A Mechanistic Evaluation Relating Microstructural Morphology to a Modified Mohr-Griffith Compression-Shear Constitutive-Model for 3D Printed Concrete (2022-02)¶

The hardened-state mechanical characteristics of 3D printable concrete (3DPC) mixtures exhibit a strong dependence on the employed extrusion-based process, material, and design parameters and are predominantly anisotropic by nature. It has been shown that at the heart of the observed mechanical anisotropy lies the microstructural morphology of the manufactured component. Additionally, it is hypothesised that a linear Coulomb friction assumption misrepresents the interfacial compression-shear constitutive behaviour exhibited in 3DPC. Thus, additional calibration of the shear model parameters is sought, forming the basis for the current investigation. In this regard, the present contribution offers a comprehensive investigation of the constant compression-shear performance of a fibre-reinforced printable concrete (FRPC) mixture via a direct shear test (DST) methodology for concrete samples additively manufactured by extrusion-based 3D concrete printing (3DCP). The anisotropic material strength is studied in the three orthogonal material planes, then suitable failure criteria are considered, and a novel modified Mohr-Griffith criterion is proposed. X-ray computed tomography (CT) is employed to explore the microstructural morphology (pore size, shape, orientation, and total porosity content), fracture surface angle, and fracture surface area of 3DCP inter and intralayers compared to specimens cast from the same FRPC mixture. A mechanistic evaluation of the constant compression-shear performances relates the ensuing shear strength to the microstructural morphology observed in the experimentally assessed samples. Thereby, this contribution provides the basis for a fundamentally more detailed understanding of the hardened-state mechanical capacity of 3DPC, which is supported by a novel failure criterion and solid theoretical explanations of the influential microstructural features affecting the mechanical characteristics. Finally, it is postulated that improved mechanical performance and reduced anisotropy, conjuring less material complexity and uncertainty, is permitted by stabilising the microstructural morphology in 3DPC.