Summary
Building structures by 3D printing concrete layer by layer is tricky because the fresh concrete has to hold up the layers above it while it hardens. This paper looks at how well different computer models can predict when these printed structures will fail under their own weight. The authors created a new model that more accurately reflects how fresh concrete gains strength and tested it on walls and cylinders made by 3D printing. They found that the model closely matched real-world failures and revealed how different sizes and hardening speeds affect a structure's ability to keep building without breaking. This helps understand when and why 3D printed concrete structures collapse, improving safety and design.
3D concrete printingbuildabilityMohr-Coulomb yield surfaceextrusion-based printingisotropic hardeningfinite element methodplasticitytangent-stiffness eigenvalueself-weight bucklingstructural failure
Authors
Saif-Ur-Rehman, Annika Robens-Radermacher, R. J. M. Wolfs, Jörg F. Unger
Abstract
Extrusion-based 3D concrete printing builds structural elements layer upon layer without formwork, so freshly deposited material must carry the growing weight of the structure while it is still gaining strength and stiffness. The largest number of layers that can be printed before failure, the buildability, is therefore central to process design. Its prediction rests above all on how accurately the constitutive model represents fresh concrete. Existing finite element frameworks pair gradual pre-failure hardening only with smooth yield surfaces, which leaves the Mohr-Coulomb surface that best describes fresh concrete restricted to perfect plasticity. This work combines a smoothed Mohr-Coulomb yield surface with nonlinear isotropic hardening of the cohesion, formulated in an updated Lagrangian setting with an activation field for layer deposition. Failure is identified from a tangent-stiffness eigenvalue criterion that matches the analytical self-weight buckling load of a slender wall and is insensitive to geometric imperfection. The framework reproduces the measured collapse of a printed straight wall within one layer and predicts the collapse of a hollow cylinder closer to experiment than existing frameworks. A study over six diameters and six hardening rates shows buildability peaking at intermediate diameters and rising with the hardening rate. Recording how much of its strength the material has used at failure, slender cylinders fail with much still in reserve while intermediate ones use nearly all of it. The two failure modes are therefore identified as the limits of one process rather than separate failure mechanisms.