Precision Estimates of AASHTO T 180: Moisture-Density Relations of Soils Using a 4.54-kg (10-lb) Rammer and a 457-mm (18-in.) Drop (2010)

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22 CHAPTER 6- CONCLUSIONS AND RECOMMENDATIONS 6.1 Summary This study was conducted to prepare precision estimates for AASHTO T180, “Moisture-Density Relations of Soils Using a 4.54-kg (10-lb) Rammer and a 457-mm (18-in.) Drop.” An interlaboratory study (ILS) was conducted to collect data from testing four aggregate -soil blends that were found suitable for construction of base and subbase. In addition, the data from the three most recent rounds of AMRL Soil Classification and Compaction Proficiency Sample Program were utilized to compute precision estimates. The four blends in 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, “Materials for Aggregate and Soil-Aggregate Subbase, Base, and Surface Courses.” 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. Since the materials used for the ILS and PSP samples were very different, the developed precision estimates cover a wide range of maximum densities and optimum moisture contents. The following presents a summary of the findings: • The average maximum density of the fine-graded blends was significantly smaller than those of coarse-graded blends. However, the standard deviations of the maximum density of the fine and coarse graded blends were the same. Therefore, the repeatability and reproducibility standard deviations of the maximum dry density of the ILS blends were combined. • The average optimum moisture content of the fine-graded blends is significantly larger than those of coarse-graded blends. However, the standard deviations of the optimum moisture content of the fine and coarse blends were the same. Therefore, the repeatability and reproducibility standard deviations of optimum moisture content of the ILS blends were combined. • The average maximum density of the ILS blends was significantly larger than that of PSP blends. In addition, the pooled standard deviations of maximum dry density from PSP were significantly smaller than those of ILS. Therefore, the standard deviations from the two sources were presented separately in the precision statement of AASHTO T180. • The average optimum moisture content of the ILS blends was significantly smaller than that of PSP. In addition, the pooled standard deviations of optimum moisture content from PSP were significantly larger than those of ILS. Therefore, the standard deviations from the two sources were presented separately in the precision statement of AASHTO T180. 6.2 Conclusions Based on the results of AASHTO T180 interlaboratory study and AMRL Soil

23 Classification and Compaction Proficiency Sample Program data a precision statement was prepared. Since the difference between the precision of maximum density and optimum moisture content from ILS and PSP were significantly different, the Sr and SR 6.3 Recommendations estimates computed from the two sources were presented separately in the prepared precision statement. The proposed precision statement is provided in Appendix E of this report. The design and control of compaction of soils is an important aspect of a pavement construction process. The level of accuracy in which a degree of compaction of soil-aggregate mixture is measured has a significant effect on the performance of a pavement as a whole. Currently, there are no precision estimates that would define the accuracy requirements for measuring maximum dry density and optimum moisture content of the soil-aggregate blends compacted according to AASHTO T 180. Therefore, it is recommended that the precision statement in Appendix E, which is prepared based on analysis of the data collected through an interlaboratory study and from the three most recent rounds of AMRL Soil classification and Compaction Proficiency Sample Program to be published in AASHTO T180 test method.