Interfacial Weakness Criterion by Indentation in 3D Printed Concrete

Three-dimensional (3D) printable concrete requires cementitious material that must have suitable but self-contradictory properties to be printable such as fluidity to facilitate pumping along with stiffness and strength to ensure buildability, both having a great influence on the cohesion of the interfacial zone. A pool of characterization tests was developed over the last decades for layered 3D printed structures to quantify and qualify the interfacial region. Although destructive tests are typically selected to capture actual interfacial bonding strength, nondestructive testings were also used. Indentation tests were preferred in this study to locally determine the mechanical properties of the center part of two consecutive layers, the edge of the layer and the interfacial zone. As results, it was found that the previously deposited layer is harder than the upper one. The hardness of the edges of the printed filament can decrease ∼50% over few hundred microns compared to the core of the material. Moreover, this decrease in hardness is also observed at the interface. From the hardness-distance profile measured perpendicularly to the plan of the interface, we propose an interfacial weakness criterion, which has been successfully applied in various conditions of 3D printed concrete elaboration.