In civil engineering, construction projects often involve assembling various components at different stages, creating interfaces where load transfer must be considered. This load transfer is governed by dowel action (reinforcement), mechanical interlock, adhesive bonding, and friction. Without reinforcement, most concrete interfaces are brittle and weak. However, interfaces don't have to be brittle and weak. Natural hard materials like bone, tooth, and seashells have tough interfaces due to geometric interlock and friction, which could be used to enhance concrete connections' performance, making them more ductile.

This study focuses on applying a natural interface design to a concrete-to-concrete connection under bending conditions. The design is inspired by jigsaw-like contours made from arcs of circles with radii R1R1​ and R2R2​, blended tangentially at specific angles θ1θ1​ and θ2θ2​. This interlock shows two stable positions during pullout, making the design bistable. The goal is to replicate this interlocking connection using Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC), known for its higher tensile strength and strain-hardening behavior compared to traditional concrete.

A literature review covered three areas: the jigsaw-like interlock's bistable behavior in various materials, the behavior of interlocks under three-point bending, and the composition and properties of UHPFRC. Following the review, a numerical study was conducted, simulating a three-point bending test on the interface. The model was used to design and optimize the interface profile, resulting in three designs connecting two UHPFRC specimens with five interlocks each. A parametric study investigated the effects of various parameters, including E-modulus, material strength, friction coefficient, and plastic strain at peak resistance.

The fracture behavior under bending revealed several phases: initial linear loading with geometric hardening, plastic deformation of the lowest tab, and eventual failure of the middle tabs at peak load. The smoothest interlocking design, with minimal surface deviation, showed the most ductile response. This design was achieved with low θ1θ1​ and θ2θ2​ and radii close to a quarter of the tab width. A more ductile behavior was obtained using materials with low elastic modulus, higher tensile strength, lower friction coefficient, or more plastic strain, although this reduced strength.

Two additional designs were investigated to improve ductility: one with a single-circle interlock and one with a gap. The single-circle design showed more ductility due to fewer contact points, while the gap design slightly improved ductility at the cost of some strength. Experimental research on scaled designs revealed discrepancies with numerical predictions, likely due to poor fiber distribution and fabrication issues. Adjusting the numerical model's friction coefficient and increasing experimental sizes could resolve these discrepancies.

Ultimately, a very smooth interlocking design was optimal for UHPFRC connections, achieved with specific angles and radii. For further research, a bent interlocking tab design is recommended to address off-center forces and improve ductility without significantly decreasing strength. Additionally, enhancing the analytical model by revising assumptions could improve its accuracy, particularly under higher displacements. This includes incorporating bending moments in the tabs, adding a plastic hardening phase, and accounting for non-rigid movements.

