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Incorporating Size Effects into Multiscale Adhesion Modelling of Bitumen-Mineral Interfaces

Lead Research Organisation: University of Nottingham
Department Name: Faculty of Engineering

Abstract

The UK's road network totals over 250,000 miles of paved roads providing a means for efficient distribution of goods and services, economic security and social prosperity. The entire road network has an asset value of £750 billion and as the UK's main transport infrastructure provides a vital service to road users, commerce and industry. However, the network requires constant upgrading, maintenance and rehabilitation with a predicted spend of £181bn required over the next 20 years.

Over 95% of these paved roads are constructed from asphalt mixtures which comprise three principal components, namely, mineral aggregates (microns to centimetres), natural or added filler (< 63 microns) and bitumen (film thickness 10-20 microns). However, in spite of their importance, the deterioration of asphalt mixtures has never been fully understood or accurately predicted. The key reason is that the current means of assessing and predicting adhesive behaviour between the bitumen and the mineral aggregates does not account for size effects at the different material dimensional scales. These size effects result mainly from the variations of the bitumen film thickness, mineral surface roughness, air void radius, bitumen polarity distribution (molecular sizing) and mineral compositional distribution. Neglect of these size effects makes it impossible to accurately predict the asphalt mixture's distresses such as material fracture (traffic load induced fatigue cracking, non-load associated thermal cracking and age related cracking), moisture damage susceptibility (material disintegration and softening, stripping and fretting), potholes and other forms of severe surface deterioration, all of which are directly affected by the bitumen-mineral interfacial adhesive properties.

The project aims to develop an overall 'adhesion analysis framework (AAF)' focusing on the measurement and prediction of interfacial adhesive properties between bitumen (binder) and mineral aggregates in asphalt mixtures using a size-affected multiscale experimental and modelling approach. Using a combination of experimental techniques, adhesion theories and material modelling approaches, size-dependent and size-independent material properties will be determined and scaled up from nano to micro to macroscale to predict the bitumen-mineral interface adhesive debonding properties of a range of asphalt mixture types. The research will use a combination of microscopy and spectroscopy imaging and molecular dynamics (MD) modelling at the nanoscale to predict bitumen-mineral interface adhesion and a range of size-independent material properties. The viscoelastic Griffith energy equilibrium principle will then be used at the microscale to produce a mechanics-based debonding initiation criterion incorporating the critical material size effects and the size-independent material properties obtained from the nanoscale MD simulations. The theoretical bitumen-mineral debonding criterion will then be verified by means of pull-off adhesion and cohesion testing incorporating different materials and size effects as well as loading and environmental conditions. The final scaling up effect will deal with crack (debonding) propagation developed through a Paris' law propagation model incorporating both size-dependent and size-independent materials parameters determined at the nano and microscales. These theoretical predictions will then be experimentally verified by a novel 'sandwich-cracking' test with prefabricated initial cracking dimensions together with material and conditioning variables. Finally, all these different multiscale effects will be incorporated into a multiscale modelling hierarchy for predicting adhesive failure and overall material response and delivered as a web-based opensource software and database. This user-friendly software will be used to design and produce better and long-lasting asphalt materials to ensure long-term sustainability of this key national asset.
 
Description This project explored how bitumen sticks to aggregates from very small scales up to the macro level. Our team first observed the bitumen-stone adhesion interface using advanced experiments to understand the fundamental adhesion mechanisms. They we used these experimental insights in molecular dynamics simulations to develop a Bitumen-Mineral Interfacial Structure (BMIS) model that predicts the adhesion strength and other intrinsic material properties regardless of size. To ensure the model was accurate, pull-off tests were conducted on a larger scale, and the model was adjusted accordingly. In addition, extensive pull-off tests on various bitumen-stone combinations were performed. These findings offer practical guidance for selecting materials in road engineering.
Exploitation Route The outcomes of this project have strong potential for both academic advancement and industrial application. Academically, the development of the Bitumen-Mineral Interfacial Structure (BMIS) model, alongside our experimental methods for characterising adhesion at multiple scales, lays a foundation for further studies in interfacial phenomena in composite materials. Researchers can expand on our work to optimise the model or adapt the techniques to other material systems.
On the industrial front, our comprehensive evaluation of adhesion properties for various bitumen-stone combinations provides direct guidance for material selection in road engineering. Infrastructure companies, material suppliers, and governmental agencies can use these insights to improve pavement design and maintenance, thereby enhancing road durability and safety. The BMIS model could also be incorporated into commercial software or consulting tools, offering predictive capabilities that aid in selecting high-performance materials. In this way, the research not only deepens scientific understanding but also offers practical, scalable solutions for the construction and transportation sectors.
Sectors Construction

