Microfibers (less than 100 μm in diameter) are commonly employed in structural applications to minimize early shrinkage cracking and lower pore pressure during fires. For any application, micro fiber-reinforced concrete (FRC) structural behavior and durability must be estimated using the mechanical constitutive law. Formulating a mechanical constitutive law for FRC presents several difficulties in terms of com-prehending the physical principles and employing suitable numerical techniques. A novel model called “Lattice Discrete Particle Model for micro-FRC (LDPM-MicroF)” is presented to simulate the fracture behavior of black micro-FRC. An equivalent fiber diameter coefficient has been defined to balance modeling accuracy and computational cost so that the LDPM-MicroF model can simulate the mechanical responses of engineered cementitious composites. The unimodal variation in tensile strength caused by the increase in microfiber dose is assessed and quantitatively reproduced by LDPM-MicroF predictions.

The Lattice Discrete Particle Model (LDPM) is highly effective in capturing the fracture behavior of concrete, especially at the scale where significant material heterogeneities, such as coarse aggregates, dominate. This model constructs a meso-structure of concrete using a stochastic approach to generate spherical particles. This process is guided by several key parameters, including cement content, water-to-cement ratio, and the size range of aggregates, from the largest to the smallest. Delaunay tetrahedralization is employed to establish the lattice framework, targeting the centers of aggregates, which results in formation of polyhedral cells surrounding each aggregate particle through a 3D domain tessellation. LDPM is integrated into Project Chrono, an open-source multi-physics simulation engine and implemented as a user element code in Abaqus. In this study, mechanical characterization of the 3D printed concrete samples will be investigated. A 3D scanner is utilized to ensure accurate geometric representation of the printed sample geometries, which are then imported into the FreeCAD preprocessor for meso-structure generation. The simulations of various mechanical tests are conducted, such as unconfined compression and three-point bending tests, with the ability to apply loads at different orientations relative to the printing direction. The model’s accuracy is validated by comparing the simulation results with experimental data, ensuring that it can accurately capture the behavior of 3D printed ultra-high-performance concrete under different loading conditions.

Lattice modeling of concrete is a discrete mesoscale representation of the material, where constitutive relations are prescribed at a smaller scale compared to traditional continuum-based models. These approaches can capture complex nonlinear behavior at the macroscale while maintaining a simpler and less phenomenological constitutive model at the mesoscale. Although these models come with a high computational cost, they are capable of accurately predicting global mechanical behavior and, in several cases, outperform continuum-based models. For this reason, they are considered valuable for generating high-fidelity databases that can be used in data-driven or coarse-graining approaches. In this study, discrete stress-strain findings from the Lattice Discrete Particle Model (LDPM) are upscaled using a coarse-graining technique based on the averaging of conservation equations. The results are used to calibrate a non-local damage model, where the non-local model’s length is prescribed by the width of the area in which energy is dissipated in the LDPM calculations. Multiple coarse-graining lengths, ranging from one to five times the maximum aggregate size, are considered. We conclude that the non-local length should better be directly related to the width of the area where the energy is dissipated in the LDPM calculation. We also observe that the calibrated constitutive model provides consistent responses on other structural geometries, including size effect studies.

The use of 3D printed polymers in the form of lattice reinforcement can enhance the mechanical properties of cementitious composites. Methods like Fused Deposition Modelling (FDM) 3D printing enable their creation, but this process has a large (negative) effect on their mechanical properties, with a large dependency on the printing direction. Continuing on our previous study concerned with modelling the anisotropic behaviour of 3D printed polymeric reinforcement, this work focuses on the reinforcement-matrix bond. Because of the layer-by-layer filament extrusion process of the 3D printing technique, the edges of FDM 3D printed polymers are typically composed of ellipses. Based on this, it is hypothesized that morphological effects as a result of the 3D printing technique enhance the bond between 3D printed reinforcement and cementitious matrix: The elliptic geometry potentially facilitates interlocking with the cementitious mortar, thereby possibly enhancing the bond behaviour in certain directions. To investigate the geometrical directional-dependent features at the edges of 3D printed polymers in more detail, micro-scale models are developed. Geometrical effects induced by different printing configurations are studied. The simulation results are verified through meso-scale pull-out experiments. The interlocking effects as a result of the 3D printing technique show to be significant seeing a bond strength increase of 56% compared to the direction without any geometrical effects.