Multi-physics Modelling for Extrusion-Based 3D-Printing: Material, Process and Applications

In face of the challenge of reducing the carbon footprint of the construction industry, 3D-printing technologies with cement-based materials have gained significant popularity in the past decade. However, ensuring a standard of quality and safety for printed objects and structures is a challenge yet to be overcome. Printability, durability and quality issues often stem from interactions between several physicochemical processes, in the material it-self and its relationship to the surrounding environment. Understanding couplings between mechanical, hydraulic, thermal and chemical processes is thus paramount to the development of predictive simulation tools, which in turn could allow for better process control and understanding of durability properties. Herein, we propose a multi-physics simulation framework, with a fully coupled material constitutive model at its core, thermodynamically derived as an extension of classical unsaturated poromechanics to chemically solidifying media. Experimental procedures from soil mechanics, such as the use of porosimetry measurements, are adapted to fresh cement-based materials and used to determine necessary model properties and their evolutions with hydration. In conjunction with this model, we introduce a finite element (FE) based framework and modelling strategy aimed at recreating the extrusion-based printing process through sequential addition of material. A systematic meshing strategy from toolpath data of the printer is also showcased. Implemented using open-source components, this framework is used to perform 2D and 3D simulations of mortar 3D-printing. This framework allows to investigate various common issues with 3D-printing of cement-based materials along with their coupled origins, ranging from printing collapse to service durability. In particular, the influence of external parameters such as layer-pressing and environmental conditions on buildability and geometric accuracy of prints are shown along with indications to mitigate them. Known durability problems induced by drying processes, such as degraded material properties at interfaces, are also presented in light of a detailed unsaturated behaviour from the constitutive model.