National Academies Press: OpenBook
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Suggested Citation:"Findings ." National Academies of Sciences, Engineering, and Medicine. 2011. Precision Statements for AASHTO Standard Methods of Test T 148, T 265, T 267, AND T 283. Washington, DC: The National Academies Press. doi: 10.17226/14481.
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Suggested Citation:"Findings ." National Academies of Sciences, Engineering, and Medicine. 2011. Precision Statements for AASHTO Standard Methods of Test T 148, T 265, T 267, AND T 283. Washington, DC: The National Academies Press. doi: 10.17226/14481.
×
Page 2
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Suggested Citation:"Findings ." National Academies of Sciences, Engineering, and Medicine. 2011. Precision Statements for AASHTO Standard Methods of Test T 148, T 265, T 267, AND T 283. Washington, DC: The National Academies Press. doi: 10.17226/14481.
×
Page 3
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Suggested Citation:"Findings ." National Academies of Sciences, Engineering, and Medicine. 2011. Precision Statements for AASHTO Standard Methods of Test T 148, T 265, T 267, AND T 283. Washington, DC: The National Academies Press. doi: 10.17226/14481.
×
Page 4
Page 5
Suggested Citation:"Findings ." National Academies of Sciences, Engineering, and Medicine. 2011. Precision Statements for AASHTO Standard Methods of Test T 148, T 265, T 267, AND T 283. Washington, DC: The National Academies Press. doi: 10.17226/14481.
×
Page 5
Page 6
Suggested Citation:"Findings ." National Academies of Sciences, Engineering, and Medicine. 2011. Precision Statements for AASHTO Standard Methods of Test T 148, T 265, T 267, AND T 283. Washington, DC: The National Academies Press. doi: 10.17226/14481.
×
Page 6
Page 7
Suggested Citation:"Findings ." National Academies of Sciences, Engineering, and Medicine. 2011. Precision Statements for AASHTO Standard Methods of Test T 148, T 265, T 267, AND T 283. Washington, DC: The National Academies Press. doi: 10.17226/14481.
×
Page 7
Page 8
Suggested Citation:"Findings ." National Academies of Sciences, Engineering, and Medicine. 2011. Precision Statements for AASHTO Standard Methods of Test T 148, T 265, T 267, AND T 283. Washington, DC: The National Academies Press. doi: 10.17226/14481.
×
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Research Results Digest 351 Responsible Senior Program Officer: E.T. Harrigan January 2011 INTRODUCTION The objective of NCHRP Project 9-26A was to develop or update precision state- ments of AASHTO standard methods of test designated by the technical sections of the AASHTO Highway Subcommittee on Materials (HSOM). To meet this objective, NCHRP Project 9-26A used both data min- ing techniques and interlaboratory studies (or “round robins,” as defined in ASTM D6631, Standard Guide for Committee D01 for Conducting an Interlaboratory Practice for the Purpose of Determining the Preci- sion of a Test Method). This research results digest summa- rizes the findings of four interlaboratory studies (ILS) conducted in 2009 and 2010 to develop precision statements for the AASHTO standard methods of test shown in Table 1. Reports were published in the form of NCHRP web-only documents (WODs) as tasks related to individual stan- dard methods were completed. Precision statements and supporting results were pro- vided to the AASHTO HSOM for review and possible adoption. A complete report of the development of each precision statement is presented in the four WODs (1, 2, 3, 4) shown in Table 1. FINDINGS AASHTO T 148, “Measuring Length of Drilled Concrete Cores” An ILS was conducted to prepare preci- sion estimates for AASHTO T 148, “Mea- suring Length of Drilled Concrete Cores.” Six drilled concrete cores with varying dimensions and surface roughness were obtained from several test sections in the FHWA’s Long-Term Pavement Perfor- mance program. The cores were deliv- ered to 11 laboratories, where the length of each core was measured using a 3-point caliper described in AASHTO T 148. The measurements were carried out at nine different locations at the center and along the circumference of the cores. A complete