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Through the work of the Strategic Highway Research Program (SHRP, 1987-1992) and the Federal Highway Administration (FHWA, 1992-present), the government has provided financial and technical assistance to develop and improve a laboratory test method to determine the resilient modulus properties of unbound materials. Although the work -- part of the Long Term Pavement Performance (LTPP) study -- has led towards the adoption of test procedure T307-99 in the current release of the American Association of State Highway and Transportation Officials (AASHTO) Tests, many skeptics insist that the method does not lend itself towards repeatable, reproducible test results.
"Resilient modulus indicates the stiffness of a soil under controlled confinement conditions and repeated loading. The test is intended to simulate the stress conditions that occur in the base and subgrade of a pavement system. Resilient modulus has been adopted by the U.S. federal highway administration as the primary performance parameter for pavement design. We thank those who prepared these papers, the reviewers who provided anonymous peer reviews, and those who participated in the symposium. We hope this STP encourages more work to improve the testing standard and the value of the Resilient Modulus test."
Numerous research efforts have been devoted to characterizing the behavior of granular materials, which is one of the main concerns of pavement engineers. For better understanding of this behavior, laboratory tests where in-situ stress conditions and traffic loads are adequately simulated are needed. This study makes use of an expanded test protocol called a performance test that includes resilient modulus as well as permanent deformation testing. This test protocol determines three nonlinear resilient modulus parameters (k1, k2, k3) and two permanent deformation parameters ([alpha], [mu]). The resilient modulus test results are required inputs in the Level 1 analysis of the proposed American Association of State Highway and Transportation Officials (AASHTO) Pavement Design Guide. In addition, both resilient modulus and permanent deformation test results provide material property inputs to pavement performance prediction models. This study also evaluated the within laboratory repeatability of the performance test and developed a within laboratory precision statement. Further, a statistical analysis was conducted on the test results to estimate the number of test specimens required for testing for specific reliability levels. Two test specimens are required for a reliability level of 15%. A within laboratory study was also conducted to investigate the influence of specimen size on test results. The specimen height was reduced from 12 in. (304 mm) to 8 in. (203 mm), and there was no difference in test results at a confidence level of 95%. The performance test was further used successfully in subsequent studies to evaluate the behavior of granular materials and the influence of various factors on their behavior. As fines content increased, the resilient modulus values decreased and permanent deformation increased. As the moisture content increased, the resilient modulus value decreased and the resistance to permanent deformation decreased. A simplified laboratory measurement tool that is repeatable, relatively cheap and easy to perform might prompt the use of laboratory measured values of resilient modulus in pavement design and facilitate correlation of these values to field measured values on a large scale. Use of measured data for the base properties rather than estimates would insure improved pavement designs and, in many cases, would save money in construction costs.
The resilient modulus test is commonly used to determine the modulus of base or subgrade materials as well as to establish their nonlinear behavior. Since the resilient modulus test is time consuming, the number of tests performed for a given project is limited. For day-to-day operation of highway agencies, a more rapid test method is needed. The stress wave (or seismic) method is being considered in Texas for this purpose. Seismic methods of testing can rapidly and nondestructively provide fundamentally correct moduli at known states of stress. Unlike the resilient modulus test, comparative field testing methods are available for seismic methods that can provide similar results under similar conditions. This paper describes the seismic test procedure and its relationship to the resilient modulus test results. Also discussed are the repeatability and reproducibility of the results as a function of operator experience, type of soil, and preparation method.
The U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) conducted resilient modulus tests on materials from the Mn/ROAD test site for the Minnesota Department of Transportation. Materials tested included samples of the lean clay subgrade at the site and the two extreme grades of base designed specifically for Mn/ROAD. Some specimens were tested in both frozen and subsequently 'thawed' conditions; others were tested at room temperature without ever having been frozen. Researchers performed linear regression analysis on the data to develop equations that predict frozen modulus based on unfrozen water content and unfrozen modulus based on stress, degree of saturation and density. We also reanalyzed data from two previously tested materials. CRREL can use the study's equations in the Mechanistic Pavement Design and Evaluation Procedure under development at CRREL to predict estimated damage in some Mn/ROAD test sections.