The particle index test provides a combined shape-texture characterization. This test requires that an aggregate sample be divided up into specific size fraction. Each size fraction is placed into a container in three layers. This is done twice; the first time, each layer is compacted with 10 blows of a tamping rod, and the second time, each layer is compacted with 50 blows of a tamping rod. The particle index is computed from the following equation:
The overall sample particle index is computed as a weighted average of the individual size fraction particles indexes based on the size fraction weights. Aggregates composed of rounded, smooth particles may have a low particle index of around 6 or 7, while aggregates composed of angular, rough particles may have a high particle index of between 15 and 20 or more.
- ASTM D 3398: Index of Aggregate Particle Shape and Texture