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.

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.

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.

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.

Concrete is one of the most widely used construction materials across the world. Although, the traditional construction techniques are being improved for efficiency, cost and material optimization, there are disadvantages as well, such as the use of formwork, highly labourintensive and limited architectural freedom for design and material savings. 3D printing of concrete structures is one of the emerging technologies promising the automation in construction industry. However, 3D printed concrete structures pose higher risks of shrinkage as compared to the conventional concrete structures due to their different construction techniques and material composition. Shrinkage can raise issues such as reduction in strength of the structures, poor durability and aesthetically unpleasant structures. The research aims to study the shrinkage behaviour of the 3D printing concrete mixture in different restrained and curing conditions. Based on the experimental data, a finite element model has been developed to predict the shrinkage behaviour over time. The free shrinkage test was performed on cast samples and 3D printed samples of dimensions 160x40x40 mm3. The specimens were kept in two curing conditions: covered with plastic sheet and (2) uncovered or exposed to environment from the time of casting. The shrinkage behaviour was studied for up to 60 days at relative humidity of 65% and temperature 20°C. A higher shrinkage values as compared to normal and highperformance concrete is observed in both 3D printed and cast samples. The 3D printed samples showed a higher shrinkage of 1518% as compared to cast samples due to the 3D printing processes. Restrained ring test were performed for three curing conditions: (1) exposed to environment, (2) covered with plastic sheet and (3) sealed with waterproof tape. The sample exposed to drying cracked within 3 days followed by covered with plastic sheets in 4 days and the specimen in third condition cracked within 8 days. The experiment indicated that the autogenous shrinkage is so high in the concrete mixture to induce shrinkage cracking. A finite element model has been developed to simulate the shrinkage behaviour of the concrete mixture based on the experiment results of the free shrinkage test of cast specimens. The capillary tension approach has been used as the theoretical framework. It uses the KelvinLaplace equation to calculate the capillary stresses and the Bentz deformation model to calculate the corresponding shrinkage strain. It is applicable for ambient relative humidity of more than 40%. The required input for the model are: degree of hydration over time, elastic modulus and distribution of relative humidity over time. The degree of hydration has been calculated using HYMOSTRUC software. The experimental results of the free shrinkage test for cast samples have been simulated in ANSYS. An inverse analysis has been used to obtain the material parameters required to model the moisture diffusion process. Transient heat equation has been used to model the diffusion process. Since the concrete mixture has a low watercement ratio, the effect of autogenous shrinkage has been taken into account as well. The calibrated finite element model gave an error of 810% as compared to the experimental results. The moisture profile obtained from the model has been compared to an analytical model to validate the model. It shows good agreement, especially for upto 28 days. Finally the developed finite element model has been applied on three case studies to assess the accuracy of the model. First, the restrained ring test has been simulated to validate the model based on the prediction of the cracking time. It showed that to accurately predict the shrinkage induced stresses, the creep and the relaxation phenomenon plays a crucial role as well. Second, the model has been used to simulate the shrinkage in a 3D printed flower pot restrained at varying time. Third, a parametric study has been designed using the developed model. Overall the study concludes that the shrinkage in 3D printing concrete mixture is quite high as compared to the normal concrete even if conventionally cast. The 3D printing processes can increase the shrinkage further. The developed finite element model can be used as a tool to estimate the shrinkage strain.

