Uncontrolled cracking in reinforced concrete structures accelerates durability issues by creating pathways for external agents to penetrate the matrix, often leading to costly repairs and reduced service life. This dissertation addresses these challenges through the development of a self-healing strain-hardening cementitious composite (SHCC) designed specifically for application in the concrete cover zone.

This thesis adopted a multi-faceted methodology. First, a self-healing SHCC material was developed, featuring bacteria-embedded polylactic acid (PLA) capsules to realize controlled microcracking and robust healing. Next, the research introduced a localized application strategy to address the cost-effectiveness of this material. By applying the self-healing SHCC exclusively to the concrete cover zone, the region most critical to durability, this approach minimizes unnecessary use of healing agents, balancing performance with economic viability. To validate the concept, experimental and numerical analyses were conducted to evaluate the performance of hybrid beams with self-healing SHCC covers. Furthermore, different manufacturing methods, including prefabrication and 3d printing, were explored. Lastly, design strategies were proposed to incorporate the self-healing benefits into structural service life models. The feasibility of the developed system was demonstrated at full scale by applying it in the construction of a tramline.

The study revealed that the incorporation of PLA capsules into SHCC significantly improved crack-healing efficiency while maintaining critical tensile properties. It was found that the fibre/matrix bond properties were enhanced by the addition of the HA. As a result, the addition of healing agents reduced residual crack widths by up to 70%, ensuring faster and more robust healing under varied conditions.

At the structural level, hybrid beams with SHCC covers exhibited enhanced performance. Beams with SHCC applied in the bottom cover zone demonstrated improved flexural behaviour, with controlled crack patterns and reduced crack widths, attributed to the optimized interface condition between the SHCC cover and concrete core. A novel type of SHCC/concrete interface that features a weakened chemical adhesion, but an enhanced mechanical interlock bonding was developed to facilitate the activation of SHCC. Similarly, hybrid beams with lateral SHCC layers showed a notable increase in shear resistance under critical loading conditions. Numerical simulations supported these experimental findings, revealing the importance of the interface condition between SHCC cover and concrete core.

For the developed self-healing cover system to be applied in structures, it is necessary to consider the implications of healing during the design process. Analysis of this thesis shows that, by refining existing engineering models to include the impact of cracks, it becomes possible to predict and design the healing effects under specific scenarios.
To further demonstrate the self-healing cover concept, the developed self-healing SHCC was applied in a full-scale construction project where stringent requirements for tensile performance and crack healing properties are essential. The project showcased the feasibility of large-scale mixing, pumping, and application of the self-healing SHCC system.

This thesis contributes to the field of self-healing concrete by advancing material performance, structural application techniques, and design integration. By focusing on localized and practical implementations, the research bridges the gap between experimental advancements and full-scale applications where traditional solutions do not meet demands. The findings underscore the potential of self-healing concrete to extend the service life of structures without imposing substantial additional costs.

Due to aging, increase in service loads, and/or upgrade of design codes, many existing concrete structures do not, or soon will not satisfy the required load-bearing capacity. Therefore, measures such as reconstruction or repair are necessary. Among various strengthening methods, ultra-high performance fiber reinforced concrete (UHPFRC), due to its high mechanical properties and superior durability, has emerged as a promising solution for strengthening concrete structures by enhancing load and deformation capacity, and improving durability. While extensive research has been conducted on the use of UHPFRC for flexural strengthening, studies focused on shear strengthening remain limited. After strengthening, the interface between UHPFRC and traditional reinforced concrete (RC) structures is formed. However, interface behavior has not been thoroughly investigated, further posing a gap in understanding the optimal strengthening design of UHPFRC in hybrid UHPFRC-concrete systems.

This dissertation focuses on the shear performance of concrete structures strengthened with UHPFRC, with a focus on the interface behavior between UHPFRC and normal concrete, and the mechanical properties of UHPFRC.

To assess the shear strengthening efficiency of UHPFRC, this dissertation starts with a literature review (Chapter 2) that addresses three major aspects. The first aspect focuses on the shear performance of hybrid UHPFRC-RC structures. Strengthening applications of UHPFRC in shear, and current analytical and numerical methods to predict the shear capacity of RC structures strengthened with UHPFRC are critically analyzed. The second aspect is focused on the UHPFRC-concrete interface behavior which is governing the response of the hybrid beams. From the review, the role of governing parameters on the interface behavior, including the effects of bonding technique, moisture exchange between the two materials, differential shrinkage and the role of coupled environmental and mechanical loads, are discussed. The final aspect deals with the application of non-destructive techniques (NDTs) to assess the strengthening efficiency of UHPFRC, focusing on evaluation of (i) UHPFRC material properties and (ii) UHPFRC-concrete interface performance in hybrid structures.

Experimental design is presented in Chapter 3, where the material and structural tests are systematically introduced. This chapter provides a series of material tests to, among others, characterize the workability, shrinkage and mechanical properties of both UHPFRC and normal concrete (NC). It also introduces the design and setup of a comprehensive structural test to evaluate the shear performance of UHPFRC-strengthened RC beams. Following the experimental methodology from Chapter 3, Chapter 4 presents the results of the material and structural tests. Through comparative analysis, this chapter examines the material and structural behavior of UHPFRC-strengthened beams and its constituents by varying different parameters, thereby setting a basis for evaluating the shear improvement in strengthened RC beams. In order to provide a deeper analysis on the UHPFRC-concrete interface quality in strengthened beams, Chapter 5 focuses on the assessment of possible delamination between UHPFRC and existing concrete by applying active infrared thermography. Combined with both experimental and numerical analysis, a systematic procedure is developed to detect subsurface delamination in hybrid UHPFRC-NC specimens. Besides the interface properties, another governing parameter, namely the material properties of UHPFRC, is examined in Chapter 6. This chapter investigates the fiber distribution and orientation within UHPFRC elements, important factors influencing material properties of UHPFRC, and therefore further affecting its shear strengthening efficiency. Using a calibrated electromagnetic method and validated through x-ray computed tomography (CT scanning), the effect of governing parameters, including casting direction and vibration time, on fiber distribution and orientation in UHPFRC elements is investigated. This chapter helps to clarify the connection between the material properties of UHPFRC and its structural performance.

Based on the structural test results (Chapter 4), evaluation of interface properties (Chapter 5) and material properties of UHPFRC (Chapter 6), in Chapter 7, a numerical model is developed to simulate shear performance of UHPFRC strengthened concrete structures, validated by the experimental results. A parametric study is further conducted to investigate key factors including interface properties, UHPFRC tensile behavior and non-uniform fiber distribution in UHPFRC. The numerical analysis offers insights on the influence of these parameters on shear strengthening efficiency.

