---
title: "Aggregate shape classification; angularity number"
subject: "Concrete Technology and Masonry Structure"
chapter: 1
key: "semester_5/concrete_technology_and_masonry_structure:angularity-number-shape"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/angularity-number-shape
site: BCT Engineering
---

# Aggregate shape classification; angularity number

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

- Asked in: 2081 Bhadra, 2+3 marks
- 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/angularity-number-shape
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1

## Question

Classify aggregate based on shape. How would you assess the shape of aggregate through angularity number? Explain taking the case of an aggregate sample with percentage of solid volume as 58.

## Short answer

Aggregate shape classes: rounded, irregular, angular, flaky, elongated. Angularity number equals 67 minus per cent solid volume. For 58% solid volume, angularity number = 9, indicating highly angular aggregate (about 42% voids), giving harsh mix but good interlock and strength.

## Answer

#### Classification of aggregate by shape (IS 383 / BS 812)
| Class | Description | Example |
|---|---|---|
| Rounded | Fully water-worn, smooth | River/sea gravel |
| Irregular | Naturally irregular, partly rounded | Pit gravel, land flint |
| Angular | Sharp edges and rough faces | Crushed rock |
| Flaky | Thickness small relative to length and width | Laminated rock |
| Elongated | Length much greater than width/thickness | Some crushed rock |
| Flaky and elongated | Both | Some crushed rock |

#### Angularity number
The angularity number measures the **voids in a compacted aggregate sample** and thus its angularity (IS 2386 Part 1). The sample is compacted in a cylinder in three layers, each given 100 blows of a tamping rod, and the percentage of solid volume in the container is calculated. The value for the most rounded gravel packed in this way is taken as 67 per cent solids (33% voids).

$$
\text{Angularity number} = 67 - \text{percentage of solid volume}
$$

The percentage of solid volume is $\frac{100\,W}{C\,G_a}$, where $W$ is the mass of aggregate in the cylinder, $C$ the mass of water filling the cylinder and $G_a$ the specific gravity of the aggregate. The range is **0 (rounded) to about 11 (most angular)**; the greater the number, the more angular the aggregate, the more voids it has, and the more paste is required.

#### Given sample (58% solid volume)
$$
\text{Angularity number} = 67 - 58 = 9
$$

The angularity number is **9**, which is high (close to the upper limit of 11), indicating a **highly angular** aggregate (crushed stone) with about 42% voids. It gives good interlock and strength, but lower workability: the mix will be harsh and will need more water and cement paste.

## Check yourself

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

1. Angularity number is:
   A. 67 - percentage of solid volume
   B. percentage solid volume - 67
   C. 100 - percentage solid volume
   D. 67 + percentage solid volume
   Answer: A. Perfectly rounded aggregate packs to 67% solids, so AN = 67 - solid volume %. (Key point: Angularity formula)

2. For a sample with 58% solid volume, the angularity number is:
   A. 125
   B. 8
   C. 11
   D. 9
   Answer: D. 67 - 58 = 9. (Key point: Numerical)

3. A high angularity number indicates:
   A. Rounded aggregate
   B. More angular aggregate with more voids, needing more paste
   C. Less voids
   D. Better workability
   Answer: B. Greater AN means more angular, more voids, more paste and harsher mix. (Key point: Interpretation)

4. The angularity number ranges from:
   A. 0 (rounded) to about 11 (most angular)
   B. 0 to 100
   C. 67 to 100
   D. 5 to 50
   Answer: A. 0 is perfectly rounded; about 11 is the most angular. (Key point: Range)

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