Subject matter

Material scientists and road pavement technologists are fully aware that end-of-life products can be engineered and re-used and/or recycled. In other words, engineering is already at service of “circular economy” and it is already turning goods at the end of their service life in resources for many other product’s cycles. However, the road pavement industry, society and governing bodies are still considering these end-of-life products as waste and secondary materials. TC 279-WMR, which is about to end its mandate, aimed at commencing fundamental research on the feasibility of incorporating waste and secondary materials in asphalt mixtures. The TC activities have seen significant participation, proving the great interest of the sector on these topics, and its outcomes did provide enormous evidence and recommendations for valorising Waste and Secondary Materials for Roads. TC 279-WMR goal was ambitious and, overall, it can be considered extremely successful. It is on this wave of enthusiasm and global participation that this proposal wants to build upon and lead towards a long-term vision contemplating the minimisation of raw materials and possible phasing-out of bituminous materials within the built environment of a post-fossil fuel society. To begin with, the proposed TC offers a clear shift in terminology. Rather than focusing on waste and/or secondary materials, the terminology will consider paving material ‘alternatives’ to the conventional components of bituminous mixtures: Alternative Paving Materials (APMs). Furthermore, this TC will aim at investigating the SUSTAINABILITY of using APMs as full and/or significant partial replacement of conventional components of bituminous mixtures, such as aggregates, bitumen, synthetic polymers and any other fossil-fuel derived additive/admixture.
Obviously, not all APMs can be considered for such an ambitious goal, hence this TC wants to focus the investigation on three specific most-promising candidates: 1) recycled tyre rubber, 2) recycled plastic 3) binders from biological sources (biobinders).
In fact, the first two APMs have higher technology readiness levels, have been investigated within TC 279-WMR and this TC could allow to complete some un-answered questions linked to their sustainability. Instead, a significant replacement of bitumen, and/or modifiers, with an alternative bio-source has not yet been investigated within the RILEM Cluster F. The initial idea of the leadership of this TC is that in order for the APMs’ replacement to be considered significant, a minimum target of 25% of each conventional bituminous mixture component would be investigated, towards producing practical recommendations.
In more pragmatic terms, this TC will aim at providing an answer to one fundamental question “Is maximising the content of APMs within the bituminous mixtures a sustainable practice?” In order to address such a complex issue, partners from both academia and industry recognise that there are at least three pressing issues to be solved before even thinking of an engineered approach that allows designing bituminous materials with significant content of APMs. These areas of research can be linked to other three fundamental questions for which a tentative answer will be given within the following task groups:

1. “What is the environmental, economic and social impact of maximising the content of APMs within the bituminous mixtures?”. TG1 will attempt to provide an answer by defining a Life Cycle Sustainability Assessment (LCSA) methodology for bituminous materials, hence by using life cycle-based techniques looking at benchmarking environmental impacts and evaluating economic and social indicators to advise on whether using APMs at the forecasted level of replacement is objectively a more sustainable option. This task will provide recommendations in terms of both methodologies and technologies.
2. “Will incorporating APMs within the bituminous mixtures affect its recyclability? A very much important question raised by most of the road authorities who are now fully aware of the tendency towards a more circular bituminous mixtures pavement industry. This task will also provide recommendations in terms of general methodologies to assess recyclability of bituminous materials and will then advise on whether APMs are an opportunity or a concern.
3. “Is there a risk of potential noxious substances released by bituminous mixtures that incorporate APMs? Road authorities and the general public want to be reassured that the incorporation of APMs in roads does not favour the generation of pollutants that can alter the environment and be toxic to humans, hence invalidating the main purpose of their initial introduction. New issues, such as microplastics, leaching of contaminants into the soil and groundwater, and ecotoxicity parameters, should be evaluated for APMs. This also includes validating or developing new testing methods specifically adaptable to road materials.

