Long Term Pavement Performance Project Laboratory Materials Testing and Handling Guide

Protocol P46
Test Method for Resilient Modulusof Unbound Granular Base/Subbase Materials and Subgrade Soils (UG07, SS07)

7. PREPARATION OF TEST SPECIMENS

7.1 GPS Materials Characterization Program - General

Unless otherwise directed by the FHWA COTR, the following preparation steps shall be followed for the GPS materials characterization program, based on the sieve analysis test results (See Form T41 or T51 as appropriate).

7.1.1 Use the 71-mm (2.8 inch) diameter undisturbed specimen from the thin walled tube samples for cohesive subgrade soils (Material Type 2). The specimen length shall be at least two times the diameter (minimum length of 142 mm [5.6 inches]) and the specimen shall be prepared as described in Section 7.2. If undisturbed subgrade samples are unavailable or unsuitable for testing, then 71-mm (2.8 inch) diameter molds shall be used to reconstitute Type 2 test specimens.

NOTE 7: If 10 percent or less of a Type 2 sample is retained on the 12.5-mm (0.5-inch) sieve, the material greater than the 12.5-mm (0.5-inch) sieve shall be scalped off prior to testing. If more than 10 percent of the sample is retained on the 12.5-mm (0.5-inch) sieve, the material shall not be tested and the material shall be stored until further notice. Instructions concerning the testing of these materials will be issued at a later date.

7.1.2 Use 152-mm (6.0-inch) diameter split molds to prepare 305-mm (12-inch) high test specimens for all Type 1 materials with nominal particle sizes less than or equal to 37.5 mm (1.5 inches).

NOTE 8: If 10 percent or less of a Type 1 sample is retained on the 37.5-mm (1.5-inch) sieve, the material greater than the 37.5-mm (1.5-inch) sieve shall be scalped off prior to testing. If more than 10 percent of the sample is retained on the 37.5-mm (1.5-inch) sieve, the material shall not be tested and the material shall be stored until further notice. Instructions concerning the testing of these materials will be issued at a later date.

7.2 GPS - Undisturbed Subgrade Soil Specimens

Undisturbed subgrade soil specimens are trimmed and prepared as described in AASHTO T234 85, Strength Parameters of Soils by Triaxial Compression, using the thin walled tube samples of the subgrade soil. The natural moisture content (w) of the tube sample shall be determined after triaxial M_r testing, following the procedure outlined in LTPP Protocol P49 (AASHTO T265 86), and recorded in the test report.

The following procedure shall be followed for the thin walled tube samples:

7.2.1 Examine the thin walled tube samples obtained from the same sampling location separately. Select the sample most suitable for testing (see NOTE 9) giving priority to samples extracted near the surface of the subgrade. That is, the sample should be taken from the top of the first tube pushed, if it is suitable for testing. If not, examine samples from increasing depths in the subgrade, selecting the first sample suitable for testing. In any case, the depth in relation to the top of the subgrade that the sample is obtained from should be noted on Laboratory Test Data Sheet T46.

NOTE 9: To be suitable for testing, a specimen of sufficient length (at least twice the diameter of the specimen after preparation) must be cut from the tube sample, and must be free from defects that would result in unacceptable or biased test results. Such defects include cracks in the specimen, corners broken off that cannot be repaired during preparation, presence of particles much larger than that typical for the material (example, +19.0-mm (+¾ inch) stones in a fine grained soil), presence of "foreign objects" such as large roots, wood particles, organic material and gouges due to gravel hanging on the edge of the tube. If the gradation and PI tests indicate that the material (from a bulk sample) corresponding to a thin-wall tube is actually a Type 1 material, the thin-walled tube shall not be used and a specimen must be recompacted (as a Type 1 material) using the bulk sample. If the gradation test indicates that more than 10 percent of a Type 2 sample is retained on the 12.5-mm (0.5-inch) sieve, the material shall not be tested and the material shall be stored until further notice. Instructions concerning the testing of these materials will be issued at a later date.

7.2.2 If a good undisturbed subgrade sample is unavailable from a particular location, a reconstituted specimen shall be prepared from the bulk sample from the same end of the test section and same layer. Select a sample for reconstitution, again giving priority to samples extracted near the surface of the subgrade.

