How Cool Pavements Work

Pavements are truly more than meets the eye. While drivers and pedestrians only see the pavement surface, a typical city street can extend about a 1 ½ feet (half a meter) below the surface. The surface layer is very strong and designed to withstand the destructive effects of weather and traffic. The lower layers are weaker than the surface layer but are designed to spread the heavy traffic loads out over the base soil. The surface and base layers are often made out of the same rock and sand, but the upper layers typically use cement or asphalt to bind the rock together. Together, these layers are designed to work as a unit to support various traffic loads in all weather and on all soil types.

“Cooling” the pavement surface

While pavement systems can be quite thick, the top surface layer plays the largest role in determining how “cool” a pavement can be. This is because sunlight meets the surface before the subsurface layers, so the surface dictates how much sunlight is absorbed by the pavement. Like conventional dark roofs, dark pavements get hot in the sun because they absorb 80-95% of sunlight. Their surface temperatures can reach as high as 150°F (67°C), warming the air and contributing to the heat island effect.[1]

A “cool pavement” has a lower surface temperature and therefore transmits less heat to the surrounding air than traditional pavements, which keeps local temperatures lower. Pavements can keep cool by reflecting sunlight (See Figure 3). Solar reflectance, or albedo, is the percentage of sunlight that is reflected by a surface. Pavements with higher solar reflectance (measured between 0 and 1) reflect more sunlight, and thus have cooler surface temperatures. A solar reflectance of 1 (100%) means that the surface is a perfect reflector, while a solar reflectance of 0 (0%) means that the surface reflects no sunlight.

Aging of the pavement surface

Newly constructed asphalt concrete pavement has a solar reflectance of around 0.05 (5%), meaning that it absorbs 95% of the sunlight that reaches its surface, causing it to get very hot on a clear summer day. However, oxidation and wear and tear from vehicle traffic cause the dark binder of asphalt concrete pavement to dissipate over time. This leads to an increase in its solar reflectance of as much as 0.20 (20%) after seven years of use.[2] Once asphalt concrete has deteriorated to this extent, it typically requires resurfacing that has conventionally been done with dark materials that approximate the original appearance of the pavement. On the other hand, newly constructed cement concrete pavement has a solar reflectance of around 0.35 (35%), absorbing roughly 65% of the sunlight that reaches its surface. As cement concrete ages, it gets darker in color from vehicle traffic and soiling, with its solar reflectance decreasing to approximately 0.25 (25%) after five years of use.[3]

Aged asphalt concrete (left), new asphalt concrete (right)	Aged cement concrete (left), new cement concrete (right)

The role of subsurface pavement layers

While the pavement surface governs how much sunlight is absorbed by the pavement system, the subsurface layers also contribute to interactions between pavements and the natural environment. This is because hard, dense surfaces like pavements have high thermal mass, which enables them to retain heat effectively. When the pavement surface absorbs energy from the sun during the day, it transmits this energy to deeper pavement layers where it is stored as heat. Once the sun goes down, pavements release the heat they have accumulated throughout the day into the air, which can affect urban nighttime temperatures. Thus, if a pavement surface is more reflective it will transmit less heat into the subsurface pavement and will keep urban temperatures lower overnight. [4]

Cool pavements in practice

Many cities are exploring the adoption of cool pavements as a heat island reduction component of climate action and/or adaptation plans. Existing dark pavements can be made “cool” with the addition of surface treatments that reflect more sunlight. New roads can also be constructed using more reflective materials or with permeable paving materials, which keep pavements cooler by absorbing and subsequently evaporating water. In addition to pavement materials, shading from vegetation can keep pavements cooler by preventing the sunlight from being absorbed by the paved surface.

Figure 4. Visible (left) and thermal infrared (right) images of a parking lot in Rio Verde, Arizona (source: Larry Scofield - APCA)

Solar reflective pavements

Cities are increasingly finding that using light-colored materials for new pavements can keep their communities cooler throughout the life of the pavement.

Figure 5. On a 72°F (23°C) June afternoon with no wind or clouds in Berkeley, CA, aged cement concrete (top right) was 21°F (11°C) cooler than new asphalt concrete (top left), and cool coatings (bottom four images) were as much as 31°F (17°C) cooler than the asphalt (source: LBNL-Heat Island Group)

When considering cool solutions for existing paved surfaces, cities can take advantage of routine pavement maintenance practices. Rather than applying a traditional asphalt seal coat or slurry seal to an existing pavement surface, cool surface treatments can repair the pavement surface while addressing the urban heat island effect. Cool surface treatments are available in a variety of materials (e.g., acrylic, cement, asphalt, rubber), for various paved surfaces (e.g., parking lots, residential streets, schoolyards), and a range of colors that can reflect more sunlight than dark asphalt. Another option to make existing pavement surfaces more reflective while repairing them is to use cement overlays (also called “whitetopping”), which places a thin layer of cement concrete over an existing asphalt surface.

Permeable pavements

In addition to solar reflective pavements, permeable pavements are another class of cool pavements. Also called pervious or porous pavements, permeable pavements are designed to allow water to pass through pores between rocks. The pavements then channel this water either to recharge local groundwater or to appropriate stormwater infrastructure, thereby diminishing runoff that can cause sewer overflow and harm local watersheds.

As water passes through the pavement, some of it adheres to the pavement and eventually evaporates. Because water requires heat from the surrounding air to transform from its liquid phase to vapor, permeable pavements can cool the surface and surrounding air as water evaporates from pavement system.

When thinking about permeable pavements, it is important to consider that some regions have relatively dry summers. When moisture is scarce, permeable pavement systems retain less water and there is less evaporative cooling of the surrounding air. Therefore, permeable pavements may confer less of a local cooling benefit where there is little summer precipitation. It should also be noted that evaporation also occurs more slowly in humid conditions, so permeable pavements cool air less effectively on very humid days.

In addition, permeable pavements have been found to cool down faster at night because of the insulating property of the air voids in the pavement structure.[5] This could help with nighttime urban heat islands but will not be as effective at cooling during the late afternoon.

Shading

An alternate option to cooler pavement materials is to provide shading from vegetation or man-made structures over paved surfaces. Shading can reduce the temperature of paved surfaces by 20-45°F (11-25°C) relative to peak temperatures in unshaded areas.[6] A study in Davis, California found that cars parked in a shaded parking lot in the summer had interior air temperatures that were 45°F (25°C) cooler than cars parked in direct sunlight.[7]

Learn more about the other cooling benefits of urban vegetation.

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