The 3D printing technique, developed by a group at Purdue University in the US, takes its inspiration from nature to control how damage spreads between the printed layers of a material. "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.
It is thought that the group’s work could lead to more damage and flaw-tolerant structural elements like beams or columns.
The idea is inspired by the crack-propagation and toughening mechanisms seen in the shells of arthropods, including the mantis shrimp which conquers its prey with a "dactyl club" appendage that grows tougher on impact through twisting cracks that dissipate energy and prevent the club from falling apart.
3D printed cement-based materials – such as cement paste, mortar and concrete – would give engineers more control over design and performance, but technicalities have stood in the way of scaling them up. "3D printing has removed the need for creating a mould for each type of design, so that we can achieve these unique properties of cement-based materials that were not possible before," said Jeffrey Youngblood, Purdue professor of materials engineering.
The team is also using micro-CT scans to better understand the behaviour of hardened 3D printed cement-based materials and take advantage of their weak characteristics, such as pore regions found at the "interfaces" between the printed layers, which promote cracking. This finding was recently presented at the 1st RILEM International Conference on Concrete and Digital Fabrication.
Elements 3D printed by the group included so-called "honeycomb," "compliant" and "Bouligand" architectures, which allowed them to engineer specific properties. The Bouligand architecture, for example, takes advantage of weak interfaces to make a material more crack-resistant, whereas the compliant architecture makes cement-based elements act like a spring, even though they are made of brittle material.
The team now plans to explore other ways that cement-based elements could be designed for building more resilient structures.