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vii ABSTRACT This report presents the results of an interlaboratory study (ILS) and data mining of Proficiency Sample Program (PSP) to prepare precision estimates for AASHTO T180 test method used for determining the relationship between the moisture content and density of soil materials. The materials for the ILS included two coarse- and two fine-grained soil-aggregate mixtures that were prepared according to Grading A and Grading E of AASHTO M147, âMaterials for Aggregate and Soil-Aggregate Subbase, Base, and Surface Courses.â Each of the blends had less than 10% material passing #200 sieve opening to represent suitable base and subbase materials. The materials used for PSP included three sets of lean clay with sand with each blend having about 85% materials passing #200 sieve opening. The comparison of statistics of the maximum dry density and optimum moisture content from ILS and PSP data indicated that the variability of the maximum density increases with the level of coarseness and the variability of the optimum moisture content increases with the level of fineness. It was also observed that the variability of optimum moisture content of clayey blends is higher than those of silty blends. A precision statement for AASHTO T180 including precision estimates computed from the ILS and PSP statistics has been prepared and provided in the report.
