AASHTO T 90:2014 Standard Method of Test for Determining the Plastic Limit and Plasticity Index of Soils.
6.4. When the diameter of the thread reaches 3 mm. squeeze the thread between the thumbs and fingers and form the mass back into a roughly ellipsoidal shape. Repeat the rolling process described in Section 6.3.1 or 6.3.2 until the soil can no longer be rolled into a thread and begins to crumble. Reform the soil into an ellipsoidal mass after each rolling of the thread to a 3-mm diameter. The crumbling may occur when the thread has a diameter greater than 3 mm. This is considered a satisfactory end point, provided the solid has been previously rolled into a thread 3 mm in diameter (Note 6). Do not attempt to produce failure at an exact 3-mm diameter by allowing the thread to reach 3 mm, then reducing the rate of rolling or the hand pressure, or both, and continuing the rolling without further deformation until the thread falls apart (Note 7).
Note 6—The crumbling will manifest itself differently with various types of soil. Some soils fall apart in numerous small aggregations of particles; others may form an outside tubular layer that starts splitting at both ends. The splitting may progress toward the middle, and the thread may finally fall apart in many small platy particles. Heavy clay soils require much pressure to deform the thread, particularly as they approach the plastic limit. These types of soils may break into a series of barrel-shaped segments each about 6 to 9 mm in length.
Note 7—For feebly plastic soils, it may be necessary to reduce the total amount of deformation by making the initial diameter of the ellipsoidal-shaped mass nearer to the required 3-mm final diameter.
6.5. Gather the portions of the crumbled soil together and place in the moisture content container. Immediately cover the container with a close-fitting lid to prevent additional loss of moisture.
6.6. Repeat the operations described in Sections 6.3 through 6.5 until the entire -g specimen is tested. Place all of the crumbled portions into the same moisture content container.
6.7. Determine the moisture content of the soil in the container in accordance with T 265, and record the results.
7.1. The plastic limit of the soil is the moisture content determined in Section 6.7, expressed as a percentage of the oven-dry mass, and determined in accordance with T 265. Report the plastic limit to the nearest whole number.
7.2. If applicable, calculate the plasticity index of a soil as the difference between its liquid limit and its plastic limit, as follows:
plasticity index = liquid limit — plastic limit (I)
7.3. Report the results as calculated in Section 7.2 as the plasticity index, except under the following conditions:
7.3.1. When the liquid limit or plastic limit cannot be determined, report the plasticity index as NP (nonpi ast Ic).
7.3.2. When the plastic limit is equal to, or greater than, the liquid limit, report the plasticity index as NP.AASHTO T 90 pdf download.