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OCR for page 22
Table 43 Results of t-test for comparison of measured and target percent round on #14 sieve of Type 5 samples Method of No. Average % % Rejection Measurement-- of Round, Round, Probability Sample Type Labs Measured Target Sx Computed t Critical t Decision (p) Roundometer-- 8 83.4 90.0 4.14 4.534 2.365 Reject 0.003 Type 5 COM-A-b/l-- 8 89.2 90.0 1.13 2.049 2.365 Accept 0.080 Type 5 COM-A-SPHT-- 4 92.1 90.0 1.13 3.621 3.182 Reject 0.036 Type 5 COM-B-NSP-- 1 91.0 90.0 -- -- -- -- -- Type 5 5 beads, the COM-A b/l parameter provided a signif- one-sample t-test. Following the ASTM E691 metho- icantly more accurate roundness measurement than dology, both within- and between-laboratory variabil- did the mechanical roundometer. However, the SPHT ity of the computerized and mechanical methods was parameter of COM-A failed to measure the roundness also determined for each sieve size of each glass of any of the glass bead types correctly. This may bead type. result from shortcomings of the parameter or a poor The computed t-values and the computed within- choice of threshold value. and between-laboratory standard deviations for each glass bead type were compared to determine the CONCLUSIONS accuracy and precision of the different methods in measuring the properties of the glass beads. For each Three methods for size and roundness measure- glass bead type, comparison was made between the ment were evaluated in this research: two comput- statistics corresponding to the sieve sizes retaining erized optical methods (COM-A and COM-B) and the highest mass percentage of the beads. This was the traditional mechanical methods--sieving and done because these sieves provided the most precise roundometer following ASTM D1214 and ASTM measurements of the glass bead type. D1155, respectively. The largest number of datasets Because only one laboratory provided size and were obtained for mechanical sieving and the roundness measurements of Type 3 and 5 glass beads roundometer: 14 datasets of size distribution and using the COM-B instrument, the discussion of pre- 11 of roundness data. COM-A users provided eight cision and bias for measurements of the properties of datasets, which included both size and roundness Type 3 and Type 5 glass beads does not apply to the data. COM-B datasets were received from four lab- COM-B results. The results of the comparison are oratories for small size glass beads (Type 1) and summarized as follows: from one laboratory for larger glass beads (Types 3 and 5). The samples upon which these tests were run Mechanical sieving, the COM-A device, and were carefully prepared via mechanical sieving and the COM-B instrument were used to measure mechanical roundness measurement, so accuracy in the size of Type 1 glass beads. Analysis of the the ILS for the roundometer, COM-A, and COM-B mass percent retained in the largest size class of results is defined as how close these measurement the Type 1 samples indicated that of the three methods came to the original mechanical sieving methods of measurement, the COM-B device and roundness measurements, allowing for the error provided the most accurate measurement of introduced in the sample preparation process. the size of Type 1 glass beads. With respect The ILS data received from participating labora- to variability, the mechanical sieve provided the tories were statistically analyzed for precision. The smallest within-laboratory standard deviation significance of the bias for each method between for measuring the size of the small beads. How- measured and target values was evaluated separately ever, the between-laboratory precisions of the for each sieve size of each glass bead type using the three methods were very similar. 22

OCR for page 22
Data from the mechanical sieve and the COM- Among the three combinations, COM-A-b/l A device were used to develop precision and provided the most accurate and precise mea- bias estimates for size measurement of the surement of the roundness of Type 3 and Type Type 3 and Type 5 beads. Between the two 5 glass beads. The mechanical roundometer did methods, the COM-A instrument measured the not provide equivalent accuracy and precision size of the Type 3 and Type 5 glass beads with in measuring the mass percent round of Type 3 more accuracy than the mechanical sieves. In and Type 5 glass beads. addition, the COM-A device provided smaller Taken together, these results suggest that com- within- and between-laboratory variability than puterized optical methods are preferable to the tra- did the mechanical sieves. ditional mechanical methods for measuring the Four combinations of methods and parameters size and roundness of glass beads. The improved were used for measuring the roundness of statistics of the b/l parameter for the larger glass Type 1 glass beads: mechanical roundometer, beads indicated the advantage of COM-A over COM-A-b/l, COM-A-SPHT, and COM-B- the roundometer for roundness measurement of T/L. Among the four combinations, the COM- the larger glass beads. Although a smaller number B-T/L provided the most accurate measurement of laboratories provided data using the COM-B of roundness of small glass beads. However, device, both size and roundness of the Type 1 glass COM-A-b/l provided the most precise within- beads were correctly measured by the COM-B and between-laboratory measurements. The instrument. mechanical roundometer did not provide equiv- A draft practice in AASHTO standard format for alent accuracy and precision for measuring the determining the size and roundness of glass beads uti- roundness of Type 1 glass beads. lized in traffic markings using the computerized opti- Three combinations of methods and parameters cal method is provided in Appendix A. This draft prac- were used for measuring the roundness of tice includes a precision and bias statement for the Type 3 and Type 5 glass beads: the mechanical computerized optical method measurements based on roundometer, COM-A-b/l, and COM-A-SPHT. the results of this research. 23