Solar Reflectance of Concrete
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Concrete Shines as Solar Reflectance Material
Concrete does a very good job of reflecting solar energy. That
is the finding from a recent PCA study which measured the solar
reflectance of 135 concrete specimens from 45 mixes representing
exterior concrete flatwork. In fact, all concretes tested in this
study would qualify for LEED® credits for heat island
effect reduction.
Solar reflectance index (SRI), a calculated value based on solar
reflectance, SR, 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 (heat
island).
In this study, the solar reflectance values of all 45 concretes
tested are 0.33 and higher, corresponding to SRIs of 36 and higher.
Solar reflectance is measured on a scale of 0 to 1: from not reflective
to 100% reflective. 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 is 0.47.
Of all the constituent materials, the solar reflectance of the
portland cement itself has the greatest effect on concrete reflectance:
the higher the cement reflectance, the higher the concrete reflectance.
Two concretes had much higher average SRs of at least 0.64 (SRI
= 78); one contained ordinary portland cement (gray) and a light-colored
slag cement with crushed limestone fine aggregate; the other contained
white portland cement and fine aggregate from crushed limestone.
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| 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. |
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. |
Supplementary cementing materials (SCMs) can also be used as ingredients
in concrete; after cement, SCMs have the next biggest effect on
slab reflectance. Fly ash can have a greater or lesser SR than gray
cement; slag has a greater SR than gray cement; and white cement
in this study has the highest SR of all materials. The lowest SRs
in this study are for concretes containing dark gray fly ash.
Fine aggregate was found to have a small, but significant, effect
on slab reflectance, but coarse aggregate had no significant effect.
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.
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 for Steep
Sloped Roofs.
For a detailed report describing test procedures, concrete mixes,
materials, and other aspects of this study, see PCA’s SN2982,
Solar Reflectance
of Concretes for LEED Sustainable Sites Credit: Heat Island Effect.
Authored by Medgar L. Marceau and Martha G. VanGeem.
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