Alkali-Silica Reaction (ASR) poses a significant challenge to the durability and structural integrity of reinforced concrete structures worldwide. This thesis presents a comprehensive modelling framework that integrates numerical simulations and phenomenological models to predict the long-term behaviour of ASR-affected structures. The framework addresses the progressive degradation of material properties and the structural implications of ASR-induced expansion, using expansion characteristics derived from experimental data.
Several numerical methods exist to simulate ASR effects at the structural scale. Among these, the Dual Mesh Method (DMM) stands out as novel pre-damage method to simulate ASR induced damage in reinforced concrete. Building upon this approach, this research develops the Modified Dual Mesh Method (MDMM), introducing key advancements such as an adapted tensile curve (ATC) to enhance its predictive capabilities.
The MDMM was validated numerically against reinforced concrete beam behaviour, demonstrating its effectiveness in simulating ASR-induced stress generation, crack propagation, and material degradation under incremental expansion conditions. A comparative analysis with the Reduced Material Properties Method (RMPM) highlighted the MDMM's effective ability to account for internal expansion forces and anisotropic behaviour influenced by structural configurations. The results confirmed the framework's accuracy in replicating critical structural responses such as crack propagation patterns and load-deflection behaviour.
Phenomenological models by Larive and Esposito were integrated into the framework, linking ASR-induced expansion with time and material property degradation. These models enabled long-term damage simulations and were applied to a hypothetical bridge pier cap. The simulation successfully captured key aspects of ASR-induced damage, including crack alignment with reinforcement, stress redistribution, and reinforcement yielding, providing insights into the service life thresholds of ASR-affected structures.
While the framework demonstrates promising capabilities, it remains in an early development stage. Limitations were observed, particularly in accurately representing the gradual reduction of elastic modulus associated with ASR progression. Further validation with complete case studies and exploration of diverse structural configurations are recommended to enhance its applicability and reliability.
This research offers a valuable tool for structural assessment, maintenance planning of ASR-affected infrastructure, bridging the gap between experimental observations and predictive modelling of long-term structural performance.

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 use of non-destructive tests for assessing the material status of concrete structures in the Netherlands remains relatively uncommon, despite the growing demand for the preservation of existing structures under the VenR policy of Rijkswaterstaat. Skepticism towards NDTs is reflected in the scarcity of documented case studies in publicly available literature and in the absence of rules and guidelines.

In this research, following an extensive literature review on the current state of assessing the material status of reinforced concrete structures using NDTs, a large-scale non-destructive inspection was carried out on the Sluinerweg viaduct. The research aimed to address the practical challenges associated with these methods. These challenges serve to formulate a practical methodology to facilitate future inspections. Limitations include the use of specific NDTs: GPR, UPE, rebound hammer, UPV, half-cell potential, resistivity, and corrosion current density. Following the inspection, a data analysis was conducted, accompanied by a destructive verification of the methods.

The integration of GPR with UPE technology showed promise for tendon duct inspections. However, a 12 mm borehole used for destructive verification proved to be too small to make accurate judgments. Additionally, the absence of grouting defects made evaluation of the method challenging. GPR provided a more accurate estimation of the cover depth compared to previous measurements conducted on the Sluinerweg viaduct using a standard cover meter. However, it was impossible to measure through the cathodic protection coating. The data's correlation with the provided drawings is promising, especially given that drawings are often unavailable. The key finding regarding the estimation of compressive strength using the rebound hammer and UPV is the strong recommendation to avoid using SonReb models unless they are specifically calibrated for the structure under inspection. No active corrosion sites were found, which posed challenges to evaluating the methods. Resistivity values measured using the Proceq Resipod consistently showed lower readings than those obtained with the Gecor-10 Wenner probe. A laboratory investigation ruled out moisture content as the cause. Fortunately, the differences are less pronounced with corroded reinforcement; however, further investigation is necessary. The previous inspection regimes in the Liggerkoppen project were found to be suboptimal in some aspects but were deemed reasonable considering the complexity of the project.