Transport
 
Description Our findings have direct practical relevance to pavement engineering. The detailed evaluation of bitumen-stone adhesion provided by our research offers industry professionals a reliable reference for accelerating and refining the material selection process in pavement construction projects. In addition, the rapid experimental methods and protocols we developed for assessing adhesion properties have strong potential for widespread adoption. These streamlined approaches not only save time and resources in laboratory settings but also pave the way for more cost-effective and durable road designs. By linking our laboratory insights and molecular simulation data to real-world applications, our work is already influencing both academic research and practical engineering guidelines, promising significant economic and societal benefits through enhanced pavement performance and reduced maintenance costs.
Sector Construction,Transport
Impact Types Economic
 
Description Proposal of a novel experimental method and the design of the instrument
Geographic Reach Local/Municipal/Regional 
Policy Influence Type Influenced training of practitioners or researchers
 
Description Green Carbon-reducing Microalgae Binder: Less Toxic and More Sustainable
Amount £44,880 (GBP)
Funding ID Project ID: 33322 
Organisation Department of Transport 
Sector Public
Country United Kingdom
Start 09/2024 
End 03/2025
 
Title A method for observing asphalt-mineral adhesion interface and the interfacial composition analysis using the ESEM-EDS test 
Description The ESEM-EDS experiment is used to identify nano/micro scale bitumen-mineral interfacial structures present for different compositional asphalt mixtures. The SEM-EDS measurement is undertaken on cross-sections of asphalt mixtures (both laboratory fabricated and field cored) using thinly sliced (cut) sections of the composite bitumen-mineral samples. Within the bitumen layer, SEM-EDS will map the location of heteroatoms such as S and N associated with high polarity bitumen molecules. These molecules are generally attracted by alkali mineral surfaces that contain Ca, K, or Na elements (also mapped by SEM-EDS), leading to a higher concentration of these bitumen heteroatoms at the alkali mineral surfaces. For neutral mineral (SiO2) surfaces, these bitumen heteroatoms will be expected to be more randomly distributed. Thus the heteroatoms (S and N) and the alkali/neutral elements (Ca, K, Na, and Si) will be selected as the 'footprint' elements for bitumen and minerals, respectively, and their spatial distributions at the interfaces will be detected and used for rebuilding the molecular structures of the bitumen-mineral interfaces for the different asphalt material cross-sections. To observe the distribution of chemicals at the bitumen-mineral adhesion interface, a cross-section of the bitumen-mineral joint is needed. The ESEM-EDS observation requires the surface of the specimen to be as flat and smooth as possible. Therefore, the freeze-fracture method is used in this research to obtain the required specimens for the ESEM-EDS test, which is a technique of physically breaking apart (fracturing) a frozen sample to reveal internal structures. 
Type Of Material Technology assay or reagent 
Year Produced 2022 
Provided To Others? No  
Impact Research on the adhesion properties between bitumen and minerals has recently been a popular topic. Still, most of the research has been concentrated on testing the adhesion properties at a macroscopic scale. A few studies have used ESEM to observe the encapsulation between bitumen and small mineral particles from a microscopic perspective. Some subjective conclusions have been drawn from the image observation. However, no study has ever directly observed the true interface between bitumen and mineral at a scale of 20-300 microns and analysed the chemical components of the bitumen layer at the interface. The method of forming samples in this project enables a flat and clear bitumen-mineral interface to be obtained, ensuring accurate data for the chemical analysis of the bitumen-mineral. Overall, the analytical approach of this project is sufficiently novel in the field of road engineering. 
 