Currently concrete is one of the most used building materials in the world. Despite its ability to withstand compressive loads, concrete has a low tensile strength, making it prone to cracking when pulled apart. Therefore, reinforcement is required to withstand the tensile stresses. This is generally achieved by applying steel rebars into the tension zone of the concrete structure. In recent years, there has been a growing trend of incorporating fibers into concrete mixtures to develop cement-based materials with properties resembling steel, such as Engineered Cementitious Composites (ECC).
This thesis explores innovative approaches to improve the mechanical properties of ECC by employing 3D-printed auxetic and non-auxetic reinforcements. Auxetic materials display a distinctive characteristic where, upon vertical stretching, they exhibit lateral expansion, and upon vertical compression, they undergo lateral contraction. In other words, when subjected to tensile loading, auxetic materials expand horizontally, and when subjected to compressive loading, they contract horizontally.
The main research question revolves around the possibility of enhancing the deformation capacity of ECC through these innovative reinforcements. To address this question, numerical simulations, experimental tests, and comprehensive analyses were conducted.
The study begin with the creation of ECC samples reinforced with 3D printed polymeric meshes, exploring different angles, volumes, and sizes of reinforcements using two distinct 3D printing materials, namely Acrylonitrile Butadiene Styrene (ABS) and Thermoplastic Poly-urethane (TPU). The mechanical characteristics of the composite materials were assessed by uniaxial tensile testing, and their response to stress was thoroughly examined.
The results conclusively demonstrate that the incorporation of 3D printed auxetic and non-auxetic reinforcements significantly increases the deformation capacity of ECC. The auxetic designs have improved deformation and flexibility, which makes them perfect for applications that value ductility and strain capacity. In contrast, non-auxetic designs, in particular honeycomb structures, exhibit higher stiffness and load-bearing capacities, making them appropriate for situations that demand structural rigidity and resistance to deformation.
Moreover, the study highlights the crucial role played by the choice of 3D printing material in influencing the strength and strain capacity of the reinforcement. ABS exhibits superior load-bearing capacity due to its high stiffness, while TPU showcases exceptional strain capacity, owing to its elastic and flexible properties. The investigation of many factors, including angles, volumes, and sizes, highlights their substantial influence on the mechanical characteristics of the ECC reinforcement. These characteristics can be changed to allow for alternatives between stiffness, load-bearing capacity, and strain capacity, which can be used to optimize the design of reinforcement for a variety of applications.
In conclusion, this thesis makes a contribution to the developing topic of "designer construction materials," where the properties of cementitious composites can be tailored and optimized through innovative reinforcement strategies. Future constructions that are durable, flexible, and sustainable will be made possible by the combination of 3D printing technology with ECC. This thesis encourages researchers to go further, where imagination and creativity meet concrete, creating a world where materials work with us to create a physical environment that is more resilient.

This research focuses on developing architected lightweight cementitious cellular composites (LCCCs) for potential thermal insulation purposes. Specifically, Voronoi cellular structures with different randomness are adopted and used as cellular structure of the LCCCs. the mechanical properties of the developed LCCCs are investigated by experimental and numerical methods. With the help of 3D printing technique, LCCCs are prepared with two mixtures: a reference mortar, denoted as REF and mortar incorporating microencapsulated phase change materials (mPCMs), denoted as PCM14. Mechanical properties of developed structures are first investigated. The compressive behaviour of the LCCCs is investigated at the age of 28 days. In general, the LCCCs show anisotropic compressive behaviour in two loading directions: the compressive strength and stiffness of the LCCCs in the out-of-plane direction is substantially higher than that of the in-plane direction. Comparing to conventional foam concrete with the same density, the out-of-plane compressive strength of the LCCCs is higher than commonly reported cases in literature. Due to the high porosity of the cellular structures, the compressive strength of the LCCCs is significantly reduced comparing to the corresponding constituent materials, as expected. In addition, this strength reduction of the LCCCs made with PCM14 is substantially lower than the that of the REF samples. Interestingly, the relative stiffness (measured stiffness of the LCCCs normalized by their porosity) of the mPCMs incorporated LCCCs is even higher than the constituent material E-modulus. Furthermore, the critical role of air voids on the compressive behaviour of the LCCCs is identified. The thermal conductivity of the LCCCs is investigated by numerical models using commercial software Abaqus and ANSYS. It is found that the thermal conductivity of the LCCCs is comparable with the conventional foam concrete of the same density. The geometry heterogeneity of the LCCCs has a minor effect on determining the thermal conductivity of the LCCCs. Moreover, increasing porosity or reducing thermal bridge on the heat transfer direction can dramatically improve the thermal insulation performance of the LCCCs. It is found that the natural convection of the air within the cellular pores and the radiation has less notable effect on the thermal conductivity of specimen. Comparing to the REF, PCM14 has enhanced thermal insulation performance. In the end, using extrusion-based 3D concrete printing technique, the LCCCs with mPCMs has been successfully printed as an implement of manufacturing LCCCs as a construction-scale brick. In general, the developed LCCCs have high porosity gives good insulation properties as well as good compressive strength. This gives them great potential to be used as novel lightweight thermal insulation materials for construction.