This research demonstrated the effectiveness of several NDTs in-situ, which should help to build trust in the reliability of these methods for future inspections. Based on the findings of this research, it is strongly recommended to conduct further large-scale inspections to improve the practical methodology, gain further experience and develop improved codes and guidelines. While there is still much to accomplish, Rijkswaterstaat's support for investigations such as the one conducted for the Sluinerweg viaduct demonstrates their commitment to a better future.

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.

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...

Concrete is widely used in civil engineering and despite its long history, accurate prediction of the hardening process, particularly in partially restrained conditions, remains a challenge. During the hardening of partially restrained concrete, residual stresses occur due to restrained deformations caused by factors such as heating/cooling and shrinkage of the concrete. Accurately predicting the development of these residual stresses during restrained hardening is critical for proper design calculations, as cracking must be controlled by reinforcement. However, predicting the development of these design stresses is complex and depends on a number of processes and parameters, including temperature development, development of mechanical properties and the degree of restraint. These stresses also decrease with time due to creep and relaxation, which further complicates predictions, especially when dealing with restrained concrete combined with shrinkage.

In addition, hardening concrete will eventually be subjected to external loads, resulting in additional stresses that may require additional reinforcement. However, when it is difficult to accurately predict the development of design stresses, approximations must be made. These approximations may either take full account of the stresses from both hardening and external loading, or reduce the residual stresses by some factor. Such factors may be based on design codes or rough estimates. Consequently, these approximations can lead to either over- or underestimation of the reinforcement required in the design, mainly due to a lack of understanding of the actual design stresses. Therefore, the research question investigated in this thesis is as follows:

What is the effect of applying time-dependent finite element analysis, including the combination of hardening processes and external loads, on improving the prediction of the development of design stresses for partially restrained concrete?

The research starts with a comprehensive literature review covering the stress development during hardening of restrained concrete, the current calculation methods and models and a validation experiment. The validation experiment, a Temperature-Stress Testing Machine (TSTM) found in literature, is modelled and the analysis results are compared with the experimental results to validate the accuracy of modelling the hardening of partially restrained concrete. A method is developed to accurately predict the hardening of restrained concrete using a combination of transient heat transfer analysis and structural non-linear analysis. This non-linear analysis uses a combination of time and load steps to apply the required degree of restraint. The finite element analysis, using measured thermal and material properties from the literature, shows good agreement with the TSTM experimental results.

In order to verify the accuracy of commonly used material models, the Eurocode and fib Model Code material models within DIANA FEA are compared with the results of the TSTM experiment. However, it was found that these material models have inaccuracies due to the use of large step sizes for the Kelvin chains that define the creep and Young’s modulus developments. Therefore, a viscoelastic material model based on the fib Model Code was developed and found to be more accurate. This material model was validated against the TSTM experiment and hand calculations. Although differences were found in the development of design stresses between the experimental results and the prescribed standards, these were mainly due to the difference in autogenous shrinkage and coefficient of thermal expansion.

Using the knowledge gained from the modelling of the TSTM experiment and the validated viscoelastic material model, a case study of the railway underpass in Leiden is performed. The chosen modelling approach is a time-dependent non-linear finite element analysis that includes all construction phases, hardening processes, and external loads within a 3D model of solid 3D elements. The model excludes concrete cracking, reinforcement and prestressing of the deck to limit complexity. This calculation method is expected to provide a more detailed insight into the development of design stresses and ensures high calculation accuracy and completeness compared to other methods. The use of a full 3D model was necessary to accurately model the temperature development in the hardening concrete, which has a significant effect on the design stresses…

The Netherlands is currently facing a significant challenge regarding its highway system due to the rise in traffic, especially in densely populated areas like The Randstad. However, constructing new infrastructure or replacing old bridges is not a practical solution due to environmental concerns. Most of the country's prestressed concrete bridges were built in the 1960s and 1970s with a lifespan of 100 years, and they are deemed incapable of handling the current traffic volume. As the existing bridges are still in good condition, recent projects have focused on widening them. Widening a bridge involves careful consideration of the behavior of all the elements in relation to each other, given the existing deck's relative stability and the inevitable shrinkage and creep of new components. To ensure a monolithic connection between the new and existing sections of the bridge, current projects aim to widen the bridges using a closure pour between the main slabs.

