Extrusion-based 3D printing technology is based on the principles of additive manufacturing. The extrusion-based 3D printing of concrete is governed by three parameters – pumpability (ability to be pumped to the nozzle), extrudability (ability to be extruded through the nozzle), and buildability (ability to withstand load of subsequent layers in fresh state). These properties are further governed by the rheological, physical, and chemical characteristics of the concrete mix. Pumpability requires the properties like flowability and segregation resistance. Extrudability needs the mix to be cohesive enough to hold its shape on extrusion. Buildability or ability to be built requires the mix to possess high shear and compressive yield strength to allow printing of layers on layers without failure (deviation from designed section) or collapse.
This article presents a detailed review on the parameters governing buildability in 3D printing of concrete. Researchers either define buildability in terms of rheological properties of the material, or in terms of failure mechanisms of elastic buckling, and plastic collapse of bottom layers during printing. Buildability in terms of rheology can be associated to structuration or structural build-up. The cementitious compound changes its flow behaviour from fluid-like to solid-like material over time, which influences the failure of a structure while printing. The failure mechanisms are addressed by researchers considering mechanical properties like the shear yield strength of layers, elastic modulus, and compressive strength of the printed structure.
While printing, the sample is under pressure and consistent motion (until extrusion) where properties like fluidity, cohesiveness, and segregation are critical. On the other hand, the layers are expected to show a solid-like behaviour to withstand step loading at the time of deposition. This anomaly needs to be studied at microstructural level. A detailed review on the relationship between rheological properties and microstructural evolution is presented. The influence of microstructural evolution on mechanical properties of concrete at early age is also highlighted. Further, the initial geometrical deformations in layers while printing may not lead to collapse but can be fatal to the structural capacity of the component in future, which will make the printed structures undesirable. This report provides a brief review on the geometrical parameters such as bond area, verticality, and dimensional tolerances, which are critical for buildability. Overall, an overview of the material and geometrical properties critical for buildability in extrusion-based 3D concrete printed structures is the focus of this report.