3D Asphalt Printer Repairs Cracks

BY EMILY NEWTON

Editor’s Note: The 2015 article, “Here’s How 3D Fabrication Enhances Asphalt Practices,” posited the asphalt paver was—and still is—the original 3D printer of roads. Now researchers at University College London have developed a drone-carried 3D printer to deliver asphalt to cracked pavements, “printing” a repair job in real time.

Researchers at University College London have developed a 3D printing system that allows the printing of asphalt. This technology steps into a new realm of possibilities for 3D printing, which has been used to manufacture everything from toys to tools. Even more fascinating is how researchers made asphalt printing possible and what it could mean for the future of construction and civil engineering.

One of the challenges that 3D printing technology has overcome is functionality with a variety of materials. Plastic worked great initially because it is easily malleable and widely available. Modern 3D printing technology can print with metals, carbon fiber and paper, among a host of other materials. Recently, asphalt joined the list.

The asphalt 3D printer the team at UCL developed had thrilling results. UCL’s project is part of a more extensive, five-year program including other universities around the U.K., called Self-Repairing Cities. The program focuses on developing drone and 3D printing technology that can autonomously repair and maintain pavements. The materials research at UCL could have a monumental impact on the materials industry at large.

OVERCOME VISCOSITY

The team at UCL published a paper titled “3D printing of asphalt and its effect on mechanical properties,” which goes into detail about their design process, the printer itself, and the findings of their research. The most significant practical problem the team faced was getting the asphalt to print reliably. Asphalt requires heat to flow, so it doesn’t naturally agree with 3D printers.

3D printing works best with materials that maintain a consistent viscosity so they can be pumped through the printer’s extruder without experiencing clogs or leaving gaps in the print.

The team at UCL found that asphalt in particular changes viscosity under pressure in a non-linear way, making it challenging to predict how viscosity will change during extrusion. To get around asphalt’s unpredictable viscosity, they molded cast asphalt into tiny, millimeter-scale pellets. These pellets are processed through a specially designed extruder (which was also 3D-printed). Inside the extruder, the pellets pass through a heat gradient that gradually softens and essentially “melts” them into a liquid form at the tip of the extruder. This way, the liquid asphalt comes out of the extruder consistently.

This design could be revolutionary for not only the Self-Repairing Cities project, but also for construction at large. While studying the 3D-printed asphalt, the UCL team discovered that it was nine times more ductile than cast asphalt (see Figure 6 in UCL’s original paper). This high ductility suggests that 3D-printed asphalt could be more durable than cast asphalt.

High ductility is helpful for the 3D printing process, but if 3D-printed asphalt is more malleable and resistant to fracturing, 3D-printed roads could experience less breakage. Ideally, the 3D-printed asphalt would flex under stress, rather than cracking. Instead of a crater-like pothole, it might bend into a shallow bowl. Additional surface treatments could be added to the road surface as well.

However, it is important to note that the UCL researchers found that 3D-printed asphalt has a similar fracture strength as cast asphalt, despite its higher ductility. Currently, 3D-printed asphalt can be stretched and shaped more, but cracks under the same amount of stress. As more research is conducted surrounding this technology, materials scientists are likely to focus on increasing that fracture strength alongside ductility.

FUTURISTIC PAVEMENT MAINTENANCE

If a high-strength, high-ductility, 3D-printed asphalt could be developed and mass-produced, it could shape the future of civil engineering. For example, combine UCL’s 3D printing technology with a manufacturing robot being developed by a Harvard graduate. The Addibot is designed to repair scrapes in ice rinks, and inventor Robert Flitsch aims to develop the robot further so that it can repair asphalt.

In the future, we could see robots like Addibot equipped with UCL’s asphalt 3D printers roaming the roads to repair potholes and cracks. These autonomous road-repair bots could function around the clock, allowing them to catch pavement failures before they achieve a low pavement condition index (PCI) score. In the future, we may see road repair robots taking care of potholes in a fraction of the time a maintenance crew or even pothole patching machines require.

The developments underway in 3D printing asphalt technology are a glimpse into the future of construction and civil engineering. 3D-printable asphalt is a breakthrough in materials science and engineering, with applications that could lead to safer, more durable roads. The potential for autonomous road repair presents advantages to explore over conventional pavement maintenance. While it may be several years or more before this autonomous 3D printing technology matures to the mainstream, it could one day revolutionize construction.

Emily Newton is a construction writer with over five years of experience covering stories in the industrial sector.

WEB EXTRA

Watch a video of the drone-with-printer in action on YouTube. https://youtu.be/mcK1xGfDWg0