The Schipholbrug, situated close to the Schiphol Airport, is a prime example of a prestressed bridge that needs to be widened, and it is the focus of this thesis. In the Netherlands, reinforced concrete is the preferred material for a closure pour due to its durability, cost-effectiveness, and established properties. However, to maintain the integration between new and old concrete, a 6-9 month delay after constructing the new bridge is necessary to build this closure pour. To minimize significant delays, it is crucial to maintain a strong connection between the original and new materials, including the closure pour. The main challenge is managing the differences in creep and shrinkage between the existing structures, fresh deck, and closure pour. These inconsistencies can cause significant tensile stresses in the closure pour, especially when delays are kept to a minimum. Therefore, identifying a cementitious material that could effortlessly create reliable bonds with the primary decks' prestressed concrete and possess a high tensile strain range property was necessary to reduce this delay.

Strain-Hardening Cementitious Composite, also known as SHCC, is a modern material that possesses an impressive tensile strain range and a comparatively lower elastic modulus. Nevertheless, what sets it apart is its strain-hardening quality, which improves its toughness even after experiencing cracks. This exceptional characteristic of SHCC allows it to offer an extended tensile strain range, making it a choice for a closure pour.

The thorough literature review investigated crucial subjects, such as the intricacies of closure pour when expanding current bridges. Moreover, it covered the fundamental attributes of concrete that are pertinent to this thesis, such as shrinkage and creep, as well as its post-crack behavior. Another segment focused on the primary material employed in this thesis, SHCC, emphasizing its fundamental characteristics, including shrinkage and crack. Lastly, the research included a section on imposed deformation that was custom-made to the specific case of this thesis.

The methodology chapter utilized analytical calculations to gain a better understanding of the deformation issues caused by shrinkage and creep and their effect on the closure pour. These calculations explored composite structure mechanics and imposed deformation to determine the longitudinal stresses present in the mid-span of the decks. To further verify the accuracy of the findings, a linear model was also developed using DIANA FEA...

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.

The soil-grout interface friction was measured to be in between 0.0 [kPa] and 14 [kPa]. To measure this friction, a special research method was invented that enabled a tunnel to be recreated. With this setup, any formation could be used. The formation could be created with a hole in the middle which simulated a borehole. Next, by filling and pressurising this hole with a bentonite-water suspension, a stable borehole could be simulated. This borehole was then filled with drillgrout to recreate a tunnel filled with grout. This grout column could then be pulled out of the soil formation so that the friction between the drillgrout and soil could be measured. In this setup, no normal forces were present. Due to the absence of normal forces no friction angle could be deducted which made it impossible to estimate the effect of the friction at the depth where a pipe laid by HDD would generally be found. It was concluded that based on the data found, it is not recommended to use grout as a borehole filling around heat transportation pipes. Although the friction would be mainly determined by the filter cake strength, which is comparable to a weak clay, effects like irregular borehole diameters and large soil stresses at the depth of the pipe, would create a frictional force that is large enough to cause buckling or failure of the insulating PUR layer.

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.