Hybrid concrete-SHCC beams are a new development in construction techniques. SHCC stands for Strain Hardening Cementitious Composite. In such beams, the tension zone could for example consist, next to the traditional reinforcement, of a material that shows strain hardening behaviour. That helps with controlling the crack width. In traditional (non-hybrid) beams, the steel reinforcement would have to control the crack width on its own. This means that there are situations that the steel reinforcement fulfills the strength requirements, but additional steel reinforcement is needed to limit the crack width. Therefore, a certain amount of steel reinforcement is needed for meeting the SLS (Serviceability Limit State) requirements, while it is not used for the ULS (Ultimate Limit State) requirements. The use of hybrid beams consisting of an SHCC layer applied in the tension zone in which the reinforcement is embedded, solves this problem. This was shown in previous MSc studies by Huang and Singh. In this study, the concept of the hybrid beam is extended. A design was made of a reinforced U-shaped SHCC mould, to be used for casting a hybrid beam. In that way, a reinforced hybrid concrete beam is created, in which the webs of the U-shape prevent the need of temporary moulds at the side of the beam, which reduces costs. A complete design is presented which is ready for further research. Next to that, it is investigated how the bending behaviour of beams, that are made using this concept, can be modelled. This was done by extending the multi-layer model, that was first proposed by Hordijk in 1991. Different from the previous built models, the model proposed in this thesis includes additional aspects, such as imposed deformations caused by drying shrinkage, and the possibility to model hybrid beams (with a U-shape). After this model was extended using VBA in Microsoft Excel, it was successfully verified by comparing its results with experimental and analytical results from previous research. Comparing the force-to-displacement curves showed that the end-resistance of the beams is predicted well by the extended multi-layer model, and that generally the same trend of the curve is followed by the model. The bending resistance of the proposed experimental setup of a hybrid beam containing a U-shape was also modelled. However, as this experiment has not been performed before, there were no experimental results to compare with. Therefore, the results for this setup are to be compared with the results that follow after experimenting with the presented design of the beam.

The demand for high-rise building has been increasing over the past decades which has induced new developments such as modular construction. Modular construction has advantages in terms increased speed, safer construction and less construction waste. Applying modular construction in high-rise however brings about great complexities for the structural design of the building. Therefore, a design aid is created which generates a structural model from any arbitrary shape and uses structural optimization techniques to explore a range of designs for the early design stage. Existing optimization techniques have been studied, including Cross-Section (CS-) optimization (size) and BESO (topology). CS-optimization adapts the size of each member, considering utilization and displacement conditions whereas BESO removes or adds elements from the model based on stress level and Target Ratio (TR). Performing the methods successively further decreases the structural weight at certain TR. Generally, weight increases as more elements are removed. By developing the two-way coupled ESO<>CS-optimization method, elements are removed by lowest strain energy density, while the cross-sections of the remaining members are updated in each iteration, thus coupling ESO (topology) and CS-optimization (size). This results in a logical load path and additional weight reduction especially in the braced frame structures loaded by lateral loads when displacement is governing. In multiple load case design, the removal of elements is somewhat hard to interpret, however the method consistently results in the lowest structural weight for the considered cases. In order to create a modular structural design, boundary conditions are assumed by standardizing column and beam dimensions and considering multiple realistic load cases (wind+vertical load). By transforming arbitrary shapes into structural models and optimizing these, a range of solutions for the structural design is presented in which the number of elements and the structural weight are generally negatively related. For the considered rectangular buildings, the results are more efficient than core or outrigger structures and almost as efficient as mega frame structures, with the added benefit that it can be applied to any shape. Exploration of these solutions is a useful contribution to the early design stage of modular high-rise buildings, while the coupled ESO<>CS-optimization method could also be useful in other optimization problems.