In the final Chapter 8, the findings regarding the overall shear strengthening performance of UHPFRC in RC beams are given, providing practical insights for optimizing UHPFRC applications in concrete structures. Finally, suggestions for future research are given.

This thesis investigates the potential of Basalt Fibre-Reinforced Polymer (BFRP) as an alternative to traditional steel reinforcement in concrete structures, with a focus on enhancing shear capacity. BFRP offers advantages such as higher tensile strength, superior corrosion resistance, and environmental sustainability. The study compares two alternative BFRP stirrup designs—braided BFRP rods and laminated unidirectional (UD) BFRP strips—with traditional steel stirrups through uniaxial tensile testing and displacement-controlled three-point bending tests on reinforced concrete beams. The results show that while BFRP stirrups improve the shear capacity of concrete beams, they do not yet match the performance of steel stirrups due to issues like reduced stiffness and stress concentrations in corner sections. However, BFRP stirrups demonstrated potential advantages in terms of weight efficiency, suggesting a promising role in sustainable construction. The findings underscore the need for further refinement in BFRP production methods to achieve consistent quality and better performance in structural applications.

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.

The emergence of innovative construction materials is dawning a new era of ambition within the civil engineering community. Among these innovative materials, Basalt Fibre Reinforced Polymer (BFRP) has recently surfaced with promising potential as a reinforcing material in concrete. Currently, in the Dutch concrete construction industry, the choice for reinforcement steel has remained unchanged for the past decades. However, the increasing availability of innovative alternatives could help the transition to a more sustainable concrete industry. Although BFRP has promising potential for application in concrete structures, the global application has not been established yet. One of the reasons for this limited research into the structural behaviour of concrete structures reinforced with BFRP-bars. Furthermore, the limited development of codes specifically designed for concrete reinforced with BFRP-bars and the modest availability compared to reinforcement steel also play into the unknowns about the material.

BFRP-bars contain certain qualities that reinforcement steel does not. One of the most prominent is resistance against corrosion due to environmental influences on concrete structures. This eliminates the requirement for the concrete cover to protect the reinforcement from corrosion. Hence, the concrete cover only serves its purpose to ensure effective bond action between the reinforcement bars and the concrete. This inherent quality of BFRP-bars eases the crack width control requirements in the codes for the design of structures reinforced with BFRP-bars to a range of 0.5 mm to 0.7 mm. Although this is a significant increase in comparison to the Eurocode for concrete structures (0.2 mm to 0.4 mm), the properties of BFRP-bars cause larger crack width development.

The aim of the experiment is to investigate the flexural behaviour of concrete beams reinforced with BFRP-bars as tensile reinforcement. The flexural behaviour of concrete structures reinforced with BFRP-bars is studied both experimentally and numerically. The research program contains 6 beams differing in reinforcement material, concrete covers, reinforcement ratio and bar diameters. To investigate the effects of the concrete cover, 2 beams are designed with concrete covers of 31 mm and 11 mm containing 3 BFRP-bars with a diameter of 8 mm in
the tension zone. To compare the behaviour of these beams, 2 identical beams with reinforcement steel are designed. To determine the effects of the reinforcement bar diameter, 1 beam is designed with 2 bars with a diameter of 10 mm. The reinforcement ratio in beams remains approximately equal, hence the only changing parameter is the bar diameter. The effects reinforcement ratio is investigated by a beam designed with 2 bars with a diameter of 8 mm. By keeping the bar diameter and the concrete cover the same, the reinforcement ratio is the only changing parameter for this beam. By subjecting the beams to a 4-point bending test, a fully developed crack pattern can be established over a certain length. By using digital image correlation (DIC), the flexural behaviour is monitored and analysed. This includes both crack width development and overall pattern forming. The results are verified with linear variable differential transformers (LVDT‘s) and a laser measuring vertical displacements. This procedure is devised to evaluate the stiffness behaviour of the beams as well as the cracking behaviours. In addition, the experimental program includes a series of direct tensile tests with reinforcement bars to determine the stress-strain behaviour of the reinforcement bars themselves...

Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) is a promising material for different applications such as bridge deck strengthening due to its improved strength and durability properties. Especially the tensile strength and ductility is heavily dependent on the distribution and orientation of the steel fibers within the element. This fiber behaviour can be unpredictable, causing a large scatter in material properties. This scatter is inconvenient for predicting the material properties. The novelty of the material is the main reason that UHPFRC is not included in the Eurocode. Certain countries like Switzerland and France have developed their own UHPFRC norms.

In order to increase the knowledge and understanding of the fiber distribution and orientation, a non-destructive testing method has been developed. Two types of magnetic probes have been developed: the C-shaped ferrite probe and the single ferrite probe. These probes are based on other probes developed at the University of Milan and Porto. The C-shaped ferrite probe is most suitable for measuring the total fiber content and fiber orientation. The single ferrite probe is able to take fiber content measurements closer to the edge making it more efficient for fiber distribution measurements. This can result in a reliable prediction of the peak-tensile strength. The addition of UHPFRC specific design rules and this non-destructive testing method will make UHPFRC and its applications more attractive.

In this research, 27 different UHPFRC plates were cast with the same dimensions of 10x200x200 mm3. These plates were cast horizontally or vertically with either 1%, 2%, 3% or 4% fiber content. Seven plates were vibrated after casting in order to analyse the influence of vibration. Each fiber content resulted in a relatively homogeneous fiber distribution and random fiber orientation. Horizontal casting of UHPFRC plates usually result in a more homogeneous fiber distribution and more random fiber orientation compared to vertically cast plates. The best distribution and orientation was reached when the elements were not vibrated at all. Vibration can result in severe fiber segregation along the element height and vertical orientation of the fibers. These conclusions hold for longer UHPFRC elements where the element height and the element thickness remains constant.

One of the main pitfalls in this research is the small thickness of the plates. The length of the fibers exceeds the plate thickness, which makes casting very difficult. During casting, fibers will get stuck at the top of the mould, which also influences the results. Moreover, it is important to continuously keep mixing the UHPFRC mixture in order to equally divide the fibers over all plates and to use a mixture that is castable yet not too flowable. High workability mixtures are more prone to fiber segregation.

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.

Recent advancements at TU Delft have investigated the potential of geometrically enhanced profiles to improve the performance of concrete-to-concrete interfaces, with one study focusing on the tensile behavior of single-tab Strain Hardening Cementitious Composites (SHCC)-to-SHCC interfaces through experimental testing. SHCC, an advanced fiber-reinforced cement-based material, is known for its distributed cracking and strain-hardening capabilities, offering superior ductility and crack control compared to traditional concrete. Building on this foundation, this thesis models the tensile behavior of geometrically profiled SHCC-to-SHCC interfaces using a continuum smeared cracking model in Abaqus and a lattice discrete cracking model.
The research aims to assess the predictive capability of these numerical models in capturing the behavior of geometrically profiled SHCC-to-SHCC interfaces. It focuses on understanding how interface parameters, such as strength and geometric profiles, influence tensile performance, particularly in enhancing ductility, controlling fracture response, and shifting failure modes from brittle interface failure to ductile SHCC material failure.