Title A novel pull-off test for the evaluation of the interficial adhesion between bitumen and mineral 
Description A new type of pull-off test machine and fixtures were designed and fabricated to measure the interfacial adhesion between bitumen and mineral. In this experiment, the film thickness of bitumen can be precisely controlled, so that the adhesion performance between bitumen-mineral can be accurately evaluated. This novel pull-off test is conducted on the UTM. The index "bond energy" derived from the loading force-displacement curve is considered the critical indicator in evaluating the adhesion properties of bitumen. The results of the pull-off test are compared with those of the standard BBS test and the Cantabro loss test, and the evaluation accuracy of this test is verified. By dividing the curve in the pull-off test into two parts, a clear distinction can be made between the adhesion and tenacity of the bitumen, allowing for a more comprehensive and accurate evaluation of the binder bonding properties, especially for the modified bitumen. 
Type Of Material Technology assay or reagent 
Year Produced 2022 
Provided To Others? No  
Impact In many studies, the binder bond strength (BBS) tests have been used to compare the adhesion properties of polymer-modified and unmodified bitumen. However, different conclusions have been reached about whether styrene-butadiene-styrene (SBS) modification can improve the adhesion properties of bitumen. Xu found that adding SBS significantly improved the adhesion of the binder, especially the effect of branched SBS. Lv and Huang suggested that SBS, polyethylene (PE), polyphosphoric acid (PPA), and gilsonite could enhance the bond strength between base bitumen and aggregate with bond strength increasing with the dosage of the SBS modifier. However, 3% (wt) modified-SBS bitumen showed a lower bond strength than the unmodified bitumen. Zhou also studied the effects of various modifiers on the adhesion performance of bitumen using the BBS test and found that SBS modification would deteriorate the bond strength of bitumen. The inconsistent findings mentioned above suggest that the accuracy of the BBS test in evaluating the adhesion properties of polymer-modified bitumen remains to be verified. In evaluating the adhesion between bitumen and minerals, more effective indicators besides a single "bond strength" are needed to comprehensively describe the adhesion properties of bitumen. In this project, a novel modified pull-off test using the universal testing machine was proposed. This instrument is expected to be promoted and widely used in pavement field. This novel experimental method can solve the above-mentioned problem of " the existing BBS test method cannot accurately distinguish the adhesion performance of the base bitumen and the modified bitumen" and provide an accurate test solution for testing material performance in pavement engineering. The achievements of this newly-proposed research method will contribute to the comprehensive understanding of the intrinsic properties of asphalt and further provide the theoretical foundation and technical supports for the asphalt modification scheme and preferable material selection to finally reduce moisture damage and enhance the endurance of asphalt pavement. 
 
Title Bitumen-Stone Adhesion Evaluation Database 
Description This database compiles laboratory evaluation results of bitumen-stone adhesion across a range of conditions, involving multiple types of bitumen and aggregates. In addition to experimental data, it also includes simulation results generated through molecular dynamics, offering insights into the adhesion mechanisms at the micro-scale. 
Type Of Material Computer model/algorithm 
Year Produced 2024 
Provided To Others? Yes  
Impact By integrating experimental and molecular simulation data, this comprehensive dataset has enabled researchers and engineers to identify optimal material combinations and predict performance under various conditions. This has led to more informed decisions in pavement design, potentially improving road durability and reducing maintenance costs. Additionally, the database has served as a valuable reference for academic studies and collaborative projects in pavement engineering. 
 
Title The molecular simulation model of bitumen volume properties and bitumen-mineral mechanical behaviours 
Description We proposed a molecular simulation model of bitumen volume properties and bitumen-mineral mechanical behaviours and simulated various results for multiple scenarios. The simulation results are uploaded to a web-based database, which is open to researchers involved in the field of bitumen-mineral interaction research. Once the basic information on bitumen and minerals is entered into the system, the database will generate the bulk properties of bitumen and the interaction properties of the bitumen-mineral interface. On the other hand, researchers can upload their data considering different types of bitumen or minerals. 
Type Of Material Computer model/algorithm 
Year Produced 2023 
Provided To Others? No  
Impact This molecular simulation model embedded in our web-based database system is able to predict the relationship between bitumen volume properties and bitumen-mineral mechanical behaviours accurately. This simulation model provides a clear physical interpretation on how bituminous binder materials change over time and with temperature, which can help researchers and engineers understand the bitumen performance under different conditions (temperature, aging conditions, etc.) and provide the technical support for pavement design. 
 
Description Collaboration on the Rees Jeffreys Road Fund (RJRF) with Teesside University 
Organisation Teesside University
Country United Kingdom 
Sector Academic/University 
PI Contribution The goal of this study is to investigate the possibility of using a bio-waste-derived, plastic waste modified bio-binder as a combined rejuvenator and binder replacement for hardened bitumen in aged asphalt mixtures. Our research team has finished the rheological testing (DSR) and various ageing (and rejuvenation) cycles on the materials. The rejuvenator characteristics (viscosity, storage stability, susceptibility to ageing) was described and the rheological and adhesion properties of aged binders before and after treating them with the bio-waste-derived rejuvenator was studied.
Collaborator Contribution Our partner at Teesside University customised the mechanical and chemical properties of a bio-waste-derived rejuvenator according to desired specifications for use in asphalt mixtures. The chemical compounds and thermal behaviour of the rejuvenator was tested.
Impact This collaboration is multi-disciplinary
Start Year 2022
 