7.3 GPS - Laboratory Compacted Specimens

Reconstituted test specimens of both Type 1 and Type 2 materials shall be prepared to approximate the in situ wet density (γ_w) and moisture content (w). These laboratory compacted specimens shall be prepared for all unbound granular base and subbase material and for all subgrade soils for which undisturbed tube specimens could not be obtained.

This protocol states that reconstituted test specimens should be compacted to in-situ moisture and density conditions as measured in the field using nuclear methods (AASHTO T239) whenever these data are available. This requirement was instituted in the protocol in an attempt to better correlate laboratory test results and those from the analysis of deflection measurements performed immediately prior to sampling. It is important to recognize that correlating the laboratory determined resilient modulus values of soils and unbound aggregate at in-situ moisture and density with that obtained from analysis of pavement deflection measurements is an important objective of the LTPP GPS materials characterization program.

However, for some samples, it may be virtually impossible to compact specimens to the measured in-situ moisture and density. In this case, the sample shall be compacted using the alternative compaction requirements of P46—compact at optimum moisture content and 95 percent of the maximum dry density of the material (section 7.3.3). The decision to use the alternate compaction procedure is at the discretion of the laboratory Supervisory Engineer and should be made on a case-by-case basis. However, every effort shall be made to compact the samples to in-situ conditions prior to electing the alternative sample compaction procedure.

In those cases where the measured in-situ properties at the time of sampling are not available, the sample should also be prepared following the alternative compaction procedure. However, the unavailability of this data must be verified with the corresponding LTPP Region prior to sample preparation. This caveat only applies to the GPS materials characterization program. For the SPS materials characterization program all samples shall be compacted to optimum moisture and 95 percent maximum dry density as described in Section 7.4 of this protocol.

7.3.1 Moisture Content - The moisture content of the laboratory compacted specimen shall be the in-situ moisture content obtained in the field using AASHTO T238 (nuclear method) for that layer. If data is not available on in-situ moisture content, then refer to Section 7.3.3.

The moisture content of the laboratory compacted specimen should not vary by more than ± 1.0 percent for Type 1 materials or ± 0.5 percent for Type 2 materials from the in situ moisture content obtained for that layer.

7.3.2 Compacted Density - The density of the compacted specimen shall be the in-place wet density obtained in the field using AASHTO T239 (nuclear method) for that layer. If this data is not available on in-situ density, then refer to Section 7.3.3.

The wet density of the laboratory compacted specimen should not vary more than ± 3 percent of the in-place wet density for that layer.

7.3.3 If either the in-situ moisture content or the in-place density data is not available, then use the optimum moisture content and 95 percent of the maximum dry density (previously determined using LTPP Protocol P44 (Base/Subbase) or LTPP Protocol P55 (Subgrade) for preparing the reconstituted specimen.

The moisture content of the laboratory compacted specimen should not vary by more than ± 1.0 percent for Type 1 materials or ± 0.5 percent for Type 2 materials from the target moisture content. Also, the wet density of the laboratory compacted specimen should not vary more than ± 3 percent of the target wet density.

7.3.4 Sample Reconstitution - Reconstitute the specimen for Type 1 and Type 2 materials in accordance with the provisions given in Appendix A. The target moisture content and density to be used in determining needed material quantities are as established in Section 7.3. Appendix A provides guidelines for reconstituting the material to obtain a sufficient amount of material to prepare the appropriate specimen type at the designated moisture content and density. After this step is completed, specimen compaction can begin.

7.4 SPS Materials Characterization Program

Unless otherwise directed by the FHWA COTR, the following preparation steps shall be followed for the SPS materials characterization program.

7.4.1 Undisturbed Subgrade Soil Specimens Undisturbed subgrade soil specimens are trimmed and prepared as described in AASHTO T234 85, Strength Parameters of Soils by Triaxial Compression, using the thin walled tube samples of the subgrade soil. The specimen length shall be at least two times the diameter (minimum length of 142 mm [5.6 inch]). The natural moisture content (w) of the tube sample shall be determined after triaxial M_r testing, following the procedure outlined in LTPP Protocol P49 (AASHTO T265 86), and recorded in the test report.

