Researchers at the Swiss Federal Laboratories for Materials Testing and Research, Empa, have been investigating a novel replacement for bitumen: string.
Alternate layers of aggregate and string, laid out in a pattern by a robotic arm, were used to form an experimental road construction system.
The idea originated from another project carried out at the Gramazio Kohler Research lab at ETH Zurich. Here string was used instead of cement to connect aggregate in pillars. Researchers discovered that 330mm-diameter pillars, 0.8m high, could withstand a pressure of 200kN, or 20 tonnes.
Empa scientists Martin Arraigada and Saeed Abbasion, who work in the institute’s concrete and asphalt laboratory wanted to find out if the idea could transfer to roads. "We want to find out how a recyclable pavement could be produced in the future. To do this, we are using digitalised construction methods in road construction for the first time," said Arraigada.
To test the aggregate-string combination, the researchers laid five layers of aggregate on a rubber mat, with string installed by the robotic arm in between each layer in a pre-programmed pattern. The construction was then loaded using a rotating plate and could withstand 5kN, or half a tonne, before the stones started to move substantially. In comparison, layers of aggregate without the string displaced far earlier.
According to Empa, there could be several benefits of such a system. First, fewer pollutants due to the production and heating of bitumen; second, a permeable road surface that allows water to pass through; and third, the possibility of recycling the material more easily.
The researchers are also computer modelling their new pavement structure using the Discrete Element Method in parallel with their laboratory tests. This should reveal the displacement of individual pieces of aggregate and the tensile forces in the string. It will also allow the impact of different patterns and mesh widths to be investigated. The next stage of laboratory testing for Arraigada and Abbasion will be dynamic load testing of the system.
Future versions of this system could see a material other than everyday string being used as the robot-laid recyclable material. Using string, the Empa researchers hope to show how the combination of digital technology and new materials could offer more sustainable road construction methods in the future.