Solar Reflectance of Concretes for LEED Sustainable Sites Credit: Heat Island Effect

Comparing the solar reflectance index (SRI), a calculated value based on measured solar reflectance, of different materials tells which ones absorb less solar radiation. This is useful because materials that absorb more heat, are generally considered undesirable for their effect on the environment. Urban areas are up to 7°F warmer than surrounding areas due to heat absorbing materials used to construct buildings and roadways. These heat islands increase the air conditioning loads in buildings and contribute to ground level ozone (smog) and related health conditions such as asthma.

CTLGroup produced a report investigating which combinations of concrete constituents will meet the SRI requirements in the Leadership in Energy and Environmental Design for New Construction (LEED-NC) Sustainable Sites Credits 7.1 and 7.2 for reducing the heat island effect.

The study measured the solar reflectance of 135 concrete specimens from 45 mixes representing exterior concrete flatwork. Measurements were made in accordance with ASTM C1549. Results revealed that concrete does a very good job of reflecting solar energy. In fact, all concretes tested in the study would qualify for LEED credits for heat island effect reduction.

Solar reflectance is measured on a scale of 0 to 1: from not reflective to 100% reflective. SRI, 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.

All concretes in this study had average solar reflectances of at least 0.30 (corresponding to an SRI of at least 29), and therefore met the requirements of LEED-NC SS 7.1. These concretes also met the requirements for steep-sloped roofs in LEED-NC SS 7.2. The lowest solar reflectances were from concretes composed of dark gray fly ash.

Two of the concretes had average solar reflectances of at least 0.64 (corresponding to an SRI of at least 78), meeting the requirements of low-sloped roofs in LEED-NC SS 7.2: Heat Island Effect – Roof.

The solar reflectance of the cement had more effect on the solar reflectance of the concrete than any other constituent material. The higher the cement reflectance, the higher the concrete reflectance. The solar reflectance of the supplementary cementitious material had the second greatest effect. 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 in this study has the highest solar reflectance of all materials. The lowest solar reflectances in this study are for concretes containing dark gray fly ash.

LEED credits for reducing heat islands with concrete are possible in the two following areas: LEED-NC SS 7.1: Heat Island Effect - Non-Roof, and LEED NC SS 7.2, Heat Island Effect - Roof.

Contact Jon Feld, PE, to learn more about CTLGroup's SRI capabilities at (847) 972-3318 or at jfeld@ctlgroup.com.