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Rubber Asphalt 

Christina Makoundou is a PhD researcher who focuses on developing advanced materials for road safety in urban infrastructures using recycled resources, especially end-of-life tyres (ELTs). Christina’s PhD project focused on incorporating large quantities of ELT rubber to design impact-absorbing asphalt pavements to protect vulnerable road users. In June 2022, she successfully completed her PhD in chemical, civil, environmental and material engineering at the Alma Mater Studiorum – the University of Bologna.

Manufacturing a softer asphalt for footpaths and cycle lanes to reduce the risk of injury for pedestrians and cyclists was one of Christina’s and her team’s goals, but when the need for virgin materials decreases so too do the climate impact and overconsumption. Christina is optimistic about the material and sees great advantages in replacing a high proportion of stone with granulates from recycled tyre rubber.

 

“Tyre rubber has many properties that make it suitable for roads in general, and using recycled material should be the goal wherever possible,” Christina says.

How did it all begin? 

After finishing her master’s thesis, “Recycled building materials reinforcement using CaCO3 nanoparticles generated by bacteria”, she decided to apply for a PhD studying innovative materials for better cities and the future, as well as fostering the use of recycled resources. Christina came into contact with the Innovative Training Networks–Horizon 2020–Marie Skłodowska-Curie Actions SAFERUP! (Sustainable, Accessible, Safe, Resilient and Smart Urban Pavements) project, which invited her to join the network to achieve the goals of her PhD project.

However, the full story of her “Vulnerable Users Protection with advanced recycling paving materials (ProtectVU) and ‘fall-proof’ pavement” project began a few years earlier. Christina partnered with her supervisors to further develop the idea (that originally comes from RISE research) that if both rubberised asphalt and impact-absorbing surfaces exist, why not use them on urban pavements and cycle lanes to protect vulnerable road users from severe injuries? 

Why use recycled tyre rubber? 

Recycled tyre rubber has been used for decades in surfacing in general, and also in pavements – mainly for its elastic properties. It was clear to Christina that if they wanted to introduce the rubber into their formulations, they should use recycled materials instead of producing new rubber for this purpose.

This also fosters the circular use of goods, recycling, and the preservation of the ecosystem. Rubber, binder and stones have been mixed successfully for years, and Christina emphasises that she was investing in an already known process – but in a new way.

Regarding Christina's focus area, two aspects stand out: 

​1. The percentage of rubber incorporated into the asphalt mix (more than 50% volume versus a few tens of per cent for the common rubberised asphalt), and 

 

2. Introducing the use of a cold binder (emulsion) instead of a hot or warm process. 

The material is a mixture of recycled rubber in a higher percentage, mineral aggregates and binder. The rubber is incorporated as an aggregate to the mix, with Christina and her team using the “dry process”. After mixing the components, the mix is compacted and cured at room temperature before being tested.

Were there any surprises during the creation process?

To their great delight, Christina and her team obtained a compacted material during the first trial that holds its form over time and looks like traditional asphalt. They also decided to use the same methods from production to construction. 

“The main surprise we encountered was the difficulty characterising the highly rubberised materials due to their high degree of softness,” she explains. “This was unusual for machines that are used to testing very stiff materials.”

For some of the formulations, they also observed a high particle loss which led them to attach great importance to this particular issue and to test the formulated materials for leaching and particle loss. Of the total mix, the creation consists of more than 50% recycled tyre rubber by volume (approximately 30–35% by weight). 

 

Christina and her team also wanted to produce a material that could be brought to market as soon as possible, and the least hazardous material possible. The use of cold asphalt was inspired by the cold processes often used for micro surfacing, leading to the following potential benefits of cold asphalt: 

  • Reducing the smell and odour 

  • Preventing mixture stickiness in the mixing tools (as often observed for hot and warm mixtures) 

  • Allowing the full process to take place at room temperature, reducing the use of heating (around 140°C to 180°C in some cases)

In addition, the team focused on three areas that benefit from using recycled tyre rubber: sustainability, economy and the work environment. Christina explains that they use an abundant recycled material and replace the aggregate in the mix, which limits the need for large-scale extraction (e.g. by excavating rock). Another innovation involves using recycled materials, and the rubber is reusable in several applications and different forms. 

What are the advantages compared to other materials? What challenges did you come up against?

One of the main challenges Christina and her team encountered was thinking about these materials as potentially commercial as soon as possible, involving the use of existing tools, processes and tests. The aim was also to propose a material that was as resistant as possible, would reduce injuries and was similar to the material used in playgrounds, but could also withstand daily traffic of bicycles or pedestrians.

 

“We still need to find the right balance between the amount of rubber and the structure of the material,” she continues. “Ultimately, we are conscious of the risk of leaching and particle loss. Some tests and methods have already been tried out, but as this topic is extremely important the use of other techniques and tests should not be neglected.”

Christina and her team were still using oil-based binders, which are responsible for most of the CO2 emissions in the process, together with their recycled mineral aggregate. An idea occurred to Christina. Why not use plant-based binders and recycled aggregates together with the recycled rubber? She suggests that additional doctoral or postdoctoral studies should examine this question. 

What were the reactions when the material was presented? 

Christina and her team have received excellent responses when presenting the material and the project, including reactions of surprise and enthusiasm regarding the injury reduction properties, as well as curiosity regarding the mechanical properties of the newly developed material and happiness when describing the cold process in its entirety. In addition to the positive reactions, there have been some worrying thoughts regarding the use of crumb rubber and the release of particles and microplastics into water and the environment. Christina explains that up until this point, they have tried (and continue to try) to find solutions that can prevent the loss of particles, such as pre-treating aggregates or rubber, or post-treating the surface.

 

“There is still a huge amount left to do in terms of research and optimisation, but think that with more fundamental research it could be possible to identify the best way to avoid this loss and leaching phenomenon as far as possible”, Christina says.