A new laboratory test protocol for the measurement of resilient modulus of unbound pavement materials is described. This harmonized protocol combines the best features from four state-of-the-art resilient modulus test procedures in current use. The harmonized procedure most closely follows the recommended protocol proposed in NCHRP Project 1-28, Appendix E, with some exceptions. The main modifications deal with revised and more rational stress sequences and a more accurate stress- dependent resilient modulus predictive equation. Thirteen different pre- dictive models and 25 sets of resilient modulus test data were evaluated as the basis for the recommended predictive equation. The determination of the elastic moduli of pavement materials is of vital importance for any mechanistically based design or analysis pro- cedure for flexible pavements. The resilient modulus (M R ), defined as the unloading modulus after many cycles of repeated loading, is commonly used in pavement engineering as an appropriate measure of stiffness for the unbound (i.e., soil) layers in a pavement structure. The characterization of unbound material stiffness using the M R was incorporated in the AASHTO pavement design guide beginning with the 1986 edition (1). The selection of an appropriate design M R value for base, subbase, and/or subgrade materials has long been complicated by various test and analysis problems. NCHRP initiated NCHRP Project 1-28, Lab- oratory Determination of Resilient Modulus for Flexible Pavement Design, to address these testing and analysis issues (2). The NCHRP 1-28 study produced an excellent set of recommendations regarding M R characterization with triaxial testing for base, subbase, and sub- grade materials. However, the NCHRP Project 1-28 recommenda- tion for yet another M R test protocol complicated the standardization of a universally accepted testing procedure that could be imple- mented throughout the pavement community. As a consequence, NCHRP Project 1-28A, Harmonized Test Methods for Laboratory Determination of Resilient Modulus for Flexible Pavement Design, was initiated to combine the best features of the various M R testing procedures in current usage (3). OBJECTIVES The major objective of NCHRP Project 1-28A (Task II: Unbound Materials) was to develop a single test method for measuring the M R of unbound granular base and subbase materials and subgrade soils that harmonized all existing testing protocols: AASHTO T 292-91, AASHTO T 294-92, AASHTO T P46-94, and NCHRP 1-28 Draft- 96 (2). The harmonized protocol developed in this research closely follows the recommendations of the NCHRP Project 1-28 Final Report, Appendix E. A flowchart for the original NCHRP Project 1-28 test protocol is presented in Figure 1. The corresponding flow- chart for the harmonized protocol developed in NCHRP Project 1-28A is presented in Figure 2. The key differences between these two protocols include the following: • Changes in material type definitions, • Changes in specimen sizes, • Changes in specimen compaction methods, • Increased loading time for subgrade soils, • Revised stress sequences, and • Revised predictive equation. TEST PROTOCOL DIFFERENCES The key differences between the original NCHRP 1-28 and harmo- nized NCHRP 1-28A test protocols are described in more detail in the following sections. Types of Materials, Specimen Size, and Compaction Methods The material type designation governs the methods for preparing and compacting the test specimens. The harmonized NCHRP Proj- ect 1-28A protocol uses gradation data and Atterberg limits to clas- sify materials as granular, low plasticity materials (Type 1) and as cohesive, plastic materials (plasticity index > 10) having a large per- centage of fines (Type 2). These two types of materials require dif- ferent compaction techniques to simulate the structure of the in situ compacted material. Once the material is classified, the next issue addressed in the protocol is the size of the test sample. The original NCHRP Project 1-28 protocol (Figure 1) recommended the use of 71-mm (2.8-in.)-, 102-mm (4-in.)-, and 152-mm (6-in.)-diameter samples, depending on the maximum particle size (d max ) of the material. Also, for both Harmonized Resilient Modulus Test Method for Unbound Pavement Materials Dragos Andrei, Matthew W. Witczak, Charles W. Schwartz, and Jacob Uzan Transportation Research Record: Journal of the Transportation Research Board, No. 1874, TRB, National Research Council, Washington, D.C., 2004, pp. 29–37. D. Andrei, Fugro Consultants LP, 8613 Cross Park Drive, Austin, TX 78754. M. W. Witczak, Department of Civil and Environmental Engineering, Arizona State University, Tempe, AZ 85287-5306. C. W. Schwartz, Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742. J. Uzan, Transportation Research Institute, Technion–Israel Institute of Tech- nology, Technion City, Haifa 32000, Israel. 29