Subject matter

Bitumen is a product derived from crude oil in a refinement process. It is commonly used as binder in asphalt pavements or as sealing and waterproofing material in roofing. Due to different origins of crude oil, the characteristics of bitumen belonging to the same class in terms of mechanical properties can considerably vary in terms of chemical composition and microstructure, which is reflected on various aspects of its long-term performance, like e.g. ageing susceptibility. Bitumen is a multiphase system with a distinct microstructure which is intrinsically associated with its composition but can also evolve depending on external conditions it is exposed to. This microstructure is responsible for the mechanical properties of bitumen, but the microstructure can also be modified by the addition of polymers. Due to the organic nature of bitumen and its additives, the material is prone to oxidation and other environmental effects over its lifetime. Ageing alters the chemical composition and functionality, microstructure, and mechanical behaviour. The ageing of bitumen is commonly quantified using several approaches based on the temporal deterioration of its mechanical properties like e.g. viscosity. Nevertheless, the spectroscopic and microscopic techniques (like infrared and fluorescence spectroscopy and microscopy, atomic force microscopy, and scanning electron microscopy) are increasingly getting into the focus of scientists because they are able to characterise the chemical and microstructural changes of bitumen form a more intrinsic aspect.

Polymers are added to bitumen with the general intention to increase the durability of asphalt layers by enhancing their resilience to mechanical and environmental effects. The traditional way of producing polymer-modified bitumen (PmB) is the mixture of a polymer with straight run bitumen. PmB is classified with the letters A, B, C, and H, where A and B stand for elastomers, C for thermoplastics, and H for higher polymers. PmB shall not exclusively mean SBS but waste and industrial by-product materials such as crumb rubber, polymerised sulphur, waste plastic (PE, PET, and similar) can also be added. While on the one hand the use of polymers in bitumen is considered critically important, on the other hand, the contribution of PmB to the global burden with microplastic in the aquatic environment could certainly become a political issue. Current European legislation demands the recycling of asphalt, but the amount is still expandable. Here, PmBs could be particularly beneficial due to their long-term chemical stability but could also experience physical instability because of the potential phase segregation and precipitation.

In general, the mixtures of polymers and bitumen are metastable and phase segregation can become a crucial issue in the case of long storage times and high temperatures. The susceptibility to phase segregation however inherently depends on the chemical composition of the two components, the addition of emulsifiers, rejuvenators, and on environmental conditions like stress, temperature, humidity, ultraviolet radiation, etc. In laboratory, the resilience of PmBs is often characterised by mechanical properties like DSR-based oscillatory shear and other methods, and BBR. Moreover, spectroscopic and microscopic techniques mentioned above are suitable to investigate the distribution of polymer and the formation of polymer micromorphological features inside specimens. Links between experiences and fundamental insights gained from other RILEM TCs (e.g. Fingerprinting bituminous binders using physico-chemical analysis). However, the microscopic and spectroscopic methods have not been standardised yet and have mostly been used for the qualitative analysis of materials. For engineering purposes, these methods are now employed for quantification purposes, and this requires calibration and standardisation.

Thus, in a first step this proposed RILEM technical committee will collect and review the knowledge on linking polymer modification and physicochemical characterisation methods developed by past RILEM TCs and the community to have a solid basis for its own work. This experimental work of the committee will include bituminous binders, different types of polymers and other additives. Based on the results obtained, recommendations for the standardised use of these methods and chemo-mechanical links will be provided.

More specifically, the objectives of this TC will be the following:

1. The research shall be focussed on the following polymers: SBS, XSB, EVA.
2. Gathering and reviewing existing knowledge on microscopy and spectroscopy as well as the EHS aspects of A review article is planned to comprehensively summarise the current knowledge for the scientific community (literature review).
3. Stability and aging tests of PmB will be conducted with the aim to identify mixtures which are particularly suitable for recycling (interlaboratory testing).
4. Identifying potential methods for the chemo-mechanical fingerprinting of PmB (experimental).
5. Assessment of the environmental effects of PmB.

RILEM Cluster F – the proposed home of this proposed TC – has already a high number of active TCs compared to the size of the active scientific community. To keep this TC focused, the TC will comprise of two task groups (TG). It is intended that his proposed TC also interacts and establishes strong links to other active TCs. For this purpose, the basis for the interlaboratory testing and other experimental activities will be the existing materials from other active TCs. The materials will focus on base and modified bitumen with polymer and rejuvenation agents

Terms of reference

The proposed TC will be active for five (5) years. The TC will perform a combination of literature review, interlaboratory testing and other experimental work. It will aim at establishing a sound foundation for subsequent scientific understanding and characterization of PmB stability as well as clarifying its sustainability. The TC was inaugurated at the RILEM Cluster F annual meeting in 2020, where a wider audience was exposed to the intentions of this TC and had the opportunity to join in on the efforts. In addition, the current experts in the field will be invited to participate through mailings and personal contacts. The TC will conclude its work in 2026 and the findings will be presented at a subsequent RILEM event.