set of measurements was repeated five times by each laboratory for the purpose of deter- mining repeatability precision estimates. The collected data were analyzed accord- ing to ASTM E691, “Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method.” Table 2 summarizes the test data from the participating laboratories and their statis- tical analysis. Analysis of the experimental data pro- vided the following findings: PRECISION STATEMENTS FOR AASHTO STANDARD METHODS OF TEST T 148, T 265, T 267, AND T 283 This digest summarizes key findings obtained in 2009 and 2010 from continuing NCHRP Project 9-26A, “Data Mining and Interlaboratory Stud- ies to Prepare Precision Statements for AASHTO Standard Test Methods.” Project 9-26A was conducted by the AASHTO Materials Reference Labora- tory under the direction of the principal investigator, Haleh Azari. This digest is based on the contractor’s task reports, which are available online as NCHRP Web-Only Documents 163 through 166. Responsible Senior Program Officer: E. T. Harrigan NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

1. The variability of the measurements signif- icantly increases as the length of the cores reaches the limits of the 3-point caliper mea- suring range described in AASHTO T 148. This was indicated by the highest repeatabil- ity standard deviation of the 12-in.-long core and highest reproducibility standard devia- tion of the 4-in.-long core. 2. The repeatability standard deviation increases with the increase in surface roughness of the cores as was indicated by higher variability of one of the 4-in.-diameter cores that had more surface irregularities than the other 4 in.- diameter cores. However, the variability was not statistically significant and the within- laboratory variability of all 4-in. diameter cores could be combined. 3. The variability of the measurements was found to be the same for cores with the same diame- ter (4-in. or 6-in.) and significantly different for cores with different diameters (4-in. and 6-in.). Therefore, the standard deviations of the mea- surements of the same diameter cores were combined to prepare separate sets of preci- sions for 4-in.- and 6-in.-diameter cores. Precision estimates for the length measurements of the 4-in.- and 6-in.-diameter cores were computed after combining the standard deviations that were not significantly different. Based on the significant dif- ference in the precision estimates of 4-in.- and 6-in.- diameter cores, repeatability and reproducibility precision estimates were reported separately for each diameter. The resulting standard deviations and the allowable range of differences between two results within one laboratory and between different labora- tories are presented in Table 3. AASHTO T 265, “Laboratory Determination of Moisture Content of Soils” An ILS was conducted to prepare precision esti- mates for AASHTO T 265, “Laboratory Determi- nation of Moisture Content of Soils.” Test data were collected for four aggregate-soil blends judged suit- able for base and subbase construction. Specifi- 2 Repeatability (S r ) Reproducibility (S R ) Sample ID # of Labs D x L (inches) Intended Height (m m) Average Measured Height (m m) STD S x (m m) CV% 1s, (m m) CV% 1s, (m m) CV% LT 659 9 6 x 12 304.80 314.47 2.40 0.76 0.29 0.09 2.41 0.77 LT 755 10 6 x 8 203.20 203.55 1.70 0.83 0.60 0.29 1.78 0.87 LT 425 10 6 x 6 152.40 152.48 1.58 1.03 0.76 0.50 1.72 1.13 LT 2894 8 4 x 9 228.60 228.23 0.43 0.19 0.28 0.12 0.50 0.22 LT 1119 8 4 x 7 177.80 177.22 1.01 0.57 0.49 0.28 1.10 0.62 LT 523 8 4 x 4 101.60 107.36 2.24 2.09 0.31 0.29 2.26 2.10 *D and L stand for diam eter and length, respectively, of the cores. Table 1 Test methods and web-only documents AASHTO Standard Method of Test NCHRP Web-Only Document T 148, Measuring Length of Drilled Concrete Cores 165 T 265, Laboratory Determination of Moisture Content of Soils 164 T 267, Determination of Organic Content in Soils by Loss on Ignition 163 T 283, Resistance of Compacted Hot Mix Asphalt (HMA) to 166 Moisture-Induced Damage Table 2 Summary of statistics of concrete core length measurements (mm)

