Embedding sustainability in paving

SAMI Bitumen technologies’ SAMIbond 007 trackless bond coat was successfully used in Western Australia’s Solomon Airport.

With a view on improving sustainability in the paving sector, Roads & Infrastructure looks at some of the latest solutions on offer, from the discussions at the AfPA 2021 International Pavements Symposium.

Australia still has over half a million kilometres of unpaved roads and over 350,000 kilometres of paved roads which will require ongoing maintenance – that’s why it’s important the pavements sector continues to optimise the materials used in roads and refines the production methods to deliver safer and more sustainable solutions to the roads network and wider communities.

Sustainable paving solutions were the focal points of discussions at the Australian flexible Pavement Association’s AfPA 2021 Symposium in August. From bio-sourced additives for binder improvement to reviving graded seals with lower environmental impact binders, a range of solutions were presented to help achieve more sustainability in the pavement sector.

Trackless bond coats

Iulian Man, Technical Service Manager at SAMI Bitumen Technologies talked about how trackless bond coats can provide longer lasting asphalt pavements.

A tack or bond coat is a very light application of bitumen emulsion used to promote the bond between the existing surface and the new asphalt application. Conventionally in Australia, the standard CRS60 has been used as a tack coat.

SAMI Bitumen has developed a trackless polymer modified tack coat emulsion, the SAMIbond 007, which improves bond strength between asphalt layers, allowing the pavement to act as a monolithic structure under very high stresses.

Due to its good bonding characteristics, SAMIbond 007 is expressly designed to withstand the high shear stresses that develop under aircraft traffic loadings. It has been successfully used in Western Australia’s Solomon Airport and on the main runway at Sydney Airport.

Road trial at Mamre Road in New South Wales’ Mount Vernon has proven a quicker curing time and higher shear strength of the SAMIbond 007 compared to the conventional CRS60, with the emulsion showing a track-free behaviour 40 minutes after spraying.

Encapsulated biochar as bitumen

Alexandru Let, Technical Manager at State Asphalt NSW, talked about the properties of asphalt with binders containing various percentages of plastic encapsulated biochar.

Biochar is a carbon rich material produced from the slow pyrolysis (heating in absence of oxygen) of any biomass, where all of the cellulose, lignin and other non-carbon materials gasify and are burned away.

According to Let, biochar has already been used as part of the mix design in a collaboration with the Austrian Road Authority, where biochar was encapsulated within recyclable plastic to prevent any dust inhalation of the biochar. The plasticised biochar was then added to the asphalt mix in a dry mix process.

State Asphalts NSW has conducted lab trials of modified bitumen samples by mixing base bitumen with plastic encapsulated biochar and asphalt mix by dry and wet procedure. Due to its carbon nature and morphology, the plastic encapsulated biochar has shown to have a twofold impact on the properties of bitumen, namely: it acts as an antioxidant thus retarding the ageing properties of bitumen and, secondly, it stiffens the bitumen by increasing its rheological (flow) properties.

State Asphalts NSW’s initial lab testings have shown that biochar acts as asphaltenes when added to soft bitumen. They also noted that plasticised biochar can be used as additive for future bio-bitumen (non-fossil bitumen). Further trials are underway to compare the lab results with trial results.

Modified graded seals

Trevor Distin, Technical and Marketing Manager at COLAS Australia, offered a presentation on the concept of surfacing in-situ gravel with modified graded seal to provide an all-weather cost-effective surfacing.

Australia has an estimate 877,000 kilometres formal road network of which only 350,000 kilometres are surfaced roads. The cost of upgrading unsurfaced roads to the same standard as surfaced roads is often financially unaffordable. This situation results in rural councils having to spend large amounts of their limited funds on ongoing grading and re-gravelling of their unsurfaced road network.

COLAS and has been surfacing gravel roads with graded seals in Southern Africa since the 1990’s and more recently in Australia since 2018. As Distin explained, modified graded seals require less base course preparation and are more tolerant of unevenness in the base surface compared to traditional seals. They are also more forgiving of base defects due to the self-healing nature of the binder.

COLAS has used modified graded seals on rural roads in New South Wales and Victoria with positive outcomes. In one project, the Western Quarries Road in Ararat, Victoria, the COLAS graded seal has been in service for two years now and has performed very well.

“This quarry carries heavy trucks laden with crushed material and [the haul roads] has stood very well to the braking and turning movements of these vehicles in this period,” Distin said.

“We just completed a 22-kilometre section where we surfaced a road in the foothills of the Gibraltar Mountain range in Northern New South Wales. This area was badly savaged by fires and this community has limited access because of the nature of the terrain and this has been successfully completed,” he added.

The next challenge for COLAS is to replace the hot cutback binder with emulsion.

“We have developed the emulsion and we are planning trials in the next month or so to prove this technology,” Distin said.

Embedding sustainability in the procurement of roads

While the solutions presented at the AfPA Symposium each offered sustainable uses of material in road pavements, embedding sustainability in the procurement of roads is a more fundamental way in which the sector can enhance its sustainability levels.

This is where associations and organisations such as the Infrastructure Sustainability Council (ISC) and the Australian Road Research Board (ARRB) are working actively with the industry to ensure just that.

ISC’s Infrastructure Sustainability Rating (IS rating) Scheme helps measure sustainability performance as a procurement requirement or as a condition of approval for project contractors. It does so by evaluating infrastructure projects against performance criteria in the quadruple matrix of governance, economic, social and environmental outcomes across the asset lifecycle.

More importantly, the IS Rating Scheme matches all of these different performance criteria with the UN Sustainable Development Goals, so project owners can know not only how an asset is performing, but also how it’s contributing to the global goals.

ARRB, in collaboration with Main Roads Western Australia and the Queensland Department of Transport and Main Roads (TMR), is developing a user-friendly Sustainability Assessment Tool (SAT) to calculate lifecycle greenhouse gas emissions and lifecycle cost benefits of innovative road pavements designs and rehabilitation treatments. A key focus of the new tool is to enable the emissions quantification of pavements using innovative materials (i.e. recycled) designs (e.g. crumb rubber asphalt), and processes (e.g. warm-mixes and in-situ stabilisation).

Once complete, the project will enable Main Roads WA, TMR and their partners to quantify and compare lifecycle sustainability and economic impacts of innovative pavements consistently and reliably.

This article is part of a series of articles covering the discussions happening around the paving industry, with reference to presentations on AfPA’s 2021 Symposium.

This article originally appeared in the October edition of our magazine. To read the full magazine, click here.