Due to an increase in paved surfaces, the urban heat island effect has become a localized problem posing a very real global threat that calls for engineers, contractors, and architects to develop innovative and immediate solutions.
The urban heat island (UHI) effect causes ambient temperatures in urban areas to increase as illustrated in Figure 1. When compared to rural areas, urban areas contain a higher density of dark pavement surfaces (DPS), which includes asphalt and concrete paved surfaces. During the day these paved surfaces act as a heat sink, absorbing radiation from the sun.
Figure 2 shows how radiant energy affects DPS. By creating higher surface temperatures, the ambient temperature during the day is increased dramatically. At night, the overheated DPS acts as the heat source releasing stored heat into the cool air above, increasing night-time temperatures.
This succession of heat being absorbed (day) and released (night) can create a wide variety of problems. Temperatures in urban areas can be increased by an incredible 50° F in a localized area, the vast majority in a thermal layer 3 to 4 feet above the ground. These extreme changes in temperature initiate a number of consequences, one being the risk of heat stroke. The Mayo Clinic defines heat stroke as a life-threatening condition when the body temperature reaches or exceeds 104° F.1
Besides health concerns, the UHI has economic effects. Ignoring the source of UHI, communities try to control the side effects through the use of cooling systems. This leads to high energy demand and an increase in the carbon footprint.
Emerging technologies can lead to less expensive solutions to the UHI problem. A solution would need to reduce DPS surface temperatures, be available locally, be easily applied, and reduce energy consumption and carbon footprint. Secondary benefits should include increased durability and service life of the DPS surface.
Emerald Cities recognized the need for a solution to the UHI effect. In the past three years, Emerald Cities has pioneered an in-depth research and development project to work with reflective experts from the Department of Energy and scientists from Lawrence Berkeley National Laboratory. Establishing a collaborative effort with Intelligent Concrete, Emerald Cities used emerging nanotechnologies to create an ultra-high-performing material. Extensive laboratory development and testing was conducted to design a nano-engineered cementitious overlay that reflects powerful solar radiation. Figure 2 illustrates how a standard DPS absorbs solar radiation versus the same DPS reflecting the sun’s rays with the Emerald Cities Cool Pavement technology.
Although engineering properties were being measured in the laboratory, Emerald Cities began field trials to refine the mixing and placing procedures. Testing in the field revealed the Nano-Crete could do more than reduce the UHI effect. Laboratory and fields samples, shown in Figure 3, determined that the nano-engineered composite resulted in flexural strength that was 17% of the compressive strength; typical concrete has a flexural strength of only 10% of compressive strength.
Another benefit of this Nano-Crete is the rolling friction of the wet surface. Figure 4 demonstrates how the Cool Pavement returns the deteriorated pavement to serviceable conditions. The overlay develops an abrasive surface similar to that of the original newly paved surface. The British Pendulum number (BPN) of the deteriorated surface was in the low 70s. The Cool Pavement brought the BPN average up to 101. The end result of the intense research, development, and testing is a material that reduces the heat island effect by reflecting sunlight, reclaims deteriorated surfaces, and can be applied with ease.
To unveil this product to the public, Emerald Cities donated material and expenses for installation to the Robert L. Duffy School in Phoenix, Ariz., shown in Figure 5. The Duffy School is an urban charter school that provides inner city youth with an exceptional educational experience. About 50% of the school property was covered with dark pavement, primarily asphalt. Surface temperatures in the asphalt parking lot exceeded 210° F, creating a thermal area that endangered students.
Application of the nano-engineered overlay took a total of three days. The result was that surface temperatures were decreased by more than 50° F and ambient temperatures were decreased by more than 30° F.
Figure 6 shows the Duffy School parking lot after project completion. The success led to initiation of the “100 Cities Project,” a program developed by Emerald Cities in conjunction with the Department of Energy. The “100 Cities Project” hopes to replicate the success at the Duffy School, reducing the UHI in 100 cities throughout the country.
The UHI effect causes dramatic temperature increases in urban areas. These temperature increases cause damage both locally and globally. Cooling systems have been used as a solution but only serve as a temporary fix for a more dramatic problem. The Emerald Cities Cool Pavement Technology is a local response to a global problem and is an innovative and easy solution to this important issue.
1 Mayo Clinic Staff. (2009). Heat Stroke. Mayo Foundation for Education and Research. www.mayoclinic.com/health/heat-stroke
2 Levinson, R. Urban Heat Island Group. Lawrence Berkley National Laboratory. http://heatisland.lbl.gov/