Requirements of aggregates; need of grading

Requirements of coarse and fine aggregates (IS 383:2016)

Common to both

• Clean, hard, strong, durable and chemically inert; free from clay, silt, organic matter, coal, mica, salts and shale (deleterious materials within the limits of IS 383).
• Free from alkali-reactive and unsound material (soundness loss in the sodium sulphate test within 12% for coarse and 10% for fine aggregate).
• Proper grading and shape.

Coarse aggregate

• Crushed stone or gravel; nominal size of 10, 20 or 40 mm chosen to suit the section (IS 456 cl. 5.3.3: not more than 1/4 of the minimum thickness of the member, and 5 mm less than the minimum clear bar spacing or cover).
• Crushing value at most 45% and impact value at most 45% (30% for wearing surfaces); abrasion value at most 50% (30% for wearing surfaces).
• Flakiness and elongation within limits (combined index not more than 40%); angular, rough texture preferred; water absorption generally below 2%.
• Specific gravity 2.5-3.0.

Fine aggregate

• Natural sand, crushed stone sand or crushed gravel sand passing the 4.75 mm sieve, in grading Zones I-IV (IS 383 Table 9).
• Fineness modulus 2.0-3.5; organic impurities checked by the colour test (the colour should not be darker than the standard); bulking must be allowed for.

Need for grading

Grading is the particle size distribution. It is needed so that the smaller particles fill the voids between the larger ones, giving the fewest voids and a dense, strong, workable and economical concrete that needs the minimum cement paste and water. It prevents segregation, harshness and honeycombing, and ensures uniform quality.