The tests were carried out in the municipality of Hengelo, where the busy Castorweg road was resurfaced last fall. As part of the project, around 1,000 square meters of the road's surface were covered with air-purifying concrete paving stones. For comparison purposes, another area of 1.000 square meters was surfaced with normal paving stones.

Researchers at Eindhoven University of Technology (TU/e) carried out three air-purity measurements on the Castorweg last spring, at heights of between a half and one-and-a-half meters. Over the area paved with air-purifying concrete the NOx content was found to 25 to 45 per cent lower than that over the area paved with normal concrete.

"The air-purifying properties of the new paving stones had already been shown in the laboratory, but these results now show that they also work outdoors", said prof. Brouwers. Further measurements are planned later this year.

Asphalt
Brouwers, who has been professor of building materials in the TU/e Department of Architecture, Building and Planning since September 2009, sees numerous potential applications, especially at locations where the maximum permitted NOx concentrations are now exceeded. The concrete stones used in the tests are made by paving stone manufacturer Struyk Verwo Infra, and are already available for use.

For roads where an asphalt surface is preferred the air-purifying concrete can be mixed with open asphalt, according to Brouwers. It can also be used in self-cleaning and air-purifying building walls.

The use of air-purifying concrete does not have a major impact on the cost of a road, Brouwers has calculated. Although the stones themselves are 50 per cent more expensive than normal concrete stones, the total road-building costs are only ten per cent higher.

Self-cleaning
Vehicle exhaust gases contain nitrogen oxides (NOx), which cause acid rain and smog. The air purifying concrete contains titanium dioxide, a photocatalytic material that removes the nitrogen oxides from the air and converts them with the aid of sunlight into harmless nitrate. The nitrate is then rinsed away by rain. These stones also have another advantage: they break down algae and dirt, so that they always stay clean.

Jos Brouwers, professor of building materials at Eindhoven University of Technology, delivered his inaugural lecture on the afternoon of Friday 2 July.

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