Existing reinforced concrete (RC) structures can be strengthened using Strain Hardening Cementitious Composites (SHCC). The ability of SHCC in exhibiting a ductile response under tensile load due to strain­-hardening after crack initiation makes it a viable material to be used in, both, construction and retrofitting of concrete structures. The main objective of this research is to study the shear behaviour of SHCC-strengthened RC beams using NLFEA. The shear behaviour of benchmark RC beams is analysed first. The analysis of the selected RC beams analysed using Damage-based shear retention function results in accurate predictions of peak load if a fine mesh size resulting in 30 or more elements in the height of the beam is used. However, the failure type is predicted inaccurately for both coarse and fine mesh sizes due to lack of consideration for aggregate interlock in Damage-based shear retention function. The analysis of the selected RC beams using Al-Mahaidi shear retention function results in accurate predictions of peak load if a coarse mesh size resulting in 20 elements in the height of the beam is used. The failure type is also predicted accurately using Al-Mahaidi shear retention function with the stated mesh size. The consideration for aggregate interlock implicitly in Al-Mahaidi shear retention function in the form of shear retention factor allows for accurate prediction of both peak load and failure type. After analysing the shear behaviour of RC beams, the shear behaviour of a reinforced SHCC beam is analysed using Al-Mahaidi shear retention function since it can predict both failure load and failure type accurately for RC beams. In comparison with experiment, the peak load for the reinforced SHCC beam is underestimated and the failure type is also incorrectly modelled. Use of embedded reinforcement results in excessive cracking along the reinforcement, causing convergence issues at a load lower than the experimental peak load. Such excessive cracking is not observed in RC beams since cracks more localized in concrete as compared to SHCC, which exhibits multi-cracking behaviour. Therefore, the shear behaviour of selected reinforced SHCC beam using Al-Mahaidi shear retention function is not accurately modelled. After the analysis of shear behaviour of concrete and SHCC separately, their behaviour is studied in the form of SHCC-RC hybrid beams. The solution strategy consisting of Al-Mahaidi shear retention function is used, and different types of hybrid interface are modelled. The results show that peak load and failure type are accurately predicted when a numerically perfect bond is modelled at hybrid interface for hybrid beams exhibiting no debonding during experimentation. This is in case of a mesh size resulting in 20 elements in the height of beam used. The peak load and failure type, however, are inaccurately predicted when delamination is modelled at the hybrid interface for hybrid beams failing due to delamination during experimentation, irrespective of the mesh size considered. This is due to the inability of the Coulomb friction interface model in recognising significant delamination at the hybrid interface as a reason for the failure of the hybrid beam.

Strain Hardening Cementitious Composite (SHCC) is a new type of concrete that is able to control the crack-width in concrete structures. The application of this material in the tension zone of hybrid concrete structures is thus of interest in practice. The interfacial behavior for these hybrid structures is something that is of importance for the composite action. Therefore, a study is performed to investigate the behavior of the interface in hybrid SHCC-concrete beams. This is done by performing an experimental study on beams with a joint at midspan. Several parameters such as the interface roughness, coupling reinforcement cover, curing method and protruding reinforcement have been investigated. The hybrid SHCC-concrete beams are tested in a four-point bending configuration to obtain a constant-bending moment region along the interface. The crack propagation along the interface and through the SHCC/concrete has been evaluated with the use of Digital Image Correlation (DIC). Furthermore, also a numerical analysis has been performed using DIANA FEA based. The numerical analysis was used to help to determine the experimental campaign. Furthermore, also several experimental samples have been modelled to study the behavior of the beam in more detail and to make recommendations for future modelling of the interface. Based on the experimental results, the influence of the interface roughness resulted in an increased bond at the interface. This is both seen in an increased bearing capacity and a reduction of the interface and joint opening at equal loads. The profiled interface and holed interface resulted in an increased capacity due to the mechanical interlocking at the interface. The bearing capacity of these samples increased by 78.4% and 54.7% respectively compared to the reference sample (13.9 kN). In case of the profiled sample, no delamination of the interface occurred as the sample failed due to a horizontal crack at the level of the coupling reinforcement. This can be attributed to the localized tensile stresses at the reinforcement level. The last sample with a different interface roughness consisted of epoxy and sand (1-2 mm). For this sample, the bearing capacity increased by 51.8%. The failure of this sample was a combination of interface delamination (initially at the joint) and a horizontal crack through the coupling reinforcement. For all the samples with a different interface roughness, no yielding of the coupling reinforcement occurred. Several beams with a smooth interface have also been tested by adjusting the reinforcement cover, curing method and applying protruding reinforcement. The influence of the reinforcement cover didn’t have an effect on the bearing capacity. However, the interface and joint opening reduced significantly as the eccentricity of the reinforcement bars also reduced. Furthermore, also the effective height increased resulting in lower reinforcement stresses. The influence of a different curing method was investigated by placing the sample in a humidity-controlled (50 %) room to investigate the effect of shrinkage. The results showed that the bearing capacity remained similar to the reference samples. This can be attributed to the fact that the bond internally was still good, caused by the restraint of the reinforcement bars. However, the deflection of the beam increased as a result of the reduced stiffness by the shrinkage induced cracks. Finally, one sample consisted of stirrups at a distance of 50 mm from the joint to take up the tensile stresses at the interface (e.g. due to reinforcement eccentricity). The results showed an increase in bearing capacity by 102.7% compared to the reference sample. However, also in this case, no yielding of the coupling reinforcement occurred. The failure of the sample is also caused by the delamination of the interface. Furthermore, as a result of this delamination, fracture of the top part of the SHCC occurred at the location of the protruding reinforcement due to the rotational restraint of the stirrup. The 2nd part of the study consisted of a numerical analysis. A Coulomb-friction model with an interface tensile strength cut-off is used to model the interface. Based on this, a good correspondence between the experimental results and FE results is found in terms of the bearing capacity and failure mode for the sample with a smooth interface(delamination). However, the crack propagation of the flexural cracks through the concrete and SHCC was substantially different.
The sample with a profiled interface has also been modelled in Diana FEA. This is done by implementing the profiled interface manually in the FE model. The model showed a good correspondence with the experimental results in terms of bearing capacity and interface and joint opening. The crack propagation in the concrete is also similar to the experimental results. The model however doesn’t show any flexural cracking in the SHCC layer as a result of the limitation of DIANA FEA. Also, an additional study is done to replicate the behavior of this beam using a smooth interface. Based on this model, it is recommended to use a perfect bond at the interface. For both these models, the FE model wasn’t able to replicate the strain hardening behavior of SHCC. This is due to the limiting material models in DIANA.

