Millions of kilometres of asphalt-paved roads provide transport routes across the globe. Construction firms lay more every day. They heat bitumen – the residue from crude-oil refining – and mix it with hot sand and gravel to make new asphalt. Often, they throw old asphalt into the hot mix.

The very high temperature needed to make these mixtures consumes a lot of energy, resulting in high CO2 emissions.

Dutch construction firm Heijmans has been using a Shell process to make asphalt at lower temperatures than conventional asphalt – using less energy and reducing CO2 emissions. And now Heijmans and Shell have patented a way to incorporate a high percentage of old asphalt into it.

“Our new product contains up to 60% old asphalt, more than the traditional approach,” says Gerbert van Bochove, Innovation Manager at Heijmans. “And we prepare it about 50°C cooler than conventional asphalt.”

Keeping cool

The key to the breakthrough lay in the composition of a soft type of bitumen that the Shell WAM (warm asphalt mix) process uses to coat the sand and gravel. “The soft bitumen chemically breaks down the recycled asphalt without the need for high temperatures,” says Gerbert. “So it allows us to recycle asphalt at lower temperatures.”

To give the resulting road surface the necessary performance qualities, a harder bitumen is also added to the asphalt mixture during its preparation. The harder bitumen mixes in easily at the lower temperature, because it is frothed up with water before it is added.

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