To evaluate the ability of numerical models to represent the tensile behavior of geometrically profiled SHCC-to-SHCC interfaces and assess the benefits of geometric enhancements for improving tensile performance, a systematic methodology was adopted. This included an analytical analysis to identify underlying failure mechanisms and develop a simplified model for quantifying their force-displacement response. A continuum smeared cracking model in Abaqus was employed to replicate experimental behaviors and investigate the influence of interface parameters through a parametric study. Finally, a lattice model was implemented to capture fracture responses in detail, enabling an in-depth analysis of the effects of interface strength and interface geometry on strength, ductility and fracture response.

The numerical models provided insights into the tensile behavior of geometrically profiled SHCC-to-SHCC interfaces, revealing the influence of interface parameters and geometric characteristics on failure mechanisms. The continuum smeared cracking model demonstrated the ability to simulate various failure mechanisms by adjusting interface parameters, but it underestimated SHCC’s strain-hardening behavior during local material-dominated failure due to the delayed introduction of damage. Conversely, the lattice discrete cracking model effectively captured distributed cracking patterns but exhibited overly brittle responses in interface-dominated failure mechanisms due to assumptions of brittle interface elements and the neglect of frictional resistance. Parametric studies confirmed that changing interface geometry and optimizing interface strength could shift failure modes from brittle, interface-dominated failure to ductile SHCC material-dominated failure.

This study highlights the capabilities and limitations of numerical models in optimizing the tensile behavior of geometrically profiled SHCC-to-SHCC interfaces. The continuum smeared cracking model in Abaqus identified critical parameters, such as tensile strength and fracture energy, influencing failure mechanisms. However, it underestimated SHCC hardening during tab failure due to delayed damage initiation, which should occur immediately after the elastic limit to accurately capture distributed cracking behavior.
The lattice discrete cracking model effectively simulated distributed cracking but exhibited overly brittle responses in interface-dominated failure mechanisms. This limitation stemmed from brittle interface assumptions and the omission of friction, resulting in a significant underestimation of displacement capacity.

Parametric studies using the analytical and lattice models revealed that modifying the interface geometry significantly reduces the interface strength required for ductile failure. For instance, the analytical model demonstrated that adjusting the geometry reduced the interface strength needed to transition from interface-dominated to material-dominated failure from 20% to 7%. In the lattice model, at low interface strengths (≤ 25%), a change in geometry of 1% enhanced SHCC damage by 3% to 6%. At higher strengths (≥ 30%), interface-dominated failure mechanisms diminished, with full SHCC material activation observed beyond 150% strength, ensuring global rather than localized failure.

These findings confirm that optimizing interface strength and geometry enables the transition from brittle, interface-driven failure to ductile, material-driven failure. Nonetheless, the models' limitations must be addressed, particularly the insufficient representation of strength-geometry-friction interactions in the analytical model and the lack of interface ductility and friction in the lattice model. Despite these challenges, the results provide a valuable foundation for refining numerical models and guiding experimental validation to optimize SHCC-to-SHCC interfaces.

Efforts are being made globally to reduce CO2 emissions, particularly within the construction industry, which is a major contributor. Cement production alone is responsible for around 6% of global anthropogenic CO2 emissions. To address this, alternative materials to Ordinary Portland Cement (OPC) are being explored, such as alkali-activated concrete (AAC), which uses industrial by-products like granulated blast furnace slag (BFS) and fly ash (FA). These by-products are activated using alkaline solutions, leading to AAC, which exhibits comparable or superior mechanical properties, alongside enhanced chemical, acid, and fire resistance. However, despite its potential, AAC has not yet seen widespread adoption due to limited understanding of its long-term behavior, particularly in larger structural components.

This study focuses on alkali-activated slag-based concrete (AAS), which has shown changes in material properties when exposed to drying, with reductions in elasticity modulus, flexural strength, and tensile splitting strength over time. The impact of these changes on the bond behavior between reinforcement and concrete is unclear, yet bond strength is crucial for structural integrity. The main aim is to evaluate how curing conditions, curing age, and reinforcement type influence the bond strength of AAS, with an emphasis on the effects of drying on material properties.

Pull-out tests were conducted to assess the bond strength, where the reinforcement is pulled from the concrete cube while measuring the applied tensile force and rebar displacement. Specimens were cured under different conditions: some under standard moisture conditions, while others were exposed to drying for up to 84 days. The tests examined two types of reinforcement: steel and prestressing strands, and compared the results to conventional concrete (CC) as a reference. A preliminary study addressed optimal test conditions, with a focus on pull-out failure, as it provides a more ductile failure mode compared to the brittle splitting failure, offering more reliable data on bond capacity.

The optimal specimen configuration was determined to be one with an unbonded concrete layer above the bonded region, which helped achieve pull-out failure. Additionally, the level of confinement during testing was optimized to prevent splitting failure, which occurs when concrete cracks due to excessive radial tensile stresses.

The main experiment revealed that bond strength in CC remained stable even under drying conditions, whereas AAS specimens showed a decline in bond strength over time, likely due to microcracking induced by shrinkage. However, the results were inconsistent, suggesting that drying may not have a definitive effect on bond strength. Prestressed strands showed weaker bond strength compared to steel reinforcement, which is attributed to differences in bond transfer mechanisms and concrete tensile strength. The smoother surface of prestressed strands relies more on friction, which is less effective in AAS due to its reduced tensile strength.

Further analysis using Distributed Fiber Optic Sensing (DFOS) showed that internal strain measurements aligned with theoretical values in some cases, though inconsistencies arose near the boundaries of the bonded region. This highlights the need for further refinement of the testing setup.

Finally, the pull-out test results were compared to semi-empirical models and code standards. These models were generally conservative, especially for AAS specimens exposed to drying, which showed reduced bond strength below the minimum requirements set by standards like AS 01 and ACI 318. While the results for moisture-cured AAS aligned with the Harajli bond behavior model, the reduced bond strength in drying-exposed AAS could lead to unsafe designs if not accounted for in structural design standards.

