Experimental 3D Printing of Overhangs by Integrated Mixing-Extrusion Process

Achieving complex geometries in extrusion-based concrete 3D printing has become a subject of growing interest, particularly in applications requiring overhangs, such as domes and arches. While most existing extrusion-based technologies are limited to printing primarily vertical walls, emerging methods are being developed to enable the printing of structures with overhangs. This paper introduces an innovative approach to 3D construction printing (3DCP) that accommodates overhangs. The approach integrates changes to both the printing process and the material formulation. A key modification to the printing process is mixing dry and liquid components immediately before extrusion, which eliminates the requirement for prolonged pumpability in the fresh-state mixture. A clay-based concrete was developed for 3D printing, where the clay content was optimized to deliver the specific fresh-state properties for the novel extrusion process, while also meeting the required hardened-state strength. The test prints demonstrated the effectiveness of the method, successfully achieving overhangs greater than 45 degrees on a simple Cartesian 3D printer. The fresh-state mixture exhibits Bingham behavior, with a yield stress between 0.2 and 0.3 MPa. The hardened mixture achieves compressive strengths between 10 and 40 MPa and flexural strengths between 3.5 and 4.5 MPa, making it suitable for creating durable formworks.