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.
A fixed diameter of 20 mm, but varying height was used to include a single filament layer (10 mm), two layers (20 mm) and four layers (40 mm). Both typical horizontal interfaces in multi-layer elements and vertical interfaces between multilaterally deposited filaments are studied.
Whilst a single 3D printable concrete mix were considered, print variables of pass time (0 to 60 minutes with 15 minute intervals) and print speed (80, 100 and 120 mm/s) are considered to investigate their potential alteration of the microstructure.
Findings were significant, indicating tri-axial spheroid shaped air voids present in printed specimens, elongated and flat in the print direction, compared to mostly spherical voids in cast specimens.
According to the authors of the study, this prompts for more research to be conducted into the effect of stress concentrations at micro-cracks or voids in 3D printed concrete, which especially impacts mechanical behavior.
Furthermore, it is found that vertical and horizontal interlayers comprise of similar porosity, and that 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.