Crack-width control of concrete structures, a serviceability limit state (SLS) criteria, could be governing in design calculations over ultimate limit state (ULS). To ensure adequate crack-width control, additional steel reinforcement is often used, which, while essential for SLS, is redundant for ULS purposes, thereby increasing the environmental impact due to excessive steel use. In efforts to reduce the amount of steel reinforcement required in RC beams, a series of MSc thesis studies at TU Delft have explored the potential of incorporating a layer of Strain-Hardening Cementitious Composites (SHCC) in the tension zone of beams, creating what are known as hybrid Reinforced concrete/SHCC (R/SHCC) beams. SHCC is a composite material that, through its specific composition and fiber incorporation, exhibits the ability to form multiple fine cracks when subjected to tensile stresses, offering an effective solution for enhancing crack-width control. Huang [1] demonstrated the effectiveness of a 70mm thick SHCC layer in a 200mm high hybrid R/SHCC beam. Singh [2] investigated the role of interface preparation on the crack-width control performance and found that both smooth and grooved SHCC-concrete interfaces provide similar flexural crack-width control in a 200mm high hybrid R/SHCC beam with a 70mm thick SHCC layer. Bezemer [3] explored the effectiveness of a 70mm thick SHCC layer in hybrid R/SHCC beams of more practical heights of 300mm and 400mm, revealing an anticipated decline in the layer’s flexural crack-width control efficiency as beam height increased.

The current study investigates the effect of three key parameters on crack width control: (1) the SHCC-concrete interface, ranging from a smooth interface to a profiled interface coated with Vaseline; (2) the rebar-SHCC bond, weakened by changing from ribbed to smooth rebars; and (3) the SHCC type, varying from PVA-based SHCC to PE-based SHCC. Additionally, the effect of curing time is examined. The beams, with a total height of 400mm and a 70mm thick SHCC layer, are tested experimentally in a four-point bending set-up to generate a constant bending moment region (CBMR), allowing for the study of flexural cracks. Digital Image Correlation (DIC) is performed to evaluate the crack patterns and the crack-widths. The performance of the beams is assessed by checking its capability to restrict crack-widths below the 0.2mm and 0.3mm crack-width limits.

The experimental results demonstrate that the use of a Vaseline-coated profiled SHCC-conventional concrete (SHCC-CC) interface enhances the crack-width controlling ability of hybrid R/SHCC beams compared to a smooth interface. The beam with this profiled interface demonstrates a significant increase in the load, expressed as a percentage of the yield load, at which the 0.2mm and 0.3mm crack-width limits are exceeded of 37.6% and 22.7%, respectively, compared to the beam with a smooth interface. This improvement is attributed to the interface’s ability to show controlled delamination (as a result of the mechanical interlock provided by the shear keys) over the full length of the region of interest (as a result of the chemical debond facilitated by the Vaseline coating).

The use of smooth rebars significantly compromises the crack-width controlling ability of hybrid R/SHCC beams, as the beam with smooth rebars experiences a decrease in the load, expressed as a percentage of the yield load, at which the 0.2mm and 0.3mm crack-width limits are exceeded of 51.0% and 35.7%, respectively, compared to the beam with ribbed rebars. This reduction in performance can be attributed to the weakening of the reinforcement-SHCC bond, which relies solely on chemical adhesion and friction with smooth rebars. This limitation hinders the activation of SHCC and leads to rapid localization of cracks within the SHCC. Despite the application of a Vaseline-coated profiled SHCC-concrete interface in both beams, the benefits of the Vaseline-coated profiled interface observed in the beam with ribbed rebars were not realized in the beam with smooth rebars, indicating that a sufficient bond strength is a prerequisite for effective crack-width control.

PE-SHCC improves crack-width control in hybrid R/SHCC beams compared to PVA-SHCC when used in combination with smooth rebars. The beam with PE-SHCC exhibits an increase in the load, expressed as a percentage of the yield load, at which the 0.2mm and 0.3mm crack-width limits are exceeded of 6.1% and 14.9%, respectively, compared to the beam with PVA-SHCC. This improvement can likely be attributed to the larger ductility of PE-SHCC and the superior reinforcement-SHCC bond strength in PE-SHCC, which facilitates greater activation of the SHCC. However, it remains uncertain whether these findings can be extended to the use of ribbed rebars, as the superior reinforcement-SHCC bond strength in PE-SHCC may lead to a excessively high bond strength which could hinder strain redistribution.

The experimental results also demonstrate that curing time significantly influences the crack-width controlling ability of hybrid R/SHCC beams, with the beam tested at a later age (85 days of SHCC age) showing superior crack-width control compared to the beam tested by Bezemer [3] at 55 days of SHCC age. Specifically, there is an increase in the load, expressed as a percentage of the yield load, at which the 0.2mm and 0.3mm crack-width limits are exceeded in the beam tested at 85 days of SHCC age of 10.6% and 5.5%, respectively, compared to the beam tested at 55 days of SHCC age. This improvement is attributed to two factors: (1) the tensile properties of SHCC degrade over time, delaying crack localization, and (2) the SHCC-CC interface bond strengthens over time, resulting in less pronounced delamination. Consequently, while less SHCC is activated, the ability of SHCC to serve as effective reinforcement is enhanced.

In conclusion, the current study demonstrates that crack-width control in hybrid R/SHCC beams can be significantly improved by employing a roughened interface and utilizing ribbed rebars instead of smooth rebars when working with PVA-SHCC. Additionally, when using smooth rebars, opting for PE-SHCC enhances crack-width control. The current study also shows that hybrid R/SHCC beams of more practical height can effectively control crack-widths beyond reinforcement yielding when implementing the proposed design adjustments.

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...

