Cool pavement is a type of pavement that helps reduce the heat island effect, according to the United States Environmental Protection Agency. This is done by using technologies that either increase the pavement's ability to reflect sunlight, which reduces heat absorption, or by using water to cool the air around the pavement. Newer methods include harvesting energy, storing heat, and using materials that change state to manage temperature. Cool pavements are often made by adding reflective coatings or materials to regular pavement or by using porous or permeable materials. While these pavements may need more frequent maintenance and might not support heavy loads as well as regular pavement, they have shown good results in lowering air temperatures and reducing energy use.
Development
The United States Environmental Protection Agency describes cool pavement as pavement that uses methods to help reduce the heat island effect. Reducing the heat island effect lowers the energy needed to keep buildings cool, which can decrease greenhouse gas emissions. Scientists at the Lawrence Berkeley National Laboratory say raising the solar reflectance of pavements from 10% to 35% could lower the air temperature by 1 degree Fahrenheit.
Cities have noticed cooler temperatures after using cool pavements and other green infrastructure. In Rome, shaded pavements with high reflectivity reduced the average radiant temperature by up to 50%. Los Angeles, Phoenix, and Tokyo have also tested cool pavements, with Los Angeles applying solar-reflective coating to 181 lane-miles of roads.
Studies on cool pavement technology are still in the early stages. Current efforts focus on how to maintain these pavements, how well they work, and how to use them on a large scale in a cost-effective way.
Types
Evaporative pavements are a common type of cool pavement that lowers surface and air temperatures by using water evaporation. This process cools both the pavement and the air around it. These pavements need water from rain or irrigation systems to work properly. They are made with materials that allow water to pass through and absorb it, such as soil additives and porous asphalt. Over time, repeated water absorption and evaporation can weaken the pavement's structure, causing it to break down. However, evaporative pavements have challenges, including needing large amounts of water, which may not be available in dry areas. They are also less strong than traditional asphalt pavements.
Reflective cool pavements work by reflecting sunlight, which reduces heat absorption. Their effectiveness depends on their ability to reflect solar radiation, known as albedo. These pavements use reflective materials like light-colored aggregates, special coatings, and advanced binders to increase reflectivity. Common coatings include clear resin, light-colored aggregates, and light-colored cement, which have different levels of reflectivity. Because they reflect sunlight, reflective pavements lower surface temperatures only during the day. Lighter cement colors improve the ability of concrete to reflect sunlight.
Other types of pavements, called "heat storage modified pavements," include energy-harvesting, high-conductive, and phase change material (PCM)-incorporated pavements. Energy-harvesting pavements use liquid circulation to collect heat energy, which can then be turned into electricity using thermoelectric generators. Photovoltaic cells can also convert sunlight into electricity. High-conductive pavements use materials that transfer heat quickly from the pavement surface to the soil below, allowing heat to dissipate faster. PCM-incorporated pavements use materials that absorb, store, and release heat as they change from solid to liquid states. This process helps control extreme heat by regulating temperature within the pavement.
Manufacturing
Evaporative porous pavements are made by drilling vertical holes into standard interlocking concrete blocks and filling the holes with gravel. These holes help water pass through the pavement and cool the area. Another method involves mixing aggregates covered with cement paste and asphalt before laying the pavement, creating connected spaces that hold water. Permeable and water-retaining pavements use materials with pores filled with blast furnace slag or pervious mortar to store runoff water in a specific layer.
Evaporative pavements often have weaker structures than other pavements because of the empty spaces inside. These pavements need more frequent maintenance and replacement because they are more likely to suffer water damage and break apart over time.
Reflective pavements are created by applying a top layer of reflective material to finished pavement or mixing reflective materials into wet concrete. Coating materials include water or solvent-based paints that reflect sunlight, infrared-colored coatings, and materials that change color based on temperature. Reflective additives can also be mixed into asphalt, such as heat-reflecting substances, or added to cement with materials like slag and fly ash.
Reflective pavements can become dirty over time, which reduces their ability to reflect heat. To keep them effective, maintenance usually involves applying a new surface coating or seal. Methods like chip seals use rollers to press gravel into the pavement, sand and scrub seals inject additives into cracks and roll them in, and microsurfacing sprays a high-friction, reflective coating onto the road.
Energy-harvesting cool pavements use fluid movement to collect heat through convection. This is done by embedding metal or concrete pipes into the pavement and allowing air or water to flow through them. Other methods include adding photovoltaic cells, thermoelectric generators, or pyroelectric materials into the pavement. So far, these pavements have not been proven strong enough for heavy traffic, as the energy-harvesting parts often break or lose efficiency during maintenance. Better ways to make these pavements stronger are still being tested.
High-conductive cool pavements can be improved by adding materials that transfer heat quickly, such as carbon or steel fibers, graphite, carbon black, steel slag, or metal rods.
PCM-incorporated cool pavements are made by placing phase change materials in protective coverings before mixing them into asphalt. Direct contact with these materials can weaken the pavement and make it more likely to crack. These pavements are often created by filling porous materials like shale or clay with phase change materials, then covering them with cement before mixing into concrete or asphalt. Another method is to cover the phase change materials with metal shells before adding them to asphalt.
Safety implications
Cool pavement helps roads let water through more easily, which makes driving safer during rainy weather. The tiny holes in evaporative cool pavement allow water to pass through the road surface. This can improve tire grip and reduce water splashing. Also, cool pavements can lower tire noise by up to eight decibels and reduce overall traffic noise to 75 decibels.
Reflective cool pavements can make roads more visible at night, which reduces the need for streetlights. This makes streets safer and saves energy. However, the higher reflectiveness of these pavements can sometimes cause glare that may make it harder to see. To fix this, roads can be treated with special anti-glare coatings.
Sunlight and heat can increase the formation of ground-level ozone, which harms people and animals. At least one in every three people in the United States has health problems linked to ozone, such as trouble breathing, asthma, and a weaker immune system. Cool pavements help reduce this issue by reflecting more sunlight and lowering road temperatures. This prevents harmful gases from combining to form ozone.
Societal implications
Installing cool pavements in urban heat islands can help reduce unequal heat distribution in cities, which affects minorities and low-income individuals more than others. These groups often do not have enough resources to deal with extreme heat. In hot weather, city areas can be 2-8 °F hotter than surrounding areas. Cool pavements lower road and air temperatures by increasing the albedo of roads, which reflects more sunlight and improves comfort.
Environmental impact
Cool pavements can lower local temperatures, which helps reduce the energy needed for air conditioning. For example, increasing the reflectiveness of pavement in Los Angeles is estimated to save more than $90 million each year in energy costs.
Using less energy can decrease greenhouse gas emissions and air pollution, depending on the type of fuel used to generate electricity. Cooler temperatures also slow chemical reactions that create smog, leading to cleaner air. In 2007, scientists calculated that raising pavement reflectiveness worldwide by 35 to 39 percent could reduce carbon dioxide emissions worth about $400 billion.
In 2022, a project in Pacoima, California, a city in Los Angeles County known for very hot summers, installed over 700,000 square feet of reflective pavement. This project studied how cool pavements affect the local climate, including changes in surface and air temperatures, pedestrian comfort, and issues like glare and air quality. On hot summer days, air temperatures dropped by 0.2 to 1.2 degrees Celsius, and surface temperatures were 2.6 to 4.9 degrees Celsius lower. Results showed that cool pavements can improve pedestrian comfort, with reductions in how hot people feel by 0.9 to 1.3 degrees Celsius and changes in temperature that make the environment feel cooler by 0.2 to 1.7 degrees Celsius.