Early Age Shear and Tensile Fracture Properties of 3D Printable Cementitious Mortar to Assess Printability Window

Digital fabrication with concrete is a giant leap forward for the concrete construction industry in achieving its sustainability goals. Concrete 3D printing, despite being the forerunner of the digital fabrication and additive manufacturing techniques, requires a significant amount of attention to be standardised. The nebulously defined parameters of pumpability, extrudability and buildability which collectively define the printability of a mix, need to be quantified. This requires the development of testing procedures to study the early age properties of concrete mixes that are presently unavailable. This study deals with the evaluation of early age tensile and shear strength of 3D printable cementitious mortars through a novel “fracture-based” methodology and tailored set-up. Direct shear and direct tensile tests have been performed on a 3D printable mix, incorporating also high proportions of Calcium Sulpho-Aluminate cement and non-structural basalt fibres, at “very early ages” namely, 30 min, 45 min, 60 min, 75 min, 90 min and 120 min from the contact between binder and water. The phase transition between fluid state to solid state of a printable mix has been observed and the energy released during the tensile and shear tests analysed with the aim of identifying the printability window from the time-evolution trend of the aforesaid parameters. The methodology has been carefully calibrated also with reference to experimental artefact, including the effect of friction among the movable mould parts, which may significantly affect the results in the case of very small values of mechanical material parameters, as it happens for concrete in very early ages.