Whilst crumb rubber asphalt products are available in Australia, they are not widely adopted by road owners for resurfacing existing pavements or constructing new pavements. A key concern leading to this low acceptance is the lack of credible evidence demonstrating that crumb rubber asphalt can meet the performance standard required by road owners.
In 2018-19 an estimated 14% of the 450,000 tonnes of waste tyres generated annually in Australia were used productively in domestic end markets; 55% were recovered via exporting shredded or whole tyres to offshore markets; and the remaining 31% went unrecovered (Department of Agriculture, Water and the Environment, n.d.). Apart from being a source of health and environmental concern, dumped and landfilled tyres also represent a loss of potentially valuable resources, as scientists have discovered many ways of modifying end-of-life (EOL) tyres for reuse. This case-in-point involves transforming EOL tyres into crumb rubber and incorporating this into asphalt mixtures for constructing pavements.
Overseas studies have demonstrated that recycling EOL tyres as asphalt material not only ameliorates an environmental problem but also, in some cases, creates no-regret benefits through pavement performance improvements. Specifically, pavements built with crumb rubber asphalt are found to produce lower road noise and exhibit higher durability, especially in a hot climate, than conventional asphalt pavements (Losa et al, 2012 and Khan et al, 2016).
Whilst crumb rubber asphalt products are available in Australia, they are not widely adopted by road owners for resurfacing existing pavements or constructing new pavements. A key concern leading to this low acceptance is the lack of credible evidence demonstrating that crumb rubber asphalt can meet the performance standard required by road owners.
Field trials – a well-accepted approach for establishing engineering requirements and performance properties of innovative pavement materials under realistic conditions – have been undertaken for crumb rubber asphalt products at several locations in Queensland and South Australia to address such uncertainties. However, these have so far been on either rural roads (Queensland) or council-owned roads (South Australia) that experience relatively light traffic. Consequently, a gap remains in the knowledge of how crumb rubber asphalt performs under heavier traffic conditions.
The crumb rubber asphalt demonstration trial (referred to as ‘the trial’ hereafter) was one such field trial designed to address this knowledge gap. It was a collaborative effort organised by Tyre Stewardship Australia (TSA), Department of Transport (DoT) Victoria and the Australian Road Research Board (ARRB). The trial was organised so that the performance of crumb rubber asphalts could be assessed in the field under real traffic and climatic conditions, compared to other asphalts under standard testing conditions, and to characterise their material properties in a laboratory. Importantly, this project represented a rare opportunity for crumb rubber asphalt suppliers to have their products independently evaluated.
The primary feature of the trial was the type of road selected for testing the crumb rubber asphalt: a state road located in metropolitan Melbourne with an annual average daily traffic (AADT) of 19,000 vehicles (6% commercial vehicles). Results obtained from this project have been used to help inform DoT of the capabilities of crumb rubber asphalts under comparable traffic conditions and will assist in generating the required information for them to be included in specifications for wider use.
Another noteworthy aspect of the trial was its highly collaborative approach, enabled by a wide range of stakeholders in the spaces of product development, environment and infrastructure. This collaborative approach lowered the project risk inherent in implementing new pavement products for individual stakeholders and enabled fast dissemination of project knowledge across the industry, governments and research bodies.
Specifically, major contributors towards the trial and their respective roles were:
The trial site is a 1.5 km section of the East Boundary Road within the suburb of East Bentleigh in Victoria, between Centre Road and South Road (Figure 1). East Boundary Road features a parking lane and two through lanes in each direction, separated by a median. The trial encompasses all three lanes of the southbound direction of East Boundary Road. This section is straight and flat.
The trial had several unique features. First, it was based on not just one type of crumb rubber asphalt product, but four supplied by three manufacturers. Having crumb rubber pavement products vary both within and between suppliers enabled the trial results to reveal which performance characteristics are common across all crumb rubber options and which are specific to brands or products.
The construction of the trial site was completed in March 2020. The 1.5 km trial site was divided into six sections and each section was paved with a different asphalt product. Four of the six sections were paved with crumb rubber asphalts and the remaining two sections were paved with two types of conventional asphalt products as the control group. Table 1 lists the specific asphalt product laid in each of the six sections.
