Learn about the applications and specifications of common recycled products with proven performance in road networks, including:
Crumb rubber is recycled rubber produced from end-of-life truck and car tyres. According to the SA EPA Guideline 183/10 Waste Tyres, end-of-life tyres can cause issues in landfills due to their relatively low-density ratio when stacked.
During the recycling process, steel and textiles are removed and the crumb rubber is formed following a series of mechanical grinding or chemical cryogenic cracking. The particle size of the crumb rubber can be adjusted depending on its intended application. A major benefit of mechanical processing is the retention of the elastomer’s properties as the polymers within the particles remain largely intact. As such, they have been identified as a source of high-value waste products which can be used within the context of circular economies.
Content in roads
For both sprayed seals and asphalt applications, crumb rubber is typically added in quantities of 10 to 20 wt.% of the binder via the wet method or up to 2.5 to 3% by mass of mixture via a dry method.
The potential applications of end-of-life tyres in road and transport infrastructure are:
- asphalt and spray seals
- embankment fill
- slope stabilisation
- stabilisation of expansive soils
- crumb rubber concrete
- drainage backfill material
- footpaths, tree protection zones, car parks, driveways
- ports, riverbank, and coastal stabilisation.
In Australia, many states and territories have developed specifications for crumb rubber used in sprayed seals and more recently asphalt mixes. Furthermore, other organisations such as Austroads and the Australian Flexible Pavements Association have developed guidelines and/or specifications on their use. See a summary of relevant guidance documents (Table 4.1).
Plastics are typically high molecular weight organic materials comprised of long chains of carbon, hydrogen, and traces of other compounds. Most plastics are derived from petrochemicals, while many, such as natural rubber, silk, and cellulose, are natural. Some plastics can be mechanically, chemically, or biologically recycled into new products.
Content in roads
Recycled plastics may be added in concentrations around 0.2 to 0.3% by mass of the asphalt mix via the wet process or around 1% by mass of the asphalt via the dry process.
Potential applications of recycled plastics in road and transport infrastructure include:
- ancillary and aesthetics
- shared user paths
- geogrids and geotextiles
- recycled plastic pipes
- concrete (as fibre reinforcement and fine aggregate replacement)
- plastic derived products
- within asphalt and spray seals.
There are currently no specifications for the use of recycled plastics in asphalt.
Recycled crushed glass
Recycled crushed glass is a mixture of different colours and types of glass derived from consumer and construction and demolition waste streams. The glass is collected, sometimes cleaned, and crushed into fine or coarse particles, depending on its intended application.
The higher quality recycled crushed glass is used for packaging, while the remaining can be used as filter media for water quality projects, abrasive material in sandblasting of ships, or road base and sub-base applications.
Content in roads
SA does not have a specification for recycled crushed glass in pavement applications. In other states and territories in Australia however, up to 20 wt.% may be used in granular pavements, a content which reaches up to 30 wt.% overseas. Asphalt applications (excluding surfacing) may contain up to 10 wt.% in Australia, and up to 15 wt.% overseas. For asphalt surfacing, different Australian authorities allow for up to 5 wt.% of recycled crushed glass.
The potential applications of recycled crushed glass in road infrastructure are:
- embankment and structural fill
- subgrade material
- drainage layers, backfill and bedding material
- asphalt and granular materials.
Table 4.4 summarises the proposed upper limits for the incorporation of recycled crushed glass in various pavement applications, as specified by Australian road agencies.
Granular and asphalt materials testing and evaluation
Recycled crushed glass should meet the aggregate testing and standard requirements to be considered for the replacement of virgin materials. It can be assessed through the methods listed in Table 4.10. In addition, it should be free from ceramics, cathode ray tubes, fluorescent light fittings, and laboratory glassware.
Austroads (2022) provides a summary of the minimum requirements for partial aggregate replacement with recycled crushed glass in various transport applications referencing relevant Australian, New Zealand, and other international specifications.