Experimental study on failure mechanism of fresh 3d printed mortar in extrusion based method

In the extrusion-based 3D printing method of cementitious materials, one of the technical challenges is about buildability; when deposited fresh mortar is not stiff enough, significant deformation or even collapse of the printed object occurs during printing. There have been several previous studies on prediction and evaluation of the buildability for a given cementitious material. When conducting printing experiment, the authors found an interesting failure mode of a printed object; well-distributed shear bands initiated and propagated on the object surface. This type of failure has not yet been reported in previous studies, which is the motivation of the present study. To investigate the failure mode and its mechanism, square-tube objects made of either limestone powder paste or cement mortar were printed by using a small-scale gantry-type 3D mortar printer. Digital images of the object surface were taken during printing and the initiation and propagation of shear bands were observed. In addition, the surface strain/deformation of the printed object was analyzed by using two-dimensional Digital Image Correlation (DIC) technique. The experimental results demonstrated that multiple shear bands could appear on the object surface during printing, followed by a “landslide” occurring along the shear band, which resulted in significant vertical and radial deformation of the printed object. In addition, the DIC analysis revealed that diagonal bands with relatively high strain levels can be observed in the strain maps before the shear band becomes visible to the naked eye. Moreover, the appearance of the shear bands precedes the remarkable vertical deformation of the printed object. This suggests that a live monitoring system to detect the imminent collapse of a fresh printed object would be possible; by taking images of the object surface and analyzing them by DIC during printing, the system can give an alert of the risk of object collapse.