National Academies Press: OpenBook

Precision Estimates of AASHTO T 242 (2009)

Chapter: CHAPTER 3- CONCLUSIONS AND RECOMMENDATIONS

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Page 30
Suggested Citation:"CHAPTER 3- CONCLUSIONS AND RECOMMENDATIONS." National Academies of Sciences, Engineering, and Medicine. 2009. Precision Estimates of AASHTO T 242. Washington, DC: The National Academies Press. doi: 10.17226/23017.
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Page 30
Page 31
Suggested Citation:"CHAPTER 3- CONCLUSIONS AND RECOMMENDATIONS." National Academies of Sciences, Engineering, and Medicine. 2009. Precision Estimates of AASHTO T 242. Washington, DC: The National Academies Press. doi: 10.17226/23017.
×
Page 31
Page 32
Suggested Citation:"CHAPTER 3- CONCLUSIONS AND RECOMMENDATIONS." National Academies of Sciences, Engineering, and Medicine. 2009. Precision Estimates of AASHTO T 242. Washington, DC: The National Academies Press. doi: 10.17226/23017.
×
Page 32

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NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 30 CHAPTER 3- CONCLUSIONS AND RECOMMENDATIONS 3.1 CONCLUSIONS This study was conducted to prepare precision estimates for AASHTO T 242, “Frictional Properties of Paved Surfaces Using a Full-Scale Tire.” The data for this study were collected by Texas Transportation Institute (TTI) and Transportation Research Center (TRC) for evaluation of state friction measurement systems. The measurements were done using state systems as they initially arrived to the center and after they were calibrated to be in compliance with AASHTO T 242. Each individual state system was operated with 12 repeats, on three different surfaces, at three different speeds. The TTI surfaces consist of seal coat over HMA, seal coat and sand over hot mix asphalt (HMA), and concrete and the speeds include 30, 40, and 50 mph (48.2, 64.4, 83.3 km/h). The TRC surfaces consist of thick coating of coal-tar emulsion over HMA, grade 5 aggregate (screened -1/4 + 10) set in epoxy over HMA, and finish coat of Ohio DOT HMA. The TRC speeds include 20, 40, and 60 mph (32.2, 64.4, and 96.6 km/h). The statistics of the friction numbers were computed for both Initial (Arrival) and Final (Departure) measurement systems. The comparison of the statistics from the initial (arrival) measurements indicated that the calibration of the state systems in addition to providing adjustments to the measurement values, would improve the precision of the measurements. The evaluation of the relation between standard deviations and averages of the friction data indicated that there is no significant correlation between standard deviations and averages of sets of friction values. This showed that standard deviations are applicable as precision estimates regardless of the average friction of the surfaces. The precision estimates of AASHTO T 242 were computed using the statistics of TTI Final and TRC Departure Sate Systems. Statistical F-test was conducted to evaluate the difference in the repeatability and reproducibility standard deviations of the friction measurements from different tire types, different surfaces, different speeds, and different state systems of the two test centers. Results of the F-test indicated that for a 1% level of significance, the difference between the standard deviations of different speeds was not significant and they were combined. However, standard deviations of the surfaces in some cases were significantly different. For the case of hot mix asphalt surface, which had significantly larger standard deviation than all other surfaces, the precision estimates were reported separately. For other cases, the standard deviations of several surfaces were either combined or the larger one of them were selected for precision estimate development. The comparison of the computed repeatability standard deviation and the current repeatability standard deviation in AASHTO T 242 indicated that the 2 FN (friction number unit) reported as the repeatability standard deviation in AASHTO T 242-96

NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 31 (2004) is smaller than the standard deviation proposed for hot mix asphalt surface (2.87 FN) and larger than the repeatability proposed for other surfaces (1.56 FN). 3.2 RECOMMENDATIONS The current version of AASHTO T 242 only includes a repeatability standard deviation, which is different from the ones computed in this study. It is recommended to adopt the precision statement provided in Section 3.3, which includes repeatability and reproducibility estimates based on a wide range of friction values measured according to the most recent version of AASHTO T 242 by state friction systems.

NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 32 3.3 PRECISION STATEMENT FOR AASHTO T 242 PRECISION STATEMENT FOR AASHTO T 242 “FRICTIONAL PROPERTIES OF PAVED SURFACES USING A FULL-SCALE TIRE” X. Precision and Bias X.1 Precision X.1.1 Single Operator Precision - The single operator standard deviation (1s limits) for friction number (FN) is shown in Table X-1. The results of two properly conducted friction test on the same surface, by the same operator, and using the same equipment, should be considered suspect if they differ by more than d2s single-operator limits shown in Table X-1. X.1.2 Multilaboratory Precision - The multilaboratory standard deviation (1s limits) for friction number (FN) is shown in Table X-1. The results of two properly conducted tests on the same surface, by different operators, using different systems, should be considered suspect if they differ by more than d2s multilaboratory limits shown in Table X-1. Table X-1- Precision Estimates a These values represent the 1s and d2s limits described in ASTM Practice C670. Note: These limits are determined based on 6696 data from evaluation of 24 state friction systems calibrated and tested at TTI and TRC test centers. The TTI surfaces consist of seal coat over HMA, seal coat and sand over HMA, and concrete and the speeds include 30, 40, and 50 mph (48.2, 64.4, 83.33 km/h).The TRC surfaces consist of thick coating of coal-tar emulsion over HMA, aggregate (screened -1/4 + 10) set in epoxy over HMA, and finish coat of Ohio DOT HMA and the speeds include 20, 40, and 60 mph (32.2, 64.4, and 96.6 km/h). X.2 Bias No information can be presented on the bias of the procedure because no material having an accepted reference value is available.

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TRB’s National Cooperative Highway Research Program (NCHRP) Web-Only Document 142: Precision Estimates of AASHTO T 242 examines precision estimates for AASHTO (American Association of State Highway and Transportation Officials) T 242, “Frictional Properties of Paved Surfaces Using a Full-Scale Tire.”

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