Researchers at the Swiss Federal Laboratories for Materials Science and Technology (Empa) are investigating how roads could be reinforced without the use of bitumen to make them more easily recyclable. Their solution? Stones, string and a robot.

The project was inspired by an experiment at ETH Zurich. In this project, pillars were piled up using only string and stones, demonstrating the stability that can be achieved by simply interlocking gravel with thread. No cement was used in the project. The gravel pillars with a height of 80 cm and a diameter of 33 cm could withstand a pressure of 200 kN, which corresponds with a load of 20 tonnes. You can read more about it here.

Asphalt also consist of rocks of various sizes, bound with a binder called bitumen. Bitumen has several downsides: it is made of crude oil, it makes asphalt susceptible to cracking and deformation, and it makes it hard to recycle the material.

The Empa researchers used various experimental setups to test solutions in order to see if string could be a suitable replacement for bitumen, making the road more water-permeable in the process. A robotic arm placed the string in a programmed pattern on the layers of gravel stacked on top of each other in a test box. The string is a common thread used by Swiss citizens for bundling waste paper. The floor was covered with a rubber mat that fixes the whole package to the ground.

Pressure was then applied to the gravel, which showed that the string entangled with the gravel in such a way it could withstand a pressure of 5 kN, half a tonne, without the stones moving much. A test with rolling pressure, like vehicles on a road, will be carried out soon.

The research does not mean the next road will be build this way, but it does have potential and may lead to a recyclable road with simple means.

Photos: Empa