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Pavements are one of the most abundant surfaces of the urban infrastructure. Though pavements have been viewed as detrimental to the fight against climate change and urban heat islands (UHI), the evolution of reflective pavements will change this perception for good.
The implementation of reflective pavements, also known as cool pavements, can help prevent extreme heat events like the ones in California, which resulted in over 9,279 wildfires across 4,359,517 acres of land in 2020 alone. Such incidents can pose a severe threat to life and property, recurring over the years and leading to long-term effects.
Reflective pavement is a paving strategy that allows more sunlight to be reflected from its surface and less to be absorbed by its mass. The technique is already being used in Los Angeles by implementing the use of reflective coatings or brighter-color materials in the pavement mixtures. The practice was found to decrease the occurrence of heatwaves by almost 40% over a period of 20 years.
This allows the pavements to lower the urban temperatures when the surrounding temperature is lower than that of the pavement surface. Not only do the reflective pavements alter air temperatures, but also influence climate change by reflecting light into building envelopes, which alter heating and cooling demands and the associated greenhouse gas (GHG) emissions in the surrounding areas.
In addition, they are capable of sending a larger amount of solar irradiation to the sky, thereby altering the planet’s energy balance. The resulting process, called radiative forcing, creates a cooling effect that can help offset climate change.
Need for Reflective Pavements
Dark pavements, like conventional dark roofs, absorb about 80-95% of the sunlight, making them extremely hot in the sun. This aggravates the condition of UHIs by heating the local air while contributing to global warming by radiating heat into the atmosphere.
Solar reflective pavements maintain a considerably cooler temperature than traditional pavements. The reflectance property of the pavements can be enhanced by the use of reflective aggregate, a reflective surface coating, or a reflective or clear binder.
Technology Behind Reflective Pavements
Reflective pavements can be made from traditional paving materials, such as cement concrete. New cement concrete has a solar reflectance (SR) of 30-50%, whereas novel cool-colored coatings are available for asphalt concrete pavements that are capable of reflecting about 50% of sunlight. Another way is by using a clear binder that divulges highly reflective (light-colored) aggregate.
The solar reflectance of pavement can change with time and use, just like all materials exposed to the atmosphere. The cement concrete pavement tends to get darker with tire and grease stains. However, the asphalt concrete lightens with age as the asphalt binder oxidizes and exposes more aggregate through wear.
Combating Climate Change
The effectiveness of reflective pavement in combating the impacts of climate change depends on a number of factors, with geographical context being one of the major factors. The local climatic conditions such as cloud factors, real-time temperature, and relative humidity play a significant role in the intensity of radiative forcing, along with changes to building energy demand (BED) due to heating and cooling.
The neighborhood layout in urban regions, like the building density and canyon aspect ratio (ratio of building heights to the adjacent pavement width), can alter the BED effect of reflective pavements.
The building characteristics, like the insulation system, the heating and cooling technology, and the ratio of the surface area to volume, can also influence the intensity of BED change. In addition, the sustainability and efficiency of the local grid also play a significant role.
Apart from surface reflectivity, the additional impacts of the pavement’s cumulative life-cycle emissions include construction and repairs of pavements, additional fuel consumption by vehicles due to pavement properties, and end-of-life landfilling or recycling. Similar to pavement reflectivity, these impacts can also differ based on the context.
For instance, the BED effect of pavements in urban neighborhoods is more evident due to the large number of apartment units whose energy demands get altered by the surface reflectivity of pavements. As these pavements also serve a relatively low volume of traffic, the contribution of their reflectivity to their total life-cycle impact is also more notable.
However, for the highways seeing considerably greater levels of traffic, the structural properties and surface roughness of the pavement result in a greater magnitude of that pavement’s life-cycle emissions by affecting the fuel consumption of vehicles. It is, therefore, necessary to consider all elements of a life cycle while deciding on an environmentally preferred option.
Benefits of Reflective Pavements
1. Reflective pavements decrease the temperature of the outside air, enabling air conditioners to operate with less energy. Such pavements also help save energy by reducing the need for electric street lighting during nights.
2. The reflective pavements cool the air, which can slow the formation of smog, reduce heat-related illnesses, and maintain a comfortable temperature outside.
3. Light-colored pavements can better reflect street lights and vehicle headlights at night, increasing visibility for drivers.
4. Reflective pavements lower surface temperatures, which causes the stormwater to cool down and lessen the damage to local watersheds.
5. Reflective pavements reduce the heat absorbed by the Earth’s surface, which can temporarily offset the warming caused by greenhouse gases.
Reflective pavement is a paving strategy that allows more sunlight to be reflected from its surface, and less to be absorbed by its mass. Solar reflective pavements maintain a considerably lower temperature than traditional pavements.
Reflective pavements can be made from traditional paving materials, such as cement concrete. Another way is by using a clear binder that divulges highly reflective aggregate.
By allowing more sunlight to be reflected from its surface, reflective pavements lower the urban temperatures when the surrounding temperature is lower than that of the pavement surface.