1、 Standard Method of Test for Determining the Plastic Limit and Plasticity Index of Soils AASHTO Designation: T 90-15 American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-1a T 90-1 AASHTO Standard Method of Test for Dete
2、rmining the Plastic Limit and Plasticity Index of Soils AASHTO Designation: T 90-15 1. SCOPE 1.1. The plastic limit of a soil is the lowest moisture content at which the soil remains plastic. The plasticity index of a soil is the range in moisture content, expressed as a percentage of the mass of th
3、e oven-dried soil, within which the material is in a plastic state. The plasticity index is calculated as the numerical difference between the liquid limit and plastic limit of the soil. 1.2. The following applies to all specified limits in this standard: For the purpose of determining conformance w
4、ith 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 E29. 1.3. Two procedures for rolling out soil samples are provided in this method: the H
5、and Rolling Method and the Plastic Limit Device Method. The Hand Rolling Method shall be used as the referee procedure. 1.4. This test method is often conducted in conjunction with T 89, which is used to determine the liquid limit of soils. 2. REFERENCED DOCUMENTS 2.1. AASHTO Standards: M 231, Weigh
6、ing Devices Used in the Testing of Materials R 18, Establishing and Implementing a Quality Management System for Construction Materials Testing Laboratories R 58, Dry Preparation of Disturbed Soil and Soil-Aggregate Samples for Test R 61, Establishing Requirements for Equipment Calibrations, Standar
7、dizations, and Checks T 89, Determining the Liquid Limit of Soils T 146, Wet Preparation of Disturbed Soil Samples for Test T 265, Laboratory Determination of Moisture Content of Soils 2.2. ASTM Standard: E29, Standard Practice for Using Significant Digits in Test Data to Determine Conformance with
8、Specifications 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 90-2 AASHTO 3. APPARATUS 3.1. DishA porcelain evaporating dish or similar mixing dish about 115 mm in diameter. 3.2. SpatulaA spatul
9、a or pill knife having a blade about 75 mm in length and about 20 mm in width. 3.3. Surface for RollingShall consist of one of the following: 3.3.1. A ground glass plate or piece of smooth, unglazed paper on which to roll the sample. Paper, if used, shall not add foreign matter (fibers, paper fragme
10、nts, etc.) to the soil during the rolling process and shall lay flat on a smooth horizontal surface. 3.3.2. Plastic Limit Rolling Device1,2A device made of acrylic conforming to the dimensions shown in Figure 1. Notes: 1. The tolerance between the width of the top plate (W) and the inside width of t
11、he bottom plate (IW) shall be such that the top plate slides freely on the rails without wobbling. 2. The top plate shall be rigid enough so that the thickness of the soil threads is not influenced by flexure of the top plate. 3. The width of the side rails shall be between 3 and 6 mm. Figure 1Plast
12、ic Limit Rolling Device 3.3.2.1. Paper for Rolling DeviceUnglazed paper that does not add foreign matter (fibers, paper fragments, etc.) to the soil during the rolling process. Attach the unglazed paper to the top and bottom plates of the device either by a spray-on adhesive or by use of a self-adhe
13、sive backing. Note 1Take special care to remove the adhesive that remains on the plastic limit rolling device after testing. Repeated tests without such removal will result in a buildup of the residual adhesive and a decreased soil thread diameter. 3.4. Moisture Content ContainerMade of material res
14、istant to corrosion and not subject to change in mass or disintegration on repeated heating and cooling. The container shall have a close-fitting lid to prevent loss of moisture from samples before initial mass determination and to prevent absorption of moisture from the atmosphere following drying
15、and before final mass determination. One container is needed for each moisture content determination. BW1LTop PlateBottom PlateH3TIW1LDimensions:IW approximately 100 mmL approximately 200 mmT 5 to 10 mmH 3.20 + 0.25 mm plus the total thickness of the unglazed paperattached to the bottom plate. See N
16、ote 3.W See Note 1.B See Note 2. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 90-3 AASHTO 3.5. BalanceA class G1 balance meeting the accuracy requirements of M 231. 3.6. OvenThermostatically c
17、ontrolled and capable of maintaining temperatures of 110 5C for drying samples. 4. CALIBRATIONS, STANDARDIZATIONS, AND CHECKS 4.1. Unless otherwise specified, follow the requirements and intervals for equipment calibrations, standardizations, and checks in R 18. 4.2. Follow the procedures for perfor
18、ming equipment calibrations, standardizations, and checks found in R 61. 5. SAMPLE 5.1. If only the plastic limit is to be determinedTake a quantity of soil with a mass of about 20 g from the thoroughly mixed portion of the material passing the 0.425-mm (No. 40) sieve, obtained in accordance with R
19、58 or T 146. Place the air-dried soil in a mixing dish and thoroughly mix with distilled or demineralized water until the mass becomes plastic enough to be easily shaped into a ball (Notes 2 and 3). Take a portion of this ball with a mass of about 10 g for the test sample. Note 2Tap water may be use
