Brief 

A Stellenbosch University study of extrusion-based 3D-printed concrete has revealed triaxial, spheroid-shaped air voids that were elongated and flat in the print direction. “It is inaccurate to qualify the homogeneity of typically fissile 3D-printed concrete elements based solely on superficial cross-sectional photographic evidence from saw-cut samples,” the researchers concluded.

 

Insight

A study published by researchers at Stellenbosch University in South Africa presented a first and comprehensive microstructural investigation of construction 3D concrete printing porosity using X-ray computed tomography to visualize and quantify porosity, pore sizes, shapes and distributions in extrusion-based 3D printed concrete.

Concrete structures additively manufactured by extrusion-based 3D concrete printing are reportedly orthotropic in mechanical behavior and exhibit relative weakness in interfacial regions. Microstructure, including porosity content, 3D porosity distribution and pore morphology presents a physical basis for these phenomena.

Orthotropic materials are a subset of anisotropic materials; their properties depend on the direction in which they are measured. Orthotropic materials have three planes/axes of symmetry (an isotropic material, in contrast, has the same properties in every direction).

3D printed plastic molds were used to sample specimens from freshly 3D printed concrete filaments, for minimum disturbance. As a reference, similar specimens of the exact same concrete mix, but cast without compaction, instead of being 3D printed were included in the study.

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