A thin layer of lighter coloured asphalt applied to road surfaces could significantly lower surface temperatures in hot urban areas, according to a recent study. This could make living conditions more comfortable and reduce the amount of energy needed to cool buildings.
Temperatures in cities can typically be warmer by several degrees than nearby rural areas due to the Urban Heat Island effect. Buildings, roads, pavements and other structures can trap heat from the sun and raise surface temperatures. This heat is transferred to the surrounding air and increases urban temperatures and air pollution problems, such as the formation of ozone and smog. It also causes discomfort, increasing the demand for air conditioning, and even heat-related illness and death, especially in vulnerable people.
Paved surfaces (e.g. roads, pavements and car parks) cover a large proportion of city surfaces and the types of materials used in paving can substantially influence local surface temperatures. These surfaces are usually paved with a mixture of asphalt (bitumen) and aggregate material. However, cool paving materials that absorb less heat and have a lower surface temperature are being developed to provide alternatives to traditional paving materials.
In this study, five different coloured samples of a specially-made thin layer of asphalt were tested for their ability to reflect the sun's radiation. More reflective materials absorb less heat, thus reducing the material's surface temperature.
The samples were made from a colourless, flexible asphalt binder and various pigments and sizes of coloured aggregates (granular material). The resulting five samples were coloured beige, off-white, green, red and yellow, which were first tested in the laboratory for their ability to reflect solar energy and were compared to a thin-layer sample of conventional black asphalt.
In the laboratory tests, all of the samples reflected back higher levels of solar radiation than conventional black asphalt. The amount of solar radiation reflected ranged from 55 per cent (off-white sample) to 27 per cent (red and green samples). Black asphalt reflects 4 per cent of solar radiation.
One possible drawback of lighter coloured surfaces is that they may cause glare, potentially a problem for drivers, for example. However, some 'invisible' reflectance is possible if the reflectance is in the 'near-infra-red' part of the spectrum. For the samples tested in this study, the amount of the sun's energy reflected in the near-infra-red part of the spectrum was far higher than the reflection of energy from the visible part of the spectrum and ranged from 63 per cent (off-white sample) to 39 per cent (green sample) compared with 4 per cent (conventional black sample).
All the samples were also tested outdoors with their surface temperature monitored over 24 hours during the summer of July 2008. The surface temperatures of the coloured samples were lower than that of the conventional black sample with the average daytime temperatures of the coloured thin-layers of asphalt ranging from 39°C (off-white sample) to 46.7°C for the black sample. The maximum recorded daily temperatures peaked at 48°C for the off-white sample, 55.8°C for the red sample and 60°C for the black sample (making the black sample 12°C higher than the off-white sample).
The thin-layers of coloured asphalt could be applied onto existing or new asphalt road surfaces. Computer modelling suggests an off-white coloured surface on a road could lower the average air temperature by 5°C in low wind speeds.
Source: Synnefa, A., Karlessi, T., Gaitani, N. (2010) Experimental testing of cool colored thin layer asphalt and estimation of its potential to improve the urban microclimate. Building and Environment. 46: 38-44.