The economic and environmental advantages of extrusion-based 3D concrete printing have made it a frontrunner in large-scale buildings. Nonetheless, this approach suffers from a contradictory rheological requirement and a high percentage of Portland cement (PC) in its mixture, which challenges its sustainability. It is claimed that the addition of supplementary cementitious materials (SCMs) to the mixture might solve this issue.
Fly ash (FA), silica fume (SF), Ground Granulated Blast furnace Slag (GGBS), limestone, and calcined clay are examples of such materials. However, the production of certain of these SCMs, such as FA, SF, and GGBS, is restricted, while others, such as limestone and calcined clay, are plentiful. To address these challenges, the purpose of this research was to investigate two developed mixtures: limestone calcined clay cement (LC3-based) and limestone slag cement (slag-based) activated with limestone-based accelerator slurry (with calcium nitrate as an accelerator).
The first phase of this work was the formulation of flowable and pumpable mixtures. The flowability of two cementitious materials (LC3 and slag-based) and limestone-based accelerator slurry was evaluated. For each mixture, the optimum superplasticizer/ accelerator dose was determined to ensure optimum flowability and pumpability. The optimal dose of superplasticizer for the LC3- and slag-based mixtures, as determined by flowability, pumping, and flow curve tests, was 0.6% and 0.3% of the binder’s mass, respectively. The recommended Ca(NO3)2 dose for limestone-based accelerator slurry was 7% of the cement weight.
Part two of this research looked at how combining limestone-based accelerator slurry with cementitious materials affected the mixture’s fresh qualities. Here, the initial setting time and buildability of the formulated mixes were investigated. At last, a printable mixture of LC3 was developed containing just 275 kg/m3 of PC with compressive strength of more than 30 MPa at 28 days of curing.
The third section of this research was devoted to material properties. Here, the development of the mixture’s compressive strength, heat evolution, and hydration product was investigated. The compressive strength development of all mixtures was assessed at 7 and 28 days of curing. In general, the compressive strength of LC3-based mixtures with various accelerator dosages was greater than that of slag-based mixtures with the same accelerator content.
Isothermal calorimetry was employed to investigate the hydration of the mixtures. The
findings for LC3- and slag-based mixtures demonstrated that a larger calcium nitrate dose significantly accelerated the hydration of the fresh mixtures. This acceleration was shown by a quicker induction time, a shifted primary hydration peak to an earlier age of hydration, increased intensity of the primary hydration peak, and greater cumulative heat. Within the first 7 days, LC3-based mixes had a greater cumulative heat evolution than slag-based mixtures.Thermogravimetric analysis (TGA) was used to analyze the amount of calcium hydroxide and hydration water in the investigated mixes at various curing periods, including 1h, 4h, and 168h. In general, the results of the TGA test and isothermal calorimetry were comparable. The final objective of this study was to examine the adaptability of the developed mixture to an actual printing structure. So, the design of a cycle arch bridge in Zaanstad was explored. The results indicate that the developed mixture will be helpful for utilizing in the 3DCP method since the mixture showed encouraging buildability and sustainability behavior.

Strain hardening cementitious composites (SHCC) can be used to reduce the amount of reinforcement needed to control crack widths. SHCC has a strain hardening behaviour with dense micro-cracking, which makes it more ductile compared to conventional concrete. Conventional concrete can be used in the non-critical locations. An interface is formed in between the two concrete layers. The interface is the weakest link in the system, governing the structural performance. Therefore, the interface is of interest in ongoing research. The lattice model allows to investigate the effect of the fundamental parameters, the interface tensile strength and interface stiffness, on the structural behaviour in different bond tests between normal strength concrete and SHCC. Direct tension, direct shear and the notched-beam test were used to investigate the effect of the individual interface properties on the global behaviour. In all bond tests a smooth and profiled interface were simulated to investigate the effect of applying a roughness to the interface.
In the small-scale tests activation of adjacent materials was found to occur at increased interface tensile strength and decreased interface stiffness. For a smooth interface a decrease in interface stiffness resulted in a scatter of failed elements over the interface in the direct tension test, increasing the ductility. Initially, a linear relation is obtained between the interface tensile strength and load capacity for the direct tension and direct shear test. With higher interface tensile strength cracking of the adjacent materials increased the load capacity, but the adjacent materials started to become governing. The application of a profile in the interface increased the capacity even further and resulted in an increased softening. For a profiled interface the effect of changing interface properties was limited in most cases compared to the effect on a smooth interface. This was due to the activation of the adjacent materials at lower interface properties compared to the smooth interface.
Similar trends were found between the structural behaviour and interface properties for the notched-beam test. Only the application of a profile did not increase the load and displacement compared to the smooth interface. With the beam simulation results a data set is generated with the information about the load, displacement, interface opening and joint opening at failure. The data is classified based on the failure mode of the beams. From the classification it was found that with a smooth interface higher interface properties can be reached before the failure mode would change from interface to partial failure or from partial to concrete failure compared to a profiled interface. The range of interface properties resulting in interface failure is higher for the notched-beam test compared to the small-scale test. The notched-beam test will, therefore, be more likely to show interface failure when used for testing the interface.

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.

Limiting maximum crack widths is a serviceability limit state, which ensures the durability of reinforced structures. In current practice, maximum crack widths are limited by using additional reinforcement, on top of the amount of reinforcement required for the designed ultimate bearing capacity. Reducing the amount of reinforcing steel used, could make the construction industry more sustainable. Recent studies showed that, hybrid reinforced beams with a 70 mm thick bottom layer of strain hardening cementitious composite (SHCC), so-called hybrid R/SHCC beams, are promising in controlling crack widths under pure bending, without using additional reinforcement (Huang, 2017; Singh, 2019). SHCC
is made from a binder, fine particles and PVA fibers, which leads to ductile material properties and crack bridging. The beams studied in these previous studies were limited to a height of 200 mm. In practice larger heights are demanded. Increasing the height of a hybrid R/SHCC beam, by keeping the
thickness of the SHCC layer constant, reduces the relative contribution of SHCC. In addition, cementitious materials exhibit strong size effects. Therefore, the effect of height scaling on the flexural crack width controlling behavior of hybrid R/SHCC beam is studied in this thesis. In addition, the optimization potential of the crack controlling behavior of the hybrid R/SHCC beams is studied, by the delamination of the SHCC-rebar interface.

The effect of height scaling on the flexural crack width controlling behavior of the hybrid R/SHCC beams is studied both experimentally and numerically, by 200 mm, 300 mm and 400 mm high hybrid R/SHCC beams, with a constant 70 mm thick bottom layer of SHCC. Reinforced concrete beams of the same heights are used as reference. The length of the higher beams are increased, to prevent
direct force transfer. The experimental results of the 200 mm high beams are used from a previous study by (Singh, 2019). To study the effect of delamination of the SHCC-rebar interface, a 300 mm high hybrid R/SHCC beam with smooth and Vaseline treated longitudinal reinforcement bars is used. The beams are tested in a four-point bending configuration, with a 500 mm constant bending moment region. All the beams have the same amount of longitudinal reinforcement. The numerical study is performed with the Delft Lattice Model. As lattice models are only recently used for the modelling of structural behavior, the use of the Delft Lattice model in this study is a contribution to the development
of lattice models. Analytical calculations, with use of the multi-layer model (Yassiri, 2020), are used in the comparison of the numerical and experimental results.