cally, there were two coarse-graded blends—one containing clay filler (Blend CC) and the other silt filler (Blend CS), and two fine-graded blends—one containing clay filler (Blend FC) and the other silt filler (Blend FS). Blends with a limited amount of materials passing the #200 sieve were selected. The ILS samples were prepared at the AASHTO Materials Resources Laboratory (AMRL) Proficiency Sample Facility using procedures developed for the AMRL Proficiency Sample Program. A total of 1,260 samples were prepared and 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 sam- ples were prepared with 4%, 6%, and 8% moisture (designated as below optimum, optimum, and above optimum, respectively); the coarse blend samples were prepared with 3 %, 5 %, and 7% moisture (designated below optimum, optimum, and above optimum, respectively). The experimental data were analyzed accord- ing to ASTM E691, “Standard Practice for Con- ducting an Interlaboratory Study to Determine the Precision of a Test Method.” Tables 4 through 7 summarize the test data from the participating lab- oratories and their statistical analysis for the four blends. Analysis of the data provided the following findings: 1. The standard deviations of the blends with clay were not significantly different from those of the blends with silt. Therefore, the standard deviations were combined. 2. The standard deviations of the coarse blends with 3% moisture (below optimum) were not significantly different from those of the blends with 5% moisture (optimum). There- fore, these standard deviations were com- bined. 3. The standard deviations of the coarse blends with 7% moisture (above optimum) were sig- nificantly different from those of the blends with 3% and 5% moisture content. Due to uncertainty in the results of 7% moisture con- tent, they were not included in the precision estimate analysis. 3 Condition of Test and Test Property StandardDeviation, mm Acceptable Range of Two Results, mm Repeatability (Sr) 4-in.-diameter 0.4 1.0 6-in.-diameter 0.7 1.9 Reproducibility (SR) 4-in.-diameter 0.9 2.4 6-in.-diameter 1.8 4.9 Table 3 Precision estimates for measurement of drilled concrete cores based on AASHTO T 148 Repeatability (S r ) Reproducibility (S R ) Sa mp le Type # of Labs Targe t % Average % S x CV % 1s, % d2s, % 1s, % d2s, % Coarse Aggregate w/ clay (3% moisture) 27 3.0 3.02 0.06 1.9 0.042 0.1 0.07 0.2 Coarse Aggregate w/ clay (5% moisture) 28 5.0 4.98 0.11 2.3 0.044 0.1 0.12 0.3 Coarse Aggregate w/ clay (7% moisture) 25 7.0 6.89 0.26 3.8 0.060 0.2 0.27 0.8 Table 4 Summary of statistics of % moisture content of coarse aggregate with clay (CC)

4. The standard deviations of the fine blends with 4% moisture content (below optimum) and those of the blends with 6% moisture content (optimum) were not significantly different. Therefore, these standard deviations were combined. 5. 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 con- tent, they were not included in the precision estimate analysis. 6. The bias and low precision of the moisture content data for the above optimum blends were speculated to be due to availability of excess moisture for evaporation. When the mixture is above the optimum moisture con- tent, free moisture is available to evaporate and escape from the pores of the bottles. How- ever, in mixtures below the optimum and at the optimum, moisture adheres to the soil- aggregate particles. 7. The standard deviations of the coarse blends were significantly different from those of fine blends. Therefore the computed preci- sion estimates from the two blends are pre- sented separately in the proposed precision statement. Table 8 presents the precision estimates for mois- ture content determination based on the results of the 4 Repeatability (S r ) Reproducibility (S R ) Sa mp le Type # of Labs Targe t % Average % S x CV % 1s, % d2s, % 1s, % d2s, % Coarse aggregate w/ silt (3% mo isture) 27 3.0 3.03 0.05 1.6 0.05 0.1 0.06 0.2 Coarse aggregate w/ silt (5% mo isture) 29 5.0 5.02 0.10 2.1 0.06 0.2 0.12 0.3 Coarse aggregate w/ silt (7% mo isture) 29 6.6 6.60 0.33 5.0 0.44 1.2 0.49 1.4 Table 5 Summary of statistics of % moisture content of coarse blend with silt (CS) Repeatability (S r ) Reproducibility (S R ) Sa mp le Type # of Labs Targe t % Average % S x CV % 1s, % d2s, % 