Cool pavement, as defined by the United States Environmental Protection Agency, is pavement that uses special technologies to help reduce the heat island effect. Most cool pavements either increase the surface's ability to reflect sunlight, which helps keep heat from reaching the ground, or use water to cool the air around them. Newer methods include collecting energy, storing heat, and using materials that change state to manage temperature. These pavements are often made with reflective coatings, special materials added to the surface, or porous and permeable materials that allow water to pass through. Although these pavements may require more frequent maintenance and could have slightly lower strength compared to regular pavement, they are effective in lowering temperatures and reducing energy use.
Development
The United States Environmental Protection Agency describes cool pavement as pavement that uses special materials to help reduce the heat island effect. By reducing the heat island effect, cool pavement helps lower the amount of energy needed to cool buildings, which can decrease greenhouse gas emissions. Scientists at the Lawrence Berkeley National Laboratory estimate that increasing the ability of pavements to reflect sunlight from 10% to 35% could lower the temperature around buildings by 1 degree Fahrenheit.
Cities have reported lower temperatures after using cool pavements and other green infrastructure. Rome has seen a 50% drop in the average temperature felt by people with shaded, highly reflective pavements. Los Angeles, Phoenix, and Tokyo have tested cool pavements, with Los Angeles covering 181 miles of roads with a special reflective coating.
Studies on cool pavement are still in the early stages. Researchers are working on how to maintain these pavements, measure their effectiveness, and use them on a large scale without being too expensive.
Types
Evaporative pavements are a common type of cool pavement that lowers surface and air temperatures by using water evaporation. This process cools the pavement and the air nearby. 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, the repeated absorption and evaporation of water can weaken the pavement, causing it to break down. However, evaporative pavements have some drawbacks, including the need for large amounts of water, which may not be available in dry areas. They are also less durable than regular asphalt.
Reflective cool pavements work by reflecting sunlight, which reduces heat absorption. Their effectiveness depends on their ability to reflect solar radiation, also called albedo. These pavements use materials like reflective aggregates, special binders, and light-colored coatings to increase their reflectivity. This helps lower surface and air temperatures. Common coatings include clear resin, light-colored aggregates, and light-colored cement, each with different levels of reflectivity. Because they rely on reflecting sunlight, reflective pavements only lower surface temperatures during the day. Light-colored cement improves the reflectivity of concrete, helping it reflect more 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 collect heat energy by circulating liquids, which can be converted into electricity using thermoelectric generators. Photovoltaic cells can also be used to turn sunlight into electricity. High-conductive pavements use materials that conduct heat well, allowing heat to move quickly from the pavement surface to the soil below, where it is released more easily. PCM-incorporated pavements use materials that absorb and store heat when they change from solid to liquid, then release it when they change back. 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 paver blocks and filling the holes with gravel. These holes help increase the pavement's ability to let water pass through and cool the surface by allowing water to collect. This effect is also created by mixing aggregates covered with cement paste and asphalt binder before laying the pavement. This process forms connected pores that hold water. Permeable and water-retaining pavements use materials with pores that are filled with blast furnace slag or pervious mortar to hold runoff water in a specific layer.
Evaporative pavements often have less strength than other types of pavement because of the spaces (voids) in their structure. They need more frequent maintenance and replacement because permeable pavements are more likely to suffer water damage and surface breakdown (raveling).
Reflective pavement is made by applying a top coat to finished pavement or by mixing reflective materials into wet concrete. Coating materials include water or solvent-based coatings that reflect sunlight well, infrared-colored coatings, and thermochromic materials that change color and light properties based on temperature. Reflective additives mixed into materials include thermochromic materials in asphalt, heat-reflective additives, and slag or fly ash in cement mixtures.
Reflective pavements are easily polluted, which reduces their effectiveness over time. Maintenance usually involves applying a new surface coating or seal to fix wear or damage. Chip seals use large rollers to press gravel into the pavement surface. Sand and scrub seals inject additives into cracks and roll them in. Microsurfacing involves spraying a high-friction, high-reflective coating over the road.
Energy-harvesting cool pavements use fluid movement through the pavement to collect heat energy. This is done by embedding stainless steel, copper, or concrete pipes into the pavement and allowing air or water to flow through them. Other methods include placing photovoltaic cells, thermoelectric generators, or pyroelectric materials into the pavement. So far, these pavements have not been proven to handle heavy traffic well because the energy-harvesting parts are easily damaged or lose efficiency during road maintenance. Better manufacturing methods to improve their strength are still being developed.
High-conductive cool pavements can be made stronger by adding materials that transfer heat well to asphalt. These materials include carbon or steel fibers, graphite, carbon black compounds, steel slag, or metal rods.
PCM-incorporated cool pavements are made by enclosing phase change materials (PCM) in protective layers before mixing them into asphalt. Direct contact with PCM can weaken the pavement and make it more likely to break. PCM pavements are often made by filling porous materials like shale or clay with PCM and then covering them with cement before mixing into concrete or asphalt. PCM can also be enclosed in metal shells before being added to asphalt.
Safety implications
Cool pavement improves a road's ability to let water through, which makes driving safer during wet weather. The porous design of evaporative cool pavement allows water to pass through the road surface. This helps tires grip the road better and reduces water splashing. Cool pavements can also lower tire noise by up to eight decibels and reduce overall traffic noise to 75 decibels.
Reflective cool pavements can improve nighttime visibility, which reduces the need for streetlights. This makes streets safer and saves energy. However, the reflective surface of cool pavements can sometimes cause glare that may make it harder to see. To address this, anti-glare coatings can be added to the road.
Sunlight and high temperatures increase the formation of ground-level ozone, which harms people and animals. In the United States, at least one in three people experiences health problems caused by ozone, such as irritated lungs, asthma, and a weakened immune system. Cool pavements help reduce ozone by reflecting more sunlight and lowering temperatures. This prevents harmful gases from reacting to form ozone.
Societal implications
Using 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 the tools or money to deal with extreme heat. In hot weather, city areas can be 2-8 °F warmer than surrounding areas, but cool pavements lower road and air temperatures by reflecting more sunlight, which makes the environment more comfortable.
Environmental impact
Cool pavements help lower local temperatures. This means buildings need less energy to stay cool. For example, increasing how much light pavements reflect in Los Angeles has been estimated to save over $90 million each year in energy costs.
Using less energy reduces pollution from power plants, depending on what fuels are used. Cooler temperatures also slow chemical reactions that create smog, improving air quality. In 2007, scientists found 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 with very hot summers, covered over 700,000 square feet of pavement with reflective material. The project studied how cool pavements affect local weather, including changes in surface and air temperatures, pedestrian comfort, 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 cooler. Results showed that cool pavements can improve comfort for people walking, with temperatures feeling 0.9 to 1.3 degrees Celsius cooler and body-heat-related temperatures decreasing by 0.2 to 1.7 degrees Celsius.