NEWS
Building in the Deep
Researchers are developing techniques to 3D print concrete underwater, enabling better construction in marine environments.
Written by Jennie M. Morton
THE NEXT FRONTIER FOR 3D PRINTING is taking shape beneath the ocean’s surface. While additive manufacturing can already produce concrete structures on land, printing underwater introduces new challenges.
DARPA, the U.S. Department of Defense’s research agency, is exploring solutions for critical infrastructure. It has tasked researchers with developing underwater 3D concrete printing (3DCP) using existing hardware and nonproprietary concrete mixes.
Building Underwater
The challenge begins with a fundamental constraint: DARPA requires the concrete formula to incorporate seafloor sediment. This approach aims to reduce the logistical, economic, and environmental costs of transporting materials by using abundant resources at the project site.
Rising to the challenge, researchers at Cornell University successfully printed underwater cement by refining the formula and adapting a 6,000-pound industrial robot. How its work overcomes the “delicate balance between pumpability, extrudability, and buildability” is detailed in Cement and Concrete Composites.
Lab investigations into 3D printing concrete underwater. Video: Vasyl Kacapyr/Cornell University
Cornell’s test prints validate the realities of underwater 3D printed concrete for marine structures. Photo: Vasyl Kacapyr/Cornell University
“This was ultimately a mixture design challenge, tailoring consistency so the viscosity is compatible with an underwater 3D extrusion system,” explained Sriramya Nair, assistant professor in the School of Civil and Environmental Engineering at Cornell University.
Marine concrete requires a specialized formula within the world of cement mixtures to endure the physical and chemical forces of seawater. It’s also ameliorated with anti-washout admixtures, which bind the cement particles so they harden even while submerged.
Additionally, underwater structures are placed with a separate pouring method than sidewalks or building foundations. The technique, called a tremie pour, directs cement through a vertical pipe into a formwork. This temporarily shields the mixture from water to minimize washout.
Subsea Conundrums
When 3D printing in the depths, things can get gummy. Consider that additive manufacturing is prized for creating objects without molds for reinforcement. What happens when its layering process is surrounded by liquid? Three complications must be overcome: washout, thickness, and pumpability.
The ocean’s buoyancy will whisk away fresh cement even when water is perfectly still. Washout jeopardizes the bottom layer’s structural integrity as well as interferes with bonding between subsequent layers.
In addition, marine cement has a thicker slurry due to its specific blend of aggregates and admixtures. Yet with 3D printing, that viscous material must flow through an extrusion nozzle that has a nearly 50 percent diameter reduction over a typical tremie pipe. The ensuing difficulties are like using a skinny straw over the jumbo one with an ice cream shake.
Lead researcher Sriramya Nair and her team overcame challenges with cement viscosity so it can be seamlessly extruded for 3D printing. Photo: Vasyl Kacapyr/Cornell University
One of the DARPA proposal requirements was to adapt land-based hardware, such as this robot, for underwater applications. Photo: Vasyl Kacapyr/Cornell University
The catch-22 is that the slurry can’t be thinned out with a plasticizer or water because it creates a delayed setting time, reintroducing the problem of washout.
“3D concrete printing is all about controlling material flow,” Nair emphasized. “The most common solution for air-based 3D printed formulas is to limit maximum particle size to around 4 millimeters, and pumpability is achieved by increasing the cementitious binder ratio. Since seafloor sediments are significantly finer, we further tweaked our mixture to balance pumpability and buildability underwater.”
Additionally, 3D printing uses a pump to move concrete since gravity alone isn’t sufficient. But if pumpability isn’t compatible with extrudability, clogs are inevitable. The result is no better than using a firehose to blast buttercream through a piping tip.
“We solved this by switching to a motor stator pump that can handle the viscous mix,” Nair said. “This also allowed us to push the limits of how much cementitious content is required despite continuous water exposure.”
Another modification was adding an accelerator that injects admixture right at the extrusion nozzle. By waiting until the last possible moment to add this agent, users can control dosage in real time.
Acing the Test
The industrial robot that directs this process prints in an area up to roughly 8-by-30 feet. The arm accepts different heads, ensuring broad compatibility with construction materials. The robot has been part of the university’s hands-on curriculum since 2022.
This trial’s tubular prints have been assessed to ensure they successfully retain the correct shape and compression.
“Concrete is dry tested by cutting into printed samples and evaluating the interlayer bonding. For 3D printing, we’re specifically looking for anisotropic behavior since mechanical performance isn’t uniform across different orientations,” Nair clarified.
Cornell’s work demonstrates that underwater 3DCP has immediate relevancy for new structures as well as reinforcement and repairs. The breakthrough is also moving toward fulfilling DARPA’s stipulated depth of several meters, which then leaves room to advance the technology and formula to match the requirements of deep sea projects like offshore energy.
“Underwater 3D printing shows that we don’t have to do things the way they have always been done,” Nair stressed. “We can place concrete underwater not only exactly where it’s needed but also in the shape needed.”
Jennie M. Morton is an engineering and construction writer based in Iowa.
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