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Effect of shape and size of aggregate on workability

Short answer

Rounded aggregates give better workability than angular, flaky ones due to lower surface area and less interlocking. Larger maximum size reduces water demand for a given slump, but is limited to 1/4 of member thickness and 5 mm less than bar spacing or cover.

How do the shape and size of aggregate affect the workability of concrete?

Shape

  • Rounded aggregates (river gravel) have the least surface area and the least interlocking, so they need less paste to coat them and give the best workability.
  • Angular, flaky and elongated particles have more surface area and voids, so they need more water and cement paste for the same slump; the mix is harsh and less workable.
  • Flaky particles also lie flat and trap water below them.

Size

  • A larger maximum size means a smaller total surface area per unit mass, so less water and paste are needed. For the same cement content, workability improves (or the water demand falls) as the size increases.
  • Size is limited by section thickness and bar spacing: IS 456 cl. 5.3.3 limits the nominal maximum size to 1/4 of the minimum thickness of the member and, for RCC, to 5 mm less than the minimum clear spacing between bars or the minimum cover, whichever is smaller (20 mm is common for RCC).
  • Very fine (small) aggregate has a large surface area and a high water demand, giving a stiff mix.

A well-graded mix of rounded aggregate of larger size gives the best workability for a given water-cement ratio.

✓ Answer checked by Claude Opus 5.5 (high effort)

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