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. In a paper entitled “Evaluation of workability parameters in 3D printing concrete,” a team of researchers measured the workability of fresh concrete for 3D printing according to four different tests: “flow table, ICAR rheometer, Vicat and an experimental applied in the laboratory by measuring the electric power consumption of the motor that rotates the screw extruder.”
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.
“Concrete mixtures produced with different aggregates, binders and different amount of water and superplasticizer were produced, tested for workability according to four different tests and printed in order to have available a wide data range of measured workability parameters and finally define their threshold values that characterize a concrete mixture as printable,” the researchers state.
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
It was difficult to find a mixture that met all four criteria. A mixture with limestone as aggregate and cement as binder was adjusted to achieve three different workability levels, high, moderate and low, but none of them were considered printable because none met all four of the established criteria.
The researchers also evaluated the loss of workability with time. Expansion of mixtures with the three different aggregates (limestone, river sand and the mixture of both) was measured 0, 15 and 30 minutes after mixing.
“Concrete with limestone filler lost workability in a higher rate than ones with river sand or combination with limestone and river sand,” the researchers state. “This can be explained by the granulometry of aggregates. Limestone filler has more fines that absorb more water from the mixture.”
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.
“The use of alternative cementitious materials such as fly ash and ladle furnace slag as a replacement of cement
(20wt.%) results to average reduction of compressive strength by 30% and density by 10%, compared to mixtures with 100% cement as binder,” the researchers conclude. “It should also be mentioned that in most cases during printing, it was observed that fly ash mixtures showed reduced values and higher loss rate of workability with time compared to other mixtures. However, lower cost and volume stability of hardened concrete are expected to be the advantages of using fly ash or ladle furnace slag in concrete for 3D printing.”
Authors of the paper include M. Papachristoforou, V. Mitsopoulos, and M. Stefanidou.
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