From the performed experiments, it is found that, the load, at which the 0.3 mm crack width limit is reached, decreases from 109% to 97% and 91% of the yielding load, upon increasing the height from 200 mm to 300 mm and 400 mm, respectively. Whereas, the beam with smooth and Vaseline treated longitudinal reinforcement bars, reached the crack width limit already at 59% of the yielding
load. Increasing the height of the reinforced concrete beams does not lead to a reduction in the crack width limit load, relative to the yielding load (76%-78%). From the cracking patterns, it is found that, upon increasing the height, the number of propagated concrete cracks decrease, both in the reinforced
concrete beams and in the hybrid R/SHCC beams, whereas the hybrid beam with smooth and Vaseline treated reinforcement bars showed a single propagated crack in the concrete layer. An effective tensile area is developed for the 300 mm and 400 mm high beams, which was not observed in the 200 mm high beams. Uniform cracking distributions are found in all the SHCC layers of the hybrid beams, except for the hybrid beam with smooth and Vaseline treated
reinforcement bars. The delamination of the concrete-SHCC interface increases, upon increasing the height, whereas the ultimate bearing capacity is similar for the 300 mm and 400 mm high hybrid R/SHCC beams. On the contrary, the hybrid beam with smooth and Vaseline treated reinforcement bars shows very limited delamination. From the out of plane measurements, it is found that, hybrid R/SHCC beams show out of plane displacements, which are highly correlated to the applied vertical forces. This is not observed for the reinforced concrete beams. The numerical models are able to simulate the trends in the cracking patterns, as observed in the experiments. In addition, the numerical models are able to simulate the trends in delamination. Increasing the concrete-SHCC bond strength, in the 400 mm high hybrid R/SHCC numerical model, leads to the formation of an additional propagated crack in the concrete layer. In addition, the deformation capacity and the delamination of the concrete-SHCC layer reduces, for the beam with the stronger concreteSHCC interface bond strength. Using a coarser 25 mm voxel size in the numerical models, instead
of the 10 mm voxel size used in previous studies (Mustafa et al., 2022), leads to similar simulated structural behavior of the beams. The voxel size limits the crack spacing, which is of larger importance for the SHCC, compared to conventional concrete. For the reinforced concrete beams, the numerical
models are able to predict the yielding load, whereas for the hybrid beams, the yielding deformation is underestimated, due to the overestimation of the ductility of the modelled SHCC. The analytical calculations show good comparison with the numerical models, both for the hybrid beams and for the reinforced concrete beams. The hybrid beam with smooth and Vaseline treated reinforcement leads
to an unreinforced hybrid beam, which is different from the experimental results. This difference is attributed to the numerical model simulating a weak bond over the full length of the beam, whereas in the experiments Vaseline is only applied over the 700 mm central span. To conclude, upon increasing the height of the hybrid R/SHCC beams, the effectiveness of the crack controlling behavior decreases. This is both found in the numerical and in the experimental results and holds both for a 0.2 mm and 0.3 mm crack width limit. However, the hybrid beams scaled in height still lead to a significant increase in the crack controlling behavior, compared to reinforced concrete beams of the same heights. Full delamination of the rebar-SHCC interface leads to worse crack controlling
behavior for the hybrid R/SHCC beams. Even more, if the full delamination occurs over the full length of the beam, the beam could be considered unreinforced. The Delft Lattice model shows large potential in the simulation of the structural behavior of both the reinforced concrete beams and the hybrid R/SHCC beams. The coarser 25 mm voxel size is found to be a time efficient and suitable modelling solution to gain insight in trends in the structural behavior of reinforced structures. The numerical model with a stronger concrete-SHCC interface showed potential in improving the crack width controlling behavior of the hybrid R/SHCC beams in height. Therefore, it is recommended
to study the effect of interface roughness for hybrid R/SHCC beams scaled in height. Additionally, determining the material input for SHCC remains a challenge in numerical simulations with the Delft Lattice Model. In order to improve the simulations of the numerical models, it is recommended to
study the material input possibilities for SHCC. Even more, before the beams are applied in practise, it is recommended to gain deeper understanding of the increased out of plane sensitivity of the hybrid R/SHCC beams. Studying the fiber dispersion would be logical start for this.

Upscaling of two geopolymer concrete mixtures in an individual prestressed girder in self-compacting geopolymer concrete (SCGC) with a topping layer cast in-situ by a ready-mix geopolymer concrete provider. The objective is to study the different material properties of the mixtures, determine their impact in the structural performance and define the extent of applicability of current methods of analysis for conventional concrete structures, as defined in EN 1992-1-1 and the Rijkswaterstaat’s Guidelines for NLFEA, for geopolymer concrete. The structural performance of the elements subjected to flexural (at 28 days and 9 months) and shear (at 28 days) tests is studied by analytical methods and 2D plane stress nonlinear finite element analyses, which are compared to experimental results in terms of deformations, load-deflection response, principal strains, normal stresses, damage evolution, cracking stages, maximum load and failure mechanism; the effect of the long-term material properties (e.g. elastic modulus, creep and shrinkage) of the geopolymer concrete mixtures in the prestressing losses, cracking load and maximum load carrying capacity is analyzed. The elastic modulus of both geopolymer concrete mixtures decreases when exposed to drying and is lower than the estimates from EN 1992-1-1 for OPC concrete of the same strength class. The increase of creep and shrinkage between 30 and 60 days suggest a pronounced viscous mechanical response. The prediction models from EN 1992-1-1 for conventional concrete to determine the material properties from the 28-day compressive strength do not capture the long-term material properties. The isotropic elasticity-based prediction models underestimate considerably the creep coefficient and shrinkage strains leading to unsafe design assumptions. The short-term flexural resistance is higher (8% by analytical model and 3% for the numerical simulation) than the maximum load during testing. The stress distributions at characteristic phases and the cracking load (3% higher than the experiment) are practically equal from the analytical and numerical model. The flexural cracking pattern in the topping in the NLFEA shows more cracks at smaller spacing because higher stresses transfer from the precast girder due to perfect bond. The short-term shear resistance was 12% higher from the numerical simulation and 16% lower from the analytical model, as compared to the experiment. The cross-section of the numerical simulation is stiffer since the precast girder and the topping are perfectly bonded, in the experiment debonding occurred. The position and orientation of the shear critical crack causing failure from the NLFEA was consistent with the DIC observations. The prestress losses at 28 days are higher than for conventional concrete and increase from 26% after 28 days to 38% after 270 days. The variation in the elastic modulus at 28 days has a marginal effect in the short-term response to the flexural test. The cracking load is decreased by 15% in the specimen after 9 months but the prestress losses will continue to increase over time and the cracking resistance will decrease which is critical for the performance over the service lifetime. The design criteria of conventional concrete result in non-conservative estimates of the cracking resistance and flexural load carrying capacity.

