Gradation of Aggregates

Gradation of Aggregates

Grading refers to the distribution of particle sizes present in an aggregate. The grading is determined in accordance with ASTM C 136, “Sieve or Screen Analysis of Fine and Coarse Aggregates.” A sample of the aggregate is shaken through a series of wire-cloth sieves with square openings, nested one above the other in order of size, with the sieve having the largest openings on top, the one having the smallest openings at the bottom, and a pan underneath to catch material passing the finest sieve(Table 1).

Fineness Modulus

Using the sieve analysis results, a numerical index called the fineness modulus (FM) is often computed. It is computed by adding the cumulative percentages

of aggregate retained on each of the specified series

of sieves, and dividing the sum by 100.

The specified sieves are 75.0, 37.5, 19.0, and 9.5 mm (3, 1.5, 3/4, and 3/8 in.) and 4.75 mm, 2.36 mm, 1.18 mm, 600 μm, 300 μm, and 150 μm (No. 4, 8, 16, 30, 50, and 100).

The coarser the aggregate, the higher the FM.

For fine aggregate used in concrete, the FM generally ranges from 2.3 to 3.1 as called for in ASTM C 33, but in some cases, fine sands are used with a FM less than 2.0 and in other cases, a coarser fine aggregate with an FM higher that 3.1.

The fineness modulus of the fine aggregate is required for mix design since sand gradation has the largest effect on workability. A fine sand (low FM) has much higher effect paste requirements for good workability.