Precision Estimates of AASHTO T265: Laboratory Determination of Moisture Content of Soils (2010)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

17 CHAPTER 4- CONCLUSIONS AND RECOMMENDATIONS 4.1 Conclusions This study was conducted to prepare precision estimates for AASHTO T265, “Laboratory Determination of Moisture Content of Soils.” An interlaboratory study was conducted to collect data from testing four aggregate-soil blends that were found suitable for base and subbase construction. Four blends, two coarse graded and two fine graded, with limited amount of materials passing #200 sieve were selected. The difference between the blends with similar gradation was in the type of filler added. One blend of each gradation was prepared with clay and the other blend was prepared with silt. The following summarizes the findings of this study:  The standard deviations of the blends with clay were not significantly different from those of the blends with silt and therefore standard deviations were combined.  The standard deviations of the coarse blends with 3% moisture (below optimum) were not significantly different from those of the blends with 5% moisture (at optimum) and therefore were combined.  The standard deviations of the coarse blends with 7% moisture (above optimum) were significantly different from those of the blends with 3% and 5% moisture content. Due to uncertainty in the results of 7% moisture content, they were not included in the precision estimate analysis.  The standard deviations of the fine blends with 4% moisture content (below optimum) and those of the blends with 6% moisture content (at optimum) were not significantly different and therefore standard deviations were combined.  The bias and low precision of the moisture content data was speculated to be due to availability of excess moisture for evaporation. When mixture is above the optimum, free moisture particles are available to evaporate and escape from the pores of the bottles. While in mixtures below the optimum and at the optimum, moisture particles are adhered to the soil-aggregate particles.  The standard deviations of the fine blends with 8 % moisture content (above optimum) were significantly different from those of the blends with 4% and 6% moisture content. Due to uncertainty in the results of 8% moisture content, they were not included in the precision estimate analysis.  The standard deviations of the coarse blends were significantly different from those of fine blends. Therefore the computed precision estimates from the two blends were presented separately in a proposed precision statement. 4.2 Recommendations The accurate determination of water content of a soil-aggregate blend is important for the proper preparation of base and subbase of a pavement. The level of accuracy in which water content of base and subbase is measured has a significant effect on the performance of the pavement as a whole. Currently, there are no precision estimates that

18 would define the accuracy requirements for water content measurements following AASHTO T265. Therefore, it is recommended that the precision statement in Appendix F, which is prepared based on analysis of the data collected through an interlaboratory study to be published in AASHTO T265.