Austroads has published ground-breaking work that enables the testing of emissions and the release of microplastics from asphalt incorporating recycled plastics.
The new testing framework is published in the second report from a project led by RMIT University which is investigating the use of recycled plastics in asphalt road applications.
The report provides a comprehensive overview of the initial development of various testing frameworks for the characterisation of road-grade recycled plastic, the incorporation of recycled plastics in bitumen and asphalt via various methods (wet, dry, and mixed), and the study of possible emissions and microplastics release.
The recycled plastic selection process used eight criteria of blendability, purity, recycling contamination, processability, storage stability, solubility, polarity and presence of hazardous additives to characterise 31 recycled plastic samples and seven virgin plastics as references for a total of more than 900 tests.
The characterisation and ranking process identified four groups of plastics for further investigation; two plastics that could be used in bitumen in a wet method (a blend of post-consumer LDPE/LLDPE and PP and a post-industrial LDPE/LLDPE source) and two that could be used in a dry method as a replacement for natural aggregates in asphalt (an ABS and PET source).
The preliminary wet method testing outcomes suggest that the plastic-modified bitumen slightly improves the physical and rheological performance of neat bitumen at low recycled plastic content, e.g. 2% (or less) by weight of the binder. A greater content of recycled plastic in bitumen has the potential to achieve superior performance but requires the addition of compatibilisers to compensate for storage stability issues.
A higher proportion of recycled plastics can be added into asphalt mixes using the dry method in comparison to the wet method and asphalt plants are likely to be able to handle recycled plastic aggregates without adjustments.
Early dry method testing outcomes found the shape and size of the plastic particles impacted on the volumetrics and compactability of the asphalt. Compactability was also influenced by the amount of recycled plastic used.
Further testing is underway to study the viscoelastic behaviour of the plastic-modified asphalt mixes, moisture susceptibility, cracking tolerance, fatigue and rutting resistance. Results will be compared against those measured for a conventional hot mix asphalt and polymer-modified asphalt.
The report also provides new testing frameworks to measure emissions and microplastics from bitumen and asphalt.
A methodology to evaluate emissions of 48 volatile organic compounds and 16 polycyclic aromatic hydrocarbons was developed for asphalt and bitumen samples. The testing framework coupled with industry-standard analytical methods produced a reliable outcome of 95 per cent recovery rate via the spike recovery method.
A new methodology was developed by adapting existing industry testing standards to assess the release of microplastics from plastic-modified asphalt by providing abrasion to asphalt samples in a controlled environment followed by a cutting-edge microplastic extraction and characterisation procedure.
The microplastic extraction procedure was optimised to successfully separate microplastics from bitumen and aggregates residues using appropriate organic solvents and separation techniques. The efficacy of this method to quantify the mass of microplastics released per surface area of tested asphalt and their size distribution was validated by fluorescence microscopy analysis.
The laboratory conditions ensuring a controlled abrasion of asphalt are currently being optimised and will be applied, in conjunction with the microplastics extraction and characterisation procedure, to different types of plastic-modified asphalt made via the wet, dry and mixed methods. Comparisons will be made with standard hot mix asphalt and polymer-modified asphalt.
Austroads expects to release the next report from the project in early 2022.
To download the report, click here.