What is the primary testing method for determining the plasticity index of an aggregate?

The primary testing method for determining the plasticity index of an aggregate is the Atterberg Limits test. This test is specifically designed to evaluate the moisture content at which soil transitions between a solid and a plastic state, and subsequently from a plastic to a liquid state.

The plasticity index itself is a numerical value that represents the range of moisture content over which the aggregate behaves plastically. It's calculated as the difference between the liquid limit and the plastic limit of the material. These limits are determined through standardized procedures in the Atterberg Limits test, making it the most relevant and widely accepted method for assessing the plasticity characteristics of fine-grained soils and aggregates.

Other methods listed do not focus on plasticity index. For instance, the Concrete Slump Test measures the workability of concrete, the Standard Proctor Test evaluates the moisture-density relationship of soils for compaction purposes, and the CBR Test assesses subgrade strength for pavement design. These tests serve different objectives and are not designed to determine the plasticity index of aggregates.