An investigation into the porosity of 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. Here, a first and comprehensive microstructural investigation is reported, using X-ray computed tomography to visualize and quantify porosity, pore sizes, shapes and distributions in extrusion-based 3D printed concrete. 3D printed plastic molds are used to sample specimens from freshly 3D printed concrete filaments, for minimum disturbance. As reference, similar specimens of the exact same concrete mix, but cast without compaction, instead of being 3D printed are included in the study. A fixed dimeter of 20 mm, but varying height is 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 are considered, print variables of pass time (0-60 min with 15 min intervals) and print speed (80, 100 and 120 mm/s) are considered to investigate their potential alteration of the microstructure. Findings are 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. 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.