In 1925, Duff Abrams introduced the concept of fineness modulus (FM) for estimating the proportions of fine and coarse aggregates in concrete mixtures.
The principle:
“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.”
Because FM is such a widely used index for aggregate proportioning, most testing labs report the FM for fine aggregate with each sieve analysis.
Limits may be taken as guidance is:
· Fine Sand - 2.2 to 2.6
· Medium Sand - 2.6 to 2.9
· Coarse Sand - 2.9 to 3.2
The higher the FM, the coarser the aggregate. FM doesn’t define the grading curve, however, since different gradings can have the same FM.
How aggregate fineness affects concrete properties?
Fine aggregate affects many concrete properties, including workability and finishability. Experience has shown that very coarse sand or very fine sand produces poor concrete mixes.
Coarse sand results in harsh concrete mixes prone to bleeding and segregation.
Fine sand requires a comparatively large amount of water to achieve the desired concrete workability, is prone to segregation, and may require higher cement contents.
Decreasing FM for sand used in mortar requires considerably more cement content when the water-cement ratio and slump are held constant. However, a changing FM has little influence on the cement content required in concrete. Usually, a lower FM results in more paste, making concrete easier to finish.
For the high cement contents used in the production of high-strength concrete, coarse sand with an FM around 3.0 produces concrete with the best workability and highest compressive strength.
In general, manufactured sands require more fines than natural sands for equal workability.
Sieve Analysis: (Calculating FM)
This is done by sieving the aggregates as per IS: 2386 (Part I) – 1963. In this we use different sieves as standardized by the IS code and then pass aggregates through them and thus collect different sized particles left over different sieves.
The apparatus used are –
i) A set of IS Sieves of sizes – 80mm, 63mm, 50mm, 40mm,31.5mm, 25mm, 20mm, 16mm, 12.5mm, 10mm, 6.3mm,4.75mm, 3.35mm, 2.36mm, 1.18mm, 600µm, 300µm, 150µm and 75µm.
ii) Balance or scale with an accuracy to measure 0.1 percent of the weight of the test sample.
The sample for sieving should be prepared from the larger sample either by quartering or by means of a sample divider.
Procedure to determine particle size distribution of Aggregates:
i) The test sample is dried to a constant weight at a temperature of 110 + 5oC and weighed.
ii) The sample is sieved by using a set of IS Sieves.
iii) On completion of sieving, the material on each sieve is weighed.
iv) Cumulative weight passing through each sieve is calculated as a percentage of the total sample weight.
v) Fineness modulus is obtained by adding cumulative percentage of aggregates retained on each sieve and dividing the sum by 100.
Sieves used for gradation test
A mechanical shaker used for sieve analysis
Reporting of Results
The results should be calculated and reported as:
i) the cumulative percentage by weight of the total sample
ii) the percentage by weight of the total sample passing through one sieve and retained on the next smaller sieve, to the nearest 0.1 percent. The results of the sieve analysis may be recorded graphically on a semi-log graph with particle size as abscissa (log scale) and the percentage smaller than the specified diameter as ordinate.
