Aggregate shape classification; angularity number

Classification of aggregate by shape (IS 383 / BS 812)

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.