Of all the man-made materials on Earth, concrete prevails. It is responsible for a majority of the critical infrastructure that modern human societies require to function and thrive. Its consumption is estimated to be more than twice that of all other building materials combined! As developing countries around the world undergo urbanization and have an increased demand for new infrastructure this consumption is only expected to rise. For example, China used more concrete for building projects between 2011-2013 than the US did in the entire 20th century!
With the knowledge that concrete holds such importance, and its use is globally widespread, it only makes sense for us to maximize its potential as a building material. Having this in mind, researchers have noticed that things in nature work seamlessly. That is why more and more often we are beginning to see scientists, engineers, architects, and designers of all kinds, creating and experimenting with biomimicry.
In traditional methods of construction with concrete, a mold or form needs to be made per element in each type of design. 3D printing removes this necessity, thus shining a new light on the unique properties of cement-based materials. There is a great potential with the method of using 3D printers. Engineers could have more control over design and performance-based materials. “3D printing cement-based materials provides control over their structure, which can lead to the creation of more damage and flaw-tolerant structural elements like beams or columns,” said Mohamadreza “Reza” Moini, a Purdue PhD candidate of civil engineering.
With this sort of forward-thinking, researchers from Purdue University have found a way to make cement (concrete’s main ingredient) that gets stronger when it cracks by using a 3D printer. “Nature has to deal with weaknesses to survive, so we are using the built-in weaknesses of cement-based materials to increase their toughness,” said Jan Olek, a professor in Purdue’s Lyles School of Civil Engineering. For example, the way a Mantis shrimp conquers its prey with a “dactyl club” appendage that strengthens on impact through twisting cracks that dissipate energy and prevent the club from falling apart; which is exactly what the team was inspired by in the first place. These designs may not only increase the strength of the material, but much less material is required due to the empty spaces between layers.
The inner cuticle of arthropod shells contain Bouligand structures – tiny microstructures composed of a series of layered fibers, with each layer rotated slightly from the one above it, reminiscent of the formation of a spiral staircase. Materials that contain this sort of structure are stronger because of the way they crack. “The cracks grow in twisted patterns following the direction of the fibers and the material’s structure has “sacrificial links” that can be broken without weakening the structure of the overall system. As a result, the layers reorient, allowing the material to achieve greater toughness. Essentially, the “crackability” of the material actually becomes a strength, spreading the damage over more of the material to prevent localized damage.”
The Bouligand architecture takes advantage of weak interfaces to make a material more crack resistant. From there, other bioinspired cement paste elements were experimented with, including the “honeycomb” and “compliant”.
Once hardened, each of these 3D printed architectures display their own unique behaviors. For instance, the compliant architecture creates spring-like cement-based elements which is very interesting because it takes what was normally a brittle material and makes it flexible.
The idea behind their technique is to control how the damage spreads between printed layers of material. Micro-CT scans are being used to examine hardened 3D-printed cement-based materials and better understand their behavior. The team’s findings make it possible to take advantage of the materials weak characteristics like pore regions found at the “interfaces” between printed layers. This approach is undergoing further research at Purdue University with the hopes of eventually contributing to more resilient structures during natural disasters.
More information on the research can be found in the paper Additive Manufacturing and Performance of Architectured Cement‐Based Materials has been published in the journal Advanced Materials on August 29, 2018.