Concrete does a very good job of reflecting solar energy. That is the finding from a Portland Cement Association (PCA) study which measured the solar reflectance index (SRI, a calculated value based on solar reflectance (SR). SRI is one way to determine how much light energy a material reflects: stated another way, comparing SRI or SR of different materials tells which ones absorb less solar radiation. This is useful because darker materials absorb more heat, which is generally considered undesirable for its effect on the environment. This may have an immediate, local effect, like heat gain in urban areas, commonly known as heat islands. 

Heat islands frequently occur in cities where up to 60 percent of the horizontal surface area is covered by buildings and paving and reduces areas of vegetation. Dark surfaces such as black roofs and dark pavements 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.

The PCA SRI study measured the solar reflectance of 45 concrete mixes 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 United States. 

Supplementary cementing materials (SCMs) can used as ingredients in concrete; after cement, SCMs have the next biggest effect on slab reflectance. Fly ash can have a greater or lesser solar reflectance than gray cement; slag has a greater solar reflectance than gray cement; and white cement has the highest solar reflectance of all materials. The lowest SRs in this study were for concretes containing dark gray fly ash.

The Test

While all materials were tested, additional concrete mixes made with the darkest  materials - sand, cement, aggregate and fly-ash combinations, that is, those with the highest solar reflectance and 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 percent for fly ash and 45 percent for slag cement were chosen because they are commonly used substitution levels for cement.

Solar reflectance index, on a scale of 0 to 100 is the relative temperature of a surface with respect to standard white (SRI = 100) and standard black (SRI = 0) under the standard solar and ambient conditions. The average solar reflectance of all mixes studied was 0.47. Therefore, they met the criteria for low-sloped roofs. 

All slab specimens received a light broom finish, which was smoother or rougher depending on other concrete ingredients. (Finish was not measured, but was determined by visual assessment.) Smooth finishes reflect better than rough finishes. Fine aggregate was found to have a small, but significant, effect on slab reflectance, but coarse aggregate had no significant effect. 

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) had the second greatest effect. Generally, the higher the solar reflectance of the cementitious material, the higher the solar reflectance of the concrete. Slag cements generally have the highest solar reflectances and white cements the lowest.

CXB-AE-CP-FDG-01: The code letters indicate concrete specimen made with a gray cement (CXB), fine Eau Claire aggregate (AE), coarse Eau Claire aggregate (CP), dark gray fly ash (FDG), and the 01 indicates that this was specimen number 1 from that mix. Three specimens were made from each concrete mix.


CW-AL-CP-01: The code letters indicate concrete specimen made with white cement (CW), fine limestone aggregate (AL), coarse Eau Claire aggregate (CP), and the 01 indicates that this was specimen number 1 from that mix. Three specimens were made from each concrete mix.
sri_CXBAECPFDG01
sri_CWALCP01

LEED Effect of Concrete

Regardless of mix constituents, concrete in the U.S. 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 percent 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 percent 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.

Reference

For a detailed report describing test procedures, concrete mixes, materials, and other aspects of the study, see Solar Reflectance of Concretes for LEED Sustainable Sites Credit: Heat Island Effect, SN2982, authored by Medgar L. Marceau and Martha G. VanGeem, Portland Cement Association, Skokie, Illinois, USA, 2007, 95 pages.