Research about alkali-activated materials or geopolymers has increased in the past decade as they are regarded as a potential replacement to cement-based materials. This is due to the fact that alkali-activated materials possess several advantages over cement-based materials, and the most important one is that they are more sustainable to the environment, as they have less CO2 emissions, less energy consumption during the production process, and the use of byproducts as a binder.
The main aim of this work is to develop a mixture that has superior buildability properties with an open time of two hours or more. In order to achieve this, the effect of admixtures, specifically retarders and viscosity modifying admixtures had to be studied. The use of admixtures in geopolymer has been regarded as a complex subject because multiple factors influence their effect on the mixture, like: binders used and their percentages, alkaline activator used, and liquid to binder ratio.
In this work, the effect of 3 retarders: sucrose, sodium chloride, and sodium borate on the setting time, flowability, and yield stress development was studied. Moreover, the effect of those retarders was studied with the addition of viscosity modifying admixtures: nano-clay (attapulgite), xanthan gum, sodium carboxymethyl starch, and sodium carboxymethyl cellulose on the same properties.
The binders and the alkaline activator used were fixed through out the entire work. the binder composed of 60% fly ash, 20% blast furnace slag, and 20% glass wool. The alkaline activator consists of sodium hydroxide and sodium silicate with a SiO2/Na2O ratio of 1.5.
The addition of sucrose didn’t lead to any delay in the setting time. While both sodium chloride and sodium borate delayed the setting time, increased the flowability, and improved the extrudability of the mixtures. Out of the viscosity modifying admixtures, attapulgite had the most positive effect as it improved the thixotropic behaviour of all the mixes and improved the yield stress. Sodium carboxymethyl starch and cellulose almost had the same effect on the all the mixtures as they both improved extrudability and the yield stress in a similar trend. Xanthan gum was considered a poor admixture to be added to this mixture as it led to significant dryness which led to loss in fluidity which affected both flowability and extrudability.
The most suitable design was the B3lbA0.5 mixture which has 3% borax and 0.5% attapulgite and a liquid to binder ratio of 0.45. It had an initial setting time of more than 8 hours, flowability was maintained for 2.5 hours and it remained extrudable for 2 hours. Its yield stress ranged from 1360.87 Pa to 1977.78
Pa. After 28 days, it had a flexural and compressive strength of 5.72 MPa and 30.42 MPa respectively. Moreover, the flexural and compressive strength of 3D printed prisms was tested in the vertical and horizontal direction on the 7th day. The results showed that the flexural strength of the 3D printed prism tested in the vertical direction was the highest compared to the cast prism and 3D printed prism tested in the horizontal direction by 0.29% and 0.43% respectively. While the compressive strength of 3D printed cube tested in the horizontal direction was higher than both the cast cube and the 3D printed cube tested in the vertical direction by 0.24% and 0.32% respectively. The strength results indicate that
the mixture could be used in structural applications. The tensile bond strength was 0.69 MPa and the failure occurred near the glue, meaning that the interface is stronger than the material. In 3D printed concrete, usually the interface between the layers is the weakest point, however due to the addition of attapulgite the interface was strengthened. This explains why the compressive strength in the horizontal direction was higher than the vertical one as the interface was no longer the weak link in the specimen. The mixture’s buildability wasn’t optimal as the bottom layer had a slight big deformation and it had only
ten layers of printed material, but that is reasonable since the open time is 2 hours. This means that for big spans this mixture can be useful as it can be printed for a long period of time and its yield stress will develop.
In order for the mixture to be printable, it had to have an initial setting time higher than three hours, a spread diameter between 150-200mm and a yield stress above 1400 Pa. The above mentioned results
fall within the range of values that were needed to have a printable mixture. However, after printing the mixture it was concluded that a spread diameter of 150-180mm and a yield stress above 1600 Pa would be more suitable for both printability and buildability, because when the spread diameter was above
180mm and the yield stress was lower than 1600 Pa, the mixture’s viscosity impacted the buildability.

Injection Bolted Connections (IBC) are connections in which the cavity between the bolts and the plates (hole clearance) is filled with two-component epoxy resin. The oversized hole clearances allow for the reuse of all the structural elements in composite structures. Due to larger hole clearance, the possibility of slip occurrence in the connection is higher and hence there is a need to study the behaviour of resin and steel-reinforced resin (SRR) under cyclic loading. A custom made setup with 3 and 6 mm oversize that replicates the nominal bearing stress experienced by the resin in a double lap shear joint was used to study the behaviour of the resin/SRR under cyclic loading. The slip obtained from cyclic loading is extrapolated for 5 million cycles to check if maximum slip exceeds the failure criterion of 0.3 mm and slip range exceeds the failure criterion of 0.1 mm over 5 million cycles. Numerical modelling was done to see if the results (initial stiffness) obtained from the experiments can be replicated by the material models for static and one cycle loading with the help of a reasonable friction coefficient between the steel and resin surfaces. Good sets of results were obtained for all the tested stress ranges. On extrapolating to 5 million cycles, resin specimens with 3 mm hole clearance failed at a stress range of 200 MPa and resin specimens with 6 mm hole clearance failed at a stress range of 150 MPa. SRR specimens with 6 mm hole clearance failed at a stress range of 200 MPa. From numerical modelling, a good correlation was found between the force vs displacement curve obtained from the experiments to the curve obtained from the numerical model.

