Design Example: Aggregate Gradation

Sand is sampled by inserting two templates into the flowing aggregate stream on the conveyor belt and collecting all material between them. This process is repeated at two additional random locations. The collected samples are then mixed to form a representative sample, labeled, and dried in an oven before further testing.

Sieve analysis determines the particle-size distribution or grading of sand, which is the primary criterion for sand selection in concrete. Well-graded sand without excessive fine or coarse particles produces optimal concrete properties. Very fine sands are uneconomical, while coarse sands result in less workable mixes, making gradation analysis essential for quality control.

A gradation curve plots particle-size distribution and must fall within prescribed limits set by concrete mix design standards. Sand from facility A met these requirements, while facility B's sand fell outside the acceptable region. A smooth curve within the bounded region indicates well-graded sand suitable for concrete, ensuring proper workability and cost-effectiveness.

A sand splitter divides the dried field sample into equal portions for testing. This ensures representative test samples are obtained from the larger collected sample. The split portions are then passed through standard sieves ranging from 150 micrometers to 10 millimeters to determine particle-size distribution and verify gradation compliance.

Standard sieves for sand gradation testing range from 150 micrometers (ASTM No. 100 sieve) to 10 millimeters (3/8 inch sieve). These sieves separate sand particles by size, allowing determination of the complete particle-size distribution. A single field sample typically represents approximately 45 to 50 tons of sand material.

Comparing gradation curves from different sand sources helps managers select material that meets concrete mix design specifications. By plotting curves and checking if they fall within prescribed limits, plants ensure consistent concrete quality and workability. This comparison prevents purchasing sand that would result in uneconomical or poorly performing concrete mixes.

Sand gradation directly influences concrete workability and economics. Very fine sands are uneconomical and difficult to work with, while coarse sands produce less workable mixes. Well-graded sand, without large deficiencies or excesses of any size, provides optimal balance between workability and cost-effectiveness, making it the preferred choice for concrete production.