These are issues that must be resolved before such a paramount change of culture within the road pavement industry can take place, leading to promoting solutions beyond fossils-based materials and allowing their widespread market uptake. To achieve that, there is an urgent need from the road pavement industry to get these answers from the international scientific community and at first. Hence, in cooperation with Cluster F leadership has been decided that the most appropriate approach in order to answer the Sustainability questions, without forgetting the importance of defining practical recommendations for engineering these solutions up to their  implementation, will be to carry out simultaneously APM-Sustainability (TC APS) and APM-Design & Performance (TC APD). These TC together will allow to couple long-term vision with feasible first steps leading towards practical recommendations defined by bituminous material technologists together with a multi-disciplinary plethora of material scientists, chemists, chemical engineers, civil engineers, biologists, etc. RILEM TC APS wants to be this kind of international cooperative platform, hence it will inherently increase RILEM membership and its reach beyond the traditional sectors of civil engineering and materials science - as demonstrated by the interdisciplinary list of interested parties at the end of this proposal – and will have the main aim of investigating whether a widespread use of alternative paving materials in road pavements is actually a good practice for the current society, as well as for the generations to come.

Terms of reference

The TC will be active for 5 years (2023-2028) and its goals will be achieved through 3 work packages:

• WP1 – Coordination: will include the day-to-day project management activities, the liaison with RILEM Cluster F board, the liaison with partners and industry to facilitate dialogues amongst the participants (i.e. shipping of materials and equipment, establish a common material database, etc.), the organisation of periodic meetings, the facilitation of reporting and overall the coordination with the activities proposed by the TC APD and other active TCs
• WP2 – Research: will include the work of the four Task Groups, detailed in section 4 
• WP3 – Communication: will foster internal communication with the creation of virtual platforms that aim to facilitate collaborations (i.e. Microsoft Teams) and promote scientific dissemination in journal papers, technical conferences, tailored industry workshop, as well as carrying out widespread public engagement with the establishment of online landing page and/or social media pages (i.e. LinkedIn)

The Chair and Deputy Chair will be in charge of WP1 and WP3, while WP2 will be carried out with the support of main TGleaders coadiuvated by TG co-leaders. This approach is expected to provide a solid leadership for each TG, to effectively carryout different tasks, as well as ensuring presence of TG co-leaders in any of the periodic meetings. The selected themes of each TG have been chosen as a starting point to achieve the long-term vision, depicted in section 2, and as an extension of the activities that were initiated by the previous RILEM TC 279-WMR. Please note that while the TG structure builds on the basis of the previous TC activity, the scope, aim and objectives of the proposed TC are significantly different than the current TCs operating in the field of asphalt materials (i.e. TC 272-PIM, 278-CHA, 264-PAR, 280-CBE and TC 295-FBB). Each TG will undertake an interlaboratory studies (ILSs) where the materials object of testing can overlap; in other words, multiple laboratories will be expected to use the same set of materials for ILSs of multiple TGs. This approach was successfully implemented in previous TC 279-WMR and TC 264-RAP by the respective TC leaders. A conservative estimate of 15-20 laboratories is expected for the participation in the interlaboratory study, once again based on past experience with TC 279-WMR. Geographically, the participating laboratories are expected to be from several continents around the world; the affiliations of the proposed TG leaders confirm such geographical diversity. Furthermore, it has to underlined that a strong connection with the TC APD is expected to take place, especially within ILSs by for instance sharing methodologies and/or materials.

A proposed timeline of TC activities is indicated below:

• September 2023: Approval of the new TC by TAC
• November 2023: Kick-off meeting and Approval of work program and assignment of group leaders at the annual meeting in Lyon, France 
• Fall 2023: Annual meeting, discussion and approval of work plan
• Spring 2024: Meeting of TGs, update of deliverables and work plan
• Fall 2024: Annual meeting, discussion of results, presentation of achievements
• Spring 2025: TG meetings, data evaluation, presentations, papers
• Fall 2025: Annual meeting, discussion of results, presentation of achievements
• Spring 2026: TG meetings, data evaluation, presentations, papers
• Fall 2026: Annual meeting, discussion of results, presentation of achievements
• Spring 2027: International symposium
• Fall 2027: Annual meeting, concluding remarks, final report, presentations of final results
• Spring-Summer 2028: Final TC APS and APD Symposium

In general, all TC meetings will be held in hybrid (in-person and online) formats to maximize attendance and participation. Further, in addition to the above work plan, TC leaders are also expected to actively disseminate TC activities and related updates at other international venues (Transportation Research Board meetings in the United States, European Asphalt Technology Association, Eurobitume, Conference on Pavements in Southern Africa, Ibero Latin American Asphalt Congress, and Australian Flexible Pavement Association events).

