To 3D print concrete, several parameters must be met. It must be able to be extruded through a nozzle, hold its shape once deposited,and also be able to hold up under the weight of successive layers.
For their materials, the researchers used crushed limestone, siliceous river sand and a combination of half and half of each. They developed a prototype 3D printing system to test the materials.
The researchers established four criteria of printability and buildability:
The mixture can be extruded through the nozzle
Good print quality meaning no voids, no dimensional variations of extruded material
Five layers of printing material can be achieved without collapse
Height of 1st layer versus height of 5th layer ~ 1
Many of the river sand and combination aggregates could be 3D printed successfully, while most of the limestone-based mixtures were proven to be not printable. The limestone mixtures also required higher amounts of water and super-plasticizer to achieve the same level of workability as the other mixtures, which led to lower values of compressive strength.