Dark manmade structures that absorb solar energy can create undesirable heat loads in cities. Precast concrete can help reduce urban heat islands with its “cooler” white surface.
By Claude Goguen, P.E., LEED AP
The term “heat island” may evoke pleasant images of summer campfires and good times. However, the “heat island effect” is anything but pleasant, causing problems in urban areas. The good news is that efforts are underway to reduce this phenomenon of spiking temperatures. Sustainable precast concrete elements with cooler, lighter-colored surfaces can reduce these undesirable heat island effects.
A heat island is an area of higher temperature, or localized temperature spike, in a region of cooler temperatures. Heat islands occur where there are dark exterior surfaces (such as black roofs and dark asphalt pavements) and a lack of vegetation. The higher temperature, or heat island effect, is the result of dark-colored constructed surfaces absorbing solar energy and retaining heat, which in turn raises urban temperatures and increases building cooling loads and energy costs. Heat islands also have an adverse effect on microclimate and local human and wildlife habitats.
Precast concrete buildings, cladding, pavers and other elements can contribute to reducing heat islands. A recent laboratory study shows that concretes typically used in the United States have a solar reflectance index (SRI) that meets the criteria of the LEED Sustainable Sites Credit for reducing heat islands. But what is solar reflectance?
Solar reflectance (SR), sometimes called albedo,1 is a measure of a material’s ability to reflect sunlight (including the visible, infrared and ultraviolet wavelengths) on a scale of 0 to 1. An albedo value of 0.0 indicates that the surface absorbs all solar radiation, and a 1.0 SR value represents total reflectivity.
Dark-colored materials generally have a low solar reflectance, meaning that dark surfaces absorb heat from the sun and warm the air through convection. Lighter-colored materials (such as precast concrete) generally have a higher SR, as they reflect heat from the sun and do not warm the air as much. Shade, from trees and buildings, and the natural process of water evaporation from the surface of plants also help keep the air cool.
A recent study conducted by scientists at Lawrence Berkley National Lab found that after parking in the sun for an hour, a silver Honda Civic (shell SR = 0.57) had a cabin air temperature about 9 to 11 F lower than an otherwise identical black car (shell SR = 0.05). However, color is not always a reliable indicator of solar reflectance, because color represents only 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.
SRI is a composite measure of a surface’s solar reflectance and emittance and is measured on a scale of 0 to 100. Emittance is a measure of how well a surface emits (or releases) heat after it has been absorbed.
In LEED-NC (New Construction and Major Renovation), two points are available for reducing heat islands. One point can be earned by using paving materials with an SRI 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 an SRI of at least 29 for steep-sloped roofs and SRI ≥ 78 (for at least 75% of the roof surface) for low-sloped roofs. This is “Sustainable Sites Credit 7.2: Heat Island Effect: Roof.”
Since concrete has an emittance of about 0.90, it needs to have an SR of at least 0.28 to meet the criterion of an SRI of 29 and at least an SR of 0.64 to meet the criterion of an SRI of 78.
How to use precast concrete for LEED credits
How can precast concrete structures help in achieving these design credits? Constructing underground or above-ground parking structures, which is a practice that lends itself to the extensive use of precast concrete materials, can substantially reduce not only the footprint of a project, but also the heat island effect of the completed project. Using high SRI precast concrete for cladding or roofing, or pavers and slabs for hardscapes, can also help. How can you increase the SRI of your precast concrete structures? Using white cement or certain cementitious materials can help.
A recent study measured the SR 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.” 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, most concrete mixes were made with the darkest (lowest SR) material combinations, that is, those most likely to fail the LEED criteria. Replacement levels of 25% for fly ash and 45% for slag cement were chosen, because they are commonly used substitution levels for cement.
Tests prove concrete’s high SR and SRI values
The study shows that all 45 concretes tested have an SR 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 containing white or slag cement also meet the criteria for low-sloped roofs.
The solar reflectance of portland cement has more effect on the SR of concrete than any other constituent material. The SR of the supplementary cementitious material (in this study, fly ash and slag cement) has the second greatest effect. Generally, the higher the SR of the cementitious material, the higher the SR of the concrete.
Slag cement concretes generally have the highest SR values. The average effect of replacing 45% of the cement in a mix with slag cement is to increase (lighten) the SR 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 SR 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 SR by 0.03.
Interestingly, the SR of fine aggregate has a very small effect on the SR of concrete, while the SR of coarse aggregate has no effect on the SR of concrete.
Take the precast advantage
Regardless of mix constituents, precast concrete 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 an SR of at least 0.3 and an SRI of at least 29.
In proclaiming the sustainableand environmental attributes of your precast concrete structures, heat island reduction may not be at the top of your list. However, depending on the type of structure and the location of the project, this attribute combined with precast concrete’s energy savings may give you an additional edge over competing materials that could help secure the job.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.
1Albedo is the fraction of solar energy reflected from surfaces on Earth back into space.