A Monash research project has highlighted the possibilities of using microwave energy to dry out unbound aggregate layers in road building to strengthen roads against climate impacts
A research team from Monash University’s Engineering department has been investigating how to improve the resilience of roads by manually drying aggregate layers before sealing.
Dr Gopoojithaa Athmarajah, Dr Amir Tophel, ARC Laureate Professor Prof Jeffrey Walker and Professor Jayantha Kodikara undertook lab tests on unbound granular materials (UGM) to explore how microwave energy, used in conjunction with hot airflow, could pave the way to building a drying technology for road construction companies.
Earthmovers & Excavators spoke with Professor Kodikara to find out why managing the moisture in aggregate is a key focus in road building research.
Subsurface strength
The Smart Pavements Australia Research Collaboration Hub (SPARC Hub) was set up with a government grant to lead university research into improving materials, designs and technologies that will make pavements safer, longer lasting and more economical.
Professor Kodikara, a SPARC Hub director, says the focus is to facilitate collaboration between universities and industry bodies to address issues facing Australian roads, particularly around how to make them more resilient in a changing climate and how to leverage the benefits of digitalisation.
This recent research project, looking at microwave convective drying of UGM, was proposed by CIMIC Group’s technical arm EIC Activities. The aim was to see if it is possible to more accurately dry loose aggregate to an optimal moisture level in order to achieve optimal compaction. With road contractors currently having to battle with the weather and whatever drying that can be achieved by the sun, a more accurate and efficient method is desirable.
“During road construction or rehabilitation, construction businesses use unbound crushed rock layers as the road bed, and on top of that is the sealed layer of asphalt,” Kodikara explains.
“This foundation layer has a significant influence on how much stress is transferred to the underlying soil.
“In some areas there could be good quality crushed rocks, but in more remote areas locally sourced marginal materials might be used.
“Eventually, depending on the quality of the road base, damage and deformation occurs, affecting the road surface. These are very moisture sensitive materials, so moisture needs to be reduced in order to have a good road and a pothole-free surface.”
If there is too much moisture saturation of the aggregate, he adds, this makes the road bed more vulnerable to deformation and failure. So, the ideal scenario is to dry the road bed until the moisture content is suitable for sealing it. However, this is easier said than done.
“If you’re relying on the sun to dry the road bed, it may not be always available, and humidity is also climate-driven and seasonal,” Kodikara says.
“This gives rise to delays and reduced productivity. Therefore, we started looking at alternative methods, including the possibility of using microwave drying.”
In the lab
To test whether microwave energy would be a feasible solution for road base drying, the team built a test bed in the lab. Here, they explored how a combination of microwave drying and hot air affects the moisture content with the potential end aim being the creation of a drying unit for road building.
“Some microwave drying is already being used in on asphalt, healing cracks and so on, in some countries, but it’s not used for road building itself,” Kodikara says.
“It was a challenging research project because we are geotechnical engineers and pavement engineers, and we had to use microwave theory and knowledge to design the experiment.
“With the budget available, we decided to use a combination microwave oven and direct it through a cone antenna onto a soil surface contained in a box. We used different classes of compacted unbound materials and tested at different moisture contents the effectiveness of the microwave. We used a single antenna, but, in the field, using an array of antennas would be more likely.
“We also tried, in addition to using microwave energy, using a hot air gun to facilitate the removal of moisture. The idea here was like a hairdryer, where air and heat are both used to dry the hair more effectively. The microwave heats the material but doesn’t remove the water vapour, so we tested a combination of microwave energy and blowing hot air across the surface.”
At the scale used, the experiment had limitations around how deep a drying effect could be achieved, but Kodikara says it has provided useful data for future research.
“With the conditions we were using we found that we couldn’t dry to a significant depth, only about 50mm, and we are talking about layers that are 100 to 120mm thick,” he says.
“The drying was also too localised and the amount of energy that might be needed is another consideration. However, we developed the theoretical aspects and used a physics-based AI model to predict the outcomes at a scaled-up level.
“At this point, we are discussing funding for further research with different parties because increasing the productivity and resilience of our roads is very important in the face of climate effects. In Australia, we use a relatively thin asphalt layer compared to other countries.
“Therefore, that asphalt layer very much depends on the support received from the underlying foundation. We propose that Australia moves towards using a thicker layer of asphalt, despite the cost, because it is needed to limit the moisture ingress and withstand our changing climate. This way, the asphalt seal and the foundation layer work in harmony to provide long-lasting resilience for the pavements.”
Overall, he says that the SPARC Hub’s work is important in tackling issues around improving roads that have been “put into the ‘too hard’ basket for 30/40 years due to the overall complexity of scientific processes involved in a pavement system”. Kodikara presented these findings to the Federal Parliamentary Committee on Resilient Roads Inquiry in 2023.
“With testing, design, construction and condition assessment, currently these are all considered as disparate processes, and we need greater connectivity to get the benefits of the impending digitalisation,” Kodikara says.
“So, we’re developing technologies and methodologies to integrate these by exploring digitalisation and smart technology to achieve greater productivity and resilience.”
For more information on SPARC Hub and its projects, visit: sparchub.org.au
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