Recommended Laboratory Test for Predicting the Initial Retroreflectivity of Pavement Markings from Glass Bead Quality
Mechanical and Civil Engineering
The initial retroreflectivity of pavement markings depends greatly on the quality of glass beads. Specifications for glass beads usually include percent rounds, gradation, coatings, and refractive index as measures of quality. However, field measurements of the initial retroreflectivity of pavement markings prepared with glass beads meeting the same specifications and applied by the same paint crew with identical equipment suggest that other bead qualities have a substantial impact on initial retroreflectivity, given that the initial retroreflectivity can vary as much as 200 millicandelas per meter squared per lux between markings. Differences in initial retroreflectivity of this magnitude can lead to a gain or loss of as much as a year or more in the useful life of a marking. Research is needed to develop a quick, reproducible, and standardized laboratory test method to predict the initial retroreflectivity of pavement markings in the field based on the quality of the glass beads. The research team developed a draw down laboratory test to determine the potential retroreflectivity in the lab. The test was calibrated and validated by conducting a field test. The draw down is rapid (preparation and testing is 24 hours), repeatable and reproducible, cost effective, and easy to use.
Smadi, O., Hawkins, N., Bektaş, B. A., Carlson, P., Pike, A., & Davies, C. (2013). Recommended Laboratory Test for Predicting the Initial Retroreflectivity of Pavement Markings from Glass Bead Quality, NCHRP Project No. 04-38, Final Report. http://www.trb.org/Publications/Blurbs/168592.aspx