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16 CHAPTER 4- ANALYSIS OF PROFICIENCY SAMPLE RESULTS 4.1 Proficiency Sample Data The results of three most recent rounds of AMRL Soil Classification and Compaction Proficiency Sample Program were analyzed to determine precision estimates for AASHTO T 180. For each round of testing, test results were obtained for a pair of samples. The samples for the first round of testing were identified as 147 and 148. The samples for the second round of testing were identified as 155 and 156. The samples for the third round of testing were identified as 157 and 158. Testing was performed using AASHTO Test Method T 180 and ASTM Test Method D 1557. The test methods are similar and both are commonly used to determine the compaction characteristics of soil. Special instructions were provided to the participating laboratories to minimize testing variations between the two methods. 4.2 Description of Samples The materials and their sources for the blends used in the three rounds of PSP Soil Classification and Compaction are provided in Table 4-1. The classification of the soils in Column 2 is determined in accordance to AASHTO Standard Practice M145. As indicated from Table 2-2 and Table 4-1, classification of the PSP and ILS materials are very different. While the ILS materials consist of only about 7% material passing #200 sieve, the PSP materials consist of about 85% filler material. This would provide a wide range of maximum dry density and optimum water content for the development of final precision estimates. Table 4-1. Sources and classifications of PSP soil blends according to AASHTO M 147 Sample # Classification Materials Source 147 & 148 Lean Clay & #30 mesh bonding clay Resco Products in Oak Hill, OH Lean Clay with Sand Ball diamond soil mix Aggtrans in Harman, MD Agricultural lime Aggtrans in Harman, MD 155 & 156 Sandy Lean Clay Core trench clay Aggtrans in Harman, MD Silica building sand Lafarge in Frederick, MD Ground fire clay Resco Products in Oak Hill, OH 157 & 158 Sandy Lean Clay Bonding clay Resco Products in Oak Hill, OH Masonry sand Lafarge in Frederick, MD
17 4.3 Testing Instructions Laboratory participants were provided with the testing instructions. The copies of the instructions are provided in Appendix C. In addition to specific instructions for AASHTO T180 and ASTM D 1557 [3], the instructions included instructions for sample preparation and a full battery of tests covered by the proficiency testing program. The testing in the AMRL proficiency testing program allowed testing to be performed using either ASTM or AASHTO test methods. It was assumed that ASTM D 1557 and AASHTO T180 are sufficiently similar that test results using either of the two methods display the same degree of testing variation. In addition, special instructions that minimize differences between the two methods were provided for laboratories using ASTM D 1557. Laboratories using ASTM D 1557 were instructed to perform testing in accordance to the test method with one exception as follows: due to restrictions inherent in the proficiency testing program, the quantity of material provided to the laboratories was limited. Therefore, there was not enough material to allow preparation of a separate specimen at each trial water content. After compaction testing at the first water content, laboratories were required to thoroughly break up the compacted soil into particles small enough to pass a 4.75-mm (No. 4) sieve, add an increment of water to attain the appropriate water content for the second trial, and then re-compact the material. This testing process of breaking up the compacted soil and adding water increments was continued until sufficient test points were acquired to draw the compaction curve. 4.4 Description of Equipment/Apparatus The equipment/apparatus used for this study is as described in the test methods, AASHTO T180 and ASTM D 1557. Testing for samples 155 and 156 was performed using either a 4-inch or a 6-inch diameter mold and either a manual or mechanical rammer having a weight of 10 lbf and a drop height of 18 inches. Testing for samples 147- 148 and 157-158 were performed using a 4-inch diameter mold and either a manual or mechanical rammer having a weight of 10 lbf and a drop height of 18 inches. 4.5 PSP Data Each laboratory was provided with a data report form for the collection of data. A copy of the data received from the laboratories is provided in Appendix D. The first three columns list the data as reported by the participating laboratories. The fourth column shows what results were identified as invalid or as outliers during the analysis. 4.6 Statistical Data Summary A summary of the statistics calculated from the data returned by the participating laboratories is provided in Table 4-2 and Table 4-3.The analysis was performed in accordance with the procedure described in NCHRP 09-26, Phase 3 report [6]. As indicated from the tables, the average maximum density and optimum moisture content values from PSP are very different from those obtained in the ILS (Table 3-1 and Table 3-2. The average maximum density of PSP materials is significantly lower than that of
18 ILS materials and the average optimum moisture content of the PSP materials is significantly higher than that of the ILS samples. Therefore, inclusion of PSP statistics in the development of precision statement provides precision coverage for a wide range of materials. Table 4-2. Summary of Statistics of maximum dry density (lb/cu. ft.) from PSP Sample ID Sample Type # of Labs Average Repeatability Reproducibility Odd Even Odd Even 147 & 148 Lean Clay with Sand 144 125.80 126.26 0.71 1.76 1.85 155 & 156 Sandy Lean Clay 253 132.23 132.33 0.52 1.17 1.13 157 & 158 Sandy Lean Clay 237 131.68 132.55 0.73 1.73 1.76 Table 4-3. Summary of Statistics of optimum moisture content (%) from PSP Sample ID Sample Type # of Labs Average Repeatability Reproducibility Odd Even Odd Even 147 & 148 Lean Clay with Sand 149 10.40 10.29 0.49 0.96 1.03 155 & 156 Sandy Lean Clay 247 8.58 8.51 0.24 0.45 0.43 157 & 158 Sandy Lean Clay 239 8.32 7.96 0.40 0.78 0.81 4.7 Precision Estimates based on the PSP data The repeatability and reproducibility standard deviations for the three sample pairs were pooled to prepare precision estimates for maximum density and optimum moisture content as provided in Table 4-4. The pooled estimates were derived using the following equation from Ku [7]: ( ) ( ) ( ) knnn snsnsns k kk p â+++ â++â+â = ... 1...11 2 22 22 2 11 1 (Equation 1) Where: =ps Pooled standard deviation =ks k th =kn standard deviation Number of laboratories analyzed resulting in kth standard deviation
19 Table 4-4. Precision estimates of maximum density and optimum moisture content from PSP data Condition of Test and Test Property 1s d2s Maximum Unit Weight (lbf/ft3 ) Single-Operator Precision 0.65 1.84 Multilaboratory Precision 1.55 4.39 Optimum Water Content (percent) Single-Operator Precision 0.37 1.05 Multilaboratory Precision 0.74 2.09
