There are several ways to reduce the carbon emissions associated with asphalt, among them the use of recycled asphalt planings (RAP), producing the asphalt mix at a lower temperature and substituting some of the bitumen with plant-derived material. National Highways is trialling all of these on two of its roads: the A2 near Canterbury in Kent and the A34 near Newbury.
Additionally, all the mixes being trialled contained polymer modified bitumen (PMB). Although this isn’t a new technology, it reduces the whole-life carbon costs of a road by increasing their longevity. This makes sense: it could be counter-productive to use an asphalt that has a lower carbon footprint up to the point of laying if it fails sooner than a standard mix once it’s in use.
On the A2, four sections of 45mm surface course were laid on stretches of road between 700m and 1,000m long: one stretch using conventional warm mix asphalt (WMA) with a standard PMB; one with a biobinder and PMB produced as hot mix asphalt; one with biobinder and PMB asphalt produced as WMA; and one that additionally contained 20% RAP.
The A34 trial added a fifth trial length containing 30% RAP. Additionally, Heidelberg reports that the RAP used in the mixes there was planed from the old road surface, making this a circular process – reusing for the same purpose in the same location.
The trials are part of National Highways’ research project Future Asphalt Surface Course Linking to NH Net Zero under its Specialist Professional and Technical Services (SPaTS2) framework. Heidelberg Materials (formerly Hanson) supplied its CarbonLock asphalt while the University of Nottingham and AtkinsRéalis Jacobs Joint Venture (JV) will monitor and assess the performance of the surfaces.
These latest trials follow on from earlier ones on the resurfacing the A30 Cutteridge to Alphington eastbound scheme near Exeter which took place in March 2023. These compared two types of biobinders in Heidelberg’s CarbonLock asphalt: Nynas’ Nypol RE and Shell’s Cariphalte CarbonSink. Heidelberg hasn’t said which one was used in these latest trials.
Biobinders are plant-based materials and have a carbon negative footprint, since the plants have sequestered carbon dioxide as they grow; hence names like CarbonSink and CarbonLock. A commonly used biobinder is tall oil pitch which is made from tall oil, a byproduct of wood pulp manufacture. Manufacturers of biobinders don’t tend to say what plants they are using, since this may differ depending on the geographical location in which they are made.
Warm mix asphalt is generally produced at temperatures between 100 and 150 degrees C, whereas traditional hot mix asphalt is produced at between 120 and 190 degrees, requiring less energy and hence lower carbon emissions. They are generally produced by adding additives to the mix.
Asphalt mixes containing PMB are used for highly trafficked roads, or to cope with extremes of climate. Usually, the choice of bitumen aims to get the right balance between a road that is stiff enough and flexible enough; polymers can allow you to have the best of both worlds. Adding polymers to the bitumen modifies it as the name suggests, to enhance the performance of the road surface so that it deteriorates more slowly than a standard mix.
The use of higher proportions of RAP also requires special additives. Rejuvenators are included to restore some of the properties of aged bitumen to make it more flexible again.
According to Heidelberg, carbon emissions associated with the asphalt for the two schemes – 1,500 tonnes on the A2 and 500 tonnes on the A34 – were 26% lower than had standard mixes been used. Of course, including all the additives needed to achieve this level of carbon savings means that the capital cost of the resurfacing works will be higher too.
As well as looking at how the roads perform in service, work to assess the feasibility of these various cocktails of low carbon technology will look at whole-life implications for both cost and carbon as National Highways looks to map out its path to net zero.
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Sources:
https://www.heidelbergmaterials.co.uk/en/news-and-events/national-highways-extends-carbonlock-trials