Issue 4
The implementation of recycled materials in road infrastructure is a significant step towards reducing our reliance on increasingly scarce quarried and extracted resources. This is an important step towards sustainability and helping develop a circular economy. Recycled materials can also help to decarbonise the construction activities of road building and maintenance.
However, recycled materials are often seen as inferior to their virgin equivalents. Many prescriptive specifications do not allow the use of certain recycled materials. For the road infrastructure construction industry to realise the benefits of utilising recycled materials, implementation pathways need to be developed.
This paper outlines the factors that need to be investigated to develop implementation pathways for recycled materials. If recycled materials are to be accepted as viable alternatives to virgin materials across the board, it is important to ensure:
Australia has an expansive road network that requires maintenance while, at the same time, the Australian Government has committed $120 billion in infrastructure projects until 2032 under the Infrastructure Investment Program. Although the program will deliver various economic and social benefits, it will require significant amounts of material and other resources. There lies a major opportunity, however, to make much greater use of recycled and replacement materials in infrastructure construction and maintenance. Using recycled materials reduces the demand for increasingly scarce and costly virgin materials, as well as diverting valued resources from landfill (Hall et al. 2022).
The National Waste Policy Action Plan identified the use of recycled materials in infrastructure as an opportunity (Australian Government 2019), while state and territory governments have also established policies to support a circular economy (Hall et al. 2022). These policies also contribute towards the achievement of Australia’s pledge to net zero emissions by 2050 (Kurniawan & Howland 2024), while enabling a growing nation.
The use of recycled materials in transport infrastructure is a sustainable solution that reduces reliance on finite resources, potentially resulting in significant environmental benefits for Australian road agencies, the broader industry and, ultimately, Australian communities.
The incorporation of recycled materials in road infrastructure needs to, for each application, consider impacts on the environment, work health and safety (WHS), engineering performance and re-recyclability (Pandelidi & Grenfell 2024). Trochez, Grenfell & Harrison (2021) specified that for their successful adoption, recycled materials:
Austroads (2022a) recognised the need to develop appropriate standards and specifications that will facilitate the safe adoption of recycled materials. These should ensure serviceability, functionality, resilience and durability.
Additionally, recycled materials can be employed as host materials in different stabilisation techniques to treat various recycled material blends to maximise their application in road infrastructure.
To ensure that recycled materials are successfully and sustainably incorporated in road infrastructure, a series of assessments need to be undertaken, as summarised in Figure 1. In certain circumstances field trials can come before or in parallel with the development of a specification. This is especially the case for recycled materials where a field trial can provide long-term evidence of performance, which provides confidence that a specification should be implemented.
Figure 1: Steps to successful recycled material adoption in road infrastructure
The variability of recycled materials, as well as that of infrastructure assets, means that assessment methodologies need to vary considering the inherent risks associated with each.
Sections 3.1-3.7 summarise various aspects for consideration and provide case studies where relevant work has been undertaken.
A market analysis is an important first step in evaluating the potential for recycled material use in infrastructure. Market analyses forecast the potential demand for materials and assess the supply chain in terms of its capacity and constraints. Results from a market analysis inform the feasibility of the material use, in terms of costs, availability and competitive uses, and identify issues that may need to be addressed to allow materials adoption to flourish.
In 2022, the NTRO delivered Infrastructure Australia’s Market Capacity report on recycled and replacement materials (Infrastructure Australia, 2022). The underpinning market analysis investigated the potential to unlock new supply chains of recycled materials for infrastructure construction.
It found that based on current technology and standards, around 27% of conventional materials used for road projects could be replaced with a range of recycled materials. This would mean substituting approximately 54 million tonnes of conventional materials used in roads infrastructure with approximately 52 million tonnes of recycled materials.
With advancements in technology and updates to standards, the tonnage of conventional materials replaced could rise to 43%, replacing 87 million tonnes of conventional materials with 80 million tonnes of recycled products.
Increasing the use of recycled materials is a cost-effective way to reduce waste and emissions and to deliver safe, sustainable and reliable infrastructure. However, in terms of embracing this opportunity, industry uptake is highly dependent on the technology, standards, market appetite and processes across the supply chain.
