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Asphalt usually is made up of high-grade chippings and crushed sand, natural sand and crushed rock. Road bitumen or polymer-modified bitumen is added as a binder. So, a lot of materials are needed to pave asphalt roads. Researchers at Empa – the Swiss Federal Laboratories for Materials Science and Technology – are consequently looking for new ways to make road construction more environmentally friendly. With the help of a robot that lays a piece of string in a mandala-like pattern on a bed of gravel, they want to test robot-assisted construction techniques for paving roads. At the same time, they also want to change the structure of the pavement by using a new type of mechanical reinforcement. This should save on materials in the future. Or even make it possible to completely recycle road surfaces.

The idea was born out of a project run by the Gramazio Kohler Research Lab at ETH Zurich, where it was pitched as an art and research project. At that time, the scientists showed how road pillars can be built up purely from string and gravel. And how these can achieve tremendous stability just by interweaving the gravel with the string. Without the addition of any cement. In a laboratory test, an 80-centimeter-high crushed stone pillar with a diameter of 33 cm withstood a pressure of 200 kN. This corresponds to a load of 20 metric tons.

Digitalized building methods in road construction

Since asphalt also consists of rock of different sizes and the binder bitumen, Empa researchers Martin Arraigada and Saeed Abbasion from the Concrete & Asphalt department have applied this concept to road construction: “We want to find out how a recyclable road surface could be constructed in the future. We are using digitalized building methods in road construction for the first time here as well,” explains Arraigada.

A road surface that is reinforced with string and does not require bitumen offers several advantages. It is more environmentally friendly because – unlike bitumen – it is not made from petroleum. As a result, no harmful emissions are released either during its production or use. Moreover, it is less susceptible to cracking and deformation. It also counteracts the effects of hydroplaning, given that bitumen makes conventional road surfaces impermeable to rainwater. The researchers believe that in the future it could also be possible to work with rock that is in less short supply but otherwise unsuitable for road construction. Last but not least, they point out that the process even makes a rollable and recyclable road surface feasible.

String instead of bitumen

The Empa researchers are now trying out the method in a variety of tests. In these tests, the robot arm places the string in a specific pre-programmed pattern on the layers of gravel stacked on top of each other. For the tests, five layers of gravel and twine fabric are layered one after the other in a test box. The bottom of the box is lined with a rubber mat to secure the entire package to the substrate. The mat plays the role of the malleable bed on which the road surface is placed.

In the end phase, the ballast-binder-thread package is loaded with a rotating plate and put under pressure. This stress test shows that the package can withstand half a ton of pressure due to how individual stones mesh with the string, without the stones being displaced significantly. The role of the string in conventional asphalt is played by the bitumen. Incidentally, the string that is used is exactly the same type that is used in your average household to tie stuff together.

A lot of innovation potential

In a 3D computer model, which the researchers are running using the Discrete Element Method (DEM) in addition to the laboratory tests, the degree of displacement of the individual stones is measured along with the tensile forces that act on the string. The team is also exploring different patterns and mesh sizes and to what extent these differences affect the stability of the road surface.

The researchers cannot yet say when this model will be ready to be applied in actual road construction. However, the basic research has a lot of innovation potential when it comes to creating a road surface that can be recycled and possibly rolled out, and that can also be made with simple materials.