Abstract
The construction sector is experiencing significant technological innovations with digitalisation tools and automated construction techniques, such as additive manufacturing. Additive manufacturing utilising cement-based materials can potentially remove the technological/economic barriers associated with innovative architectural/structural shapes which are not suitable for conventional formworks adopted for concrete material. However, in the “free-form” digital fabrication with concrete, the mechanical properties prediction of the material in the fresh state is essential for controlling both the element deformations and overall stability during printing. In this paper, the authors explore the critical aspects related to the determination of the early-age creep properties of a 3D printable cement-based material, particularly investigating such a behaviour at different resting times. The experimental results are used to calibrate the Burgers’ analytical model to consider both the elastic and the viscous response of the 3D printable mortar investigated in the fresh state. The visco-elastic model is validated by comparing the analytical total strain vs time curve with the corresponding experimental counterpart replicating the layer-by-layer stacking process in the 3D concrete printing process. It was found that the Burgers’ model represents a valuable numerical approach to evaluate the overall accumulation of layer deformation of a 3D printed element, since it is capable of taking into account the time dependency due to the time gap and the variable material stiffness over the process time.
Highlights
The 3D concrete printing (3DCP) [1, 2] technique consists of extruding and depositing fresh concrete filaments without formworks that are generally adopted to confine and stabilise the poured material in the traditional formative processes
The 3DCP technique starts with the pumping of the cementitious mortar, whose composition is designed to provide a thixotropic behaviour: the shear-thinning property of thixotropic materials implies that the viscosity and the yield stress decrease if external energy is applied
The optimal and calibrated displacement rate adopted in this study is congruent with other works available in literature focused on cement-based printable mortar [30,31,32]
Summary
The 3D concrete printing (3DCP) [1, 2] technique consists of extruding and depositing fresh concrete filaments without formworks that are generally adopted to confine and stabilise the poured material in the traditional formative processes. The 3DCP technique starts with the pumping of the cementitious mortar, whose composition is designed to provide a thixotropic behaviour: the shear-thinning property of thixotropic materials implies that the viscosity and the yield stress decrease if external energy is applied. The fresh concrete mix should ensure adequate yield stress, stiffness, and stability to sustain its self-weight and the weight of the filaments above it; the printable cementitious mortar must satisfy specific rheological and mechanical requirements resulting into specific buildability properties (just after the extrusion) [4, 5]. The prediction of the actual behaviour of such innovative printable materials is not straightforward, especially in terms of deformations during printing; such an issue is associated with the technological novelties of digital fabrication with concrete and there is limited knowledge in this field
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