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3 CHAPTER 2- DESIGN AND CONDUCT OF THE ILS The AASHTO T180 standard test method is intended for determining the relationship between the moisture content and density of soils when compacted with 4.54-kg (10-lb) rammer, dropped from a height of 457 mm (18 in.). Four alternative procedures, Methods A through D, are provided in the test method for determining the soil moisture content- density relationship. The differences between the four procedures are in the size of the molds and the gradation of the soil. Methods A and B applies to fine-graded soil blends passing 4.75-mm sieve, and Methods C and D applies to coarse- graded soil blends passing 19.0 mm sieve. Methods A and C provides instructions for compacting each of the soil types in a 4â mold and Methods B and D provides instructions for compacting each of the soil types in a 6â mold. The AASHTO Subcommittee on Materials has requested the precision estimates of AASHTO T180 to be determined for Method B and Method D, which are specific to compacting fine- and coarse-grained soil blends in 6â mold. The following sections will report the details of the design of an ILS specific to the two methods. The approach used for the design of the ILS was based on ASTM E691 [4], âStandard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method.â The development of the precision statement for T 180 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 M 147, âMaterials for Aggregate and Soil- Aggregate Subbase, Base, and Surface Coursesâ [5]. The Grading A, which is a coarser gradation was used for preparing samples for Method D of AASHTO T 180. The Grading E, which is a finer gradation, was used for preparing samples for Method A of AASHTO T180. Four blends were prepared according to the two fine and coarse gradations. The fine-graded blends had a 4.75-mm nominal maximum aggregate size and the coarse blends had a 19.0-mm nominal maximum aggregate size. The two fine blends were similar in gradation but different in type of mineral filler (material passing # 200 sieve) used. While one blend had silt as a mineral filler, the other blend had clay. Similarly, the two coarse blends, which were similar in gradation had different mineral fillers, either silt or clay. The amount of filler was limited to 7% in all four mixtures to meet a requirement for good quality subbase and base materials. The gradations of the four mixtures as well as Gradings A and E from AASHTO M 147 are provided in Table 2-1. The sources of aggregate materials utilized in the study and their classifications according to AASHTO M 145 [5] are provided in Table 2-2.
4 Table 2-1. Gradation 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 Table 2-2. Sources and classifications of ILS soil-aggregate blends according to AASHTO M145 Soil- Aggregate- Type Soil-Aggregate Classification (AASHTO M145) 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 Preliminary Study of AASHTO T180 A preliminary study was conducted at the AMRL laboratory to examine the compactibility of the selected materials and the rationality of the measured density and optimum moisture contents. Three replicates of each of the four materials were compacted using a 4.54-kg manually-operated rammer according to procedures B and D of AASHTO T 180. Prior to the compaction, specific gravity of the soil-aggregate blends were determined according to AASHTO T 84 and T 85 [1]. The specific gravity values were used to calculate the percent moisture that results in 100% saturation of the blends. The measured specific gravities are provided in Table 2-3.
5 Table 2-3. Specific gravities (Gsb) of the soil-aggregate components and blends Materials Specific Gravity Fine Crushed Limestone (+ # 4) 2.765 Sand (- #200) 2.680 Sand (- # 4 to + # 200) 2.682 Silt and Clay (- #200) 2.675 Blend 2.682 Coarse Limestone Buell Dust (+ # 4) 2.722 Limestone Buell Dust (- #200) 2.675 Limestone Buell Dust (- # 4 to + # 200) 2.714 Silt and Clay (- #200) 2.673 Blend 2.715 The soil-aggregate blend samples for compaction were prepared by first drying the blends in the oven at 60°C (140°F). Representative samples of appropriate size, 7 kg (16 lb) for fine-graded blend and 11 kg (25 lb) for coarse graded blend, were taken to be moistened for compaction. Each sample was prepared at initial moisture content of about 2%. The test specimens were then compacted in approximately five equal layers. After compaction and testing of the first moisture content trial, the compacted samples were broken up into particles small enough to pass a 4.75-mm (No. 4) or 19-mm sieve depending on the gradation of the blends. An increment of water (about 1.5%) was added to obtain the appropriate water content and to re-compact the material for the second trial. The testing process involving breaking up the compacted soil and adding water increments for further re-compactions was continued until sufficient test points were acquired to draw the compaction curve. The compaction curves resulted from compactions of the four soil-aggregate blends in the preliminary study are demonstrated in Figure 2-1. The maximum density and optimum water content of the four blends are shown in Table 2-4. As indicated from the compaction curves and the values in the table, the coarse blends achieved higher maximum density at lower optimum moisture content than the fine blends. In addition, the blends with silt demonstrated higher optimum moisture content than the blends with clay. Based on the rationality of the compaction results, the tested blends were selected for the interlaboratory study.
6 128 130 132 134 136 138 140 0 2 4 6 8 10 12 D ry D en si ty , lb /c u ft Moiture Content, % Fine-Graded w/ Silt 130 131 132 133 134 135 136 137 0.0 2.0 4.0 6.0 8.0 10.0 12.0 D ry D en si ty , lb /c u ft Moiture Content, % Fine-Graded w/ Clay 143 144 145 146 147 148 149 150 151 0 2 4 6 8 10 D ry D en si ty , lb /c u ft Moiture Content, % Coarse-Graded w/ Silt 143.5 144.0 144.5 145.0 145.5 146.0 146.5 147.0 147.5 148.0 148.5 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 D ry D en si ty , lb /c u ft Moiture Content, % Coarse-Graded w/ Clay Figure 2-1. Compaction curves and 100% saturation line of samples in the preliminary study Table 2-4. Optimum moisture content and maximum density values from the preliminary study Soil-Aggregate Type Dry Density, lb/ft Moisture, % 3 Fine-Graded w/ Clay 136.4 5.6 Fine-Graded w/ Silt 136.1 6.2 Coarse-Graded w/ Clay 147.6 4.3 Coarse-Graded w/ Silt 149.3 5.4
7 2.3 Participating Laboratories Hundreds of laboratories that are certified by the AASHTO Accreditation Program (AAP) [2] for soil and aggregate testing were contacted and invited to participate in the T180 ILS. The laboratories were ranked by their scores earned through accreditation process. Thirty-five laboratories including commercial, governmental, and research laboratories with the maximum score of 5 were selected to participate in the study. 2.4 Interlaboratory Sample Preparation and Shipping The ILS samples were prepared by 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 [2]. A total of four hundred twenty samples were prepared to be sent to the 35 selected laboratories. Each laboratory received 12 samples that consisted of three replicates of each of the four soil- aggregate blends. The coarse blend samples weighed about 14 kg and the fine blends samples weighed about 9 kg. The laboratories were asked to take representative samples of 11 kg (25 lbs) and 7 kg (16 lbs) for testing from the coarse and fine blends, respectively. 2.5 Interlaboratory Study Instructions Laboratory participants were provided with the testing instructions and data sheets to record the data. The laboratories were requested to follow Method B and Method D of T 180 to compact the three replicates of the four soil-aggregate blends at 5 different moisture contents. In addition to maximum density and optimum moisture content, the laboratories were asked to report all the measured weights and computed dry density and moisture content values. The moisture contents used in the study were selected based on the optimum moisture content of the blends that were determined as part of the preliminary study. The laboratories were asked to prepare the blends at 5 different moisture contents; two below the optimum, two above the optimum, and one at about the optimum. The interval between the consecutive moisture contents was asked to be kept at 1.5%. The specific gravity values of the blends (Table 2-3) for calculating the 100% saturation line were given to the laboratories. The instructions and the data entry sheet are provided in Appendix A.
