Rubber Asphalt

By adding recycled tyre rubber particles to bicycle lanes, their performance features are enhanced, reducing the risk of serious injuries from falls and adjusting rolling resistance for sharp curves, crossroads, and more. This is a fantastic innovation with a relatively long history. The use of rubber in road applications has been evaluated in Sweden since the mid-1970s and internationally, primarily in southern Europe and in California and Arizona, with excellent results, including reduced cracking, noise reduction, less rotting, better drainage, and more. Recently, new initiatives are underway to match and outperform pure polymer-modified bitumen, particularly for niche applications

Christina Makoundou, a PhD researcher partnered 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 severe injuries? Take part of her journey.

After completing her master’s thesis, “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, as well as fostering the use of recycled resources. She encountered 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 if both rubberised asphalt and impact-absorbing surfaces exist, they could be used on urban pavements and cycle lanes to protect vulnerable road users from severe injuries.

One reason Christina chose recycled tyre rubber was its elastic properties. She realised that if rubber was to be introduced into their formulations, using recycled materials was preferable to producing new rubber. This approach supports the circular use of goods, recycling, and ecosystem preservation. Rubber, binder, and stones have been mixed successfully for years, and Christina emphasised that she was investing in a known process, but in a new way.

A version of the material was laid and used in Imola, northern Italy, by Christina and her team during her PhD.

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.

Result of the samples after Addition of pigments to the mix to color the matrix of the material

Uniaxial compression test on Impact-Absorbing Pavement samples made with cold binder.

A version of the material was laid and used in Imola, northern Italy, by Christina and her team during her PhD.

In some formulations, they observed high particle loss, which led them to prioritise testing for leaching and particle loss. The mix consists of more than 50 percent recycled tyre rubber by volume (approximately 30-35 percent 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 used 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.

A significant challenge Christina and her team faced was considering these materials for commercial use as soon as possible, involving existing tools, processes, and tests. The goal was to propose a material that was as resilient as possible, reducing injuries and resembling playground materials, but also capable of withstanding daily bicycle or pedestrian traffic. Christina notes that they still need to balance the amount of rubber and the structure of the material; “We are conscious of the risk of leaching and particle loss. While some tests have been tried, this crucial issue requires further techniques and tests.” They also considered using oil-based binders with recycled mineral aggregate, sparking the idea of combining plant-based binders with recycled aggregates and rubber. Christina suggests this as a topic for doctoral or postdoctoral studies.

The project and material have received excellent responses, with reactions of surprise and enthusiasm regarding injury reduction properties, curiosity about mechanical properties, and appreciation for the cold process. Despite positive feedback, there are concerns about crumb rubber usage and the release of particles and microplastics into the environment. The team has tried and continues to seek solutions to prevent particle loss, such as pre-treating aggregates or rubber and post-treating surfaces. They acknowledge that much research and optimisation remain, but believe that with more fundamental research, it is possible to minimise leaching and particle loss.

The benefits of using the material outweigh the drawbacks. Christina recommends continued research to optimise the proposition, aiming to introduce more recycled materials without sacrificing the benefits, including reduction, suitability for pavements and cycle lanes, and traditional asphalt methods.

A version of the material was laid and used in Imola, northern Italy, by Christina and her team during her PhD. This trial aimed to test the materials in real conditions and gather data on their reactions to traffic and weather. Christina and her team’s material is designed for footpaths and cycle paths, but she hopes it will eventually be used for motorways, elderly neighbourhoods, personal use, platforms, and more. She believes tyre rubber’s properties make it suitable for roads in general, and using recycled material should be the goal wherever possible.

Christina Makoundou is a PhD researcher focusing on developing advanced materials for road safety in urban infrastructures using recycled resources, particularly end-of-life tyres (ELTs). Her PhD project aimed to incorporate large quantities of ELT rubber to design impact-absorbing asphalt pavements for protecting vulnerable road users. In June 2022, she successfully completed her PhD in chemical, civil, environmental, and material engineering at Alma Mater Studiorum – University of Bologna. Currently, she lives in Antwerp, continuing her research as part of the YUFE4PostDocs programme at the University of Antwerp, focusing on developing recycled and bio-based materials for urban roads.