Laboratory Determination of Resilient Modulus for Flexible Pavement Design (2004)

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National Cooperative Highway Research Program RESEARCH RESULTS DIGEST January 2004—Number 285 Subject Areas: IIB Pavement Design, Management, Responsible Senior Program Officer: Edward T. Harrigan and Performance and IIIB Materials and Construction Laboratory Determination of Resilient Modulus for Flexible Pavement Design This digest presents key findings from NCHRP Project 1-28A, “Harmonized Test Methods for Laboratory Determination of Resilient Modulus for Flexible Pavement Design,” conducted by the University of Maryland – College Park. The digest is an abridgement of portions of the project final report by the principal investigator, Matthew W. Witczak, Ph.D., Professor Emeritus, University of Maryland—College Park, and Professor, Arizona State University, Tempe, Arizona. This digest presents the two key products from research conducted under NCHRP Project 1-28A, “Harmonized Test Methods for Laboratory Deter- mination of Resilient Modulus for Flexible Pave- ment Design.” The objective of Project 1-28A was to develop (1) a test method for measurement of the resilient modulus of hot mix asphalt (HMA) that harmonizes the procedure proposed by NCHRP Project 1-28 with the existing AASHTO TP31 method and the FHWA LTPP Laboratory Start- Up and Quality Control Procedure and (2) a test method for measurement of the resilient modulus of unbound granular base and subbase materials and subgrade soils that harmonizes the procedure proposed by Project 1-28 with the existing AASHTO TP46, T 292, and T 294 methods and the FHWA LTPP Laboratory Start-Up and Quality Control Procedure. The resilient modulus, MR, of unbound base, subbase, and subgrade materials is a key input property for the mechanistic-empirical pavement design procedure developed in NCHRP Project 1-37A, “Development of the 2002 Guide for the Design of New and Rehabilitated Pavement Struc- tures: Phase II.” Inaccurate determination of the resilient modulus of the unbound materials in the pavement structure will contribute to erroneous predictions of overall pavement response and pave- ment performance. NCHRP Project 1-28, “Laboratory Determi- nation of Resilient Modulus for Flexible Pavement Design,” completed in 1997 by the Georgia Institute of Technology, produced an excellent set of findings relative to the MR characterization of (1) HMA materials through the indirect tensile (diametral) test and (2) unbound base and subbase materials and subgrade soils through the triaxial test. The test procedures recommended by Project 1-28 more accurately accounted for the effects of varying field conditions, such as temperature of the HMA surface layer or the moisture content of the base, subbase, and subgrade, on the resilient modulus. The resilient modulus procedure proposed by NCHRP 1-28 for measuring MR of HMA mixtures enhanced earlier ASTM, AASHTO, and SHRP procedures to achieve a reduced testing time and more reproducible test results. The resilient modu- lus procedure proposed for unbound materials and soil made use of the grain size distribution and the plasticity index to classify the materials for test purposes. Axial deformation is measured on the sample and the load cell is located inside the tri- axial cell. For very soft specimens that may be damaged by clamps, the measurements are made between the top and bottom platens. Besides clamp-mounted LVDTs, more accurate sensors (e.g., optical extensometers and non-contact sensors) could be used to measure the axial deformations. Three alternative test procedures that take into account both the behavior of the material (granular or fine-grained) and its function within the flexible pavement structure are provided. Scalping and replacing techniques are used to reduce variability in test results. The predictive equation takes into account the effects of both the deviatoric and the volumetric component of the loading in the form