Denmark is running out of raw materials. Three students point to a possible solution
The shortage of raw materials is one of the construction sector’s biggest challenges. Now, a new bachelor’s project shows that the shape of sand particles may play an important role in the roads of the future – and the findings are attracting considerable interest in the industry.
Stone and gravel are becoming scarce in Denmark. This is a problem for the construction industry, which uses these materials in stabilised base course – the layer beneath the asphalt.
Both authorities and companies are therefore looking for new and better ways to make use of the raw materials already available.
And perhaps the answer lies in sand.
Three newly graduated civil engineering students from the Department of Civil and Architectural Engineering, Nikolaj Dybdal Storm, Rasmus Rask Hansen and Karl Sandvad Sønderskov, have investigated whether the shape of sand particles affects the strength of stabilised base course.
“Many gravel pits today contain far more sand than stone and gravel. This means that large quantities of sand accumulate without being used,” says Nikolaj Dybdal Storm.
“If we can find a way to use the sand – for example, by increasing its proportion in stabilised base course – we can make smarter use of our national raw materials and remain self-sufficient for longer.”
Otherwise, the industry may eventually have to import large quantities of stone and gravel from abroad as domestic resources run out – with both economic and climate-related costs.
The project has therefore attracted considerable interest in the industry.
“Resource scarcity is an urgent challenge today. There is a lot of money at stake, and that is why there is considerable interest in projects of this kind,” says Kristian Rosted Brødbæk, Head of Business Development at GEO.
The key lies in the angularity of the sand particles
Today, regulations for stabilised base course specify the quantities of gravel, stone and sand that the mixture must contain.
There are also requirements for the angularity of the stones – but not for the sand. The three civil engineering graduates therefore investigated whether the angularity of sand particles affects the bearing capacity of stabilised base course.
In the laboratory, they produced different stabilised base course mixtures in which the only significant difference was the angularity of the sand particles.
“Our results showed, among other things, that the most angular sand particles produced the highest bearing capacity,” says Karl Sandvad Sønderskov.
The explanation is that angular sand particles interlock better than more rounded particles and can therefore achieve a higher bearing capacity.
The project thus points to untapped potential in the shape of sand particles.
“There are exciting perspectives in the results the students have achieved. This is important knowledge and really, really strong work,” says Kristian Rosted Brødbæk.
The report shows that the relationship is not entirely linear, but that the angularity of the sand is an important parameter for the bearing capacity of the stabilised base course mixture as a whole.
The results have also impressed Hans Christian Korsgaard, Chief Consultant at Pavement Experts and external supervisor on the project.
“Angular sand particles appear to improve the bearing capacity of stabilised base course. This means that more sand can be used without compromising quality. That is surprising and really interesting,” he says.
“Foolish” use of resources today
Another challenge is that the same basic recipe for stabilised base course is used regardless of whether a motorway, residential road, cycle path or pavement is being constructed.
And that is foolish, the three newly graduated engineers argue.
“It’s like building your child’s playhouse out of high-strength concrete. It’s total overkill,” says Rasmus Rask Hansen.
Using the same recipe everywhere causes raw materials to run out faster, which is why Kirstine Skov Nielsen also questions the current regulations.
She is a raw materials planner at Central Denmark Region and project manager for the Raw Materials Initiative.
“The requirements for stabilised base course were developed in 1977, but we’ve also been baking bread since 1977 – and that recipe has been revised hundreds of times since then. There has been no development in this area when it comes to stabilised base course, and that doesn’t make sense,” she says.
The bachelor’s project will not change the regulations overnight. But Kirstine Skov Nielsen believes the project contributes knowledge that is needed to challenge the current standards and represents a step in the right direction.
“Change requires documentation. These young people have delved deeply into an issue that has been underexplored for a long time and have contributed new knowledge,” she says.
The Raw Materials Initiative hopes to challenge the way we construct pavements, cycle paths and roads today. Can we do it more intelligently and sustainably? she asks rhetorically.
“We can. The students show that the angularity of the raw materials is an important parameter for the quality of stabilised base course,” she says.
“If rounded materials are also to be used more effectively in the future, we do not necessarily need to use the same high-quality materials in the stabilised base course for a pavement as for a motorway. We need to move away from that as part of a more sustainable use of Denmark’s raw materials.”
If climate and resource considerations are given greater political priority in the future, the results could provide a stepping stone towards developing the current requirements for base layers – and, in the long term, contribute to better use of Denmark’s limited raw material resources.
What is stabilised base course – and why is it important?
Stabilised base course consists of stone, gravel and sand. It lies beneath the asphalt and keeps roads stable and able to withstand heavy traffic for many years.
High bearing capacity is essential to prevent roads from deteriorating over time or under heavy loads.
Today, we use the same “recipe” for stabilised base course on both cycle paths and motorways – but if the composition were adjusted to the type of road, scarce resources could be reserved for, for example, railways and airports, where the loads are greatest.
Denmark has long been self-sufficient in raw materials for road construction, but today, coarse stone and gravel particles are becoming increasingly difficult to source locally.
When raw materials have to be transported over longer distances or imported, both costs and CO₂ emissions increase.
That is why authorities, companies and the industry are working to make better use of Denmark’s raw materials – so that local resources will last longer.
What did the laboratory tests show?
They tested three stabilised base course mixtures, where the angularity of the sand was the primary difference. Other factors that also affect bearing capacity were controlled for, including particle strength, particle size distribution, water content, fines content and density.
The results showed that the mixture with the most angular sand:
- achieved the highest CBR value (bearing capacity)
- had the highest friction angle
- but at the same time had the lowest maximum dry density
This suggests that angular sand particles provide better mechanical interlocking between the particles and can therefore improve bearing capacity – potentially also allowing for a higher sand content.