By Robert L. Reid

Adding plant-based biogenic components to the binders used to create bitumen for asphalt can help decarbonize road construction by capturing an increasing amount of carbon in road surfaces or other asphalt-based products as they are recycled and reused over time.

Shell plc has developed one such binder method, known as the Shell Bitumen CarbonSink, which can include varying amounts of biogenic components based on local requirements and supplies. Research by the company’s Shell Construction and Road global business unit indicates that this method of creating a binder can lock as much as 6 mt of CO2 into each kilometer of road surface. This assumes a model single surface layer of asphalt with a depth of 50 mm, a width of 3.5 m, and a 5% binder content in the asphalt mix.

Typically, when an asphalt road’s surface ages, it is milled, recycled, and reused. This occurs periodically to create a new road surface at the same site. But “you don’t lose the carbon you put in originally,” explains Jean-Nicolas Desprez, Shell Construction and Road’s regional technology manager for Europe and South Africa, who also leads a Shell laboratory in Strasbourg, France, that conducts binder analysis and helps conduct trials on new products. And, in fact, “you are cumulating it every time you add fresh bitumen to your asphalt mix when recycling the road surface,” Desprez says.

Thus, the amount of CO2 locked into the road will keep increasing to an estimated 8 mt per kilometer by year 15, 17 mt by year 50, and 19 mt by year 100, according to the company’s projections. Nearly all roadway asphalt is recycled into new asphalt, explains John Read, Ph.D., Shell’s general manager of technology.

Multiple formulations, applications

Road asphalt is generally a mix of aggregates, sand, filler, and binder — and the quantities of each will vary, depending on what type of asphalt you want to make, says Desprez.

For example, there are different formulations for the bottom layer, intermediate layer, and top layer of a roadway, Desprez explains, and the percentage of binder in each also varies but generally falls within a range of 4%-6%, he notes. When Shell tested its CarbonSink binder method, the firm worked with multiple asphalt formulations and percentages of binder content to consider the widest possible usage.

“Shell Bitumen CarbonSink is not limited to any type of application,” Desprez explains, but can meet the performance requirements of highways, country roads, airfields, racetracks, and even other industrial applications such as roofing materials. “Anywhere bitumen is used, you can apply the concept of CarbonSink,” Desprez stresses.

Seeking sources

Variability is also key to the plant-based content in the binder method, notes Read. In developing it, Shell “looked at a vast gamut of different biogenic components and assessed their feasibility for use in asphalt,” Read explains. “Not everything worked. There are some biogenic components that don’t give you the properties you’re looking for — but many do.”

Rather than selecting just one source of plant-based biogenic material to use worldwide, however, Shell has tested multiple potential sources in different regions and determined multiple alternative sources from a variety of agricultural and industrial products that are not part of the food chain, Read says. Being able to use multiple sources is critical to the goal of decarbonization, he adds, because if a single source of plant material was used, it would have to be transported all around the world. Doing so would potentially eliminate the benefit from decarbonizing the asphalt itself because of the increased carbon generated through the extended supply chain, Read explains.

Likewise, because each individual source of plant-based material has its own growing season, that specific material might not always be available at the times and in the quantities needed. And customers might insist on specific biogenic components in their specifications — which, again, might not always be available, Read says. “So we tend not to talk about the specifics because we’ve looked for many different sources in every location where we operate,” Read says, and have found multiple potential sources of biogenic material that “works best for each location.”

Applications in action

Although the binder is commercially available in Europe, it is still undergoing trials in other parts of the world, including India and China, Read notes. In the United Kingdom and France, binder created using the method has been used in several asphalt projects. In June, for example, heavy building materials supplier Hanson UK used one such binder as part of a new warm-mix asphalt product known as Duralayer Bio to resurface a road in Shaftesbury, England. The section of road measured roughly 560 m long by 6.5 m wide, says Desprez.

Because Duralayer Bio is manufactured at a lower temperature than traditional hot-mix asphalt, it also requires less energy to produce, which results in fewer carbon emissions, Hanson noted in a June 1 press release.

Likewise, 20 mt of a binder created using the method was used to construct a new surface parking lot at the Brumath Enrobés asphalt production plant in Bas-Rhin, France, according to an Aug. 1 press release from Shell.

Although the United States is not currently a focus market for Shell Construction and Roads, Read is optimistic that the CarbonSink binder method could help the firm reenter the American market.

Shell is also developing another product — known as Shell Cariphalte AgeSafe — that is designed to inhibit the aging of bitumen to extend the useful life of asphalt surfaces. Although this product is not expected to be on the market until next year, it has undergone testing at several locations by Hanson in Cumbria County in northwest England.

Combining the aging inhibitor with the CarbonSink binder would ideally increase the decarbonization of roads even further, Read stresses.