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.

The current structural demands include complex designs, efficient utilization of material resources, and maintenance of existing structures and infrastructure. In all these cases, the connections between structural components are the main focus of design since they are widely considered the weakest link in a structural system. The demand for strong and durable connections with cementitious materials is higher than ever. Creating reliable connections is largely connected to material reuse, waste reduction, ease of disassembly, and the ability to extend the life cycle of structures. These principles contribute to a more sustainable approach to construction.

Recent research shows that by implementing intricate interlocking geometries, toughness can be added to inherently brittle materials like ceramics or polymers. With the concept of "toughness by segmentation," new metamaterials emerge with enhanced properties compared to the monolithic material they are made of. In this study, the focus is on bistable interlock, a new type of connection. Inspired by nature, the connection is based on double-radii morphologies that geometrically lock into two equilibrium positions under tensile load, exhibiting two distinctive peaks in their force-displacement diagram. When the bistable interlock mechanism was applied to Acrylonitrile Butadiene Styrene (ABS), a relatively brittle yet strong polymer, the sutured material was up to 10 times tougher than monolithic ABS. The focus of this research is to manufacture the bistable interlock mechanism with cement-based materials and specifically, Strain Hardening Cementitious Composite (SHCC). SHCC belongs to the category of fiber-reinforced concrete and is distinguished by its tensile hardening behavior and pseudo-ductility stemming from its fiber-bridging property. Combined with the geometrical hardening of bistable interlock, the ultimate goal is to create resilient connections that balance toughness and strength.

The performed literature study was focused on three areas: the bistable interlock mechanism, interfacial load transfer mechanisms in concrete-to-concrete interfaces such as friction, chemical bond, and mechanical interlock, and the material and mechanical properties of Strain-Hardening Cementitious Composites (SHCC).

Two main areas of interest were the objects of the experimental study. The first was to understand the tensile behavior of bistable interlocks, and the second was to optimize it by appropriately tailoring the interface and geometry. The design of the experiments featured three parameters: the key shape (straight & curved keys), the interface treatment (untreated & lubricated interface between the two parts), and the geometry (based on width-to-height ratios for straight keys & radii ratios for curved keys).

From the experimental results, it was found that the shape of the keys changed the tensile response of the specimens greatly. The influence was different for untreated and lubricated interface specimens. For the untreated specimens, the complex shape of the bistable interlocked geometry combined with interface adhesion led to 78% of the untreated specimens rupturing at the interface. Only 44% of the straight keys showed failure under the same conditions. In this application of bistable interlock, no benefits of geometrical hardening could be exploited due to the strong adhesive bond causing premature failure of the keys at the interface. For the lubricated specimens, shifting from straight to curved geometry brought simultaneous increases in force and energy (i.e. defined as the area under the force-displacement diagram) for all the specimens, fully exploiting the benefits of the frictional contact of the bistable interlock mechanism. The increase in force documented ranged from 41-62% and in energy from 9–96%.

The aforementioned difference in tensile response highlights that the interface treatment is a governing parameter. Only 56 and 22% of untreated straight and curved specimens fully delaminated (e.g. instead of breaking) in comparison to 89% of their lubricated equivalents. The rest of the specimens exhibited (localized) SHCC failure due to the strong interface bond. Untreated specimens showed a higher resistance force (approximately 20% for straight and 10% for curved keys) but a more brittle response, resembling a monolithic connection, while lubricated specimens showed less resistance to tension, resembling a sliding connection. This trend is consistent with broader findings in the literature: inherently brittle monolithic materials compared to their architectured counterparts exhibit greater strength but lower toughness. For the straight keys, lubrication made the failure mode more uniform but decreased the strength and energy. The strong bond of untreated specimens, accompanied by a hardening response due to fiber activation against torsion and/or bending, was responsible for this result. Specimen imperfections caused this state of combined loading. Curved lubricated keys showed an enhancement in energy absorption (i.e. area under the force-displacement diagram) due to the exploitation of the bistable interlocks. Special curved keys made of assembled parts were investigated, simulating a precast-to-precast connection. The assembled keys did not outperform the lubricated and untreated curved keys in terms of strength and energy absorbed. Their benefits lie in two areas: they were easier to manufacture, and they attained a larger second peak than the first in the force-displacement diagram. This characteristic is beneficial for the mechanical stability of the system.

To optimize the response, the specific geometry of the specimens was analyzed (w/h and 𝑅1/𝑅2). The influence of the geometry on the tensile response was not as prominent as the interface treatment. However, improvements were noticed when increasing geometry parameters. For untreated and lubricated straight keys, increasing the length led to a proportional increase in absorbed energy but not in strength. For the curved untreated specimens, the increase in geometry yielded no major differences since the interface treatment governed the response. Conversely, for the lubricated specimens, with a geometry increase, the response was enhanced in both strength and energy and eventually, a design threshold at 𝑅1/𝑅2 = 1.10 was noticed. A clear trend of an increase in the first peak, and a decrease in the second peak as the geometry increased, existed. Extensive cracking and loss of stiffness after the second equilibrium position due to the geometrical interference of larger keys were responsible for that.

Overall, the architectured SHCC material, straight or curved, attained 1/3 of the strength of the monolithic SHCC. This was even lower for lubricated keys. When it came to energy absorption, the lubricated curved keys with bistable interlocks performed better, reaching up to 75% of the SHCC’s energy. This is contrary to the literature findings, where bistable interlocked materials made of ABS were tougher than monolithic ABS. In the case of SHCC, the material properties were different. Due to the extensive cracking of the key, reduced frictional contact occurred, and reduced energy was absorbed. However, a beneficial characteristic of the architectured SHCC keys was their sustained resistance to tension at higher strain levels. That makes them beneficial for many engineering applications where energy absorption and resistance to impact loads and thermal and/or hygral effects are prioritized. Another benefit exists in the customization of their tensile response by fine-tuning geometrical parameters. For a radii ratio of 1.10 in bistable interlocked keys, a satisfactory balance of strength and toughness was achieved, showing that with appropriate design, the connections have promising results.