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.

Energetic materials have widespread applicability in various fields of engineering as propellants, explosives, and pyrotechnics. The Netherlands Organisation for Applied Scientific Research (TNO) is investigating additive manufacturing of solid propellants. As part of this investigation, TNO developed a new polymer binder mix for solid propellants. The thermomechanical behavior of the solid propellants is not well understood yet. Therefore, TNO is interested in developing computational modelling schemes for characterizing their behavior. Computational schemes based on the finite element method (FEM) are developed in this thesis. They are calibrated with matrix-only and solid propellant uniaxial tension experiments provided by TNO. The developed schemes are compared with the experiments to better understand the materials behavior and improve the computational models.

The macro-structural behavior of solid propellants is significantly affected by the behavior of the micro-structure. It is computationally unfeasible to directly consider the micro-structure in macro-structural FEM. Therefore, computational homogenization (CH) is employed for idealizing the micro-structure with representative volume elements (RVEs) at a finite number of macroscopic locations. Micro-structural behavior consists of numerous nonlinear thermomechanical processes. This thesis focuses in characterizing micro-structural matrix viscoelasticity, continuum matrix damage, and debonding. The brittle and rate dependent behavior of the matrix and the solid propellants is described with these thermomechanical processes. Other relevant processes such as temperature effects, nonlinear elasticity, anisotropy, and large strains are not considered.

The experiments exhibit a clear rate dependence throughout the entire loading process, and viscoelasticity is hypothesized to play a major role in this dependence. The experimental samples experience a brittle failure that is believed to be caused by matrix micro-crack damage. Particle debonding is observed in the failure planes of the solid propellants and is hypothesized to significantly affect their behavior.

Matrix-only and solid propellant computational results confirm that viscoelasticity is a major source of rate dependence and that (continuum) matrix micro-crack damage causes brittle failure. They also suggest that continuum matrix damage is a significant source of rate dependence in the post-damage regime, and matrix-only experiments agree. Comparing the results for perfectly bonded solid propellants to the results for solid propellants with debonding and to the solid propellant experiments, it is clear that debonding indeed has a large effect in reducing the ultimate strength of the material. The full CH damage-debonding-viscoelastic scheme shows promise as a first step towards full characterization. Future work can improve the scheme by addressing the issue of free surface propagation and can build on it by including relevant thermomechanical processes that are not yet implemented. Recommendations on how to achieve this are given in the final chapter.

