Pavements are an abundant urban surface, covering about 40 percent of US cities. But in addition to carrying traffic, they can also emit heat.
Due to what is known as the urban heat island effect, impermeable and densely built surfaces like sidewalks can absorb solar radiation and warm their surroundings by re-emitting that radiation as heat. This phenomenon poses a serious threat to cities. It increases the air temperature by up to 7 degrees Fahrenheit and contributes to health and environmental risks – risks that climate change will amplify.
In response, researchers at the MIT Concrete Sustainability Hub (MIT CSHub) are studying how a surface that usually increases urban heat islands can reduce their intensity instead. Their research focuses on “cold pavements,” which reflect more solar radiation and emit less heat than conventional paving surfaces.
A recent study by a team of current and former MIT CSHub researchers in the journal of Environmental sciences and technologies describes cold pavements and their implementation. The study found that they could lower air temperatures in Boston and Phoenix by as much as 1.7 degrees Celsius (3 F) and 2.1 C (3.7 F), respectively. They would also reduce greenhouse gas emissions, reducing total emissions by up to 3% in Boston and 6% in Phoenix. Achieving these savings, however, requires that cold pavement strategies be chosen based on the climate, traffic and building configurations in each neighborhood.
Cities like Los Angeles and Phoenix have already had significant experiments with cold pavements, but the technology is still not widely implemented. The CSHub team hopes their research can guide future cold paving projects to help cities cope with climate change.
Scratch the surface
It is well known that darker surfaces get hotter in the sun than lighter ones. Climatologists use a metric called “albedo” to help describe this phenomenon.
“Albedo is a measure of surface reflectivity,” explains Hessam AzariJafari, lead author of the article and post-doctoral fellow at MIT CSHub. “Low albedo surfaces absorb more light and tend to be darker, while high albedo surfaces are brighter and reflect more light. “
Albedo is at the heart of cool pavements. Typical paving surfaces, like conventional asphalt, have low albedo, absorb more radiation, and emit more heat. Cold pavements, however, have brighter materials that reflect more than three times as much radiation and, therefore, re-emit much less heat.
“We can build cold pavements in different ways,” explains Randolph Kirchain, a researcher at the Materials Science Laboratory and co-director of the Concrete Sustainability Hub. “Brighter materials like concrete and lighter colored aggregates offer a higher albedo, while existing asphalt pavements can be made ‘cool’ with reflective coatings. “
CSHub researchers looked at these different options in a study from Boston and Phoenix. Their analysis took into account different results when concrete, reflective asphalt and reflective concrete replaced conventional asphalt pavements, which make up more than 95 percent of pavements worldwide.
Knowledge of the situation
For a comprehensive understanding of the environmental benefits of cold pavements in Boston and Phoenix, researchers had to look beyond paving materials alone. In fact, in addition to lowering the air temperature, cold pavements have direct and indirect impacts on climate change.
“The only direct impact is radiative forcing,” AzariJafari notes. “By reflecting radiation back into the atmosphere, cold pavements exert radiative forcing, which means they alter the Earth’s energy balance by sending more energy out of the atmosphere, like polar ice caps.”
Cold pavements also exert complex and indirect impacts on climate change by modifying energy consumption in adjacent buildings.
“On the one hand, by lowering temperatures, cool pavements can reduce some air conditioning needs [air conditioning] in summer while increasing the demand for heating in winter ”, explains AzariJafari. “Conversely, by reflecting light – called incident radiation – onto neighboring buildings, cool pavements can heat structures, which can increase air conditioning use in summer and reduce heating demand in winter. “
In addition, albedo effects only represent a part of the overall life cycle impacts of a cold pavement. In fact, the impacts of construction and material extraction (together called embodied impacts) and pavement use both dominate the life cycle. In addition to the albedo effects, the main impact of the use phase of a pavement is the overconsumption of fuel: pavements with smooth surfaces and rigid structures result in less overconsumption of fuel in the vehicles traveling on them.
Evaluating the impacts of cold pavements on climate change is therefore a complex process, involving many compromises. In their study, the researchers sought to analyze and measure them.
To determine the ideal cold pavement implementation in Boston and Phoenix, the researchers studied the life cycle impacts of switching from conventional asphalt pavements to three cold pavement options: reflective asphalt, concrete, and reflective concrete.
To do this, they used coupled physical simulations to model buildings in thousands of hypothetical neighborhoods. Using this data, they then trained a neural network model to predict impacts based on building and neighborhood characteristics. With this tool in place, it was possible to estimate the impact of cold pavements for each of the thousands of roads and hundreds of thousands of buildings in Boston and Phoenix.
In addition to the albedo effects, they also examined the intrinsic impacts for all pavement types and the effect of pavement type on excessive fuel consumption of vehicles due to surface qualities, stiffness and rate. deterioration.
After assessing the lifecycle impacts of each type of cold pavement, the researchers calculated which material (conventional asphalt, reflective asphalt, concrete and reflective concrete) benefited each neighborhood the most. They found that while cold pavements were advantageous in Boston and Phoenix overall, ideal materials varied widely within and between the two cities.
“The impact of radiative forcing was one of the universal benefits for all types of neighborhoods and paving materials,” notes AzariJafari. “This was especially the case in areas with shorter, less dense buildings, where the effect was most pronounced.”
Unlike radiative forcing, however, changes in the energy demand of buildings differed by location. In Boston, cold pavements have reduced energy demand as often as they have increased it in every neighborhood. In Phoenix, cold pavements negatively impacted energy demand in most census tracts due to incident radiation. However, taking into account the radiative forcing, the cold pavements ultimately had a clear advantage.
It was only after taking into account the intrinsic emissions and the impacts on fuel consumption that the ideal type of pavement emerged for each neighborhood. After controlling for life cycle uncertainty, the researchers found that reflective concrete pavements performed best, found to be optimal in 53% and 73% of neighborhoods in Boston and Phoenix, respectively.
Once again, uncertainties and variations have been identified. In Boston, replacing conventional asphalt pavements with a cold option has always been preferred, while in Phoenix, concrete pavements – reflective or not – performed better due to high temperature stiffness that minimized fuel consumption of vehicles. And despite the predominance of concrete in Phoenix, in 17% of its neighborhoods all reflective paving options were found to be more or less effective, when in 1% of cases conventional pavements were in fact superior.
“Although the impacts of climate change we have studied have been found to be numerous and often at odds with each other, our conclusions are unambiguous: Cool pavements could offer immense climate change mitigation benefits for the two cities, ”says Kirchain.
Improvements in air temperature would be noticeable: the team found that cool pavements would reduce peak summer temperatures in Boston by 1.7 C (3 F) and Phoenix by 2.1 C (3.7 F) ). The reductions in carbon dioxide emissions would also be impressive. Boston would reduce its carbon dioxide emissions by up to 3% over 50 years, while reductions in Phoenix would reach 6% over the same period.
This analysis is one of the most comprehensive cold pavement studies to date, but there is still a lot to study. As with pavements, it is also possible to adjust the albedo of the building, which can lead to changes in the building’s energy demand. The intensive decarbonization of the network and the introduction of low carbon concrete mixes can also modify the emissions generated by cold pavements.
There is still a long way to go for the CSHub team. But by studying cold pavements, they’ve come up with a brilliant solution to climate change and opened up avenues for new research and future mitigation.
The MIT Concrete Sustainability Hub is a team of researchers from several MIT departments working on the science, engineering, and economics of concrete and infrastructure. His research is supported by the Portland Cement Association and the Ready Mixed Concrete Research and Education Foundation.