In the last two weeks, both Tarmac and Aggregate Industries announced that they are switching to warm mix production as standard across all their asphalt plants. Warm mix is good news because it allows the temperature of the plant to be lowered by 20 to 40 degrees C, cutting both carbon emissions and energy bills.

The suppliers are following the lead of National Highways which, in August last year, said that it would be asking contractors to use warm mix across the strategic road network, as part of its commitment to achieving net zero road maintenance and construction by 2040. According to National Highways, the benefits of using warm mix are bountiful: cutting carbon by up to 15%, faster construction times, lower risk of early damage to freshly laid pavements and fewer nasty fumes for workers and road users to inhale.

Given all these benefits, some may wonder why warm mix hasn’t been adopted sooner and more widely around the UK. There are two answers to that question: lack of specifications and cost.

Warm mix was invented in the early 1990s and, after some success in Europe, it really took off in the US where it now accounts for around 40% of production. US Departments of Transport mandated it, with a big benefit being that it led to better compaction, which in turn helps with the longevity of the road.

Warm mix can be made in two different ways, by foaming the asphalt or by using chemical additives. Production plant foaming involves injecting small amounts of water into the liquid asphalt to create a foam and requires a significant but one-off investment to adapt the asphalt plant. With additives, no adaptions to the plant are required – but there is always the additional cost of the additive.

In the US, around 60% of warm mix is made through the foaming method. In the UK, we use mostly additives. So, although UK suppliers are producing warm mix as standard, they could switch back to ‘bespoke’ hot mix without too much fuss or cost.

Although National Highways has allowed the use of warm mix since 2015, contractors and designers have had to apply for a special dispensation, or Departure from the specification, to do so which added bureaucracy and cost to the process. Never-the-less, National Highways says that it received 250 applications to use warm mix at 300 locations including the M1, M4, M5, M6, A36 and A303. And in July 2019, National Highways added a section on warm mix to its Specification for Highway Works.

Another force for change in the UK was a report from the UK’s All Party Parliamentary Group on Highways, published in September 2019, which called on councils to switch to warm mix to help them meet their climate change goals. The forward to the report said: “This publication is designed to encourage those authorities which have responsibility for highways to put their theoretical support for environmental measures into practice.”

In those countries where use of warm mix is higher, it has been clients who have driven its adoption (with the possible exception of France). In the UK, local authorities are the most important influencers since they control the majority of the country’s roads. National Highways’ change in strategy will help, since local authorities – and the engineering companies that advise them – often use the Specification for Highway Works for their projects.

Finally, there is the issue of cost. Warm mix additives do increase the cost per tonne of an asphalt mix. But, if more authorities are specifying it, there may be benefits from economies of scale. And then there is the cost of energy: when energy prices are high, as they are, the balance of the cost equation changes. So, perhaps this is the perfect time to switch to warm mix all round.


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Sources:

https://tarmac.com/news/tarmac-leads-the-change-to-low-temperature-asphalt-in-net-zero-drive/

https://www.aggregate.com/news-and-resources/press-releases/Aggregate-Industries-Switches-To-Warm-Mix-Asphalt-As-It-Continues-With-Sustainability-Pledge

https://www.gov.uk/government/news/highways-england-accelerates-switch-to-lower-carbon-asphalts

https://www.standardsforhighways.co.uk/ha/standards/mchw/vol1/pdfs/2989369%20MCHW%20Vol%201%20Series%20900_web.pdf

https://www.asphaltuk.org/wp-content/uploads/aia-warm-mix-document-Fl.pdf