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“By mid-century, two out of every three persons on the planet will be living in urban areas. These growing urban populations face two simultaneous climate challenges; extreme heat events attributed to Urban Heat Island (UHI) effects, and Global Climate Change (GCC).”

These projections come from a co-authored by MIT researchers , , Xin Xu and Jeremy Gregory evaluating the impact of cool pavement on urban centres.

Several measures are being undertaken to address UHI mitigation around the world. These include increased urban vegetation and the installation of “cool roofs.” However, not all these strategies are within the control of municipal decision-makers, making it difficult for cities to implement effective solutions.

Yet, pavement material change is one strategy over which cities have a strong influence, if not total control, the researchers write, and it could make a significant difference.

The MIT study focussed on Boston and Phoenix, chosen for their differing climates and urban characteristics, thus providing a comprehensive assessment of the context-sensitivity of cool pavement strategies.

The study’s bottom line is that suitable pavement selection could, over a 50-year period, offset the equivalent GHG emissions of driving 5,500 to 17,800 vehicles in Boston and 13,700 to 96,000 vehicles in Phoenix.

Studies like this must be conducted properly and thoroughly. MIT sought to address gaps in earlier investigations.

“A change in one component of the built environment not only affects its own lifecycle but also alters the environmental performance of other components,” the MIT study says.

Pavement surfaces and thermal properties can interact and affect the energy use of nearby buildings by changing local ambient temperature and irradiance. The structural properties of the surface material can alter fuel use of passing vehicles by changing the rolling resistance and wasted motion energy. Different materials have different supply chains and therefore produce varying amounts of emissions.

“Previous studies have not had consistent scope and did not explore the impact of local urban context. The microclimate, morphology (building configuration) and prevailing traffic vary significantly within a city. These differences play an important role in the net impact of pavement albedo (solar absorption) modification.”

The urban environment is highly diversified, both in terms of morphology and traffic patterns.

For example, the “incident radiation-induced cooling burden” on road-adjacent buildings is much higher in Phoenix than in Boston. Therefore, it is unlikely that any single cool pavement solution will be optimal for all cities across the country or continent.

“Different designs deflect and deteriorate at different rates according to traffic volume and speed. Both effects are time-dependent and location-specific. Spatial variation within a city and consequent shading from adjacent building will vary by city area, affecting the amount of incoming and outgoing solar radiations. Pavement materials affect building energy demand by altering both the local ambient temperature (the UHI effect) and the magnitude of radiative energy incident on the building.”

The MIT framework addressed three of these interactive effects: excess fuel consumption in vehicles due to both pavement roughness and stiffness; excess building energy demand; and direct radiative forcing.

In addition, the MIT study sought to eliminate previous study shortcomings by “capturing the impacts of all activities that current knowledge allows attributing to the materials choice associated with a pavement’s surface.”

Included was a framework to model the life cycles of the road materials and the entire transportation and construction process, including best-estimate allowances for maintenance over a 50-year period.

More than 95 per cent of current road surfaces in Boston and Phoenix are covered with flexible pavements, ie. asphalt. The MIT study suggests the overall excess fuel consumption benefits of shifting from existing flexible surfaces to a rigid pavement (ie; concrete) far exceeded the implied added embodied carbon burden both cities, despite variations between each city’s urban design and climate.

The potential for dramatic reductions in overall GHGs, as well as a net positive for lifetime carbons, makes a compelling argument for concrete roads.

John Bleasby is a freelance writer. Send comments and Climate and Construction column ideas to editor@dailycommercialnews.com.