Extrudability analysis of 3D printable concrete as a two-phase discrete flow

3D printing with concrete is primarily achieved through an extrusion process, where the material is precisely deposited layer by layer to construct structures. Proper control of extrusion ensures consistent material flow, accurate layer formation, and the overall stability of the 3D-printed structure. However, clogging during the extrusion process can occur as the nozzle size is too small relative to the aggregate size. To fundamentally understand the clogging mechanism and optimize the extrusion process, a precise two-phase discrete element model was established to simulate the extrusion process. A cluster of particles was used to model fresh concrete, with hard cores representing the aggregates and concentric soft shells representing fresh cement paste surrounding them. Five nozzles with different outlet diameters were designed and 3D printable concrete with different fineness modulus levels was formulated. The parameters of the two-phase discrete element model were determined based on rheological properties and calibrated using slump flow test results. The simulation results showed good agreement with the experimental pressure values and the quality of extruded filaments. Based on these results, it suggested that ensuring a proper balance between the aggregate fineness modulus and the outlet diameter of printing nozzles is essential for the extrusion process.