Advantages of Light-Colored “Cooler” Aggregates in Roadway Maintenance
Posted by Don Barrett on Mar 18, 2015
Selecting Aggregates for Hot Mix Asphalt and Street Maintenance Materials for Maximum Durability and Safety
Based on research conducted by the University of California, Lawrence Berkeley Laboratories, among others, there are many advantages to public agencies using slurry and chip aggregate that is light in color (i.e., grey instead of black). While some people may prefer a black roadway color to lighter colors for aesthetic reasons, there is no question that the use of lighter colored aggregates results in increased durability of the underlying pavement, safety for nighttime driving, environmental benefits, and reduced cost.
Durability—does aggregate color impact service life? In research conducted by Lawrence Berkeley National Laboratories and the Institute of Transportation Study at U. C. Berkeley, Cooler Reflective Pavements Give Benefits Beyond Energy Savings: Durability and Illumination, the authors found that light-colored pavements are cooler, because the light color increases sunlight reflection. Cooler pavements result in longer pavement service life (a more than 10 fold increase in pavement life), and improved streetlight effectiveness (discussed below). Because hot mix asphalt pavements, as well as slurry, chip and cape seals, all incorporate asphalt (black) materials in their production, light-colored aggregate is the remaining way to improve surface reflectivity and lower temperatures. Studies have shown that light-colored aggregate does indeed improve reflectivity and lower pavement temperatures.
Safety—what effect does light-colored aggregate have on streetlight and headlight effectiveness? Light-colored aggregate roadways also improve the effectiveness of streetlights and headlights. Both types of safety lighting depend upon both direct and reflective lighting for their effectiveness. Pedestrians crossing a street at night are illuminated both by the light source, such as a streetlight or headlights, and by light reflected from the street. Pedestrian fatalities are 12% of all motor vehicle deaths. The need for better lighting has become greater because of the aging of the population. “Enhanced visibility due to reflective pavements will help to avoid accidents and reduce the costs of automobile insurance. In addition, better illumination probably reduces auto theft and other street crimes.” (Pomerantz, Akbari, & Harvey)
Increased roadway reflectivity helps headlights and streetlights perform better. In a study commissioned by Graniterock, two streets were examined for reflectivity and nighttime pedestrian visibility. The streets were slurry sealed with black aggregate (Mountain View) and grey granite (Cupertino). Dr. Kenneth Ziedman, Ph.D., concluded that the grey aggregate improved streetlight and headlight effectiveness by increasing roadway luminance (brightness) by a factor of two to three times. This substantially increases pedestrian contrast against the roadway background resulting in improved visibility of pedestrians.
Environmental Benefits—considering pavement reflectivity impacts on heat island effects in urban communities. Scientists are concerned with a phenomenon called the “heat island effect.” This is the tendency of air temperatures to be significantly higher in large cites compared with the surrounding countryside. As cities act as “heat islands,” hot weather health hazards are exacerbated, energy demand is increased, and it becomes more difficult to meet air quality goals. One way to reduce the heat island effect is to use pavements with higher reflectivity—i.e., those made with lighter colored aggregates.
Newly paved streets are black when they are first paved because the aggregates are coated with black asphalt. However, with the passage of time and vehicle use, the asphalt coating is worn away from the aggregate exposing the constituent aggregate’s color. After five years, hot mix reflectivity improves by 15% if a light-colored aggregate is used. Slurry seals and chips seals with light-colored aggregates will become much lighter in color the first year.
Reduced “Heat Island Effect.”—Public streets cover a significant portion of the land surface in most cities. Public streets can cover 10% of the land area in a city. Darker colored streets increase pavement temperatures. As street pavement temperatures are increased, outdoor temperatures are also increased due to the transfer of pavement heat into the atmosphere. People respond with air conditioning further raising outdoor temperatures. The result is increased energy usage and cost.
The Bottom Line
If color is to be specified, only light-colored aggregates should be specified for chip, slurry and hot mix asphalt
Benefits of Cooler Roadways from Light-colored Aggregate Cooler pavement temperatures result in a ten-fold reduction in roadway rutting and shoving deformation; these are the significant causes of roadway failure.
Light-colored aggregate use results in cooler pavement surface temperatures, slowing the aging effects of sunlight on emulsion and asphalt materials. Emulsion and asphalt provide the “glue” in slurry seals and hot mix asphalts. With aging from higher temperatures, the “glue” becomes brittle or stiff shortening the pavement’s life.
Streetlight and headlight effectiveness is improved by a factor of two to three times when street pavements are more reflective. Improved lighting is likely to reduce pedestrian and vehicle accidents.
With an aging population, increased lighting is important to an expanding constituent group. Local governments should implement policies now that improve roadway lighting effectiveness.
Graniterock has aggregates for your roadway project that fully meet Caltrans Specifications for physical properties and gradations. These aggregates are light in color and in some cases will help your project to qualify for LEED credits.
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