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

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3 CHAPTER 2- DESIGN AND CONDUCT OF THE ILS The development of precision estimates for AASHTO T265 required conduct of an interlaboratory study (ILS) involving measurement of moisture content of selected soil-aggregate blends prepared with known levels of moisture. The following sections will report the details of the design of the ILS. The approach used for the design of the ILS was based on ASTM E691-07, Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method [2]. The development of the precision statement for T265 required participation of a minimum of 6 laboratories with a preferred number of 30 as specified in E691. 2.1 Materials Selection The materials used in the study were blended according to the Grading A and Grading E requirements of AASHTO M147, “Materials for Aggregate and Soil- Aggregate Subbase, Base, and Surface Courses” [3]. Grading A was used to create a coarse gradation with a 19.0-mm nominal maximum aggregate size and Grading E was used to create a finer gradation with a 4.75-mm nominal maximum aggregate size. Four soil-aggregate blends were prepared, two fine graded and two coarse graded. The two fine and the two coarse graded blends were similar in gradation but differed by the type of mineral filler (passing # 200). Two of the blends, one coarse and one fine, included silt as mineral filler, and the other two blends included clay as mineral filler. The amount of filler was limited to 7% in all four mixtures to meet the requirement for good quality subbase and base materials. The gradations of the four mixtures as well as Gradings A and E from AASHTO M147 are provided in Table 2-1. The sources of aggregate materials utilized in the study and their classifications according to AASHTO M 145 [4] are provided in Table 2-2. Table 2-1: Gradations of ILS fine and coarse blends and Grading E and A of AASHTO M147 Sieve Size Fine w/Clay Fine w/Silt Grading E Coarse w/Clay Coarse w/Silt Grading A 1"  100.0  100.0  100 100.0 100.0  100 1/2"  100.0  100.0  100 90.0 90.8  ‐ 3/8"  100.0  100.0  100 64.0 64.0  30‐65 # 4  99.8  99.8  55‐100 45.9 46.9  25‐55 #8  45.2  46.2  ‐ 29.8 30.8  ‐ # 10  41.6  42.5  40‐100 23.6 24.6  15‐40 # 40  22.5  23.0  20‐50 11.3 11.8  8‐20 # 200  7.1  6.9  6‐20 7.0 7.0  2‐8

4 Table 2-2: Sources and classifications of ILS soil-aggregate blends according to AASHTO M145 Soil‐ Aggregate‐  Type  Soil‐Aggregate  Classification  (AASHTO M  145)  Materials  Source  Fine‐Graded  (Grading E of  AASHTO  M147)  A3  Crushed Limestone (particle size  passing #4 and retained on #8 )  Lafarge Frederick, MD Washed Concrete Sand (Natural Sand  Passing #8)   Aggtrans in Hanover, MD Lean Clay (CL) Aggregate Transport  Corporation in Harwood, MD  Silt (ML)  U.S. Army Corps of Engineers,  Waterways Experimental  Station in Vicksburg, MS  Coarse‐ Graded  (Grading A  of AASHTO  M147)  A1 Crushed Limestone     Lafarge Frederick, MD Manufactured Fine Aggregate  (Limestone Buell Dust)  Lafarge Frederick, MD Lean Clay (CL) Aggregate Transport  Corporation in Harwood, MD  Silt (ML)  U.S. Army Corps of Engineers,  Waterways Experimental  Station in Vicksburg, MS  2.2 Participating Laboratories Hundreds of laboratories that are certified by the AASHTO Accreditation Program (AAP) [5] for soil and aggregate testing were contacted and invited to participate in the T265 ILS. The laboratories were ranked by their scores earned through the accreditation process. Thirty-five laboratories including commercial, governmental, and research laboratories with the maximum score of 5 were selected to participate in the study. 2.3 Interlaboratory Sample Preparation and Shipping The ILS samples were prepared by the AMRL staff in the Proficiency Sample Facility located at the National Institute of Standards and Technology (NIST) using procedures developed for the AMRL Proficiency Sample Program [6]. A total of 1260 samples were prepared to be sent to the 35 selected laboratories. Each laboratory received 36 samples that consisted of three replicates of each of the four soil-aggregate blends prepared at three different percentages of moisture. The coarse blend samples weighed about 350 g and the fine blends samples weighed about 150 g. The fine blend samples were prepared with 4%, 6%, and 8% moisture and the coarse blend samples were prepared with 3 %, 5 %, and 7% moisture. Tables 2-3 through 2-6 provide the weight of the components of each blend.

5 Table 2-3: Weights (g) of various components of the coarse-graded samples with clay (CC) ID  #1/2  #3/8  #4  #8  Buell Dust  Clay  Water  CC (3 %)  35.0  91.0  59.5  35.0  105.0  24.5  10.5  CC (5 %)  35.0  91.0  59.5  35.0  105.0  24.5  17.5  CC (7 %)  35.0  91.0  59.5  35.0  105.0  24.5  24.5  Table 2-4: weights (g) of different components of the coarse-graded samples with silt (CS) ID  #1/2  #3/8  #4  #8  Buell Dust  Silt  Water  CS (3 %)  31.5  94.5  56.0  35.0  105.0  28.0  10.5  CS (5 %)  31.5  94.5  56.0  35.0  105.0  28.0  17.5  CS (7 %)  31.5  94.5  56.0  35.0  105.0  28.0  24.5  Table 2-5: weights (g) of different components of the fine- graded samples with clay (FC) ID  #8  Sand  Clay  Water  FC (4 %)  76.5  60.0  13.5  6.0  FC (6 %)  76.5  60.0  13.5  9.0  FC (8 %)  76.5  60.0  13.5  12.0  Table 2-6: weights (g) of different components of the fine graded samples with silt (FS) ID  #8  Sand  Silt  Water  FS (4 %)  75.0  60.0  15.0  6.0  FS (6 %)  75.0  60.0  15.0  9.0  FS (8 %)  75.0  60.0  15.0  12.0  2.4 Interlaboratory Study Instructions Laboratory participants were provided with the testing instructions and data sheets to record the data. The laboratories were requested to follow AASHTO T265 to determine the moisture content of the four soil-aggregate blends, each prepared with three different moisture percentages. The instructions and the data entry sheet are provided in Appendix A.