult to place and finish properly. A low FM indicates finer aggregate that will require additional cement and increase water demand. This will be a mix easier to place and finish, but costlier, with increased potential for early-age cracking.
As with many things, there is a sweet spot that gives optimum results. For most mixes, an FM of fine aggregate between 2.5 and 3.0 yields a concrete that is easy to place, easy to finish and not as likely to crack. ASTM C 33 requires the FM of fine aggregate to be between 2.3 and 3.1, and that the final concrete batch not vary from the design mix more than 0.2. Fine aggregate FM’s in the upper end of the spectrum, closer to 3.0, have been noted to produce concrete with good strength and workability in mixes with high cement contents.
Once the fineness modulus of the fine aggregate is known, it can be used to select proportions of coarse aggregate. The American Concrete Institute (ACI) includes Table A18.104.22.168 in its standard practice 211.1-91 Selecting Proportions for Normal, Heavyweight, and Mass Concrete that gives the volumes of coarse aggregates for various fineness moduli of fine aggregates. If the maximum size of the coarse aggregate and the fineness modulus of the Fine Aggregate are known, the volume of the dry rodded coarse aggregate can be obtained from this table. In an example, the standard states that, “For a fine aggregate having a fineness modulus of 2.8 and a 37.5 mm nominal maximum size of coarse aggregate, the table indicates that 0.71 m3 of coarse aggregate, on a dry-rodded basis, may be used in each cubic meter of concrete. The required dry mass is, therefore, 0.71 x 1600 = 1136 kg.”
To sum up, fineness modulus is not only required for complete, workable concrete mix designs, it is a tool to use in evaluating aggregates and adjusting proportions and provide properties to characterize the completed mixes.