It all began with a cup of coffee.
During regular visits to cafés around Melbourne Arul Arulrajah took note of what happened to the remnants of the ground coffee used to create his morning pick-me-up.
“I see the baristas throwing away the used coffee grounds and I think, ‘why not look at this as an engineering material?’” he says.
Prof. Arulrajah leads the geotechnical group in the university’s Centre for Sustainable Infrastructure at Swinburne University of Technology’s Hawthorn campus and has been investigating the use of recycled materials in road construction.
“I’ve done a lot of research on recycling materials for road construction and the different types of waste products that can potentially be used in pavements or geotechnical applications. Coffee just came about because I’m an avid coffee drinker,” Prof. Arulrajah tells Roads & Civil Works Magazine.
Using his own expertise in the area and embracing his love for coffee, Prof. Arulrajah and PhD candidate Teck-Ang Kua have been investigating the potential of used coffee grounds as a material in road construction, specifically subgrades.
Cafés in the local Hawthorn area contributed used coffee grounds to the research, which have been used extensively in early experimental phases to test the suitability of the organic waste as a component in the non-structural embankment fill material.
The material underwent extensive laboratory testing until Prof. Arulrajah and Mr. Kua came up with a suitable solution.
The coffee was dried in a 50 degree Celsius oven for five days, then sieved to filter out lumps. They then mixed seven parts coffee grounds with three parts slag, followed by a liquid alkaline solution to bind everything together.
The mixture was compressed into cylindrical blocks that proved strong enough to use as a subgrade material due to its high organic content, low maximum dry densities and high optimum moisture content.
“We’ve got it up to the level where it compares to other road binder materials. We’re moving into tests to simulate how the material would behave under true traffic conditions.
The question now is: what do we have to do to make sure it doesn’t break down over time?”
He explains that the type of coffee bean, whether it is from Africa or South America, didn’t affect the results as there were little to no variants between the different types from an engineering material perspective.
Prof. Arulrajah admits that recycling coffee grounds into road construction is a “novel” idea and one he has not seen or heard of before. However, it is a sustainable concept that he is passionate about.
“On average, the cafés we collect from dispose of about 150 kilograms of coffee grounds per week. We estimate that the coffee grounds from Melbourne’s cafés could be used to build five kilometres of road per year. This would reduce landfill and the demand for virgin quarry materials,” he says.
“This subgrade has very low carbon properties because there’s no cement in there. It’s a fairly green solution and that’s another angle I’m looking at in terms of its use.”
Prof. Arulrajah says that with the number of cafés already in Melbourne, and more opening regularly, it means that there are always large amounts of used coffee that can be diverted from landfill.
He adds that there is a lot of potential to take the research further, and he is exploring options.
“Right now, we’re just looking at subgrades, but we have a few ideas within the road construction space down the track,” he says.
“What we can do is tick all the boxes from an experimental point of view, but I hope to possibly get a commercial entity on board.”
The research is a collaboration with Suranaree University of Technology, Thailand; Southeast University, Nanjng, China; and Shanghai Jiao Tong University, China. The results were published in the journal Construction and Building Materials.
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