The research found that there is general lack of awareness of recycled and replacement material uses and benefits across the infrastructure value-chain. This can lead to low market confidence and risk aversion that inhibits market opportunities, including improving the cost-effectiveness of emerging materials and markets.
Government and industry stakeholders are calling for improved education and awareness to help participants along the supply chain understand recycled materials and their opportunities in road infrastructure. To support awareness a strong, contemporary regulatory system, including product specifications and standards, is needed to facilitate industry development and advancement of market opportunities whilst protecting against adverse impacts.
Industry engagement also revealed strong themes of risk-aversion, misinformation and a negative perception towards recycled materials. While education and practical demonstrations are a key action, there is a broader cultural issue at play affecting the implementation of government policies aimed at increasing the use of recycled materials. A targeted program to recognise and reward innovation and incentivise behavioural changes should also be considered.
The recycled material supply chain is diverse and the market is fragmented, servicing a mix of customers and locations across Australia. As such, recycled and replacement products are not always readily available where they are needed, resulting in high transportation costs that can offset cost or environmental benefits. Governments should consider ways to support the development of recycled material markets in under-served areas.
The supply of recycled materials is varied and many recycled materials, such as recycled plastics and glass, have competing uses that may limit their availability. Other materials, such as ground granulated blast furnace slag, reclaimed asphalt pavement and fly, have few competing uses and are mostly used for infrastructure applications.
With demand for materials for use in road construction projects expected to grow, there are substantial economic, social and sustainability benefits in utilising more recycled materials in delivering the infrastructure pipeline. Challenges, however, remain in navigating the pathway to increased adoption.
The next steps in the evaluation process will help address many of these challenges.
The assessment of WHS impacts needs to not only regard the form in which the recycled materials are available but also additional considerations relating to any processing required for their inclusion in the selected infrastructure asset. Some potential WHS hazards for consideration include but are not limited to:
For instance, the incorporation of recycled polymeric materials, such as crumb rubber and plastics, in bituminous binders, needs to consider the size of the particles in which the materials are available (powder, pellets etc.) and potential fumes and emissions generated during their blending with bitumen at elevated temperatures.
Pandelidi and Grenfell (2024) evaluated the generation of volatile organic compounds (VOCs), bitumen fumes (BF), polycyclic aliphatic hydrocarbons (PAHs) and total suspended particles (TSPs) when blending recycled low-density polyethylene (LDPE) and comingled high-density polyethylene (HDPE)/polypropylene (PP) with bitumen and asphalt in a laboratory environment.
Measurements were undertaken within an enclosed chamber, statically and on the operator’s person, during binder blending and asphalt mixing activities. The results of the plastic-containing materials were assessed against conventionally used polymers and unmodified bitumen in all conditions, while the fumes and emissions from static and in-person measurements were also assessed against exposure standards as set by Safe Work Australia (2022).
In-person and static sampler results showed that, even though in some cases the incorporation of recycled plastics resulted in greater release of fumes and emissions when compared to conventionally used materials, in all cases they were below the Safe Work Australia limits, where such limits are set.
To establish that recycled materials are safe for the environment, their propensity to release certain harmful contaminants to the soil, atmosphere and the aquatic environment, needs to be quantified. The introduction of recycled materials in road infrastructure, however, potentially introduces new risks not previously considered. As such, relevant acceptable limits are not available. Therefore, the necessity of benchmarking against accepted practice is essential in such scenarios.
Pandelidi and Grenfell (2024) developed a protocol for the quantification of microplastics release and evaluated different methods for the quantification of leachates from recycled plastics-containing asphalt.
To quantify the release of microplastics, asphalt slabs containing LDPE and HDPE/PP were manufactured via both the wet and dry method. Slabs containing conventional materials were also prepared to set the baseline. After preparation, all slabs were aged by exposure to elevated temperatures and then abraded using a grooved steel surface. The released particulate matter was collected and the concentration of microplastics released was quantified using thermogravimetric analysis (TGA).
