Research Summary Report of B06

M. Sc. Quoc Tuan La                                                     Researcher, quoc-tuan.la@tu-braunschweig.de

Prof. Dr.-Ing. Ralf Jänicke                                          Project leader, r.janicke@tu-braunschweig.de

Technische Universität Braunschweig, Institute of Applied Mechanics

Prof. Dr.-Ing. habil. Stefan Kollmannsberger        Project leader, stefan.kollmannsberger@uni-weimar.de

Bauhaus-Universität Weimar, Professorship of Data Engineering in Civil Engineering

The main goal of the project is to simulate the deposition process of additive manufacturing for complex geometries.  The simulation focuses on Shotcrete 3D Printing (SC3DP) and includes identifying and calibrating a thixotropic material model. Additionally, Variationally consistent Computational Homogenization is employed to minimize computational costs.

Summary

Shotcrete 3D Printing (SC3DP) is a soft material that hardens over time through structure formation processes like flocculation. When subjected to loads exceeding its time-dependent yield strength, the material transitions back to a liquid state and exhibits pronounced shear-rate-dependent behavior. These characteristics are commonly modeled using a combination of thixotropy and Bingham-type rheology.

Once the material model is identified and implemented to the Finite Element Method, it is calibrated against experimental results from tests conducted on freshly printed SC3DP. These tests include triaxial and indentation tests, which measure elastic and plastic properties, plastic flow, and yield strength. Furthermore, tilting tests are performed to investigate the interactions between printed layers.

To simulate complex geometries using a high-fidelity material model, Variationally consistent Computational Homogenization is combined with an advanced Finite Cell discretization strategy to overcome existing computational challenges. First, a Representative Volume Element is created to act as a constitutive driver for the macroscopic Boundary Value Problem, which is solved using the nested FE² method. However, this procedure is computationally expensive and typically reserved for academic problems. To improve efficiency, Numerical Model Reduction is employed. This method collects snapshots during an offline phase using Proper Orthogonal Decomposition, allowing the macroscopic problem to be solved more efficiently during the online phase. Finally, the Finite Cell Method is utilized to create Finite Element models with complex geometries on a part-scale.

The resulting macroscopic models are validated against SC3DP structures with unique geometry, such as double curved wall and vaulted ceiling.

Current state of research

Cementitious materials develop strength through both irreversible structural changes, such as those caused by hydration mechanisms, and reversible changes primarily driven by flocculation within the microstructure. While the hydration process is slow and contributes to long-term strength development, this project primarily focuses on flocculation. During the build-up phase, cement particles suspended in water form weak physical bonds and aggregate into larger clusters when the material is at rest. These agglomerations provide structural stability but can break down into smaller clusters under constant shear stress. This reversible behavior is characteristic of thixotropy, which describes the reduction in viscosity under constant shear and the recovery of viscosity once the shear stress is removed. To model cementitious materials, particularly SC3DP in this project, thixotropic rheology is used in conjunction with Bingham-type rheology.