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September 2010 NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Responsible Senior Program Officer: E. T. Harrigan Research Results Digest 346 OPTICAL SIZING AND ROUNDNESS DETERMINATION OF GLASS BEADS USED IN TRAFFIC MARKINGS This digest summarizes key findings from NCHRP Project 20-07, Task 243, "Development of a Test Method for Optical Sizing and Roundness Deter- mination of Glass Beads Utilized in Traffic Markings," conducted by the AASHTO Materials Reference Laboratory (AMRL) and the National Institute of Standards and Technology (NIST), Gaithersburg, Maryland. The digest was prepared from the project final report authored by Haleh Azari (AMRL) and Edward Garboczi (NIST). INTRODUCTION which is very time consuming if determined traditionally using the manual sieve and Glass beads are used to enhance the roundometer. nighttime and wet visibility of traffic mark- The majority of glass bead manufac- ings such as paints and thermoplastics. The turers and distributors and a number of state size and shape (i.e., the roundness) of the highway agency laboratories have pur- beads are significant determinants of their chased computerized optical equipment ability to reflect the light they receive from to expedite the quality assurance of glass a source. Perfectly round and well graded beads used in traffic markings. Despite the C O N T E N T S beads most effectively reflect light back to increasing popularity of such equipment, Introduction, 1 the source; this retroreflectivity is critical for no standard test method exists for their use Experimental Design, 2 visibility in low-light situations. Due to the with glass beads. This creates confusion ILS Sample Preparation, 2 Test Methods Evaluated in the ILS, 3 great effect of the size and roundness of glass when comparing the results of one labo- Participating Laboratories, 3 ILS Instructions, 3 beads on their reflectivity, AASHTO M 247 ratory with the results of another. In addi- specifies requirements for the size distribu- tion, the accuracy and precision of the data ILS Test Results and Analysis, 3 Test Results, 3 tion and degree of roundness of glass beads obtained with the computerized methods Method of Analysis, 4 Analysis of Results from used for traffic markings. are not yet known. Finally, the results from Traditional Mechanical Measurement of bead size and round- computerized optical equipment have not Methods, 4 Analysis of Results from ness has traditionally been performed using been compared with those obtained using COM-A Measurements, 7 sieves following ASTM Method D1214, the traditional ASTM methods, and the Analysis of Results from COM-B Measurements, 13 the roundometer following ASTM D1155, correlation between mechanical and com- Comparison of Precision and manual microscopy. Computerized puterized methods is not known. Estimates of Various optical methods have been used for quite In NCHRP Project 20-07, Task 243, an Measurement Methods, 16 Size Measurements, 16 some time for characterization of fine par- interlaboratory study (ILS) was designed Roundness Measurements, 18 ticles. Several manufacturers of comput- and conducted to (1) determine the precision Comparison of Bias of Various erized optical equipment have developed and bias of both optical and traditional Measurement Methods, 19 Size Measurements, 19 applications for measuring the size and mechanical methods for different glass bead Roundness Measurement, 20 shape of translucent glass beads. The types (as defined in AASHTO M 247), (2) Conclusions, 22 main advantage of this approach is faster compare the precision and bias of various Appendix A, 24 measurement of the glass bead properties, measurement methods, and (3) develop a