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1 Predicting the Initial Retroreflectivity of Pavement Markings from Glass Bead Quality Pavement markings contribute to motorist safety by providing much needed guidance along the roadway under both daytime and nighttime conditions. Basic pavement mark- ing characteristics such as color, width, and placement are defined clearly in the current Manual on Uniform Traffic Control Devices (MUTCD). However, roadway authorities are left to decide the minimum retroreflectivity (nighttime visibility) thresholds with which they are comfortable, if retroreflectivity is monitored at all, and achieving these thresholds is an ongoing challenge. This project developed a recommended laboratory test to predict the initial retroreflectiv- ity of pavement markings in the field based on the quality of the glass beads. The test was expected to be rapid (i.e., preparation and testing complete in 24 hours or less), repeatable and reproducible, cost-effective, practical (i.e., suitable for routine use in a state materials testing laboratory), and verified and validated through measurements of the initial retro- reflectivity of pavement markings applied in the field. The work plan for this research included two key components: a laboratory test and a field verification test, along with the necessary data collection efforts for monitoring and evaluation purposes. The laboratory component addressed the characterization of glass beads and the development of a test method to determine potential initial retroreflectivity. This process included identifying key issues that relate specific bead properties to pavement marking retroreflectivity. The field component served as a verification of the laboratory test results. The field component addressed in-place initial retroreflectivity as a function of bead properties, placement, and the interaction of beads and paint. Data collection included bead and paint properties, pavement marking installation information, retroreflectivity measure- ments, pavement marking images, and video from a high-speed camera. The research effort was categorized by the following major tasks: ⢠Proof-of-concept testing ⢠Laboratory testing procedures ⢠Field testing procedures ⢠Laboratory versus field analysis ⢠Field implementation ⢠Interlaboratory study ⢠Drawdown testing procedure A proof-of-concept test consisted of conducting a small-scale experiment using the rec- ommended drawdown laboratory test procedure on two bead samples. This work was com- pleted prior to the full-scale laboratory evaluation. Each bead sample was evaluated at two laboratories in terms of gradation, roundness, coating, color, and air inclusions. The two S u m m a r y
2bead samples were used in a small-scale field test to compare lab and field retroreflectivity and to determine optimal installation requirements. Modifications to both the laboratory and field test procedures were considered based on the results from the proof-of-concept testing. The laboratory portion of this project consisted of using the drawdown method to pro- duce sample plates for a number of different bead packages, which, after 24 hours, were measured in terms of resulting retroreflectivity. The overall goal was to assemble bead pack- ages that give a wide range of gradation, color, presence of coating, and air inclusions, which should result in a wide range of retroreflectivity values. The research team worked with industry to obtain 30 bead packages, which originated from seven manufacturing sources. This was done to create a range of bead quality and physical characteristics. The bead char- acteristics evaluated included the following: ⢠Gradation ⢠Roundness ⢠Color ⢠Air inclusions ⢠Coating The 30 bead packages had a wide range of rounds (68% to 90%), a range of color (26 to 38 in terms of luminance or L values), a range of air inclusions (0.95% to 7.78%), and very different gradations. The resulting retroreflectivity ranged from a minimum of 290 milli- candela (mcd) to a maximum of 680 mcd. The field testing portion of the research was conducted using 15 different bead packages (as recommended from the laboratory testing) applied on both concrete and asphalt sur- faces. The retroreflectivity of these stripes was measured after 24 hours. A comparison of retroreflectivity readings for the 15 bead packages was completed based on the laboratory and field testing results. A statistical analysis was completed to compare the laboratory and field retroreflectivity data. With two exceptions on concrete, all of the field values were lower than those in the laboratory, which is intuitive given that the laboratory represents ideal conditions. Retroreflectivity values for the markings on concrete were closer to the laboratory values (averaged 17% lower) as opposed to asphalt (averaged 26% lower). At the completion of the laboratory and field testing, the research team worked with a strip- ing contractor to apply the developed drawdown test procedure using a long-line paint truck. The predicted laboratory pavement marking retroreflectivity values were checked against initial field retroreflectivity, with the difference between the predicted versus observed retroreflectivity values being only 13 mcd in both cases. These results verify that the devel- oped drawdown procedure can predict the retroreflectivity potential of a bead package. This information was used to assist in finalizing the recommended laboratory test procedure. The variability of the developed drawdown test method was investigated using a modified interlaboratory analysis. This included five different laboratories, which used the same set of beads and the same paint to conduct the drawdown test independently. The drawdown test method developed was proven to be repeatable and reproducible based on an interlabora- tory study of five labs. The research team developed a drawdown laboratory test to determine potential retro- reflectivity. The test was calibrated and validated by conducting a field test. The drawdown procedure met the project objectives given that it was rapid (preparation and testing is 24 hours), repeatable and reproducible, cost effective, and easy to use.
