The Interfacial Transition Zone (ITZ) in Fiber Reinforced Concrete (FRC) fibre-reinforced concrete is the microscopic zone between the fibres and the cement matrix, which is essential to the overall performance of the composites. To explore the micro-mechanical properties of the ITZ, a combined experimental and numerical study was carried out targeting this microscopic area. Specifically, an indentation splitting test was developed and the single fibre pullout test was conducted. Those two test methods were then simulated to give a comprehensive understanding of the mechanical behaviour and fracture patterns of the interface.

For the indentation splitting test, micro-cubes (300 μm long) containing a segment of vertically aligned microfibre were prepared using CEM I, CEM III and CEM III mixed with limestone powder at varying water-to-cement (w/c) ratios from 0.3 to 0.5. These specimens were then subjected to a splitting test under a nano-indenter equipped with a wedge tip, and the results were compared to those of microcubes without fibres. Mechanical properties including load capacity, deformation at peak, stiffness and fracture energy were analysed. Following the experiment, lattice models with simplified and realistic microstructure were built. For the lattice model with simplified microstructure, the mechanical properties of local phases were calibrated with the experimental results. The modelling result shows that the material properties of the ITZ are approximately equivalent to 20% of the paste properties regardless of the w/c ratio. The lattice models with realistic microstructure were built based on 3D CT scans of the micro prisms with. For the 3D reconstructed digitallattice model with real microstructure, two material assignment methods, thresholding and greyscale mapping, were compared. The simulation results showed significant differences between the two methods, which may be attributed to the empirical parameters used to calculate tensile strength.

Furthermore, the single fibre pullout test was conducted to examine the interphase properties and compared with the experimental result of the indentation splitting tensile test. Results show that CEM I demonstrates the highest chemical bond energy (Gd) among the tested materials, decreasing significantly with an increase in the w/c ratio. In contrast, CEM III+L displays an inconsistent trend, with the highest Gd observed at a w/c ratio of 0.4. However, the experimental results for frictional bond strength (τ0) and slip-hardening coefficient (β) do not exhibit significant trends or differences. The simulation of single fibre pullout accurately captures the initial load phases up to full debonding.

The developed indentation splitting test offers a valuable method for assessing the tensile properties of the ITZ. The modelling results indicate that the mechanical properties of the ITZ are approximately 20% of those of the bulk paste. This finding can be integrated into larger-scale simulations by incorporating the effects of the ITZ. Coupled with the lattice model, this approach provides an effective way to enhance the fibre-matrix ITZ of FRC. Additionally, the insights from this study can be leveraged to optimize the design and performance of FRC, potentially leading to cost savings and extended durability in construction projects.