Description

AASHTO T 90 – Standard Method of Test for Determining the Plastic Limit and Plasticity Index of Soils

The plastic limit of a soil is the lowest water content determined in accordance with this procedure at which the soil remains plastic. The plasticity index of a soil is the range in water content, expressed as a percentage of the mass of the oven-dried soil, within which the material is in a plastic state. It is the numerical difference between the liquid limit and plastic limit of the soil.

The following applies to all specified limits in this standard: For the purpose of determining conformance with these specifications, an observed value or a calculated value shall be rounded off “to the nearest unit” in the last right-hand place of figures used in expressing the limiting value, in accordance with ASTM E 29.

Two procedures for rolling out soil samples are provided in this method: the Hand Rolling Method and the Plastic Limit Device Method. The Hand Rolling Method shall be used as the referee procedure.

AASHTO T 90-20 is often conducted in conjunction with T 89, which is used to determine the liquid limit of soils.