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From page 4...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 4 CHAPTER 2- RESULTS OF ANALYSIS AND ESTIMATES OF PRECISION This chapter provides statistics of the friction measurements of TTI and TRC friction test centers. Also provided in this chapter are the precision estimates for AASHTO T 242 computed based on the statistics of the TTI Final and TRC Departure State System data.
From page 5...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 5 2.2.2 Analysis of Final State System Measurements The Final State System measurements were made after the adjustments were applied to the state systems to put them into compliance with ASTM E 274. The Final State Systems were operated on Pad 1, Pad 2, and Pad 3 at 30, 40, and 50 mph (48.3, 64.4, 80.5 km/h)
From page 6...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 6 2.2.3 Comparison of TTI Initial and Final Precisions The comparison of the initial and final statistics would indicate if the calibration process improves the precision of the frictional measurements. The statistics in Table 2-1 and Table 2-2 of initial and final measurements at 40 mph shows that 5 out of 6 repeatability and reproducibility standard deviations or coefficient of variations have improved upon calibration of the systems.
From page 7...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 7 Table 2-3- Computed F- and critical F- values for comparison of precision estimates for different surfaces of TTI measurements Table 2-4- Combined standard deviations for friction numbers of different surfaces (Pads) at TTI 2.2.4.2 Test of Significance on Standard Deviations from Different Speeds The evaluation of differences in variability of TTI friction measurements at different speeds would determine if statistics from different speeds could be combined.
From page 8...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 8 Table 2-5- Computed F- and critical F- values for comparison of the precision estimates for different speeds of TTI friction measurements Table 2-6- Combined repeatability and reproducibility precisions for frictional properties of TTI measurements
From page 9...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 9 Figure 2-1- Box plots and h and k statistics for friction numbers of TTI Initial State System at 40 mph
From page 10...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 10 Figure 2-2- Box plots and h and k statistics for friction numbers of TTI Final State System at 30 mph
From page 11...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 11 Figure 2-3- Box plots and h and k statistics for friction numbers of TTI Final State System at 40 mph
From page 12...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 12 Figure 2-4- Box plots and h and k statistics for friction numbers of TTI Final State System at 50 mph
From page 13...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 13 2.3 ANALYSIS OF TRC FRICTION DATA 2.3.1 Analysis of Arrival State System Measurements The Arrival (ARR) State System measurements were made upon arrival of the systems to the TRC test center prior to any adjustments.
From page 14...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 14 Figure 2-5- Box plots and h and k statistics for friction numbers of TRC Arrival State System at 20 mph
From page 15...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 15 Figure 2-6- Box plots and h and k statistics for friction numbers of TRC Arrival State System at 40 mph
From page 16...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 16 Figure 2-7- Box plots and h and k statistics for friction numbers of TRC Arrival State System at 60 mph
From page 17...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 17 2.3.2 Analysis of Departure State System Measurements The Departure (DEP) State System measurements were made after the adjustments were applied to the state systems to put them into compliance with ASTM E 274 (AASHTO T 242)
From page 18...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 18 Figure 2-8- Box plots and h and k statistics for friction numbers of TRC Departure State System with 501 tire at 20 mph
From page 19...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 19 Figure 2-9- Box plots and h and k statistics for friction numbers of TRC Departure State System with 501 tire at 40 mph
From page 20...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 20 Figure 2-10- Box plots and h and k statistics for friction numbers of TRC Departure State Systems with 501 tires at 60 mph
From page 21...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 21 2.3.2.2 Data from Smooth 524 Tire The DEP State System data from 524 tire at 20, 40, and 60 mph (32.2, 64.4, and 96.6 km/h) are shown on box plots in Figure 2-11, Figure 2-12, and Figure 2-13.
From page 22...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 22 Figure 2-11- Box plots and h and k statistics for friction numbers of TRC Departure State Systems with 524 tires at 20 mph
From page 23...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 23 Figure 2-12- Box plots and h and k statistics for friction numbers of TRC Departure State Systems with 524 tires at 40 mph
From page 24...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 24 Figure 2-13- Box plots and h and k statistics for friction numbers of TRC Departure State Systems with 524 tires at 60 mph
From page 25...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 25 2.3.3 Comparison of TRC Arrival and Departure Precisions The comparison of the ARR and DEP statistics would indicate if the calibration process improves the precision of the frictional measurements. Table 2-13 provides the pooled standard deviations of the ARR and DEP State Systems with 501 tire.
From page 26...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 26 Table 2-14- Computed F- and critical F- values for comparison of precision estimates of 501 and 524 tires for different speeds Table 2-15- Computed F- and critical F- values for comparison of precision estimates of 501 and 524 tires for different surfaces Table 2-16- Pooled repeatability and reproducibility standard deviations of 501 and 524 tires 2.3.4.2 Test of Significance on Standard Deviations from Different Speeds The evaluation of significance of differences in variability of TRC pooled friction measurements at 20, 40, and 60 mph (32.2, 64.4, and 96.6 km/h) in Table 2-16 would determine if statistics from different speeds could be combined.
From page 27...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 27 at 20 mph is significantly larger than that at 60 mph. Therefore, for Pads 5 and 6 the standard deviations of the three speeds were pooled and for Pad 4 the largest repeatability and reproducibility values would be selected for precision estimate determination as shown in Table 2-18.
From page 28...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 28 Table 2-19- Computed F- and critical F- values for comparison of precision estimates of different TRC surfaces Table 2-20- Combined repeatability and reproducibility precisions for frictional properties of TRC measurements 2.4 COMPARISON OF TTI AND TRC PRECISIONS The comparison of the repeatability and reproducibility standard deviations of TTI (Table 2-6) and TRC (Table 2-20)
From page 29...
... NCHRP Web-Only Document 142: Precision Estimates of AASHTO T 242 29 Table 2-21- Computed F- and critical F- values for comparison of variances of TTI and TRC friction measurements 2.5 PRECISION ESTIMATES FOR AASHTO T 242 Since the variances of frictional measurements from TRC were significantly different from those of TTI test centers, the larger variances of the two sets were used to develop the precision estimates for AASHTO T 242. The pooled variances of Pads 4 and 5 of TRC were also kept separate from those of Pad 6.

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