Portland cement concrete can reduce heat islands and minimize the impact buildings have on microclimate, and human and wildlife habitats. A recent laboratory study shows that concretes typically used in the US have a solar reflectance index (SRI) that meets the criteria of the LEED Sustainable Sites Credit for reducing heat islands.
The Leadership in Energy and Environmental Design Green Building Rating System for New Construction and Major Renovation (LEED-NC) is a voluntary rating system for designing, building, and certifying green buildings. Download LEED-NC from the U.S. Green Building Council website: Leadership in Energy and Environmental Design Green Building Rating System for New Construction and Major Renovation (LEED-NC)] LEED is transforming the marketplace because architects increasingly specify materials that qualify for LEED points.
In LEED-NC two points are available for reducing heat islands. One point can be earned by using paving materials with a solar reflectance index of at least 29 for 50% of the site hardscape (including roads, sidewalks, courtyards, and parking lots). This is “Sustainable Sites Credit 7.1: Heat Island Effect: Non-Roof”. Another point can be earned by using roofing materials with a solar reflectance index of at least 29 for steep-sloped roofs and 78 for low-sloped roofs over at least 75% of the roof surface. This is “Sustainable Sites Credit 7.2: Heat Island Effect: Roof”.
A heat island is an area of higher temperature in a region of cooler temperatures. Heat islands occur where there are dark exterior surfaces (such as black roofs and dark pavements) and a lack of vegetation. Dark-colored materials generally have a low solar reflectance, so they absorb heat from the sun and warm the air through convection. Lighter-colored materials (such as concrete) generally have a higher solar reflectance, so they reflect heat from the sun and do not warm the air as much. Shade, from trees and buildings, and the natural process of evaporation of water from the surface of plants also help keep the air cool.
|Figure 1. This drawing shows how heat islands
raise the air temperature in urban areas relative
to the surrounding countryside.
[Figure copied from The Urban Heat Island Group,
lasted visited 2007 March 30.]
Heat islands usually occur in urban areas, so they are sometimes called urban heat islands. Figure 1 shows how heat islands raise the air temperature in urban areas relative to the surrounding countryside. This temperature difference can be as much as 4°C (7°F). Urban heat island effects are local and have a negligible influence on climate change, according to the Intergovernmental Panel on Climate Change. However, in places that are already burdened with high temperatures, the heat island effect can make cities warmer, more uncomfortable, and occasionally more life-threatening, according to the U.S. Federal Emergency Management Agency. Air temperatures greater than 24°C (75°F) increase the probability of the formation of ground level ozone (commonly called smog), which exacerbates respiratory conditions such as asthma. Higher temperatures also lead to a greater reliance on air conditioning, which leads to more energy use.
In any area with buildings and other human-made structures, choosing exterior building materials with low solar reflectance, such as concrete, can reduce heat islands.
Solar Reflectance and Solar Reflectance Index
Solar reflectance (sometimes called albedo) is the ratio of solar energy that falls on a surface to the amount reflected. It is measured with a solar spectrum reflectometer on a scale of 0 to 1: from not reflective (0) to 100% reflective (1.0). Generally, materials that appear to be light-colored have high solar reflectance and those that appear dark-colored have low solar reflectance. However, color is not always a reliable indicator of solar reflectance because color only represents 47% of the sun’s energy at the earth’s surface. The remaining 53%, which is invisible to the human eye, consists of ultraviolet (3%) and infrared (50%) radiation.
The solar reflectance index (SRI) is a composite measure of a surface’s solar reflectance and emittance. Emittance is a measure of how well a surface emits (or lets go of) heat after it has been absorbed. Like solar reflectance, it is also measured on a scale of 0 and 1. Since concrete has an emittance of about 0.90, it needs to have a solar reflectance of at least 0.28 to meet the criterion of an SRI of 29 and at least 0.64 to meet the criterion of an SRI of 78.
Figure 2. These cementitious materials
were used to make the concretes.
The labels, which are explained in the
full report,describe either the
material source or its relative color.
A recent laboratory study measured the solar
reflectance of 45 concretes in accordance with ASTM C 1549, Standard Test Method for Determination of Solar Reflectance Near Ambient Temperature Using a Portable Solar Reflectometer. These concretes were selected because they represent the range of concrete and concrete constituents typically used in exterior flat work in the US. The concrete constituents consist of six portland cements, six fly ashes, three slag cements, four fine aggregates, and two coarse aggregates. While all materials were tested, more concrete mixes were made with the darkest (lowest solar reflectance) material combinations, that is, those most likely to fail the LEED criteria. Test specimens were proportioned, mixed, fabricated, and finished like typical exterior flatwork with a light broom finish. Replacement levels of 25% for fly ash and 45% for slag cement were chosen because they are commonly used substitution levels for cement.
The study shows that all 45 concretes tested have a solar reflectance of at least 0.3 and an SRI of at least 29. Therefore, they all meet the LEED criteria for non-roof surfaces and steep-sloped roofs. Two of the concretes studied also meet the criteria for low-sloped roofs. These contain either white cement or slag cement.
The solar reflectance of portland cement has more effect on the solar
reflectance of concrete than any other constituent material. The solar reflectance of the supplementary cementitious material (in this study, fly ash and slag cement) has the second greatest effect. Generally, the higher the solar reflectance of the cementitious material, the higher the solar reflectance of the concrete. Slag cement concretes generally have the highest solar reflectances. The average effect of replacing 45% of the cement in a mix with slag cement is to increase (lighten) the solar reflectance of the concrete by 0.07. The average effect of replacing 25% of the cement in a mix with dark gray fly ash (defined in this study as fly ash with a solar reflectance of 0.28) is to decrease (darken) the solar reflectance by 0.02. The average effect of replacing 25% of the cement in a mix with the other fly ashes is to increase (lighten) the solar reflectance by 0.03.
Figure 4 - A sample of the
concretes tested shows
that concrete can be a
variety of shades of light
The solar reflectance of fine aggregate has a very small effect on the solar reflectance of concrete, while the solar reflectance of coarse aggregate has no effect on the solar reflectance of concrete.
Regardless of mix constituents, concrete in the US can reduce heat islands and qualify for points in the LEED Green Building Rating System. The study shows that all 45 concretes tested according to ASTM C 1549 have a solar reflectance of at least 0.3 and an SRI of at least 29. Therefore, using concrete on at least 50% of a site’s hardscape (including roads, sidewalks, courtyards, and parking lots) can earn one point under “Sustainable Sites Credit 7.1: Heat Island Effect: Non-Roof”. Using concrete on at least 75% of steep-sloped roofs can also earn one point under “Sustainable Sites Credit 7.2: Heat Island Effect: Roof”. Further, two of the concretes tested can also earn one point for low-sloped roofs under “Sustainable Sites Credit 7.2: Heat Island Effect: Roof” because these concretes have a solar reflectance index of at least 78.
[Include the reference: Marceau, Medgar L. and VanGeem, Martha G., Solar Reflectance of Concretes for LEED Sustainable Site Credit: Heat Island Effect, SN2982, Portland Cement Association, Skokie, Illinois, USA, 2007, 94 pages.]