Combining the high strength of Ultra High Performance Concrete (UHPC) and the strain capacity of Strain Hardening Cementitious Composites (SHCC), Strain Hardening Ultra High Performance Fibre Reinforced Concrete (SH­UHPFRC) could be a promising material for the application of strengthening RC elements. This research describes the development of an SH­UHPFRC mixture, using Ultra High Molecular Weight Polyethylene (UHMWPE) fibres. The benefits of using this material as a strengthening material were analysed using a numerical model and the environmental impact of the SH­UHPFRC was evaluated. During the material development the effects of different material types and the applied ratios were considered. The flowability of the mixture, compressive strength and tensile response were tested to determine the mixture design. The effect of different cement types and amount of superplasticizer played a significant role in the increasing of the workability of the mixture. The effect of using UHMWPE fibres over steel fibres was investigated, as well as the effect the amount of UHMWPE fibres had on different properties. The material properties of the material research were implemented in a numerical model using ATENA software, representing a strengthened reinforced concrete (RC) beam subjected to a three­point bending test. In addition to the modelling of a RC beam strengthened with SH­UHPFRC, a parameter study was executed to determine the effect of increased strain capacity in the strengthening material. This was done by adjusting the tensile stress­strain relation in the material model. Three levels of strain were tested and the capacity and cracking behaviour of the strengthened beam were compared. The environmental impact was evaluated using the CUR Groen Beton calculation tool. The mixture design of SH­UHPFRC was compared to that of relevant concrete types. The environmental impact per volume is high for SH­UHPFRC, but the mechanical properties are superior which leads to a lower required volume to achieve similar mechanical results. Comparing a strengthened beam to a RC beam demonstrated the contribution of SH­UHPFRC, proving that, including the environmental impact, SH­UHPFRC could outperform NC as a strengthening material. The developed mixture granted a compressive strength of nearly 120 MPa, a tensile strength of 8.9 MPa and a tensile strain capacity over 2%. The use of this material for the strengthening of a RC beam lead to an increase in shear capacity of 78%, following from the numerical model. Evaluating the environmental impact, the use of SH­UHPFRC overcomes the use of NC to strengthen a shear­deficient beam. It is recommended to conduct further research on specific aspects of the material optimization of SH­UHPFRC. For the numerical modal the resemblance to practice could be improved and the range of parameters expanded. The environmental impact of a beam strengthened with SH­UHPFRC proved to be lower compared to an RC beam with equal shear capacity, showing the benefit of this superior material. An analysis of the full life cycle of an element strengthened with SH­UHPFRC could be done to get a better estimation of the environmental effect of using this material for strengthening purposes over NC.

When concrete is subjected to imposed deformations, stresses may develop. If at any point in time this stress exceeds the tensile strength of the material, the concrete will crack. Early-age cracking of concrete structures may lead to problems with durability, serviceability and aesthetics. During hardening of concrete the material properties are still in development. Therefore, to be able to predict the crack width, understanding of the stress- and strength development is required. In addition, concrete is a visco-elastic material which means that stresses are affected by phenomena such as creep or relaxation. If the design codes predict the crack width in concrete structures under imposed deformations accurately remain subject of debate. The CROW report published in 2021 [9] provided the starting point for this research. The aim of this study was to gain more insight on the background of the design codes, and to explain the fundamentals of the crack width prediction in case of imposed deformations. For this purpose, the following research question was formulated:
”What is the applicability of the design codes regarding the crack width prediction of reinforced concrete structures under imposed deformations?” From the literature study it became clear that the main difference between the design codes was related to which boundary conditions and cracking theory were applied. The majority of the design codes applied the well known tension bar model theory where both ends are fully restrained. In addition, there were also codes using the continuous base restraining theory in which the tensile member is continuously restrained along one edge. This made a huge difference in the prediction of the crack width. From the finite element analysis which was performed, it turned out that there is a difference between the steel stress development of imposed loading and imposed deformations. In addition, in case of imposed loading less cracks were developed than under imposed deformations. However, from this specific numerical analysis it turned out that the maximum crack spacing and crack width is smaller in
the imposed loading model in comparison to the imposed deformation model. The only explanation for this is that in case of imposed loading the cracks are more evenly distributed. A case study was used to simulate the hardening process of concrete in combination with autogenous shrinkage as imposed deformation and compared with a finite element analysis. The goal was to determine a set of model properties that are useful for future engineers. The accuracy of the input parameters was verified with the experimental work carried out by M. Sule. Overall, when performing a non-linear finite element analysis a lot of knowledge was required. It turned out that specific choices such as the constitutive model type, the kinematic and equilibrium condition have a major impact on the outcome. Using the modified Bar model of Lokhorst in combination with this numerical approach resulted in good agreement with the experimental findings.

The parameter study showed that regarding the tension bar models, the bar diameter has the largest influence on the crack width prediction. While with respect to the continuous models there was no onesided answer to the question which parameter had the largest influence on the crack width prediction. In one of the design codes based on the continuous model theory named CIRIA [8] the degree of restraint clearly stands out as the most important parameter. Whereas in the another design code based on this theory, namely the ICE [4], all parameters that were investigated in this thesis have limited effect on the crack width prediction. The design codes considered in this master thesis do not yet fully represent the crack width due to the hardening of concrete or due to autogenous shrinkage. The design codes are applicable for the
crack width prediction of reinforced concrete structures under imposed deformations if conservative assumptions such as a weak bond between concrete and steel reinforcement are taken into account. It is clear that the problem treated in this report has more complexity than what one initially may think.
Opinions on how to determine the crack width of reinforced concrete tensile members under imposed deformations differ. The difference is related to the type of restraint and the cracking theory which are suggested in most used analytical design models. This report contributes to a better understanding of
the crack width development under imposed deformations through non-linear finite element analyses and verification with experiments. The results from this master thesis suggest that an approach to a more consistent crack width prediction under imposed deformations should investigate how bond in the interface between concrete and steel influences cracking. This has lacked attention in the current formulas in the design codes.

Concrete elements, partially or entirely prefabricated are becoming more and more popular in engineering practice. The biggest challenge in this process is the realization of connections between components. These connections fall within two categories: Precast-to-precast connections with hardened parts and
precast-to-in situ connections, where an in-situ concrete element needs to adhere to a precast element. Some kind of interlocking surface might be effectual in both situations. Geometrical interlocking that exists in natural materials (turtle shells, diatoms), is the main inspiration for interlocking mechanisms in engineering practice. The concept of bistable interlocks has been introduced in the past few years to theoretical mechanics. These interlocks could allow many stable positions before the connection collapses, and may provide an additional ”safety boundary” in brittle concrete-to-concrete connections by spreading the nonlinear deformations through the material and making the transformed material less brittle and more damage tolerant. In this research, the properties of bistable interlocked materials will be explored based on the numerical models built. These properties are immediately correlated to the experiments performed. Sensitivity analyses are performed to investigate the parameters that can lead to the optimal behavior of the sutured geometries.

