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practice in AASHTO standard format for the use of selected according to the Type 1, Type 3, and Type 5 computerized optical methods and equipment to mea- gradings in AASHTO M 247. The roundness of the sure the size and roundness of glass beads used in traf- three blends was selected as 70%, 80%, and 90% by fic markings (see Appendix A). mass, respectively. Although the original glass beads This digest summarizes the results of NCHRP in the 23 kg bags had the overall gradation that was Project 20-07, Task 243, as presented in the full proj- required for the ILS samples, the test specimens were ect final report titled "Optical Sizing and Roundness not sampled directly from the bags for two reasons: Determination of Glass Beads Used in Traffic Mark- (1) the beads might have become segregated during ings." The project final report is available as NCHRP transportation and (2) the test specimens were required Web-Only Document 156 at and to have specified roundness levels that could not be contains nine appendices: Appendix A, Instructions achieved by direct sampling. and Data Sheet for Interlaboratory Study; Appendix B, Sets of glass bead samples each including three Results of Percent Retained by Mechanical Sieve; replicates of the three blends were prepared for each Appendix C, Results of Roundometer; Appendix D, participant in the ILS. The first blend, referred to as Results of Percent Retained Using COM-A; Appen- Y, was prepared to meet the Type 1 gradation. Each dix E, Results of SPHT Roundness by COM-A; size class in the Y samples was a blend of 70% round Appendix F, Results of b/l Roundness by COM-A; and 30% non-round glass beads, where the roundness Appendix G, Results of Size Measurements by level was determined from roundometer and spiral COM-B; Appendix H, Results of Roundness by separator results. The second blend, referred to as P, COM-B; and Appendix I, Recommended Test Method was prepared according to the Type 3 gradation. Each for Measurement of Size Distribution and Roundness size class in the P samples was prepared with 80% of Glass Beads Using Computerized Optical Equip- round and 20% non-round beads. The third blend, ment. The main text of the project final report also pre- referred to as C, was prepared according to the sents the results of a complementary study to evaluate Type 5 gradation. Each size class in the C samples the effectiveness of various computerized optical was prepared with 90% round and 10% non-round method size and shape parameters in discriminating glass beads. Table 1 provides information on the real shape and size through an analysis of X-ray micro- tomography images of various glass bead types. Table 1 Percent passing of the three glass bead types EXPERIMENTAL DESIGN Sieve U.S. An ILS was conducted to evaluate the precision Opening Sieve Type 1 Type 3 Type 5 and accuracy of both the optical and traditional Size (Y) (P) (C) mechanical methods. Glass bead samples with spe- 2350 #8 cific size distribution and percent roundness were prepared and sent to participating laboratories for 2000 #10 100% property measurements. The ILS was designed in 1700 #12 95% accordance with ASTM E691-09, Standard Practice 1400 #14 100% 40% for Conducting an Interlaboratory Study to Determine the Precision of a Test Method. ASTM E691 specifies 1180 #16 100% 95% 5% that development of a precision statement requires 1000 #18 40% 0% participation of a minimum of 6 laboratories with a 850 #20 100% 5% preferred number of 30. 710 #25 0% 600 #30 95% ILS Sample Preparation 500 #35 Commercial glass beads of Types 1 (fine grada- tion), 3 (medium gradation), and 5 (coarse gradation) 425 #40 as specified in AASHTO M 247 were obtained in 300 #50 35% 23 kg bags from two manufacturers. Three blends of 180 #80 fine, medium, and coarse glass bead samples were 150 #100 0% prepared. The gradations of the three blends were 2