TGA was found to be an effective method to quantify the presence of microplastics in the abraded matter. The results revealed that the content of microplastics was relating to their concentration in the binder, in the case of wet mixes, while relatively greater content of microplastics was measured for slabs manufactured via the dry method.
To measure the leaching potential of asphalt containing recycled plastics, various methods were evaluated. During one method, binders were submerged in deionised water, agitated and exposed to elevated temperature. The water was then assessed for leachates using gas chromatography/mass spectroscopy (GC/MS). Another approach investigated was the employment of the Australian Standard Leach Procedure (ASLP), typically used to quantify the presence of leachates in soil. The ASLP method used loose mix asphalt samples, both crushed to size and uncrushed.
Results from the ASLP method were found to be influenced by the presence of aggregates and, therefore, the method was not found to be suitable. On the other hand, the method employing GC/MS failed to detect any leachates. This could, on the one hand, be due to the lack of any leachates or, more likely, an indication that the method needed to be further calibrated.
Road infrastructure materials can, in most cases, be recycled at the end of their lives. It is, therefore, imperative that the introduction of recycled materials does not change that fact. Considerations for re-recyclability may include all aspects discussed in Sections 3.1-3.4.
Rice and Harrison (2021) assessed the re-recyclability of crumb rubber-containing asphalt back into asphalt as reclaimed asphalt pavement (RAP). This investigation included a practicality study and a laboratory assessment.
The practicality study assessed paving, milling, reclamation, processing and paving activities. The laboratory assessment evaluated the properties of binder extracted from CRMB-containing RAP.
This research found that the presence of CRMB in RAP did not notably affect current processes or equipment. However, the laboratory studies revealed challenges associated with measuring the viscosity of the extracted RAP binder. Rice and Harrison (2021) attributed those to the presence of crumb rubber particle, which affected repeatability.
Recognising the economic and environmental advantages, Australian road agencies are increasingly focusing on reducing reliance on non-renewable resources by incorporating recycled materials into road infrastructure. Additionally, there are opportunities to further enhance the use of recycled materials in road pavements. For example, they can be used as host materials in stabilisation techniques such as foamed bitumen and cement stabilisation.
Zhalehjoo et al. (2024) assessed the performance of recycled material blends stabilised with foamed bitumen and cement using a comprehensive laboratory testing program. Several laboratory tests were undertaken to characterise the physical, engineering and mechanical properties of the stabilised blends.
This NACOE project investigated whether different recycled blends could be utilised as host materials for both cement and foamed bitumen stabilisation. A total of 8 recycled material blends were selected for the laboratory testing assessments over 2 years. Blends including recycled crushed concrete, reclaimed asphalt pavement, recycled crushed glass and recycled crushed brick were investigated. The standard Department of Transport and Main Roads (TMR) mix design procedures for lightly cement-treated and foamed bitumen stabilised bases and subbases were utilised to assess whether conformance could be achieved when treating recycled materials blends.
The research project found that both the innovative foamed bitumen stabilisation and the widely used cement stabilisation techniques can meet TMR specification requirements when applied to recycled material blends. This finding suggests an opportunity to utilise these blends in plant-mix stabilisation and as top-up materials for in-situ stabilisation.
Additionally, the work has demonstrated the potential for improved performance by optimising blends through materials engineering expertise, allowing for the tailoring of grading envelopes to suit the specific stabilisation method.
Building on the laboratory findings from Zhalehjoo et al. (2024) on the stabilisation of recycled materials, this research (NACOE O24 project Year 3) recently advanced to field trials construction and monitoring to validate the laboratory testing results and increase practitioners' confidence in using recycled materials within the pavement stabilisation sector.
A field trial was constructed on the Brisbane Valley Highway, Queensland, under this NACOE project, to assess and monitor the performance of stabilised recycled materials in a real pavement structure. The project used recycled crushed concrete (TMR: Type 2.1) with lightly bound stabilisation application.
The in-situ stabilisation was carried out in July 2024, followed by various assessments including laboratory and field testing conducted before and after compaction. Long-term performance monitoring, including visual inspections and comparisons with the adjacent lane, will be undertaken in the future.