A parking garage of Eindhoven airport partially collapsed in 2017. It was caused by failure of the longitudinal joint (long-side) of the composite plank floor or breedplaatvloer on the roof level. Experimental and numerical research showed that the joint suffered high positive bending moment in its transverse direction. However, from the investigation results, there was not enough resistance at the concrete-to-concrete interface around the joint to transfer the tensile force from the precast to the cast-in-situ section, which led to delamination of the two layers of concrete and resulting in failure when the delamination crack reaches the end of the coupling reinforcements. As happened in that case, the concrete-to-concrete interface usually is the weakest link and has a critical role on behaviour of a composite concrete system, especially the one which has unreinforced interface (no stirrup near joint). Further studies have been conducted to understand the influence of various details around the joint on the interface behaviour. In this thesis, details in spacing between lap splices (coupling reinforcements in cast-in-situ layer and bottom reinforcements in precast layer), spacing between connecting reinforcements (stirrups crossing the interface near the joint), the role of connecting reinforcement, and the sensitivity of interface parameters were studied numerically using DIANA finite element analysis software. Since the spacings are in direction of the specimen’s width, interface behaviour was analysed in both longitudinal and transverse directions. An additional study about compressive membrane action or arching was also conducted to understand the influence of lateral restraint, which usually occurs in composite plank floor systems used in buildings, including the one used in Eindhoven parking garage, to the capacity of the structure. This action was suspected of providing additional strength to the existing composite plank floor

Two composite SHCC-concrete beam specimens from the experimental research by Harrass [1] were used in this numerical study since the experiment had both unreinforced and reinforced interface specimens which were important for this study. The unreinforced interface beam (Sample 1) was suitable for the study of lap splice spacing and lateral restraint without any influence from reinforcement crossing the interface, while the reinforced interface beam (Sample 7) was suitable for the study of stirrups spacing. The specimens are solid beams (without weight-saving element) consisting of a SHCC (Strain Hardening Cementitious Concrete) precast layer with a joint in the mid-span, and a regular concrete cast-in-situ layer. By using DIANA 10.4 finite element analysis software, this study is able to simulate both specimens in 2D and 3D numerical models. The models represented Sample 1 failed with a horizontal crack along the interface and a flexural crack at the end of coupling reinforcement reaches the top of the cast-in-situ layer, while the models represented Sample 7 failed with a horizontal crack along the interface, a flexural crack at the end of coupling reinforcement, and a crack at the stirrup location in precast layer. From the verification study, the reinforcement bond-slip function (CEB-FIB 2010) was chosen not to be used for the rest of the study since it did not affect much the load capacity and the failure mechanism of the specimens. Consequently, pull-out failure of the stirrup is excluded for the rest of this study.

Prior to the main study, the influence of each interface parameter was studied in both unreinforced and reinforced interface models by varying the interface parameters. It was observed that interface tensile strength and stiffness are governing in the unreinforced interface model. Within the range of those parameters, the load capacity was increased and decreased by more than 50% in compared to the reference model verified by Sample 1. By adding a rectangular stirrup near the joint of unreinforced interface model, the model with reinforced interface has a different governing parameter, the cohesion, and the variability of the results decrease. Within the range of the cohesion, the load capacity was increased and decreased up to 30% in compared to the reference model verified by Sample 7. Since the interface parameters influence the capacity of both unreinforced and reinforced interface beams, two different interface types are used for the whole of the study. They are known as “smooth interface” which uses the parameters obtained from the verification with the experimental specimen, and “perfect bonded interface” which use rigid connection between the elements of the two concrete layers.

To study the influence of lap splices spacing, models with three lap splices setup from Harrass’ experiment (three coupling reinforcements and three bottom reinforcements) were compared to models with a single lap splice (one coupling reinforcement and one bottom reinforcement) with the same total reinforcement area. As a result, with perfect bonded interface, model with single lap splice has higher load capacity by more than 10% in compared to model with three lap splices though both models failed with the same failure mechanism which was the horizontal crack along coupling reinforcement and a flexural crack at the end of coupling reinforcement reaches the top of the cast-in-situ layer. Stress concentration around coupling reinforcements were observed in all models, especially in model with single coupling reinforcement. However, different horizontal crack propagation occurred in each models. Uniform horizontal crack propagation along the interface were observed in models with smooth interface, while more concentrated crack propagation around the coupling reinforcements were observed in models with perfect bonded interface, especially in model with single lap splice. This different crack propagation in models with perfect bonded interface could be the cause of different load capacity since more uncracked elements could provide more tensile force transfer from precast layer to cast-in-situ layer.

In the study of the influence of stirrups spacing, models with rectangular stirrup (two legs) setup were compared to models with a single leg vertical stirrup setup with the same total reinforcement area. In both cases the presence of the stirrup near the joint stopped the propagation of the horizontal crack. As a result, all models could reach yielding of the coupling reinforcement for both interface types although different structural stiffness is observed. The plausible cause for this difference in structural stiffness was the higher tensile stress in stirrups of model with two legs stirrups compared to model with single leg stirrup. This higher tensile stress might be resulted by the more distributed stirrups across the width of the beam.

In the additional study, models with full height lateral restraint at the support were compared with models with simple support. As a result, with perfect bonded interface, model with lateral restraint has higher load and displacement capacity by more than 14 and 2.5 times consecutively compared to the model without lateral restraint. In compared to the collapse load, the numerical result of model with lateral restraint has higher load capacity by almost 4 times. With smooth interface, model with lateral restraint has higher load and displacement capacity by more than 10 and 5.5 times consecutively compared to the model without lateral restraint. In compared to the collapse load, the numerical result of model with lateral restraint has higher load capacity by more than 2 times. These increases are in accordance with a research by Ockleston [2] which found a considerable increase of load capacity on concrete slab with lateral restraint compared to the yield line theory. Part of the increase of capacity was resulted by the fix boundary action due to bending moment at support. However, it was observed that compressive membrane action started to develop after the first flexural crack as the horizontal force at support rapidly increased after that crack. Although the models with lateral restraint of both interface types had a different failure mechanism compared to the models without lateral restraint, they have a similar final stage of the failure mechanism, which is the flexural crack at the end of coupling reinforcement. This flexural crack propagation can be prevented from occurring earlier due to the high compressive stress at the top part of the cast-in-situ layer. When the concrete crushed at the support due to limited rotation capacity of the concrete, the compressive stress dropped causing the flexural crack at the end of coupling reinforcement to develop.

In conclusion, there was an influence of the interface behaviour to the failure and the capacity of composite SHCC-concrete beam. However, the influence was varied, depending on the coupling reinforcement spacing, presence of stirrup crossing the interface near the joint, spacing of stirrup, and interface type. It is also concluded that compressive membrane action in addition to fix bending action, which occurred due to the lateral restraint, increased the capacity of the structure. This increase depended on the interface type and rotation capacity of the concrete. A wider range of the influencing parameters are needed in future studies to get a more robust results which are beneficial for a more general conclusions. However, the series of experimental research based on this study are essential to provide a verification on the results of this numerical study.