The following two task groups (TG) are intended:

TG 1, Stability of polymer-containing bitumen (Miomir Miljković, University of Niš)

On the basis of the literature review, an interlaboratory testing will compare the results from different laboratories on the differences in phase segregation and the impact of aging. Before the interlaboratory testing, this TG will review the state of the art from past RILEM TC STAR reports and scientific publications. This work will be done in 2022 and finished in 2023. The interlaboratory testing will be organised in 2023 and executed in 2023 and 2024. Data evaluation will be carried out in 2025, and the recommendations will be developed and published in 2025 and 2026.

The questions to be answered by this TG are:

• What are the most promising microscopic techniques to monitor the polymer network formation in polymer-containing bitumen.
• What are the optimal procedures to test stability of the system?
• Which spectroscopic techniques can identify all additives in polymer-containing bitumen?

Basic methods which should be used by each participating laboratory will combine the benefits of mechanical and chemical information. In addition, participating laboratories are welcome to add any other methods, e.g. DSR, BBR, GPC, NMR, other spectroscopic and microscopic methods, etc. New techniques and methods shall be invited.

The goal of this TG is to provide recommendations on how to prepare polymer-containing bitumen samples, and process and analyse data in a standardised way. These recommendations should have a potential to be considered by international standardisation committees and set a common ground for the future work with polymer-containing bitumen and make results from different groups more comparable.

TG 2, Aging susceptibility and sustainability of polymer-containing bitumen (Xueyan Liu and Peng Lin, TU Delft)

The idea of this TG is to distinguish additives from different sources or refining processes and quantify additive contents (defined polymers, and rejuvenation agents, etc.) by suitable methods. The work in this TG includes a review of the state of the art (2022 and 2023), experimental activities (2024), evaluation and recommendation (2025, 2026)

The questions to be answered by this TG are the following:

• What is the impact of photo-chemical aging on polymer-containing bitumen?
• What problems could occur in the recycling of polymer-containing bitumen?
• Which compounds could be transferred from polymer-containing bitumen into the environment?
• Can the sustainability of polymer-containing bitumen be quantified?

The goal of this TG is to identify compare available methods for fingerprinting and quantification of certain additives and judge their benefits. This can also be seen as a pre-standardisation activity to ensure that the future research in this area can provide reliable and comparable results.

Detailed working programme

Fall 2021: Approval of the new TC by TAC

2021/23: Pre-TC-work and official kick-off meeting in winter 2022. Approval of the work programme and the assignment of TG leaders.

Spring 2023: Meeting to discuss the opinions and ideas with the other asphalt scientists during the TC annual meeting.

During 2023: Individual TG meetings, finalising proposal of work plan, evaluation of other possible synergistic activities, identification of missing expertise/experts that could join this TC.

2022 to 2023: Review and gathering of state of the art for TGs.

During 2023: Individual TG meetings, finalising proposal of work plan, starting of interlaboratory testing (TG 1) and experimental work (TG 2)

Winter 2023/24: Annual TC meeting in the course of the Annual Cluster F meetings

2023 – 2024: Executing and finalising interlaboratory testing and experimental work. The beginning of data evaluation.

During 2024: Workshop of the TC: Bringing together the experts from bitumen and asphalt community and the key experts from materials science, polymer science, chemistry, and environmental science to define best candidate laboratory characterisation techniques for binders and mixtures, along with the optimal specimen preparation methods for the chemo-mechanical characterisation of polymer-containing bitumen.

Winter 2023/24: Annual TC meeting in the course of the Annual RILEM Asphalt TC meetings.

2025 – 2026: Data evaluation and developing recommendations for both TGs as the basis for the further use in standardisation.

Winter 2025/26: Annual TC meeting in the course of the Annual RILEM Asphalt TC meetings.

Spring/Summer 2026: Presentation of results at RILEM Annual meeting

Winter 2026: State-of-the-art report on physicochemical testing methods for polymer-containing bitumen binders, Final Report and Recommendations and closing of the TC during the Annual RILEM Asphalt TC meetings.