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Pervious Concrete Research Facility;

Winter Performance and Enhancement of Pollutants Removal

steven_kohm@mail.uri.edu

    

Courtesy of Gaia Engineering

 

Heat Island Effect

 

What is the heat island effect?

    
The urban cityscape is filled with man-made materials that absorb the suns light.  Darkly colored roads and roofs have replaced surface area which was once predominantly vegetated lands.  Many of these man-made surfaces are also impermeable and typically dry.  For these reasons, summertime ambient temperatures in cities are typically warmer than those of rural areas.  On a sunny summer day city temperatures can be up to 8°F higher than rural areas [1]. Figure 1 shows the differences in ambient temperatures relative to ground cover.  This phenomenon is known as the heat island effect.

 

Figure 1 – showing the relationship between surface cover and ambient temperatures for different land cover areas.  [6] (Courtesy of Environmental Protection Agency)

 
How is the heat island effect measured?
 
The ability of a material to reflect the sun’s light is measured in a unit less parameter known as an objects albedo.  Albedos can range from 0 for very dark energy absorptive surfaces to 1 for lightly colored reflective surfaces.  Fresh asphalt has an albedo of 0.04 because of the bituminous material used [1].  Commonly used darkly covered roofs also tend to absorb sunlight and therefore, have low albedo values.  Albedo is measured using a device called a pyranometer.
 
What are the impacts of the heat island effect?
 
Heat island can lead to increased air conditioning use which puts a strain on a city’s energy grid.  This increased demand for cooling can amount to a 5-10 percent increase in peak electricity [6, 3] See figure 2.  One estimate shows that the temperature increase from heat island can account for up to 1.5 gigawatts of energy [1].  The Hoover Dam has a power capacity of 2.08 gigawatts.  This increase correlates with increased emissions from municipal power plants.  Therefore, the heat island effect contributes to problems with air quality including smog formation.  Also, the rainwater runoff originating from city pavements and roofs can disrupt aquatic ecosystems.  For example, pavement with surface temperatures of 100°F is able to heat rainwater from 70°F to 95°F [6, 7]. 

 

Figure 2 – relating electricity used per hour in New Orleans, LA to outdoor temperature.  As temperature increases, more people are using cooling technologies, which causes energy use to increase. [2, 9] (Courtesy of Environmental Protection Agency)

 

What can be done?
 

By lowering the albedo factors within a city, heat island effects can be avoided.  One way of accomplishing this reduction is targeting darkly colored pavements.  Pavement makes up a large amount of urban surface area, typically 30-45%, see figure 3 [6, 2].   Asphalt’s high albedo factor indicates a huge capacity to store the sun’s heat.  Cool pavements are a class of materials that increase reflectivity while still serving as structurally sound roads and parking lots.  There are many other benefits associated with cool pavements as well.

 

Figure 3 – showing the amount of paved surfaces in  four large U.S. cities.  [4, 6] (Courtesy of Environmental Protection Agency)

 

 

 

Cool Pavements

Modified Asphalt Pavement – Asphalt can be modified to increase its typical albedo of about 0.04.  Using lightly colored aggregate helps to increase the reflectivity of asphalt’s black bituminous surface.  Chip seals are another way of increasing reflectivity [10].
 
Modified Concrete – Concrete has a moderate albedo of 0.4.  However, this number can be increased to 0.7 simply by using white cement instead of the more common gray mixtures [10]. 
 
Resin Based Pavement – This technology uses clear tree resin to bind aggregate and the albedo is a function of aggregate color. The resin usually comes from pine trees. This method uses no petroleum-based products and is pH-neutral, non-corrosive, non-toxic and non-hazardous. This might be suitable for areas that do not experience a high volume of traffic, such as walkways or private roads [10].
 
Permeable Pavements – This technology includes several different types of modified pavements including porous asphalt, rubberized asphalt, or pervious concrete. These technologies increase evaporation and convection. When the surface is wet, cooling is provided through evaporation.  When dry, the subsurface of the material is cooler than non-permeable alternatives [10].
 
Chip Seals – When asphalt needs to be resurfaced, chip seals containing low albedo, lightly colored aggregate can help to reduce the heat island effect.  Some chip seals boast albedos of 0.2 compared to fresh asphalt’s albedo of 0.04 [10].
 
