Albedo is the percentage of incoming radiation reflected off a surface. An albedo of 1 means that 100% of incoming radiation is reflected (no radiation is absorbed); an albedo of 0 means that 0% of incoming radiation is reflected (all radiation is absorbed).
The more radiation reflected the less global warming that occurs. You have probably heard how the ice caps reflect solar radiation and hence why their melting is such a big issue. Fresh snow has an albedo of approximately 0.9.
Generally, lighter coloured surfaces have a greater albedo effect. Hence changing from dark asphalt to light coloured concrete can greatly increase the reflection of incoming radiation and result in a reduction in global warming. Using GGBS in your concrete makes it lighter again and hence increases its albedo number. Research shows that an increase in the albedo value of 0.01 of a surface is the equivalent of 2.55 kg CO2 saved per m2 (Akbari, Menon & Rosenfeld (2009) Global cooling: increasing world-wide urban albedos to offset CO2. Climate Change Vol. 95, Issue 3–4).
Hence if you insist on concrete roads with GGBS instead of asphalt a significant contribution can be made to reducing global warming. For every 25 m2 of GGBS concrete paving you can save over 1 tonne of CO2 per annum.
For more information, please see “Global cooling: increasing world-wide urban albedos to offset CO2”, by H. Akbari, S. Menon, and A. Rosenfeld.
|Albedos: Reflectance of Pavement Surfaces|
|Gray Portland Cement Concrete||0.35–0.40 (new)|
|White Portland Cement Concrete*||0.70–0.80 (new)|
*Concrete made with GGBS has a similar albedo to White Portland Cement Concrete (WPC) but unlike WPC it does not cost extra. For more information, please see “Albedo: A Measure of Pavement Surface Reflectance”, American Concrete Pavement Association.
This image illustrates that lighter coloured surfaces reflect most of the incoming solar radiation as light which can pass through the greenhouse gases that surround the earth. With increased solar radiation being reflected outside the earth’s atmosphere there is a reduced contribution to global warming. On the other hand the darker coloured surfaces absorb a lot of the solar radiation and radiate it back out as heating leading to an increase in global warming.
The term “heat island” describes built up areas that are hotter than nearby rural areas. The annual mean air temperature of a city with 1 million people or more can be 1–3°C warmer than its surroundings. In the evening, the difference can be as high as 12°C. Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water quality.
Increasing the albedo of surfaces in cities will reduce the heat island effect, resulting in significant, cost, comfort and health benefits.
The images to the right support the proposition made above that lighter coloured surfaces reflect more solar radiation and hence remain cooler and darker coloured surfaces absorb more solar radiation and hence become hotter. The infrared image demonstrates that the light coloured concrete remains cooler than the darker coloured asphalt road. It can easily be appreciated that a city with darker roads and roofs would become much hotter leading to local heat island effects and wider global warming effects. The local heat island effect can have significant cost implications. K. Boriboonsomsin et al tell us that “One of earlier studies demonstrated the simulation results that increasing the albedo of 1,250 sq km (483 sq mi) of roadways in Los Angeles by 0.25 would save cooling energy worth $15 million per year, and would reduce smog-related medical and lost-work expenses by $76 million per year.”
The infrared image demonstrates that the light coloured concrete remains cooler than the darker coloured asphalt road.
The following extract summarises the finding in the paper which you will find in the library section of the website.
“The use of cool paving materials or ‘cool pavements’ has been identified as one strategy that can help mitigate urban heat island effect. One method of creating a cool pavement is to increase the solar reflectance or albedo of its surface. This can be achieved by many existing paving technologies. This study explores alternative ways of creating high albedo concrete for use in pavement applications. The key approach is to make concrete whiter by replacing cement with whiter constituents. Fly ash and slag are used as the main constituents since they are environment-friendly, readily available, and already accustomed with by the concrete industry. Compared to a conventional concrete mix, concrete mixes containing fly ash have lower albedo whereas concrete mixes containing slag have higher albedo.
“Among all the mixes tested, the mix with 70% slag as cement replacement achieves the highest albedo of 0.582, which is 71% higher than the conventional mix. It also has better compressive strength as tested at 7 and 28 day and modulus of rupture as tested at 7 days. In addition to consuming 43.5% less energy, the production of the high solar reflectance concrete produces less pollutant emissions and greenhouse gases on the order of 20–60%. Furthermore, the analysis of some urban cities shows that the implementation of this high solar reflectance concrete can possibly increase the city albedo by 0.02–0.07. This amount of albedo modification has potential to provide great benefits to economics and the environment in many ways ranging from decreasing energy demand to improving air quality.”
Abstract from paper entitled “Mix Design and Benefit Evaluation of High Solar Reflectance Concrete for Pavements” by Kanok Boriboonsomsin and Farhad Reza (January 2007). Please see the library for this and many more technical papers.
The difference in luminescence between asphalt and lighter coloured concrete.
These images illustrate that there is a reduced requirement for artificial lighting when surfaces are lighter coloured. These two images are of similar shopping centres with the same degree of artificial lighting. The difference in luminescence is because at one centre the car park is asphalt and in the other it is lighter coloured concrete. Because of the increased visibility as a result of the lighter coloured concrete there is a considerable safety benefit. Using lighter coloured surfaces can result in up to a 30% reduction in energy usage because reduced artificial lighting is required to achieve a similar brightness.