Paving Materials for Heat Island Mitigation
M. Pomerantz, H. Taha, A. Chen,* A. RosenfeldÝ
One of the causes of higher temperatures in cities (compared to their environs) is the absorption of sunlight by dark pavements. In warm climates, this absorption can contribute damage to the environment by adding to air-conditioning electrical demand and smog. In this work, the dollar value of potential savings from lighter pavements is estimated.
We examined the effect of paving urban roads, driveways, and walkways with materials that are lighter colored than black asphalt. The lighter color means that the sunlight is reflected rather than absorbed, and so the pavement is cooler. If a pavement is cooler, its lifetime is longer and the environmental damage of disposing of it is lessened. Reflectivity is also a safety factor in visibility at night, and this factor reduces the demand for electric lighting for streets. The drawback of high reflectivity is glare, which may be a limiting consideration. We considered new pavements, which are made of cement concrete; more flexible surfaces, like asphalt concrete; and porous pavements, such as grass pavers. We also reviewed materials for resurfacing existing pavements. We assembled information on the compositions of the materials, their suitabilities for particular applications, and their approximate costs.
We have shown by computer simulation that, if all possible pavements were whitened to an albedo of 0.35, the cooling of the city would save electricity and reduce smog damage. The present dollar value of these savings in Los Angeles is estimated to be $1.80/m2 ($0.17/ft2) of pavement. The cost of pavement depends on the amount of material used. If the surface layer is thin enough, its cost may be sufficiently low that the savings from a lighter color would pay for any extra cost. Thus, it may happen that a lighter pavement is overall less costly than a dark one. We illustrate this in the Figure. If the savings is $1.80/m2, for a 6-mm (1/4-in.) thick resurfacing, money would be saved, as long as the extra costs of the aggregate and binder are within the triangle to the left of the filled squares. For example, an increase of binder price by $1.64 per liter ($6.25 per gallon) and no increase in aggregate price will not increase the pavement cost more than $1.80/m2. Or, aggregate priced at $138 per Mg ($126 per ton) more than present cost, and no increase in binder price, could be used without exceeding the benefits of the cooler surface. Such aggregate can be quite white and yield a more reflective surface, with no overall extra cost to society. If the pavement is 25 mm (1 in.) thick, the range of affordable price increases is confined to the area below the line defined by the open squares. The four times thicker surface implies a four times smaller range of affordable price increases.
Figure. The savings resulting from a cooler pavement are greater than the extra costs, if the extra costs of the aggregate and binder are within the triangle to the left of the solid line for a 6-mm thick resurfacing. The savings exceed the extra cost as long as the materials prices are within the triangle to the left of the line for the 25-mm thick resurfacing.
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