Juxtaposing fresh material characterisation methods for buildability assessment of 3D printable cementitious mortars

Both industry and academia are rapidly developing processes, materials, and projects to explore the potential of extrusion-layering additive manufacturing of cementitious materials, generally known as 3D concrete printing (3DCP). Because the lack of supportive formwork makes objects prone to failure during printing, a key aspect remains the so-called ‘buildability’, a qualitative descriptor to indicate the resistance against such failures. Obviously, the material characteristics of the applied print mortar are an important (although not sole) parameter to determine buildability. However, it is not yet clear which material properties are the most suitable, and how they should be determined experimentally. In literature, a range of approaches has been suggested, but comparative studies are very few in number and limited in scope. This paper presents a juxtaposition of fresh material characterisation methods by subjecting four different mortars to a range of tests related to buildability, including rotational rheometry, unconfined uniaxial compression tests, direct shear tests, and ultrasonic wave transmission tests. For reference, some hardened state properties were also determined, and a printing trial was performed on one mixture. Significant differences between the mixtures were found, including different development characteristics, even though three of the four mixtures were composed of different proportions of the same 4 dry materials. Furthermore, it was shown that strength values from different experiments could only be correlated by assuming significant friction angles associated with Mohr-Coulomb failure behaviour. We propose this could be established relatively easily through a novel method, by combining rheometry-shear and uniaxial compression test results. The data seem to indicate this would be a valid approach. Normalized but physically different parameters, such as compressive strength and pulse velocity, could not be consistently correlated. Their proportions are time and mixture-dependent, which adds significant complexity to quality control and the development of generalized methods to characterize and compare buildability of cementitious mortars.