Experimental Study on Achieving Overhangs in 3D Concrete Printing Through Printhead Redesign

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 concrete printing (3DCP) that accommodates overhangs. The approach integrates changes both in the printing process and the material formulation. A key modification of the printing process is mixing dry and liquid components immediately before extrusion, eliminating the requirement for prolonged pumpability in the fresh-state mixture. We select a cement-based clay-concrete mixture optimized for 3D printing, focusing on both fresh-state rheology and hardened-state strength. The test prints demonstrated the effectiveness of the method, successfully achieving overhangs greater than 45 degrees on a simple gantry 3D printer. The fresh-state mixture exhibits Bingham behavior, with a minimal shear rate 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.