---
title: "Effect of shape and size of aggregate on workability"
subject: "Concrete Technology and Masonry Structure"
chapter: 1
key: "semester_5/concrete_technology_and_masonry_structure:aggregate-shape-size-workability"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/aggregate-shape-size-workability
site: BCT Engineering
---

# Effect of shape and size of aggregate on workability

Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials

- Asked in: 2081 Bhadra, 2 marks; 2080 Baisakh, 2 marks (asked 2 times)
- Video: [Aggregates || Shape of Aggregate || Angularity number || Concrete Technology || CH 02 E 01](https://www.youtube.com/watch?v=aQx7cABe8nY) — TRICKY DOSE
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/aggregate-shape-size-workability
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1

## Question

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

## 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.

## Answer

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which aggregate shape gives the best workability?
   A. Angular
   B. Flaky
   C. Elongated
   D. Rounded
   Answer: D. Rounded has the least surface area and interlock. (Key point: Shape)

2. Increasing maximum aggregate size generally:
   A. Reduces water demand for the same cement content
   B. Increases water demand
   C. Makes mix stiffer
   D. Has no effect
   Answer: A. Less surface area per mass needs less paste and water. (Key point: Size)

3. IS 456 limits maximum aggregate size to:
   A. Half the member depth
   B. 1/4 of minimum member dimension and 5 mm less than clear bar spacing
   C. Twice bar spacing
   D. 40 mm always
   Answer: B. Size is limited by section and bar spacing. (Key point: Size limit)

## Related in Concrete Technology and Masonry Structure

- [Aggregate shape and texture: strength, workability](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aaggregate-shape-texture-strength) — Concrete Technology and Masonry Structure, chapter 1, asked 2×
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