With rapid industrialisation, the infrastructure sector has seen exponential growth in prefabricated concrete elements due to their speedy construction and efficient usage of material. Precast concrete elements have thus observed some deterioration due to increased internal temperature as a result of rapid curing, and also through deliberate heat curing techniques. This has led to researchers in the past to study the effect of such curing conditions on the durability aspect of the binders, especially their impact on Delayed Ettringite Formation. Precast elements such as railway sleepers, exposed to in the humid environment were thus prone to internal sulphate attack and needs to be investigated. Use of Ground Granulated Blast Furnace Slag (GGBFS) as a substitution for binder content in conventional portland cement in the Netherlands has been prevalent since the early 1900s primarily because of its abundant resource from iron industry. The benefits of slag have been then exploited as it was one of the supplementary cementitious systems which along with being a sustainable solution, provides good resistance to environmental degradation such as chloride penetration resistance. However, their advantages surrounding extreme curing conditions have to be studied, unless used at optimised quantities. The research focused on the potential of blast furnace slag systems to undergo internal sulphate attack due to high internal temperatures. The simulation of high internal temperature was done through heat curing inside an oven following which continuous storage under lime solution was carried out in order to saturate the system. Slag systems at low and high substitution levels (20% & 50%) were used along with a combination of coarser and finer surface areas, to investigate their subsequent influence was chosen for the study. Also, since infrastructure industry adopts CEM II & CEM III-A cement type, where the former was low slag concentration with moderate fineness and the latter with higher substitution level of slag in combination with high overall fineness, their potentials for DEF have also been studied. For all the mixes, the influence of high curing temperature and exposure to moisture was studied through microstructural changes, pore size variations and mineralogical composition effect along with fineness on paste specimens. All studies were compared to reference systems of CEM I, which was observed to be the most detrimental due to DEF. Test results indicate that at lower substitution levels of slag secondary ettringite forms in significant quantities in neat systems along with traces of carbo-aluminate phases in the case of slag systems. Also, higher substitution levels does not appear to completely suppress the formation of ettringite after exposure. Its formation in both the cases showed more or less no influence of fineness of slag added, except in the case of pore size distribution. The significant presence of carbo-aluminates was observed in the case of all slag systems that could prove to be beneficial as they do not translate to deteriorative expansion.

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.

