Concrete pavements versus asphalt pavements is an important topic in the construction industry. Pavement reflectivity—how much of the sun’s rays are either absorbed or reflected and thus how the material affects the temperature of an urban area—is a key component in that debate.
Unfortunately, the National Asphalt Pavement Association (NAPA) asserts that there is a lot of flawed or invalidated science that is going into calls for legislation and building codes that mandate which material can be used based on reflectivity, despite which material would be best for the project. We explore the issue below.
Pavement Reflectivity and the Urban Heat Island
Asphalt is assumed to have a higher pavement reflectivity because it is darker, meaning concrete would be the go-to choice if legislation is passed concerning pavement reflectivity, even though other materials might be better suited to meeting the engineering requirements of the project.
At the heart of the matter is the urban heat island (UHI) effect. The UHI effect is where large amounts of non-green spaces, such as concrete and pavement-covered areas, are believed to increase the overall temperature of urban areas in comparison with surrounding rural areas. Many believe that lighter-colored concrete is less likely than darker asphalt to increase UHI, but NAPA maintains it’s a flawed conclusion for many reasons.
First, conclusions concerning pavement reflectivity’s impact on UHI was correlated from data concerning roofing materials, and “pavements are not roofs and they serve a different function. Pavements are constructed at ground level, unlike roofs, which are built atop buildings and may act as either a heat collector or an insulator for the facility,” according to a NAPA release on pavement reflectivity.
Secondly, NAPA maintains that “Although the surface temperature of darker objects is warmer than lighter surfaces, there is no evidence that a pavement’s surface temperature can increase the cooling load of adjacent buildings, let alone contribute to the overall UHI effect. However, ongoing research indicates that reflective pavements may actually increase the cooling loads of adjacent buildings due to reflected sunlight.” Basically, there is no research to show that pavement reflectivity increases UHI, but there is evidence that highly reflective pavements can increase the heat of surrounding buildings.
Lastly, NAPA maintained that research has shown that asphalt can have a positive impact on UHI and other factors, saying “that objective research has proven the benefits — including verified reduction in UHI and storm-water pollution control — of porous, permeable, and pervious hardscape surfaces,” like porous asphalt and pervious concrete. This is the research that needs to be taken into consideration when legislating what materials can and can’t be used on a building site where UHI is a concern.
Case in point: a study in Phoenix, Arizona compared the heat signature of Sky Harbor International Airport—the largest heat island in the city—and its thick concrete runways with that of the asphalt of nearby North Scottsdale Road using NASA ASTER Night Surface Temperature imaging. The study showed the road’s dark asphalt to be considerably cooler than the airport’s lighter-colored concrete.
We echo the sentiments shared by NAPA in support of “a robust, rigorous research program to understand the real impacts that pavement color, mix design, porosity, and depth have on urban heat islands and to explore potential cost-effective solutions.”