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Road Services - Porous concrete

Military Road South at South 272nd Street Project

What is porous concrete sidewalk?

Porous concrete sidewalk is a cement mixture that contains voids when hardened. These voids allow water to drain through the sidewalk surface and infiltrate directly into the soil below. This project will be using porous concrete sidewalks on all four legs of the intersection.

Porous concrete specifications from other agencies

Porous concrete helps mitigate the 'urban heat island' effect

According to the U.S. Environmental Protection Agency, an "urban heat island" (UHI) refers to an urban area that is significantly warmer than its surroundings. The temperature difference usually is larger at night and during the winter; however, it primarily depends on local conditions. Urban heat islands are caused by land surface modifications from urban development and waste heat from energy usage. Many U.S. cities and suburbs have air temperatures up to 10°F warmer than the surrounding natural land cover.1

Porous concrete helps reduce local UHI effects in several ways. Foremost, its relatively light color has a higher albedo, or reflectance, than darker pavements such as asphalt. Additionally, the pores associated with porous concrete allow it to store relatively less heat than typical concrete.2 Consequently, porous concrete absorbs less solar radiation, stores less heat, and transfers less heat to its surroundings than most paving materials.

Water infiltration associated with porous concrete limits UHI effects following periods of rainfall by keeping the recesses of the pavement cool.3 Additionally, by eliminating stormwater pooling, porous concrete dries faster and restores its surface albedo more quickly. Infiltration associated with porous concrete also provides nearby trees and plants relatively better access to oxygen and nutrients from soils beneath the pavement while reducing the temperatures near trees' upper root zone. As a result, vegetation grows faster and larger. This reduces UHI effects in the long-term by providing more shade and increasing local evapotranspiration.4

Porous concrete is better for vegetation

Many trees and plants have difficulty growing in areas covered by impervious pavements, sidewalks, and landscaping, because air and water have difficulty getting to the roots. Unlike typical concrete, porous concrete pavements and sidewalks allow local vegetation to receive more air and water, allowing vegetation to grow faster and larger.5

Related links

The following links will take you outsideExternal link. of the King County website.

1 EPA, “Heat Island Effect, Basic Information.” (accessed Aug. 7, 2007)
2 Tennis, et al. "Pervious Concrete Pavements." Portland Cement Association and National Ready Mixed Concrete Association. (accessed Aug. 7, 2007)
3 EPA, “Heat Island Effect, What Can Be Done.” (accessed Aug. 7, 2007)
4 Golden, Jay and Kamil Kaloush. “Alternative pavements ease urban-heat effect.” The Arizona Republic, Aug. 4, 2007. (accessed Aug. 7, 2007)
5 Tennis, et al. "Pervious Concrete Pavements." Portland Cement Association and National Ready Mixed Concrete Association. (accessed Aug. 7, 2007)

Information from the Road Services Division's website is available to people with disabilities in alternate formats upon request by calling 206-477-3839 or 711 for the TTY relay service.

External link = External link

Photo: Porous concrete
Porous concrete sidewalk.
Enlarged view, 151KB .jpg
(Photo courtesy of Greg McKinnon, Stoneway Concrete.)

Photo: Porous concrete subbase
Porous concrete subbase.
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Photo: Porous concrete adjacent to curb
Porous concrete adjacent to curb.
Enlarged view, 261KB .jpg