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.

Strain Hardening Cementitious Composite (SHCC) is an innovative type of fibre-reinforced cement-based composite that has superior tensile properties. Because of this, it holds the potential to enhance the shear capacity of reinforced concrete (RC) beams, if applied properly. Experimental research was thus carried out with the purpose of investigating the shear behaviour of reinforced concrete beams enhanced with thin SHCC laminates (10 mm in thickness) in their webs (henceforth referred to as hybrid SHCC-concrete beams). This research distinguishes itself from other studies by the fact that the hybrid beams were manufactured by casting conventional concrete inside pre-cast SHCC laminates, consequently, forming an interface between concrete and SHCC. Moreover, two different types of SHCC-concrete interface designs have been used in the hybrid beams, namely smooth and profiled ones. Furthermore, beams with and without transverse reinforcement (stirrups) were both prepared in order to investigate the effect of two specific methods of shear reinforcing (i.e., SHCC and stirrups) on each other. On top of that, one of the hybrid beams with stirrups has been supported only at its normal concrete core to uncover any irregularities in the shear behaviour compared to a beam supported at its full-width. Conventional RC beams (without SHCC laminates) were also prepared as references. All beams were tested in a three-point bonding set-up while monitored by two separate systems: Digital Image Correlation (DIC) and Linear Variable Data Transformers (LVDTs). The camera images were analysed by the software package, GOM Correlate 2019. During the casting of the beams, samples of all materials were taken and tested to establish their mechanical properties for quality control purposes. Results show that the hybrid beams have obtained higher shear capacity than the control group. Only the hybrid beams with a minimum amount of shear reinforcement were capable to activate SHCC web laminates to their full extent. In the case of hybrid beams without transverse reinforcement (TR), only half of SHCC laminate potential was utilised approximately. This study proves that, by applying advanced material (i.e., SHCC) properly, an efficient way of improving the shear capacity of an RC beam can be achieved, so long there is minimum TR provided. If a minimum transverse reinforcement is not present, the benefits of shear enhancement by SHCC become less effective due to the low Young’s modulus of SHCC.

Cracks that develop in concrete due to tensile stresses can lead to the corrosion of embedded reinforcing steel, which is a primary cause of concrete deterioration. It's common practice to introduce additional reinforcement to limit crack width in concrete. Another potential solution involves partially substituting concrete with alternative materials. Strain-hardening cementitious composites (SHCC) display ductile behavior, forming multiple fine cracks under tension, making them a potential candidate for use in conjunction with traditional concrete. This study aims to explore the flexural behavior of SHCC-RC hybrid beams and the shear behavior of SHCC-RC hybrid beams without transverse reinforcement. This investigation encompasses load-bearing capacity, crack patterns, crack width control, and post-cracking shear ductility. The SHCC-RC hybrid beams consist of a SHCC U-shaped formwork with lower-quality concrete cast inside. The feasibility of 3D printing the stay-in-place formwork is also under examination. Two types of experiments were conducted to examine the structural response of the SHCC-RC hybrid members: a four-point bending test to assess flexural behavior and a three-point bending test to study shear behavior. Digital image correlation (DIC) was used to evaluate cracking patterns and crack widths, and these measurements were corroborated with linear variable differential transformers (LVDTs). Four specimens underwent bending tests: a control beam, a hybrid beam with a transverse profiled interface, a hybrid beam with both transverse and longitudinal profiled interfaces, and a hybrid beam with a 3D printed SHCC formwork. The experimental results of the bending tests reveal that SHCC-RC hybrid beams exhibit significantly higher bending capacities compared to the control beam. Specifically, the control beam reached a capacity of 98.3 kN, while the hybrid beams with transverse, transverse and longitudinal interfaces, and a printed SHCC formwork had capacities of 145.1 kN, 159.1 kN, and 152.4 kN, respectively. The interface properties between SHCC and concrete were robust enough to prevent complete delamination of the SHCC formwork. Additionally, the hybrid beams demonstrated superior crack width control. The hybrid beams with precast SHCC U-shaped formwork, with or without a longitudinal profiled interface, exceeded a maximum crack width of 0.3 mm at 85.9% and 86.3% of their capacity after the reinforcing steel yielded, respectively. In the case of a printed SHCC formwork, the maximum crack width exceeded 0.3 mm at 78.1% of the capacity, while the control beam exhibited this at 49% of its capacity, well before the reinforcement's yield point. Furthermore, four specimens without transverse reinforcement were subjected to shear tests: a control beam, a hybrid beam with a transverse profiled interface, a hybrid beam with both transverse and longitudinal profiled interfaces, and a hybrid beam with a 3D printed SHCC formwork. The shear test results suggest that a stay-in-place formwork can marginally enhance the shear capacity of an RC beam. The control beam and the hybrid beam with a transverse profiled interface exhibited capacities of 103.7 kN, while the hybrid beam with both transverse and longitudinal profiled interfaces had a capacity of 113.5 kN. The composite beam with a printed SHCC lost formwork achieved a capacity of 124.5 kN, possibly due to unintended increases in SHCC formwork thickness. All hybrid beams displayed greater energy absorption capacity and superior post-cracking shear ductility compared to the control beam. Specifically, the energy absorption capacity for hybrid beams without a longitudinal profiled interface, with a longitudinal profiled interface, and with 3D printed formwork was 26.7%, 106.5%, and 160.5%, respectively...