British researchers at University College London (UCL) have broken new ground with what they believe is the first application of 3-D printed asphalt.
The technology won’t replace full-sized paving equipment any time soon, but opens a world of opportunities to repair smaller cracks with tougher asphalt — or even deploy a fleet of drones equipped with miniature asphalt printers to seek and destroy small cracks before they graduate to potholes.
The project required the research team to build a 3-D printer capable of heating miniature asphalt pellets to just the right temperature so they could be applied to test surfaces.
The team adapted an existing 3-D printer to apply the asphalt.
Richard Jackson, materials scientist and 3-D printing engineer at UCL, notes that the 3-D-printing process isn’t that much different from applying asphalt on a large scale — it’s just more precise, covering a specified area layer by layer.
A tiny screw inside the extruder nozzle propels the asphalt to the target surface.
“From the perspective of a materials scientist, bitumen is a funny old material,” he says.
“It’s a non-Newtonian substance in that if you speed up the screw that’s extruding the material, it actually slows the material extrusion down. For most other materials it comes out faster.”
Earliest iterations of the printer used a resin screw, which sometimes broke during printing.
When temperatures in the extruder were raised above 150 C, the asphalt simply flowed out of the printer nozzle without the use of the extruder screw.
Ultimately, temperatures were lowered to a more optimal 130 C and a more robust design incorporated a metal screw.
The device could scan the crack and then provide exactly the material required to perform the repair
— Richard Jackson
University College London
“The first successful applications of asphalt were printed on paper over an area about as large as my finger,” Jackson says.
“It took about 10 to 15 minutes to cover that area. The very unexpected result was that the 3-D-printed asphalt was nine times more ductile than asphalt applied using traditional methods. It looks quite different under the microscope. The increased ductility is due to microstructural changes in the asphalt which result in crack-bridging fibres that increase toughness.”
The process uses material precisely, filling only the crack required. 3-D asphalt printing is also energy efficient.
While full-scale paving requires significant heat applied to asphalt materials over a long period of time, asphalt is heated only briefly in 3-D printing applications, just before it reaches the extruder.
“Some companies are very interested in additive applications because they change the cost proposal for materials,” Jackson says.
“You could print the cheapest material at the bottom of the crack and then, as you approach the road surface, you could switch to a more expensive material. You could also add features like capsules of sunflower oil that would help asphalt to reflow and heal a crack. You could lay down a cooler polymer layer and use it to imbed chips that would act as road sensors and provide information about the repair.”
The newest iteration of the printer features a needle-nosed extruder that can reach further into cracks. It’s also much faster.
Jackson reckons that the latest model can repair a crack 10 to 15 centimetres long and two centimetres deep on the university parking lot in about five minutes.
The research team’s next challenge involves adding sand and other small aggregates to the asphalt mix to further test its properties at the molecular scale.
Taken to its limits, Jackson says that 3-D asphalt printing technology could help to enable a fleet of autonomous robots, traveling by ground or air, to first identify cracks, clean them and then perform the repair.
“In the shorter term it might result in something a little less romantic, with crews placing a black box over a small area of the road surface,” he says.
“The device could scan the crack and then provide exactly the material required to perform the repair.”
While autonomous 3-D printing robots might make some members of the public feel a little unsettled. Jackson says this research project has met with wider acceptance.
“Drivers may not love autonomous robots,” he says. “But they hate potholes more.”