Federal Highway Administration
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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-07-052
Date: September 2007
This LTPP program protocol describes the laboratory preparation and testing procedures for the determination of the Resilient Modulus (Mr) of unbound granular base and subbase materials and subgrade soils under specified conditions representing stress states beneath flexible and rigid pavements subjected to moving wheel loads. This protocol is based partially on the test standard AASHTO T292-91I, Resilient Modulus of Subgrade Soils and Untreated Base/Subbase Materials. The test shall be carried out in accordance with the following protocol procedure.
The methods described are applicable to: undisturbed samples of natural and compacted subgrade soils, and to disturbed samples of unbound base and subbase and subgrade soils prepared for testing by compaction in the laboratory.
In this protocol, stress levels used for testing specimens for resilient modulus will be based upon the location of the specimen within the pavement structure. Samples located within the base and subbase will be subjected to different stress levels as compared to those specimens that are from the subgrade. Generally, specimen size for testing depends upon the type of material based upon the gradation and the PL of the material as described in a later section.
The value of Mr determined from this protocol procedure is a measure of the elastic modulus of unbound base and subbase materials and subgrade soils recognizing certain nonlinear characteristics.
Mr values can be used with structural response analysis models to calculate the pavement structural response to wheel loads, and with pavement design procedures to design pavement structures.
A repeated axial cyclic stress of fixed magnitude, load duration (0.1 second), and cycle duration (1 second) is applied to a cylindrical test specimen. During testing, the specimen is subjected to a dynamic cyclic stress and a static confining stress provided by means of a triaxial pressure chamber. The total resilient (recoverable) axial deformation response of the specimen is measured and used to calculate the resilient modulus.
The resilient modulus test provides a basic constitutive relationship between stress and deformation of pavement construction materials for use in structural analysis of layered pavement systems.
The resilient modulus test provides a means of characterizing pavement construction materials, including subgrade soils under a variety of conditions (i.e., moisture, density, etc.) and stress states that simulate the conditions in a pavement subjected to moving wheel loads.
Thin-walled tube samples of the subgrade for use in resilient modulus testing shall be kept in an environmentally protected (enclosed area not subjected to the natural elements) storage area at temperatures between 5°C (41°F) and 21°C (70°F). They shall be stored on their ends in the same orientation as retrieved in the field.
Bulk samples of base/subbase and subgrade materials should be kept in an environmentally protected storage area at temperatures between 5°C (41°F) and 38°C (100°F).
Each sample shall have a label or tag attached that clearly identifies the material, the project number/test section from which it was recovered and the sample number, as a minimum. Bulk granular samples shall be marked with two tags. One shall be placed inside the bag and one attached to the outside.
In this protocol, the International System of Units (SI - The Modernized Metric System) is regarded as the standard. Units are expressed first in their "soft" metric form followed, in parenthesis, by their U.S. Customary unit equivalent.
Resilient modulus testing shall be conducted after; (1) approval by the FHWA COTR to begin unbound material resilient modulus testing, (2) approval of Form L04 by the FHWA-LTPP Region, (3) appropriate material classification tests are completed and (4) final layer assignments (corrected form L04, if needed) have been completed. To attain approval under item (1), the laboratory must; (a) submit and obtain approval of the QC/QA plan for the unbound materials resilient modulus testing, (b) demonstrate that their testing equipment meets or exceeds the specifications contained in this protocol, and (c) successfully complete all applicable requirements of the Start-up and QC Procedure for LTPP P46 Resilient Modulus Testing.
The test shall be performed on the test specimens prepared from bulk samples of the unbound granular base and subbase materials retrieved from BA-type, 305-mm (12-inch) diameter, boreholes from the test pit(s) or from other bulk sampling locations as dictated by the sampling plans for the particular LTPP section.
For the subgrade soils, the test shall be carried out on undisturbed thin-walled tube samples retrieved from A-type, 152-mm (6-inch) diameter, boreholes and other sampling areas; if available. If the thin-walled tube samples are unavailable or unsuitable for testing, or if directed by the FHWA COTR, then bulk samples of subgrade soils shall be used to remold test specimens for resilient modulus tests. Bulk samples of subgrade soils are retrieved from BA-type, 305-mm (12-inch) diameter boreholes, test pit(s) or from other bulk sampling locations as dictated by the sampling plans for the particular LTPP test section.
The test results shall be reported separately for test samples obtained from the bulk samples collected at the beginning and end of the test section as follows:
(a) Beginning of the Section (Stations 0-): Bulk and thin-wall tube samples of each layer that are retrieved from areas in the approach end of the test section (stations preceding 0+00) shall be assigned Laboratory Test Number '1'.
(b) End of the Section (Stations 5+): Bulk and thin-wall tube samples of each layer that are retrieved from areas in the leave end of the test section (stations after 5+00) shall be assigned Laboratory Test Number '2'.
(c) Within the Section (Stations 0+00 - 5+00): Bulk and thin-wall tube samples of each layer that are retrieved from areas within the test section shall be assigned Laboratory Test Number '3'.