A Contractor's Guide to Aggregates (Jalli) — Sizes, Uses, and Quality

Aggregates (known as jalli or blue metal in Tamil Nadu) are crushed rock particles used as the coarse component in concrete. While fine aggregate (sand) fills the voids between aggregate particles, the aggregate itself provides the bulk strength and volume. Together, fine aggregate, coarse aggregate, cement, and water form concrete. The quality and size of aggregates have a significant impact on the compressive strength, workability, and durability of the concrete.

6mm aggregates: Used for precast concrete elements, thin slabs, and applications where minimum voids are needed. Also used as a sub-base material in tile-laying.

12mm aggregates: Common for slabs, beams, and columns in residential construction. Good balance of workability and strength.

20mm aggregates: Standard for large-scale structural concrete — foundations, large columns, road base concrete. Lower surface area means less cement paste required.

40mm aggregates: Used for large foundation concrete, road sub-base layers, and industrial flooring where high volume and mass concrete is required.

Shape: Angular and cubical aggregates (from VSI technology) give better interlocking and higher concrete strength than flaky or elongated shapes. Eswara Sand supplies VSI-manufactured aggregates for consistent shape.

Hardness: Test by scratching with a steel nail — good granite aggregate should not scratch easily. Soft or weathered rock significantly reduces concrete strength.

Cleanness: Aggregates should be free of dust, clay, and organic matter. A simple water rinse should produce clear water within a few minutes.

Size consistency: A good supply should have the stated size as the majority (>85%) of particles.

A common mix for general residential concrete (M20 grade, 20MPa design strength) is 1:1.5:3 — 1 part cement, 1.5 parts fine aggregate (M Sand), 3 parts coarse aggregate (20mm jalli). The exact ratio should be designed by a structural engineer for critical applications. Using the wrong aggregate size or poor quality aggregates will reduce the achieved strength regardless of the cement quantity.