Description Research Collaboration with SUPAR at University of Antwerp 
Organisation University of Antwerp
Country Belgium 
Sector Academic/University 
PI Contribution Our research team hosted a three-month visiting PhD student from the Sustainable Pavements and Asphalt Research (SUPAR) group at the University of Antwerp in February 2025. During her stay, we have been providing extensive academic guidance and hands-on experimental training inbitumen-stone adhesion studies. We granted full access to our specialised equipment and facilities, assisted in designing experimental protocols, and supported data collection and analysis.
Collaborator Contribution The SUPAR group contributed by facilitating academic exchange and sharing their expertise in bitumen adhesion research. Their involvement has enriched the scientific dialogue and opened up opportunities for subsequent collaborative funding applications.
Impact Expected outcomes from this collaboration include one to two journal papers. This collaboration is not multi-disciplinary.
Start Year 2025
 
Title A Data Sharing Platform for Experimental and Simulation Data on Bitumen-Aggregate Adhesion 
Description This website includes physical/chemical information for pavement materials, along with multiscale adhesion metrics (e.g., surface energy, mechanical adhesion strength, and adhesion work) and molecular simulation outputs. It enables global access to existing datasets and new contributions, promoting transparency, reproducibility, and innovation in pavement research. 
Type Of Technology Webtool/Application 
Year Produced 2025 
Open Source License? Yes  
Impact The development of this research tool has significantly advanced pavement research by providing a centralized, open-access platform for comprehensive physical and chemical data on pavement materials. It has enabled researchers to integrate multiscale adhesion metrics and molecular simulation outputs into their analyses, which has improved the understanding of adhesion mechanisms and material performance. These capabilities accelerate the design of more durable, sustainable pavement systems and encourage interdisciplinary approaches that drive further scientific and engineering breakthroughs. In addition, the tool has fostered enhanced transparency, reproducibility, and collaboration across the global pavement research community. 
URL https://dev-adhesion.azurewebsites.net/
 
Description Academic Alliance for Pavement Engineering - Launch Workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact This workshop of the Academic Alliance for Pavement Engineering brought together pavement research academics from various UK universities. The event was organised to share ideas, discuss current research challenges, and explore potential collaborations and funding opportunities. During the event, participants engaged in dynamic discussions and networking, which led to the identification of several promising research collaborations and joint funding proposals. Overall, the workshop has laid a strong foundation for an ongoing series of events, fostering a closer, more collaborative research community in the field of pavement engineering.
Year(s) Of Engagement Activity 2024
 
Description Orgnazation of the International Academic Workshop "The 7th Chinese-European Workshop on Functional Pavement (CEW 2023)" 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact The 7th Chinese-European Workshop (CEW2023) on Functional Pavement, co-hosted by Aston University and The University of Nottingham, UK, took place from July 2nd to July 4th, 2023, in Birmingham. This event, founded in 2010, has continually fostered advancements in experimental characterization, advanced modelling, material development, and the design and construction of functional pavements. CEW2023 successfully extended this tradition, providing an invaluable international forum for the dissemination and exchange of emerging ideas within the pavement engineering community. The workshop aimed to promote cutting-edge research and development in the field of functional pavements, focusing on the integration of green, intelligent, durable, and safe pavement technologies. These themes are pivotal for advancing circular and low-carbon economies, supporting future and smart cities, ensuring long-life infrastructures, and creating user-friendly built environments. More than 200 participants from over 16 countries attended CEW2023, demonstrating the workshop's global reach and its effectiveness in bridging the gap between functional and structural pavement requirements.
This diverse international attendance facilitated a rich exchange of ideas, fostering dialogue and collaboration among academics, researchers, practitioners, and administrators worldwide. The event was notably chaired by Dr. Yuqing Zhang and Airey Gordon, who are Principal Investigators (PIs) of our EPSRC project. Their leadership in organizing CEW2023 significantly contributed to promoting our current research endeavors. The workshop served not only as a platform for showcasing the latest developments from our project but also as an opportunity to discuss potential future cooperations with international experts in the field. The outcomes and impacts of CEW2023 have been profound. The workshop succeeded in creating and strengthening scientific communities within the broader research fields of pavement engineering, laying the groundwork for future collaborative efforts. Moreover, the discussion and dissemination of our project's findings and objectives have enhanced its visibility and influence across the global pavement engineering community, potentially leading to new research partnerships and furthering the project's goals.
Year(s) Of Engagement Activity 2010,2012,2014,2016,2018,2020,2023
URL http://www.cew2023.com
 
Description Workshop Program University of Nottingham- CD Lab Bitumen TU Wien 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Four Postdocs and two Ph.D. students attended a workshop at Technische Universität Wien with colleagues from CD Lab Bitumen TU Wien. Our team presented results from NTEC Lab for bitumen adhesion, ageing and modifications. Discussions and the starting point for further collaboration between the University of Nottingham and Technische Universität Wien were made.
Year(s) Of Engagement Activity 2022