20、d for routine testing if comparative tests indicate no differences in results between using tap water and distilled or demineralized water. However, use distilled or demineralized water for referee or disputed tests. Note 3The objective is to add enough moisture to a plastic soil sample so that the
21、3-mm thread described in Section 6 does not crumble on the first roll. For a nonplastic soil, this will not be possible. 5.2. If the plasticity index (both liquid and plastic limit) is to be determinedTake a test sample with a mass of about 10 g from the thoroughly wet and mixed portion of the soil
22、prepared in accordance with T 89. Take the sample at any stage of the mixing process at which the mass becomes plastic enough to be easily shaped into a ball without sticking to the fingers excessively when squeezed. If the sample is taken before completion of the liquid limit test, set it aside and
23、 allow to season in air until the liquid limit test has been completed. If the sample taken during the liquid limit test is too dry to permit rolling to a 3-mm thread as described in Section 6, add more water and remix (Note 3). 6. PROCEDURE 6.1. Determine and record the mass of the moisture content
24、 container. 6.2. Select a 1.5- to 2.0-g portion from the 8-g mass of soil prepared in accordance with Section 5. Form the selected portion into an ellipsoidal mass. 6.3. Use one of the following methods to roll the soil mass into a 3-mm-diameter thread at a rate of 80 to 90 strokes per minute, count
25、ing a stroke as one complete motion of the hand forward and back to the starting position again. 6.3.1. Hand Rolling MethodRoll the mass between the palm or fingers and the ground-glass plate or unglazed paper with just sufficient pressure to roll the mass into a thread of uniform diameter throughou
26、t its length. Deform the thread further on each stroke until its diameter reaches 3 mm. Take no more than 2 min to roll the soil mass to the 3-mm diameter. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law
27、.TS-1a T 90-4 AASHTO Note 4The amount of hand or finger pressure required will vary greatly, according to the soil. Fragile soils of low plasticity are best rolled under the outer edge of the palm or at the base of the thumb. 6.3.2. Alternate Procedure, Plastic Limit Device MethodPlace the soil mass
28、 on the bottom plate. Place the top plate in contact with the soil mass. Simultaneously apply a slight downward force and back and forth motion to the top plate so the plate comes in contact with the side rails within 2 min. During this rolling process, do not allow the soil thread to contact the si
29、de rails. Note 5In most cases, more than one soil mass (thread) can be rolled simultaneously in the plastic limit rolling device. 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 r
30、olling 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
31、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 contin
32、uing the rolling without further deformation until the thread falls apart (Note 7). Note 6The 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 bo
33、th 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 segm
34、ents each about 6 to 9 mm in length. Note 7For 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 plac
35、e 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 8-g specimen is tested. Place all of the crumbled portions into the same moisture co
36、ntent container. 6.7. Determine the moisture content of the soil in the container in accordance with T 265, and record the results. 7. CALCULATIONS 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
37、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= (1) 7.3. Report the results as calculat
38、ed 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 (nonplastic). 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Du
39、plication is a violation of applicable law.TS-1a T 90-5 AASHTO 7.3.2. When the plastic limit is equal to, or greater than, the liquid limit, report the plasticity index as NP. 8. PRECISION STATEMENT 8.1. This precision statement applies to soils with a plastic limit range between 15 and 32, tested u
40、sing the hand rolling method. 8.2. Repeatability (Single Operator)Two results obtained by the same operator on the same sample in the same laboratory using the same apparatus should be considered suspect if they differ by more than 10 percent of their mean. 8.3. Reproducibility (Multilaboratory)Two
41、results obtained by different operators in different laboratories should be considered suspect if they differ from each other by more than 18 percent of their mean. 1The plastic limit rolling device is covered by a patent (U.S. Patent No. 5,027,660). Interested parties are invited to submit informat
42、ion regarding the identification of an alternative(s) to this patent to AASHTO Headquarters. Your comments will receive careful consideration at a meeting of AASHTO Technical Section 1a on Soil Materials Tests. 2Bobrowski, L. J., Jr. and D. M. Griekspoor, “Determination of the Plastic Limit of a Soil by Means of a Rolling Device,” Geotechnical Testing Journal, GTJODJ, Vol. 15, No. 3, September 1992, pp. 284287. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.