1s, % d2s, % Fine Aggregate w/ clay (4% moisture) 30 4.0 4.04 0.14 3.4 0.18 0.5 0.20 0.6 Fine Aggregate w/ clay (6% moisture) 29 6.0 5.92 0.20 3.4 0.17 0.5 0.25 0.7 Fine Aggregate w/ clay (8% moisture) 30 8.0 7.39 0.63 8.5 0.73 2.0 0.87 2.4 Table 6 Summary of statistics of % moisture content of fine blend with clay (FC) Repeatability (S r ) Reproducibility (S R ) Sa mp le Type # of Labs Targe t % Average % S x CV % 1s, % d2s, % 1s, % d2s, % Fine Aggregate w/ silt (4% moisture) 29 4.0 3.97 0.11 2.9 0.17 0.5 0.18 0.5 Fine Aggregate w/ silt (6% moisture) 30 6.0 5.97 0.16 2.7 0.12 0.3 0.19 0.5 Fine Aggregate w/ silt (8% moisture) 30 8.0 7.69 0.46 6.0 0.60 1.7 0.68 1.9 Table 7 Summary of statistics of % moisture content of fine blend with silt (FS)

ILS conducted in this study. The standard deviations corresponding to coarse and fine blends were used to compute the allowable differences between two moisture content measurements. AASHTO T 267, “Determination of Organic Content in Soils by Loss on Ignition” An interlaboratory study was conducted to pre- pare precision estimates for AASHTO T 267, Deter- mination of Organic Content in Soils by Loss on Ignition.” Samples from three types of soils (clay, silt, and sand) were each blended with three differ- ent percentages (2%, 5%, and 8%) of fine walnut shell grits as organic material and sent to 30 labora- tories for organic content measurement. The labora- tories were instructed to test three replicates of each organic content level of each soil type. Results were obtained from 27 different laboratories. ILS test results were analyzed for precision in accordance to ASTM E 691. Before the analysis, any outlier data were eliminated by following the procedures described in ASTM E 691 for determin- ing repeatability (Sr) and reproducibility (SR) esti- mates of precision. For each set of data, the h and k statistics, representing the between and within lab- oratory consistency, were used to identify the out- lier data. Data exceeding the critical h and k values were eliminated; once identified for elimination, the same data were eliminated from any smaller subsets analyzed. Multiple sets of data in each soil type were elim- inated based on the critical h and k values. After eliminating the outlier data, the averages and the repeatability and reproducibility standard deviations of the data were determined. The Sr and SR precision estimates were determined using the remaining data in conformance with ASTM E 691. A summary of statistics of the measurements is shown in Table 9. The comparison of the design and measured organic content values in the table indi- cates that every soil has a certain percentage of intrin- sic organic material; clay has the greatest amount of intrinsic organic material, whereas sand has the least amount. Upon subtracting the intrinsic organic con- tents from the measured organic contents, the aver- age of the measured values agree closely with the design values as shown in Table 10. In addition to the adjusted averages, Table 10 also provides the adjusted variability of the measurements. The table shows that the standard deviation of the measurements for sand increases with the increase in the percentage of organic material. The increased variability of the sand blend with higher percentages of organic material indicates segregation of organic material during shipment. This could be explained by the non-cohesive nature of sand that does not allow the ground walnut shell grits to adhere to sand particles. Analysis of the data provided the following findings: 1. Clay has the greatest amount of intrinsic organic material and sand has the least amount. 2. The within-laboratory and between-laboratory standard deviations were very consistent for different organic content levels of clay or silt blend. Therefore, for these two blends, the standard deviations corresponding to 2%, 5%, and 8% organic material were combined. 3. For the sand blend, the within-laboratory and between-laboratory standard deviations of 5 Material and Type Index Standard Deviations (1s) Acceptable Range of Two Results (d2s) Single-Operator Precision: Coarse blend Fine blend 0.05 0.16 0.14 0.46 Multilaboratory Precision: Coarse blend Fine blend 0.12 0.21 0.33 0.58 Table 8 Combined standard deviations of the blends with various moisture contents

