Episode 22: Additive manufacturing enhances toughness in formable cementitious materials

In this podcast episode, MRS Bulletin’s Laura Leay interviews Reza Moini of Princeton University about his group’s development of an enhanced additive manufacturing technique to fabricate cementitious materials with excellent fracture toughness. They based their design of the material on the double-helical or double-bouligand structure of coelacanth fish scales that resist deformation. In order to fabricate the material, Moini’s research team used a two-component robotic additive manufacturing process. The extrusion system was controlled using specialist algorithms. This work was published in a recent issue of Nature Communications.

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. The natural world has developed some sophisticated structures and mechanisms. The scales of an ancient fish – once thought to be extinct – exhibit excellent fracture toughness. Probing the mechanism behind this fracture toughness enables improved concrete structures to be developed using advanced additive manufacturing.

REZA MOINI: The materials and structures that we design in the next ten, twenty, thirty years won’t be the same as the ones we design today and so the meaning of concrete may change in almost every aspect we know today, given the advanced manufacturing capabilities but also the advanced computational and design capabilities that are only going to grow. In other words, as we advance our tools of design, computation, and fabrication, we may be redefining what concrete is as a material and what properties it can be bestowed upon that may be strange today. 

LAURA LEAY: That was Dr. Reza Moini, Assistant Professor of Civil and Environmental Engineering at Princeton University. Fish scales are comprised of bundles of collagen known as fibrils that produces sheets or lamella. In most fish each lamella is rotated relative the one they are adjacent to, forming a simple helical structure. The fish scales that inspired the work on concrete are different: they have a unique double-helical or double-bouligand structure.

REZA MOINI: In the coelacanth fish scale, the fibril bundles in the adjacent two layers form an orthogonal bilayer as one unit. These orthogonal bilayer units then progressively rotate through the whole thickness of the scale known as double-helical architecture. Between these fibril bundles also exists inter-fibril bundle arrangements that are more loosely packed leading to the presence of a weak interface between the fibril bundles.   

LAURA LEAY: The double-helical structure together with the soft interface between the hard lamella lead to a structure with improved resistance to deformation, including resistance to delamination of the lamella. Compared to traditional formable materials such as cement and concrete, which have limited ability to dissipate energy at the microstructural level, the structures in the fish scale lead to enhanced fracture toughness. Applying the same architecture to concrete, the bilayer appears to promote crack shielding and this, along with interlocking behind the crack tip as well as deflection ahead of the crack tip, leads to enhanced fracture toughness compared to cast counterparts. In order to apply the same structural motifs to concrete, Dr Moini’s team needed to develop an enhanced additive manufacturing technique. They used a two component extrusion system where a concrete mix of cement paste and fine aggregate was dosed with an accelerant to control the curing process. The extruder was manipulated using a robot arm and controlled using specialist algorithms. These sorts of developments take time, perseverance, and inspiration. Often they only move forward after an unintentional discovery, as Dr. Moini explains: 

REZA MOINI: The unknown unknown that we stumble upon, sometimes in the lab due to an accident or due to an error in the experiment which then reveals questions to us that we have not been asking, we’re always fascinated and surprised by coming across one of those which don’t necessarily happen every day. It’s important to allow researchers some time – to carve out some time – to explore and be ready to be surprised and allow for some time to treat the research and the science as a playground and not only to formalize research but to allow for some ability and some time to facilitate discoveries.

LAURA LEAY: This work was published in a recent issue of Nature Communications. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.