Recently, new initiatives have been launched to match and outperform pure polymer-modified bitumen, particularly for niche applications. Christina Makoundou, a PhD researcher in partnership with RISE, Sweden’s research institute and innovation partner, developed the idea that if both rubber asphalt and impact-absorbing surfaces exist, why not use them on urban pavements and cycle lanes to protect vulnerable road users from serious injuries? After completing her master’s thesis, titled “Recycled Building Materials Reinforcement Using CaCO₃ Nanoparticles Generated by Bacteria,” Christina decided to pursue a PhD focusing on innovative materials for better cities and the future, while also promoting the use of recycled resources. She became involved 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 project, “Vulnerable Users Protection with Advanced Recycling Paving Materials (Protect VU) and ‘Fall-Proof’ Pavement,” began a few years earlier. Christina, along with her supervisors, further developed the idea originating from RISE that, since both rubberised asphalt and impact-absorbing surfaces exist, they could be used on urban pavements and cycle lanes to protect vulnerable road users from serious injuries.
One reason Christina chose recycled tyre rubber was its elastic properties. She realised that if rubber were to be introduced into their formulations, using recycled materials would be preferable to producing new rubber. This approach supports the circular use of goods, recycling, and ecosystem preservation. Rubber, binder, and stones have been successfully mixed for years, and Christina emphasised that she was investing in a known process, but in a new way. Two aspects stand out in her focus area: the percentage of rubber incorporated into the asphalt mix (more than 50% by volume, compared to a few tens of percent for common rubberised road asphalt) and the introduction of a cold binder (emulsion) instead of a hot or warm process. The material is a mixture of recycled rubber at a higher percentage, mineral aggregates, and binder. The rubber is incorporated as an aggregate using the “dry process.” After mixing the components, the mix is compacted and cured at room temperature before testing. To their delight, they obtained a compacted material in the first trial that holds its form over time and resembles traditional asphalt. They also used the same methods from production to construction. The main surprise was the difficulty in characterising the highly rubberised materials due to their softness, which was unusual for machines designed to test very stiff materials. In some formulations, they observed high particle loss, leading them to prioritise testing for leaching and particle loss. The mix consists of more than 50% recycled tyre rubber by volume (approximately 30–35% by weight). Christina and her team aimed to produce a material that could be marketed as soon as possible while being the least hazardous.
The use of cold asphalt was inspired by cold processes often employed for micro-surfacing, offering potential benefits such as reducing smell and odour, preventing mixture stickiness in mixing tools (common in hot and warm mixtures), and allowing the entire process to occur at room temperature, thus reducing the need for heating (typically around 140°C to 180°C). Additionally, the use of recycled tyre rubber benefits sustainability, economy, and the work environment. It replaces the need for large-scale aggregate extraction (e.g., excavating rock) with an abundant recycled material. The rubber is reusable in several applications and forms.
Two aspects stand out in Christina’s focus area: the percentage of rubber incorporated into the asphalt mix (more than 50 percent by volume, compared to a few tens of percent for common rubberised asphalt) and the introduction 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 using the “dry process.” After mixing the components, the mix is compacted and cured at room temperature before testing. To their delight, they obtained a compacted material in the first trial that holds its form over time and resembles traditional asphalt. They also used the same methods from production to construction. The main surprise was the difficulty in characterising the highly rubberised materials due to their softness, which was unusual for machines designed to test very stiff materials.
Christina and her team faced the challenge of considering materials for commercial use as soon as possible, utilising existing tools, processes, and tests. Their goal was to propose a resilient material that mimics playground surfaces while withstanding daily pedestrian and bicycle traffic. Christina emphasises the need to balance the amount of rubber and the structure of the material, being mindful of leaching and particle loss. Although some tests have been conducted, further research is required. They also explored the use of oil-based binders with recycled mineral aggregates and considered combining plant-based binders with recycled aggregates and rubber as a potential topic for doctoral studies.
The project has received positive feedback, with surprise and enthusiasm regarding its injury-reduction properties and the cold process used. However, there are concerns about crumb rubber usage and the potential release of particles and microplastics into the environment. The team is actively seeking solutions to prevent particle loss, such as pre-treating aggregates and post-treating surfaces. While they acknowledge that further research and optimisation are needed, they believe the benefits of the material outweigh the drawbacks. Christina advocates for ongoing research to maximise the use of recycled materials without compromising the advantages, including suitability for pavements and cycle lanes.
A version of the material was trialled in Imola, northern Italy, during Christina’s PhD to test its performance under real conditions. Although designed for footpaths and cycle paths, she envisions its future use in motorways, elderly neighbourhoods, and personal applications, arguing that tyre rubber is generally suitable for road construction and that prioritising recycled materials should be a goal. In June 2022, Christina completed her PhD in chemical, civil, environmental, and materials engineering at Alma Mater Studiorum – University of Bologna. Currently based in Antwerp, she continues her research as part of the YUFE4PostDocs programme at the University of Antwerp, developing recycled and bio-based materials for urban roads.