The project will focus on additional field trials, specifically plant-mixed stabilisation, in the coming years.
The research is expected to offer new insights into the suitability and optimisation of different recycled material blends for foamed bitumen and cement stabilisation. This work will help reduce reliance on finite quarried materials and aligns with broader goals of sustainability and enhanced resilience in road construction.
To support the implementation of recycled materials into road infrastructure, specifications need to be developed. Ideally, the industry needs to move towards performance-based specifications that are materials agnostic, provided the performance requirements are met.
Sometimes it is important to develop new specifications for the supply of recycled materials where controls on consistency of supply did not previously exist. This can provide confidence to contractors incorporating recycled materials as part of their product to utilise them more readily. In recent years NTRO has worked to develop specifications for the supply of recycled materials, such as for recycled crushed glass.
NTRO was engaged by Austroads to develop a specification for the supply of recycled crushed glass sand for use us in road infrastructure. This work was undertaken against the backdrop of the National Waste Policy (DAWE 2018) and the National Waste Policy Action Plan (DAWE 2019) with a view to reducing waste glass stockpiles. It looked at several end applications for recycled crushed glass including in bedding and backfill, embankments and earthworks, landscaping and as a fine aggregate replacement in concrete. It also considered other work that had been undertaken to look at the use of recycled crushed glass within unbound granular pavement layers and as a partial aggregate replacement in asphalt. As a result of the work, Austroads Technical Specification ATS3050 (2023) Supply of Recycled Crushed Glass Sand was developed. This specification covered quality system requirements, the glass source, contamination and requirements for its use as a granular material and within concrete. It developed a quality plan for ensuring consistency of supply and defined the testing and conformance requirements. This specification was supported by a technical basis report Development of a Specification for Recycled Crushed Glass as a sand aggregate replacement (2022b) and a guideline on Crushing, Processing and Cleaning of Recycled Crushed Glass for Transport Infrastructure (2022c).
Australian road agencies are increasingly incorporating recycled materials into road infrastructure to reduce reliance on finite resources, recognising the associated economic and environmental benefits. Opportunities exist to further enhance this approach, particularly by using recycled materials as host materials in stabilisation techniques including foamed bitumen and cement stabilisation. These methods offer a sustainable solution to improving the performance and resilience of road pavements.
This paper highlights the complexity surrounding the comprehensive evaluation of recycled materials for use in road infrastructure.
While laboratory testing and initial field trial have shown promise in using different recycled material blends for pavement stabilisation, further research, particularly additional field trials and long-term performance monitoring, is essential to build more confidence and maximise their use in transport infrastructure.
There is, therefore, an evident need to develop a carefully considered, fully comprehensive framework for the assessment and implementation of recycled materials in road infrastructure.
As well as development and implementation of performance-based specifications, one of the biggest challenges to implementing recycled materials is procurement. This is especially the case with recycled materials, as until they become ‘business as usual’, there is usually a cost premium associated with them. The development of sustainable procurement systems is seen as a way to overcome this hurdle by ensuring the process is tied to carbon emissions and not purely on cost.
Allied to sustainable procurement systems, a one-size fits all carbon accounting tool for Life Cycle Assessment (LCA) is required. Over the last few years, NTRO has been developing the sustainability assessment tool for pavements in collaboration with TMR and MRWA. If this tool can be released to a wider audience and gain traction, it could be linked with developing sustainable procurement systems, which in turn will encourage the uptake of recycled materials and sustainable solutions where long-term performance can be guaranteed.
In order to make the outputs from LCA tools more accurate, more data is required on long-term performance of different material types and a better understanding of the maintenance regimes required to get the best out of the road asset. With recycled materials implementation, long-term field trials and monitoring are seen not just a way of providing confidence to the road asset owners. They are also a pathway to develop the field performance data required to provide lifetime estimates and maintenance strategies associated with the operational use phase. This information can then be fed into LCA tools to improve the accuracy of the whole-of-life embodied emissions calculations.
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