Influence of thermo-hygrometric conditions on the interface bond in 3D printed concrete

For the additive manufacturing in civil engineering, the cementitious ink must have contradictory properties to be printable, indeed it must be initially fluid enough to be pumpable and extrudable, and also should stiffen quickly after deposition to be buildable. These can influence the mechanical properties and the behavior of the printed structure. This work is focused on the role of the printing conditions, mainly time gap between successive layers and environmental conditions, on the quality of the interface between printed layers. The mechanical properties of the interface were studied by means of classical and instrumented indentation tests at micro and macroscopic scales jointly to bidirectional macro compression tests. In addition to the macrohardness tests, microindentation allows to study the role of the interface at a local scale by applying the interfacial weakness criterion based on a hardness profile established on a cross-section in the neighborhood to the plane of the interface. The influence of the printing conditions on the mechanical behavior of the interface is clearly highlighted. As an example, this criterion shows a degradation of the interface property with an increase in the time gap between layers in addition to the influence of the thermo-hygrometric conditions. For a better understanding of the mechanical behavior at the interface, additional instrumented indentation tests in the plane of the interface using macro-loads are carried out until the rupture. The critical load of fracture confirms the role of the printing conditions, whereas the compression tests are not able to show significant differences between the elaboration conditions. The indentation test, which is not widespread in the field of civil engineering, proves here that it can be very useful for a finest mechanical characterization of the material, especially for the characterization of the interface at a local scale.