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Aggregate sizes and importance of grading

Short answer

Nominal size is the sieve designating the aggregate (e.g., 20 mm graded passing 40 mm and 20 mm limits). Single size has most particles between two adjacent sieves (e.g., 20 mm single size passing 40 mm, retained on10 mm). Maximum size is the smallest sieve with100% passing. Grading reduces voids, saving paste, and improves workability, strength, density, and economy.

Define nominal size, single size and maximum size of aggregate with example. Why is grading of aggregate important with regard to the properties of concrete?

Definitions (IS 383, IS 2386)

  • Nominal size: the size by which the aggregate is designated, i.e. the sieve size through which most of it passes. A "20 mm nominal size graded aggregate" (IS 383 Table 7) has 100% passing 40 mm, 95-100% passing 20 mm, 25-55% passing 10 mm and 0-10% passing 4.75 mm.
  • Single size aggregate: most particles pass one sieve and are retained on the next smaller one. E.g. 20 mm single size: 100% passes 40 mm, 85-100% passes 20 mm, 0-20% passes 10 mm and 0-5% passes 4.75 mm. It is used for making up a grading by blending.
  • Maximum size: the smallest sieve on which practically no material is retained, i.e. the sieve through which 100% of the aggregate passes. E.g. an aggregate with 100% passing 40 mm and 95% passing 20 mm has a maximum size of 40 mm and a nominal size of 20 mm.

Importance of grading

Grading is the distribution of particle sizes (found by sieve analysis, IS 2386 Part 1).

  • A well-graded aggregate has the fewest voids, so less cement paste is needed to fill them: economy.
  • Better workability, with less segregation and bleeding.
  • Higher density and strength and lower permeability, so more durable concrete.
  • Less shrinkage, because less paste and water are required.
  • Poor grading (too much of one size, or gap-graded) gives harsh mixes, high water demand, honeycombing and higher cement use.
  • Grading uniformity is checked by the fineness modulus and the grading limits of IS 383 Tables 7 and 9.

✓ Answer checked by Claude Opus 5.5 (high effort)

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