The primary objective of the trial was to demonstrate the capability of the selected crumb rubber asphalt products under real traffic and climatic conditions. Data required to support this objective was collected in a two-year site monitoring program that ended in June 2022. Specifically, surface performance data measured in levels of cracking, roughness, rutting, texture and skid resistance were collected every 6 months by the ARRB Network Survey Vehicle (NSV) vehicles.
Figure 2 provides an example of the cracking data collected one month after the construction of the trial site, showing that cracking was consistent throughout the site prior to the trial and that the new surface eradicated the surface cracking.
The trial also included environmental emission monitoring during the construction phase of trial roads. The monitoring was designed to measure the fuming exposure experienced by pavement installers. The emission data collected provided a relative measure on the fuming hazard associated with the four crumb rubber asphalt products. Specifically, monitored hazardous emissions included volatile organic compounds, benzothiazole, total suspended solids, bitumen fumes and polycyclic aromatic hydrocarbons.
To assess the potential fuming exposure from crumb rubber asphalt mixes, emissions monitoring was undertaken by personal sampling for several airborne contaminants in the breathing zones of operators involved in the paving, including paving operators, screed operators and rake hands.
The monitoring devices were assembled into small backpacks worn for the extent of the paving operations by the nominated members of the paving crew (Figure 3). The devices collected samples for all the asphalts paved, both crumb rubber asphalts and control mixes, allowing comparison.
The results of the monitoring were compared against established workplace exposure standards and guidelines, such as those published by SafeWork Australia and the American Conference of Governmental Industrial Hygienists (ACGIH), as applicable.
The trial also included laboratory performance tests, which took place during construction of the pavement. The purpose of the laboratory tests was to establish an expected performance benchmark, albeit under laboratory conditions. Since real road conditions cannot be perfectly replicated in a laboratory, the comparison between laboratory test results and in-field performance data can be used to highlight any difference between expected and actual performance for crumb rubber asphalt products. Specifically, the three types of testing undertaken for the laboratory testing program were binder testing, plant mix testing and core testing.
For binder testing, samples of all bituminous binders used in the crumb rubber asphalt trial, including control mixes, were collected at the asphalt mixing plant to check their adherence to their relevant specifications.
For plant mix testing, samples of each trial asphalt mix were collected at the plants from the delivery truck at the time of production, as they were being prepared for delivery (Figure 4). The samples were prepared and tested in the laboratory to verify the manufacturer-supplied designs for bulk density, average maximum density, and air voids by either the Marshall method or Gyratory method.
For core testing, asphalt cores were collected (Figure 5) from the paved asphalt sections to verify layer thickness and test the strength capabilities in situ.
After the trial construction and laboratory assessment were completed, the remaining project activities consisted of site monitoring. This was undertaken frequently by the ARRB NSV and DoT SCRIM vehicles over the first two years of life of the trial sections, with results detailed in the final project report issued in June 2022.
Assessing the inherent properties of the crumb rubber asphalt mixes in the laboratory, and their performance in the field, informs the Department of Transport as to how the mixes may be implemented in its specifications. Incorporating these mixes into the specifications will allow their use alongside and potentially in place of traditional mixes, to maximise the use of innovative recycled materials and improve the sustainability of road building.
Beyond crumb rubber, other recycled waste materials can be used as road construction materials, such as plastics and glass. The processes and outcomes of the trial may inform a framework for implementing the use of these recycled and innovative materials not included in traditional specifications.
Experiences gained in the topics of project management, design, construction, performance monitoring and data analysis from this trial are expected to be valuable and transferable to the trial of other innovative pavement technologies that promote sustainability in the circular economy.
Of particular relevance to this trial was the use of recycled plastic in asphalt mixes. Crumb rubber and recycled plastics are similar in chemical structure as they are both polymers. Waste plastic is a source of another environmental crisis Australia and the world are currently facing. Success in this trial of crumb rubber road can potentially expedite the investigation on the viability of recycled plastic modified roads under a range of traffic conditions.