White Topping – White topping refers to placing four or more inches of fiber enhanced concrete on top of the asphalt pavement.  The same effect can be reached using a similar technique known as ultra thin white topping where 2-4 inches of lower albedo strength enhanced concrete is placed atop the pavement [10].
 
Microsurfacing – By using an extremely thin sealing layer on top of asphalt pavement, albedo can be reduced.  The sealing layer is typically a combination of cement, sand, and a liquid blend of emulsified polymer resin. This method has been found to increase albedo to 0.35. However, the usefulness of this method is diminished in high traffic areas where constant use causes the sealing layer to wear, thereby decreasing the albedo [10].
 
Grass Paving – This system works by combining a base course of sandy gravel material with a plastic grid mat that evenly disperses automobile surface loads. The plastic mat prevents compaction from vehicles, which allows grass to grow. This system can improve runoff and trap contaminants.  Some systems offer as much as 5721 psi of strength [10].
 

 

New Porous Pavement System

The benefits of permeable, or porous, pavements include much more than the reduction of the heat island effect.  The increased void spaces can help to reduce tire noise by up to 8 decibels [5, 6].  Gap graded porous concrete created with white cement and lightly colored aggregate may also promote street light reflectivity at night.  This translates to improved nighttime driving visibility and decreased street light output requirements.  The permeable properties of these materials also help to increase safety by reducing hydroplaning effects [6].
 
Permeable pavement systems promote infiltration of storm water into the water table while simultaneously improving runoff quality.  Research has demonstrated the ability of a permeable pavement to retain suspended solids[6]. Limited research also has been conducted on the water-purification properties of pervious concrete.  If these pavements are incorporated with specially designed media with the ability to remove metals, oxygen demand, and other common storm water contaminants, the pollutant load on the environment could be drastically reduced.  One such material might be quaternary ammonium cations, QAC’s, attached to a native Rhode Island soil with high organic content.  These materials typically referred to as organoclays, have been used with to treat contaminated runoff in the past with great success.  Never have these QAC’s been attached to a native Rhode Island soil with the purpose of treating storm water runoff.
 

 

Works Cited

[1] Akbari, Hashem . Heat Island Group. 30 Aug 2000. Lawrence Berkley National Laboratories, Web. 15 Oct 2009. <http://eetd.lbl.gov/HeatIsland/>.

[2] Akbari, H., L. Rose, and H. Taha. 1999. Characterizing the Fabric of the Urban Environment: A Case Study of Sacramento, California (PDF) (65 pp, 6MB). Paper LBNL-44688. Lawrence Berkeley National Laboratory
[3] Akbari, H. 2005. Energy Saving Potentials and Air Quality Benefits of Urban Heat Island Mitigation (PDF) (19 pp, 251K). Lawrence Berkeley National Laboratory.
[4] EPA’s Urban Heat Island Pilot Project Lawrence Berkeley National Laboratory (LBNL)
[5] Glazier, G. and S. Samuels. 1991. Effects of Road Surface Texture on Traffic and Vehicle Noise. Transportation Research Record 1312:141-44.
[6] "Heat Island Effect." U.S. EPA. 16 Sept 2009. The Environmental Protection Agency, Web. 15 Oct 2009. <http://www.epa.gov/heatislands/index.htm>.
[7] James, W. 2002. Green roads: research into permeable pavers. Stormwater 3(2):48-40.
[8] Rose, L., H. Akbari, and H. Taha. 2003. Characterizing the Fabric of the Urban Environment: A Case Study of Greater Houston, Texas (PDF) (65 pp, 4.5MB). Paper LBNL-51448. Lawrence Berkeley National Laboratory.
[9] Sailor, D. J. 2002. Urban Heat Islands, Opportunities and Challenges for Mitigation and Adaptation. Sample Electric Load Data for New Orleans, LA (NOPSI, 1995). North American Urban Heat Island Summit. Toronto, Canada. 1–4 May 2002. Data courtesy Entergy Corporation.
[10] Cambridge Systematics, Inc., "Cool Pavements." Reducing Urban Heat Islands: Compendium of Strategies June 2005: 1-35. Print.