6Repeatability ReproducibilitySoil Type Design Organic Content No. of Labs Average Measured Organic Content Sx 1s (Sr) d2s 1s (SR) d2s Clay 0% 27 3.03 0.981 0.277 0.785 1.018 2.880 Clay 2% 25 5.38 0.925 0.287 0.813 0.966 2.735 Clay 5% 26 8.29 0.985 0.259 0.732 1.017 2.879 Clay 8% 24 11.16 0.787 0.233 0.661 0.819 2.319 Silt 0% 26 0.95 0.369 0.122 0.346 0.388 1.098 Silt 2% 25 6.06 0.378 0.129 0.366 0.544 1.540 Silt 5% 25 2.92 0.529 0.155 0.437 0.408 1.154 Silt 8% 25 8.93 0.379 0.195 0.551 0.424 1.199 Sand 0% 25 0.32 0.140 0.052 0.147 0.149 0.422 Sand 2% 26 5.55 0.362 0.219 0.621 0.363 1.027 Sand 5% 25 2.43 0.292 0.430 1.216 0.555 1.570 Sand 8% 26 8.59 0.631 0.396 1.120 0.741 2.097 Table 9 Summary of statistics of organic content measurements after elimination of outlier data Repeatability ReproducibilitySoil Type Source- Design No. of Labs Average Sx 1s (Sr) d2s 1s (SR) d2s Adj. Clay 2% 26 2.25 0.505 0.282 0.798 0.576 1.630 Adj. Clay 5% 25 5.32 0.498 0.246 0.697 0.554 1.567 Adj. Clay 8% 24 8.28 0.519 0.232 0.655 0.566 1.602 Adj. Silt 2% 25 1.97 0.313 0.129 0.366 0.338 0.956 Adj. Silt 5% 25 5.05 0.262 0.155 0.437 0.302 0.856 Adj. Silt 8% 26 7.93 0.368 0.196 0.556 0.415 1.176 Adj. Sand 2% 25 2.07 0.262 0.216 0.610 0.337 0.953 Adj. Sand 5% 25 5.14 0.534 0.397 1.124 0.660 1.869 Adj. Sand 8% 25 8.24 0.683 0.372 1.054 0.774 2.192 Table 10 Summary of statistics of organic content measurements after subtracting the intrinsic organic content

5% and 8% organic content were significantly larger than those of 2% organic content. There- fore, the standard deviations corresponding to different organic content levels were not combined. 4. The large variability in measurement of organic content of sand blends with 5% and 8% organic material is speculated to be caused by segregation of organic material during shipment as a result of the non-adhesive nature of sand. 5. Since sand has typically less than 2% organic material in its natural state, the precision estimates for sand were prepared based on the standard deviations of the blend with 2% organic content and the standard devia- tions corresponding to 5% and 8% organic content were not included in precision esti- mate development. 6. The within-laboratory and between-laboratory standard deviations of the silt and sand blends were statistically similar and they were combined. 7. The within-laboratory and between-laboratory standard deviations of the clay blend were significantly different from those of sand and silt blends and were reported separately. Table 11 presents single operator and multi- laboratory estimates of variability (1s) and the allow- able difference between two results (d2s) for organic content measurements of the soil blends. AASHTO T 283, “Resistance of Compacted Hot Mix Asphalt (HMA) to Moisture- Induced Damage” An interlaboratory study was conducted to pre- pare precision estimates for AASHTO T 283, “Resistance of Compacted Hot Mix Asphalt (HMA) to Moisture-Induced Damage.” Two different sources of aggregates—limestone and sandstone— with varying levels of moisture resistivity and two methods of compaction—gyratory and Marshall— were selected for the study. The combination of aggregate types and compaction methods resulted in four sets of specimens to be evaluated in the study. Before conducting the ILS, the FHWA conducted a preliminary study in which the moisture susceptibly of the four selected specimen types was evaluated using AASHTO T 283 test methods and Hamburg wheel track testing. A total of 40 laboratories par- ticipated in the ILS and provided complete sets of data from testing either gyratory, Marshall, or both specimen types. Detailed volumetric and mechanical data were collected from the laboratories in the ILS. In addi- tion to tensile strength ratios (TSR), laboratories provided the individual indirect tensile strength val- ues of the dry and conditioned specimens. Tables 12 through 15 summarize the test data from the partic- ipating laboratories and their statistical analysis for the four blends. These results and those of the preliminary FHWA study indicated