Quantifying Pavement Albedo
Start date: 08/17/12
End date: 11/30/16
Sponsor(s): Federal Highway Administration
About the research
This project evaluated, characterized, and modeled albedo as well as heat sorption and release in real-world paving materials. Field-measured pavement albedo and thermal properties were used to develop predictive models for albedo and thermal heat transfer relative to pavement type (i.e., asphalt or concrete), age, mix properties (e.g., color), structural characteristics (e.g., roughness), solar radiance, and local climate.
Key pavement characteristics to measure in the field were identified, and seven city-level field testing sites in the central and eastern United States were selected to represent a range of local aggregate types, pavement types and ages, and climates. Ten locations were investigated at each site, including five portland cement concrete (PCC) and five asphalt concrete (AC) pavements of ages from less than 1 to more than 30 years and representing a range of aggregate colors. Ten percent of the locations were not evaluated due to incorrect surfaces, insufficient data, or safety concerns. Albedo, thermal properties, and pavement surface characteristics data were collected, and cores were obtained to measure thermal properties in the laboratory. Test tracks at Auburn University’s National Center for Asphalt Technology (NCAT) and Minnesota’s MnROAD facility were used to collect 24-hour measurements for thermal model validation. An albedo model and a thermal model were developed using the field and laboratory data. The resulting models were compared to existing pavement design systems, such as AASHTOWare Pavement ME Design software, and sustainability rating systems, such as Greenroads or GreenPAVE.
The albedo data showed that different parameters influence albedo for AC and PCC pavements, albedo approaches a steady value over time, and the albedo trends for each site differ. Pavement thermal modeling required an understanding of the surface and thermal properties, small incremental units of time and layer thicknesses, 10 to 20 days of simulation to achieve balance throughout the pavement and subgrade system, and continuous data over an extended period.
The AC albedo model reasonably predicted albedo over time using pavement age and coarse aggregate color. However, the PCC albedo model did not predict field albedo using pavement age, coarse aggregate color, and surface texture; additional field study is needed. Climate-related factors, particularly winter maintenance activities, may also play a role in pavement albedo. The thermal model predicted pavement thermal response in warm, dry conditions but did not account for the influence of moisture and freezing conditions. Asphalt and concrete thermal properties vary and may have up to a 15% influence on AASHTOWare Pavement ME Design results. Most highway sustainability rating systems have recognized the complexity of pavement albedo and have removed this parameter.