13.3 : Design Example: Aggregate Gradation

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The manager of a concrete plant needs to choose between two manufactured sand production facilities, A and B, to purchase sand that meets the required gradation for use in concrete.

Samples of manufactured sand from the conveyor belts at both facilities are collected after halting the belts.

Two templates are spaced and inserted into the sand stream.

The complete material in the space between the template is transferred into a container.

This is repeated at two more random locations on the belt.

The collected samples are then mixed to form a representative sample, labeled, and kept in an oven to dry.

The dried samples are split using a sand splitter into equal portions, which are then subjected to sieve analysis by passing through a set of standard sieves.

The corresponding gradation curve is plotted.

The gradation curve for the sand from facility A is observed to be within the region bound by the prescribed limits as per the concrete mix design.

Contrarily, the sand from facility B falls outside this region. As a result, the manager decides to buy sand manufactured at facility A.

The right type and quality of aggregates are crucial for concrete as they significantly influence its properties, mix proportions, and cost-effectiveness. If different sources are available for sand, the commonly used fine aggregate in concrete, the selection of sand is primarily based on its gradation.

The grading, or particle-size distribution, of sand is determined using sieve analysis, with standard sizes ranging from 150 μm to 10 mm (ASTM No. 100 sieve to 3⁄8 in. sieve). Sand is sampled for gradation from a flowing aggregate stream, conveyor belts, or stockpiles. A single sample typically represents about 45 to 50 tons of sand. Finally, the test portions for sieve analysis are obtained from the field sample using a sample splitter.

Concrete using very fine sands can be uneconomical, and coarse sands can result in less workable mixes. Generally, well-graded sand, without a large deficiency or excess of any size and giving a smooth grading curve within the region bound by the grading curve for fine and coarse sand, is preferred in concrete.

Tags

Aggregate Gradation Concrete Properties Fine Aggregate Sieve Analysis Particle size Distribution Sand Selection Concrete Mix Proportions Sample Splitter Well graded Sand Economical Concrete

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