With the importance of sustainable practices ever growing, the Australian flexible Pavement Association (AfPA) have endorsed a five-year strategy to improve environmental and health and safety standards in Australia’s paving sector through the reduction and removal of hydrocarbon cutters in spray sealing operations.
Australia produces an estimated 510.1 million tonnes of greenhouse gas (GHG) emissions per year, according to the federal government’s most recent quarterly report. There is an increasing push to adopt alternate paving technologies that have the potential to reduce environmental impacts.
The Australian flexible Pavement Association (AfPA) have continuously championed sustainability in the flexible paving industry, including solutions such as the use of reclaimed asphalt in new asphalt pavements and the use of crumb rubber and secondary materials as alternative pavement component.
Energy savings have also been targeted by the AfPA through recommending the reduction of asphalt manufacturing temperature. What these initiatives aim to achieve is to reduce the impact the flexible pavement industry has on the environment while shifting towards a circular economy.
To understand the strategy and the importance of reducing the use of bitumen cutters, it’s important to consider the two types of flexible roads used in Australia: sprayed bituminous seals and asphalt.
Current Flexible Roads
According to Anna D’Angelo, AfPA Director of Technology and Leadership, asphalt makes up less that 20 per cent of all pavements surfacing across Australian roads with spray sealing accounting for the remaining 80 per cent.
“Spray sealing is a low cost and sustainable surfacing option, considering Australia’s large size as a country and low traffic density,” says D’Angelo. “Approximately 340,000 kilometres of Australia’s surfaced road network have been sealed by this method with 10 to 15 per cent annually resealed.”
Unlike most international spray sealing practices, Australia still uses hydrocarbon cutters, such as kerosene, to cutback hot binders when constructing road spray seals.
These cutters are primarily used to change the viscosity of the bitumen temporarily to facilitate spraying and promote initial wetting and effective adhesion of the aggregate. These cutters evaporate into the environment in the months after spraying.
“When and how these cutters should be used is currently mandated in the contract specifications of the state road authorities,” says D’Angelo. “However, the practice of cutting back bitumen presents safety and environmental concerns.”
The strategy endorsed by the AfPA board focuses on reducing the use of cutters in spray sealing while investing in alternative products like bitumen emulsion or technologies which can allow the elimination of cutters.
This endorsement is part of the AFPA strategic objectives to support industry to continuously improve the health and safety of people, promote sustainability and introduce best practice and continual improvement.
“The strategy aligns with AfPA’s vision to support a healthy and safe industry while being adaptive to change,” says D’Angelo. “Through introducing this strategy, we are doing both, and that’s really how this strategy came around. We wanted to improve working conditions, safety and sustainability.”
The problems posed by the use of kerosene-type hydrocarbons, according to D’Angelo, is related to the products’ low flash point: cutting back bitumen for sprayed sealing purposes requires managing the addition of this product into bulk bitumen at high temperatures. Due to these high temperatures, there are risks of fire or explosion due to hot volatile vapours.
The hot nature of the bitumen itself, poses the risk of burns to users if an accident should occur.
“And from a sustainable and environmental point of view, the emissions released contribute to the generation of GHG and generate photochemical smog,” says D’Angelo.
As part of research for sustainable alternatives to the use of cutback bitumen, D’Angelo organised a webinar in March 2021.
The webinar shared the practices and trends within the construction industry in other countries such as UK, France, Mexico and South Africa, that showed the commitment worldwide to reduce the carbon footprint.
Several sustainable technologies to improve spray seals construction practises were presented, such as the utilisation of synchronised spray/spreader machines that allow instantaneous wetting of the aggregate with the hot binder and the use of breaking agents when using bitumen emulsion to accelerate the curing.
Among the different solutions available, D’Angelo believes that bitumen emulsion represents a strong opportunity in the Australian market due to its potential to reduce both safety and environmental hazards, through the reduction of energy consumption and GHG while removing hot bitumen from the process.
According to D’Angelo, the last ten years have also seen an increase in international markets in the use of specialised emulsions such as polymer modified emulsion (PME) and thixotropic emulsion that offer improved performance when compared to traditional emulsion.
“With the styrene-butadiene-styrene (SBS) polymer, you increase the elasticity and performance of the bitumen and extend the range of operating temperatures,” explains D’Angelo. “And the adoption of PME not only offers the opportunity to move away from the use of cutter but presents a more sustainable, safer solution while creating durable roads.”
“For example, in the UK in the last ten years they have completely switched to the use of PME,” cites D’Angelo. “This product is sprayed cold at 80 or 90 degrees without the need of cutters, and it can be used on high traffic roads ensuring high performance and durability.
In addition to spray seal operations, bitumen emulsion also offers the advantage of preserving natural resources through recycling and use of cold mixes techniques that further reduce the carbon footprint.
Achieving the Strategy
The first steps in the strategy have already been taken, according to Kevin McCullough, AfPA board member and chair of National Technology and Leadership committee.
“We’ve been discussing with Asset owners’ top management on the need to reduce cutter use and are expecting technical specifications to follow,” says McCullough. “And we support the provision of polymer modified emulsion in urban areas as this is the short-term solution to the challenges of the current practice.”
Changes to cutting bitumen will be targeted firstly at urban areas due to the density of population.
“If you consider the hazards and emissions associated with the use of cutters, you’d want to start by selecting safer technologies to use in highly populated areas,” says D’Angelo. “So as an industry we want to improve safety for people and to reduce our impact on large-scale.”
This story originally appeared in the June 2021 edition of our magazine. To read the magazine, click here.