that AASHTO T 283 is, in general, very variable and may provide erroneous results. The limestone mixture, which was known to be highly moisture resistant, was indicated as moder- ately resistant to moisture while the sandstone, which was known to be moisture sensitive, showed rela- tively good moisture resistance. Moreover, the results demonstrated that while the repeatability standard deviations of dry and wet strength measurements and their corresponding TSR values were very sim- ilar, the reproducibility standard deviations of the reported strength measurements were significantly larger than those of their corresponding TSR values. Therefore, as suggested by a number of highway agencies, while the wet strength values can be used 7 Condition of Test and Test Property Standard Deviation, % (1s) Acceptable Range of Two Results, % (d2s) Single-Operator Precision: Clay 0.25 0.72 Silt and Sand 0.19 0.54 Multilaboratory Precision: Clay 0.57 1.60 Silt and Sand 0.35 1.00 Table 11 Precision estimates for measurement of organic content of soil

in place of TSR for comparison of moisture suscep- tibility of various mixtures within one laboratory, their use for between-laboratory comparison is not advisable. The ILS test results were analyzed for precision in accordance with ASTM E 691. Before the analy- sis, any partial sets of data were eliminated by fol- lowing the procedures described in ASTM E 691 in determining repeatability (Sr) and reproducibil- ity (SR) estimates of precision. Data exceeding the critical h and k statistics, which represent the within and between variability, were eliminated. Once iden- tified for elimination, the same data were eliminated from any smaller subsets analyzed. The precision estimates of AASHTO T 283 are presented in Table 16. Statistical comparisons showed that the repeatability and reproducibility of TSR values of gyratory and Marshall specimens of the limestone and sandstone mixtures were not signif- icantly different. Therefore, the repeatability and reproducibility statistics for AASHTO T 283 were determined by combining all appropriate within- and between-laboratory standard deviations. As indicated by Table 16, the acceptable range of TSR values 8 Repeatability Reproducibility Property # of Labs Average STD CV% STD CV% Dry Tensile Strength, kPa 18 647 28.64 4.4 135.97 21.0 Wet Tensile Strength, kPa 18 616 24.08 3.9 108.43 17.6 TSR 19 0.95 0.030 3.1 0.091 9.6 Table 12 Statistics of dry and wet indirect tensile strength and TSRs of gyratory compacted limestone mixtures Repeatability Reproducibility Property # of Labs Average STD CV% STD CV% Dry Tensile Strength, kPa 15 1205 67.01 5.6 381.35 32.2 Wet Tensile Strength, kPa 15 1013 55.11 5.4 332.43 35.2 TSR 16 0.88 0.035 4.0 0.087 10.6 Table 15 Statistics of dry and wet indirect tensile strength and indirect TSR of Marshall compacted sandstone specimens Repeatability Reproducibility Property # of Labs Average STD CV% STD CV% Dry Tensile Strength, kPa 14 970 57.74 6.0 163.79 16.9 Wet Tensile Strength, kPa 14 852 56.76 6.7 182.56 21.4 TSR 13 0.87 0.035 4.1 0.082 9.4 Table 13 Statistics of dry and wet indirect tensile strength and indirect TSRs of Marshall compacted limestone specimens Repeatability Reproducibility Property # of Labs Average STD CV% STD CV% Dry Tensile Strength, kPa 21 956 49.99 5.2 286.87 30.0 Wet Tensile Strength, kPa 19 785 36.08 4.6 158.67 20.2 TSR 19 0.89 0.031 3.5 0.088 9.9 Table 14 Statistics of dry and wet indirect tensile strength and indirect TSRs of gyratory compacted sandstone mixtures

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TRB’s National Cooperative Highway Research Program (NCHRP) Research Results Digest 351: Precision Statements for AASHTO Standard Methods of Test T 148, T 265, T 267, AND T 283 summarizes the findings of four interlaboratory studies conducted in 2009 and 2010 to develop precision statements for the AASHTO standard methods of measuring length of drilled concrete cores, laboratory determination of moisture content of soils, determination of organic content in soils by loss of ignition, and resistance of compacted hot-mix asphalt to moisture-induced damage.

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