Mechanical characterization of 3D printable concrete

The current study deals with test methods for characterizing the mechanical behaviour of 3D printed wall elements. At first, the effect of interfaces present in a printed concrete element was examined through an assessment of porosity and bond shear test. The porosity was evaluated using specimens extracted from the bulk as well as from the interfaces between horizontal and vertical layers of printed concrete. It was found that the porosity in the bulk was lower by 6–8% as compared to the mould cast concrete. On the other hand, the porosity at the interface between horizontal and vertical layers was found to be higher by 11–14% and 10–16% respectively. Therefore, although the bulk in 3D printed elements is more densified, the interfaces are weak with higher porosity. The interfaces were further evaluated by a direct bond shear test. Compared to the shear strength of the mould cast concrete, the shear strength at interface between the horizontal and vertical layers was found to be lower by 24–25% and 22–30% respectively. The compressive strength of the printed concrete was similar when tested in different loading directions but lower by 12–22% as compared to the mould cast concrete. This reduction in compressive strength may be due to the higher porosity at the interfaces present in printed concrete elements. The flexural strength was found to depend on the region subjected to the maximum bending moment. When tested along the direction where the maximum bending moment and therefore the maximum bending stress occurs at the weaker interfaces between layers, the flexure strength was lower by about 32–40%. On the other hand, the flexural strength was found to increase by 13–20% when tested in directions where maximum bending stress is induced in the bulk concrete, having a lower porosity as compared to the mould cast concrete. Finally, some perspectives are provided on formulating a structural design procedure for a 3D printed wall element.