Calculating Coarse Aggregate Angularity Number from Void Percentage

The angularity number is a property of aggregates that indicates their angularity or sharpness. It is an important factor in determining the workability of concrete or asphalt mixes and the strength of the compacted layers.

A higher angularity number indicates more angular aggregates, while a lower number indicates more rounded aggregates. Aggregates that are perfectly rounded are considered to have an angularity number of 0.

The angularity test for coarse aggregates involves measuring the percentage of voids in a sample of the aggregate after it has been compacted in a standard container in a specific way.

Given Data

• Percentage of voids in the coarse aggregate sample = 40%

Angularity Number Formula

The angularity number is calculated based on the difference between the actual percentage of voids in the tamped aggregate and the percentage of voids that would be expected for perfectly rounded aggregates of the same size range. Perfectly rounded aggregates typically have a void content of about 33% when tested under standard conditions.

The formula to calculate the angularity number (AN) is:

\( \text{Angularity Number} = \text{Percentage of Voids} - 33 \)

Where:

• Angularity Number is the value representing the angularity.
• Percentage of Voids is the void content found in the standard test.
• 33% is the assumed void content for perfectly rounded aggregate.

Calculation Steps

Using the given percentage of voids from the test, we can substitute this value into the formula:

Given Percentage of Voids = 40%

Angularity Number \( = 40 - 33 \)

Angularity Number \( = 7 \)

Result and Conclusion

Based on the angularity test results showing 40% voids for the coarse aggregate sample, the calculated angularity number is 7.

This calculated value of 7 falls within the typical range for angularity numbers of coarse aggregates used in construction.

Revision Table: Coarse Aggregate Properties

Additional Information on Aggregate Angularity Test

The angularity test provides valuable information about the shape characteristics of coarse aggregates. Here are some additional points:

• The test method typically involves a standard cylindrical container and a specific tamping procedure to ensure consistent compaction.
• The percentage of voids is determined by filling the container with aggregate, weighing it, and knowing the specific gravity of the aggregate and the volume of the container.
• Aggregates with higher angularity numbers (more angular) generally lead to mixes with higher internal friction and shear strength when compacted, which is beneficial for pavement layers.
• However, highly angular aggregates can also reduce the workability of concrete or asphalt mixes, requiring more effort to place and compact, or potentially needing more binder.
• The angularity number is one of several properties considered when selecting aggregates for specific construction applications, alongside grading, strength, durability, and surface texture.