The following procedure shall be followed for the thin walled tube samples:

Examine the thin walled tube samples obtained from the same sampling location separately. Select the sample most suitable for testing (see NOTE 10) giving priority to samples extracted near the surface of the subgrade. That is, the sample should be taken from the top of the first tube pushed, if it is suitable for testing. If not, examine samples from increasing depths in the subgrade, selecting the first sample suitable for testing. In any case, the depth in relation to the top of the subgrade that the sample is obtained from should be noted on Laboratory Test Data Sheet T46.

NOTE 10: To be suitable for testing, a specimen of sufficient length (at least twice the diameter of the specimen after preparation) must be cut from the tube sample, and must be free from defects that would result in unacceptable or biased test results. Such defects include cracks in the specimen, corners broken off that cannot be repaired during preparation, presence of particles much larger than that typical for the material (example, +19.0-mm [+¾ inch] stones in a fine grained soil), presence of "foreign objects" such as large roots, wood particles, organic material and gouges due to gravel hanging on the edge of the tube. If the gradation and PI tests indicate that the material (from a bulk sample) corresponding to a thin-wall tube is actually a Type 1 material, the thin-walled tube shall not be used.

7.4.2 Laboratory Compacted Specimens Reconstituted test specimens of both Type 1 and Type 2 materials shall be prepared to the optimum moisture content and 95 percent of the maximum dry density (previously determined using LTPP Protocol P44 (Base/Subbase) or LTPP Protocol P55 (Subgrade)). Use 71-mm (2.8-inch) diameter molds to reconstitute Type 2 test specimens and 152-mm (6.0-inch) diameter split molds to reconstitute Type 1 materials.

NOTE 11: If 10 percent or less of a Type 2 sample is retained on the 12.5-mm (0.5-inch) sieve, the material greater than the 12.5-mm (0.5-inch) sieve shall be scalped off prior to testing. If more than 10 percent of the sample is retained on the 12.5-mm (0.5-inch) sieve, the material shall not be tested and the material shall be stored until further notice. Instructions concerning the testing of these materials will be issued at a later date.

NOTE 12: If 10 percent or less of a Type 1 sample is retained on the 37.5-mm (1.5-inch) sieve, the material greater than the 37.5-mm (1.5-inch) sieve shall be scalped off prior to testing. If more than 10 percent of the sample is retained on the 37.5-mm (1.5-inch) sieve, the material shall not be tested and the material shall be stored until further notice. Instructions concerning the testing of these materials will be issued at a later date.

The moisture content of the laboratory compacted specimen should not vary by more than ± 1.0 percent for Type 1 materials or ± 0.5 percent for Type 2 materials from the target moisture content. Also, the wet density of the laboratory compacted specimen should not vary more than ± 3 percent of the target wet density.

7.4.3 Sample Reconstitution - Reconstitute the specimen for Type 1 and Type 2 materials in accordance with the provisions given in Appendix A. The target moisture content and density to be used in determining needed material quantities are as established in this section. Appendix A provides guidelines for reconstituting the material to obtain a sufficient amount of material to prepare the appropriate specimen type at the designated moisture content and density. After this step is completed, specimen compaction can begin.

7.5 GPS and SPS - Compaction Methods and Equipment for Reconstituting Specimens

7.5.1 Compacting Specimens for Type 1 Materials The general method of compaction for Type 1 materials will be that of Appendix B of this protocol.

7.5.2 Compacting Specimens for Type 2 Materials The general method of compaction for Type 2 materials will be that of Appendix C of this protocol.

7.5.3 The prepared specimens should be protected from moisture change by applying the triaxial membrane and tested within 5 days of completion. Prior to storage and directly after removal from storage, the specimen shall be weighed to determine if there was any moisture loss. If moisture loss exceeds 1 percent for Type 1 material or 0.5 percent for Type 2 materials, then the prepared specimens will not be tested. However, a new specimen will need to be prepared for testing. Material from the specimens not tested may be reused.