Starting from the late eighties induced seismic activity is present in the Northern part of the Netherlands. Cause of these earthquakes is the extraction of natural gas. In this region, most of the building stock comprises out of unreinforced masonry (URM) buildings. Multiple experimental campaigns are performed since 2014 to investigate the structural integrity of these type of structures. Both static and dynamic, full-scale and smallscale, experiments are executed to identify both in-plane and out-of-plane failure mechanisms. Strengthening, or retrofitting, of these already existing structures, is of interest and different researchers propose multiple techniques. From the literature, it is found that the application of strain hardening cementitious composites (SHCC) results in the required improved structural performance of the retrofitted masonry. The SHCC material is characterised by high ductility in the range of 3-7%, tight crack widths of around 60 μm and a relatively low fibre content equal to 2% (maximum by volume). In this experimental campaign small-scale, both inplane and out-of-plane (i.e. shear strength and flexural strength, respectively), experiments are performed on masonry samples retrofitted using a single-sided applied SHCC overlay. Different thicknesses of this overlay, masonry surface preparations, and overlay curing conditions are considered. With the help of these experiments the material properties of retrofitted masonry are investigated. From the initial shear strength experiments, it is concluded that the retrofitting approach did not work as intended. Results of all the different specimen examined shows similar capacities to the non-retrofitted masonry triplets. With the help of digital image correlation, it is shown that for all of the retrofitted specimen, the overlay material is not activated during the tests. Additional experiments and numerical simulations are performed to investigate the several parameters possibly influencing these experiments. The bond strength of the interface between the SHCC and the masonry substrate is determined to be too low. This is one of the main factors resulting in the absence of cracks in the retrofitting overlay. Additionally, the high cracking strength of the SHCC mixture considered, and the applied pre-compression, have influenced the cracking behaviour of the retrofitting overlay. Besides the shear strength of the retrofitted masonry material also the flexural strength is investigated with multiple out-of-plane four-point bending tests. Retrofitted masonry beams are used for these experiments. The plain masonry beams are not able to carry their self-weight, therefore all of the additional capacity is accommodated to the applied overlay. The experimental results are governed by shear failure. For eight out of ten specimens debonding of the masonry units is prevalent. Two of the retrofitted masonry beams showed flexural failure. A thicker overlay will result in higher flexural strength. Again, the bond strength between the overlay and the masonry substrate seems to be governing. No numerical analyses are performed to analyse these experiments in more detail. From this research, it is concluded that a single-sided SHCC overlay can be a very attractive method for retrofitting unreinforced masonry. However, both the interfacial bond strength and the material properties of the SHCC material must be taken into account. The bond strength must be sufficiently high, and the cracking strength of the overlay material sufficiently low, for the overlay to be activated. With the help of numerical simplified micro-models, the requirements of both parameters can be estimated.