Detailed working programme

Brief descriptions of each task groups of the “Research” work package and their corresponding activity are provided next.

TG1: Life Cycle Sustainability Assessment
This TG will extend the investigation performed by TG4 of TC 279-WMR to advance the benchmarking of the environmental impact of bituminous mixtures with and without recycled tyre rubber, selected bio-binders, recycled plastics. Hence, this TG will build on an existing methodology and will enrich it further with data collection and life cycle assessment studies, possibly implementing a cradle-to-grave approach. TG1 will also create data collection tools and methodologies to assess the economical and social impact of the use of APMs within bituminous materials. With regards to the latter, this TG will be a first international effort proposing social-LCA indicators for bituminous mixtures. For this purpose, the TG has been divided in three subtasks, one for each of the investigated aspects.
The coordination has already identified the main TG-leader: Ana Jimenez Del Barco Carrion from University of Granada (Spain) and other co-leaders with relevant experience on the following subtasks have been identified:

• TG1.1 Environmental LCA (extending benchamarking values of environmental impact indicators)
• TG1.2 Economical LCA (assessing potential convenience by creating economical industry average values)
• TG1.3 Social LCA (creating databases related to newly proposed social impact indicators for bituminous mixtures)

TG2: Recyclability
The research will address the recyclability of bituminous mixtures containing APMs. The group will at first define a methodology for evaluating recyclability of any bituminous mixture to then evaluate recycling potential of bituminous mixtures incorporating three selected APMs. In this sense, the group will start from the materials already investigated during TC 279-WMR (i.e. recycled tyre rubber and/or recycled plastic) and will investigate appropriate ageing procedures, mix design and expected performance of recycled bituminous mixtures incorporating tyre rubber, plastics and bio-based materials. This approach facilitates the organization of the experimentation, as materials and initial design will be known, and it will also create direct synergies with TC APD.
The coordination has already identified the main TG-leader: Kamilla Vasconcelos from University of Sao Paulo (Brazil) and other co-leaders with relevant experience on the following subtasks have been identified:

• TG2.1 Laboratory aging (towards recommendations for laboratory aging of bituminous materials with APMs)
• TG2.2 Recycling aged APM mixtures - Design (focus to be decided in collaboration with TC APD)
• TG2.3 Expected performance of recycled mixtures (focus to be decided with TC APD)

TG3: Ecotoxicity and potential release of pollutants and microplastics

TG3 will focus on evaluating whether the introduction of APMs into the bituminous mixtures has the potential of generating additional burden to the environment and its living species. Microplastics generated by the traffic abrasion on APM-modified materials and the potential leaching of noxious substances into the pavement sublayers and subgrade will be evaluated using novel approaches that incorporate several disciplines. The production of potentially hazardous compounds will be investigated
via ad-hoc testing methods to be developed or refined, since limited studies are currently available that specifically tackle leachates and microplastics from bituminous mixtures. The magnitude of pollutants released will be used to evaluate the potential ecotoxicity of APMs when used in road applications. TG3 will deploy three subgroups, each of which will tackle one of the three topics highlighted below: microplastics, leaching, and ecotoxicity.
The coordination has already identified the main TG-leader: Filippo Giustozzi from Royal Melbourne Institute of Technology (Australia), and other co-leaders with relevant experience on the following subtasks have been identified:

• TG3.1 Potential release of microplastics (procedures to artificially produce micro-plastics from mixtures)
• TG3.2 Analysis of leachates from bituminous mixtures (assessing micro-plastics content through analyses of leachates)
• TG3.3 Ecotoxicological impacts on biodiversity (assessing the impact of microplastics on living beings)