Laying roads with a lighter carbon footprint
Dec 13, 2010
Road builders around the world face increasingly tighter environmental regulations, including related to CO2 emissions.
National and local governments in some countries are also mandating the recycling of asphalt pavement. These two trends tend to be in conflict with each other. Recycling pavement can require high-temperature asphalt mixes, which are energy intensive. But one European paver has recently adapted a Shell asphalt-mixing process to reconcile these conflicting trends in one road-paving product.
In 2000, the world had more than 15 million kilometres of paved roads, the majority of which were covered in asphalt. Since then, many more kilometres of asphalt have been laid – and many more are yet to come. Fortunately, some of the old asphalt pavement can be recycled as new pavement. However, when this is re-used in conventional hot asphalt mixtures it generally requires the new asphalt mixture to be heated to a higher temperature than normal, which is usually accompanied by greater emissions of CO2 and other air pollutants.
Recently, however, Gerbert van Bochove of the Dutch construction company Heijmans and Karel Poncelet from Shell have patented a way to prepare asphalt at a temperature that is some 50°-60°C lower than conventional asphalt mixing temperatures without compromising quality.
The lower temperature reduces energy use during the asphalt manufacturing process by between 25% and 35%, and also results in lower emissions of CO2 and of other air emissions known to add to air quality problems (e.g., particulates, CO). But what is most remarkable about the new product is that it consists mainly of old asphalt. Marketed by Heijmans under the trade name Greenway LE, it contains as much as 60% recycled material.
Asphalt – new, old and recycled
Asphalt is a mixture of sand, gravel and bitumen – the heaviest fraction of refined oil. The bitumen binds the mixture into a weather-proof and wear-resistant material. But bitumen is solid under ambient conditions. In order to be combined with the granular material, it needs to be heated until it liquefies. For the harder grades of bitumen typically used in roads, this change occurs at about 160°C.
At such temperatures old asphalt paving tends to break down into its constituent parts, which can then be reused in hot asphalt mixture. So it is common practice for asphalt companies to incorporate some old asphalt into new batches during the mixing process. Companies in the Netherlands take this practice further than those in other countries. That’s because the Netherlands has no gravel quarries from which to obtain virgin material. So it’s not surprising that between 40% and 50% of Dutch high-temperature asphalt mixtures typically consists of recycled paving.
The Shell WAM Foam process
In 2008, however, Gerbert realised that there was a way to avoid the high temperatures in asphalt recycling. His idea was based on the characteristics of the Shell WAM Foam asphalt-preparation process. (WAM stands for “warm asphalt mixture”.)
The Shell WAM Foam process makes use of two types of bitumen: a soft one that “melts” at low temperatures, and a conventional hard one. The soft bitumen coats the granular material at a low temperature before the hard bitumen is added. That’s when the foam comes into the process. The hard bitumen is frothed up with water – like milk at an espresso bar – just before it is added into the low-temperature mixture.
The soft bitumen was the key to Gerbert’s proposal. “Soft bitumen acts as a natural rejuvenator”, he explains. “It makes the aged bitumen pliable, enabling the recycled asphalt to break apart without the need for high temperatures.”
Gerbert shared his idea with Karel, Shell’s sales development manager in Europe for Shell Specialities technologies, which include bitumen. To adapt the Shell WAM Foam process for high levels of reclaimed asphalt, they had to experiment like a baker perfecting a new cake recipe – by adjusting the amounts of ingredients as well as the timing and temperatures. “We had to look to see what was available in Shell’s portfolio of bitumen grades”, says Karel. “And Heijmans had to do some testing in a laboratory to see if the different combinations were working.”
Their collaboration ultimately was successful, resulting in the Greenway LE asphalt.