When being used to 3D print structures like bridges and houses, cement needs to be able to sustain the weight of the layers being extruded on top of it. Additionally, in order to be delivered through a pumping system, 3D printable cementitious materials need to have special rheological performance properties, such as plastic viscosity and yield stress, that can help determine their pumpability and buildability.

There’s been plenty of research conducted that looks into the relationship between a cementitious material’s rheological performance and how pumpable and buildable it is, a lot of which has determined that it’s important to optimize these properties in order to satisfy pumpability and buildability requirements.

According to new research from Nanyang Technological University (NTU) in Singapore, a variety of different aspects, like particle size gradation, chemical additive, and materials constituents, can affect a cementitious material’s rheological properties. Cementitious composite materials reinforced with fiber are often used to prevent cracks and increase tensile properties, which is why the NTU research team believes that a kind of 3D printable fiber reinforced cementitious composites (3DPFRCC) should be created, based on rheological performance, for large-scale 3D printing.

In their paper, titled “3D printable high performance fiber reinforced cementitious composites for large-scale printing,” the NTU researchers explain how they developed a novel mixture of 3DPFRCC, which featured rheological properties “characterized by yield stress and plastic viscosity.”

The abstract reads, “3D printing cementitious composites require special rheological properties, which are affected significantly by the materials constituents. In this work, a novel mixture of 3D printable fiber reinforced cementitious composites (3DPFRCC) was developed. The rheological performance, setting-time, and mechanical properties were characterized, a printing test was carried out as well to test the buildability and pumpability. Results indicate that the new material possesses appropriate rheological and mechanical performances. Rheological properties are designed based on previous practical printing test. The static and dynamic yield stress are 3289 Pa and 314.7 Pa, respectively. The plasticity viscosity is 32.5 Pa·s. The initial setting time is 59.2 minutes. The flexural strength and compressive strength are 8.6 MPa and 71.2 MPa, respectively at 28 days. Then, a 78 × 60 × 90 cm (L × W × H) structure was printed successfully in 150 minutes, which demonstrates that this novel 3DPFRCC possesses excellent buildability and pumpability, which is capable of large-scale printing.”

Large-scale 3D printing: 78 × 60 × 90 cm (L×W×H)

The mixture was a combination of natural river sand, Ordinary Portland Cement type I 42.5, silica fume, PVA fiber with a 0.8% oil coating, and class F fly ash. Flexural specimens were manufactured on a gantry-style concrete 3D printer. The researchers tested the mechanical performance (compressive and flexural strength) of the material, and evaluated how it performed during the actual 3D printing, and “open time was characterized by setting time.”

“Build-up and pumping pressure models indicate that printing height and pumping pressure are corresponding to the yield stress and plastic viscosity of materials, respectively (Weng et al., 2018). A meter-level printing was carried out by Weng et al. via optimizing the materials’ rheology,” the researchers wrote. “Based on the rheological properties of materials used in his meter-level printing, the materials used in this work were designed by optimizing materials constituents to meet the requirement of rheology for large-scale (meter-level) printing. Besides, setting time indicates the workable time for 3D printing process, which should be enough for 3D printing process to ensure the pumpability and appropriate bonding performance.”

After characterizing their material’s mechanical performance, among other qualities, the researchers concluded that, in order to satisfy 3D printability requirements, cementitious composites do require a special rheological performance.

3D printing model. a) top view of 3D printed part; b) overall view of 3D printed part

“The results show that the static yield and dynamic yield stress of this 3DPFRCC are 3289 Pa and 314.7 Pa, respectively; the plastic viscosity is 32.5 Pa·s; initial setting time is 59.2 minutes; the flexural strength and compressive strength are 8.5 MPa and 71.2 MPa respectively at 28 days,” the researchers wrote. “Finally, a 78 cm × 60 cm × 90 cm (L × W × H) structure was printed in the printing test, which demonstrates that this new material possesses excellent printability and is capable for large-scale printing.”

Co-authors of the paper are Yiwei Weng, Shunzhi Qian, Lewei He, Mingyang Li, and Ming Jen Tan, all with the university’s Singapore Centre for 3D Printing.

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