When hardening concrete is subjected to imposed deformations, stresses maydevelop. If at any point in time this stress exceeds the tensile strength ofthe material, the concrete will crack. Early-age cracking of concretestructures may lead to issues related to durability, serviceability and aesthetics.During hardening of concrete the material properties are still in development.This means that to be able to assess the risk of early-age cracking for aspecific case, understanding of the hydration process and the stress- andstrength development is required. Besides, concrete is a viscoelastic materialwhich means that stresses are affected by mechanisms of creep- and relaxation. Theright approach for assessing the risk of early-age cracking in concretestructures is yet to be determined and the effects of creep and relaxation onthe stress development during hardening remain subject of debate. The aim of this study was to gain more insight in the effect of stressrelaxation on early-age cracking of concrete structures under imposeddeformations. For this purpose, the following research question was formulated: “How can early-age cracking inconcrete under imposed deformations be analysed taking into account stressrelaxation and what is the applicability of the models used?” To be able to answer this research question, first the relevant processesand mechanisms that play a role in the hardening of concrete had to bediscussed in more detail. The state-of-the-art of the subject was discussed aswell as the methods to take into account viscoelastic material behaviour in theanalysis. Next, the research was narrowed down by considering a single case fromengineering practice. It was decided to consider a dive-under that was beingconstructed near Zwolle, the Netherlands. This dive-under consists of multiplesegments which are all constructed in the same manner by first casting the slaband subsequently casting the walls on top. The hardening process of the wallscorresponds to a typical imposed deformations case as is also described inliterature.  In order to assess the risk of early-age cracking for the selectedcase, finite element software was used. The hardening process of the walls ofthe dive-under was modelled by considering a cross-section of the wall andslab. A parameter study was carried out to gain more insight in the effect ofaltering the different input parameters of the analysis on the resultingstress- and strength development during hardening.  Also, creep data which could be found in literature was analysed andthe Maxwell chain model was adopted to be able to model the viscoelasticmaterial behaviour. The Maxwell chain model consists of units of springs anddampers connected in series. When using several of these units in parallel, theviscoelastic material behaviour can be simulated. Evaluation of the Maxwellchain model was needed to be able to use the available creep data in theanalysis.  Because the outcome of the analysis was dependent on many inputparameters, several laboratory tests were carried out at the Stevin laboratoryof the TU Delft. This was done for the specific concrete mixture that was also beingused in the construction of the walls of the dive-under. The aim of these testswas on the one hand to simulate the hardening process of the wall of thedive-under and on the other hand to determine the strength- and stiffnessdevelopment of the material over time. Also, the autogenous deformations weremeasured in a ADTM test. The results of the tests were analysed and couldsubsequently be used to improve the accuracy of the finite element modelregarding the risk of early-age cracking.  Then, by making use a finite element model of the TSTM test and comparingthe outcome of this analysis with the results of the actual TSTM test, theviscoelastic material behaviour of the concrete could be derived. It was foundthat the effects of creep- and stress relaxation at early-ages were initially underestimatedand new creep curves were derived. An average early-age creep factor for thestress in a governing point in the bottom of the wall of around 2.4 was found. Next to the laboratory testing and computational models, temperaturemeasurements and visual inspections were done in practice on a segment of thedive-under. The temperature measurements could directly be used to improve themodel. The aim of the visual inspections was to determine whether early-agecracking would actually occur in the walls of the dive-under. During theinspections it was found that early-age cracking did not occur in any of theinspected walls. Cracking of the walls was eventually observed, however notwithin the time period that was regarded for early-age cracking. The results ofthe inspections were then used to be able to make judgements on the accuracyand suitability of the model for the assessment of the risk of early-agecracking. By making use of the collected data, the derived material propertiesand the temperature measurements from practice, the assessment of the risk ofearly-age cracking could be performed more accurately. This resulted in afigure which showed the stress- and strength development over time for agoverning point in the cross-section of the wall. Based on information found inliterature, a global risk of early-age cracking of the wall could then bedetermined.  When comparing the results of the above described finite elementanalysis with the results of the visual inspections, it was found that theresults of the model did not correspond to the observations from practice. The outputof the analysis suggested a high risk of early-age cracking, while in realityearly-age cracking did not occur in any of the inspected walls. Possible causesfor this difference were subsequently discussed. In the end, conclusions were drawn on the accuracy of the analysis, thematerial properties that were determined through laboratory testing and theeffect of viscoelastic material behaviour on the risk of early-age cracking ofthe walls. It was found that the material behaviour at early-ages (the first 48hours) is very important for the overall stress development. Also, the resultsof the laboratory testing suggested that the effects of creep- and stressrelaxation are generally underestimated in this period. Moreover, it wasconcluded that early-age autogenous swelling is of significant influence on thestress development over time. This period of swelling prior the autogenousshrinkage is however not taken into account in the current Eurocode.  More research is needed into the (viscoelastic) material behaviour atearly-ages. In this way the creep curves as proposed in this research can beverified. The fact that cracking of the walls only occurred at higher ages,suggests that the effect of stress relaxation at higher ages is limited. Thisbehaviour should be investigated in more detail. Also, the research methods thatare used in this study should also be combined for more cases in the future.Using the combination of a computational model, laboratory tests and observationsfrom practice creates the necessary conditions to be able to draw well-foundedconclusions on the accuracy of the used models and the material behaviour in practice.Testing methods should be developed/improved to be able to measure materialbehaviour at very early ages.  

Concrete is one of the most widely applied construction materials worldwide. Although traditional construction processes are optimized in terms of efficiency, costs and material use, the main disadvantages are still related to the use of formwork, labour and limited freedom of shapes in architectural design. The future of concrete structures looks promising with the development of 3D concrete printing techniques. Research to 3D concrete printing systems and printable concrete mix designs led to the development of 3D printed bridges, houses and other pioneer structures. On the other hand, the influence of printing process parameters on the mechanical properties of printed elements is still a relevant research topic. The layer-wise construction process inevitably introduces interfaces between adjacent filaments. Therefore the experimental research of this master thesis focussed on the influence of printing process parameters. Samples were tested in tension, compression and tensile splitting. The time interval between printing two filaments was the first parameter to investigate. The results showed significant strength decrease with increasing time intervals. The second variable was therefore the effect of different curing conditions applied to printed filaments with a time interval of three hours. Results showed that it is possible to limit the strength decrease when curing of the printed filaments is applied. Finally, the nozzle standoff distance with respect to the printed filament was a variable in this research, as a nozzle with backflow was used. No significant differences were observed between different nozzle standoff distances. The anisotropic compressive strength and the tensile splitting strength were hardly influenced by the different printing process parameters. 3D concrete printing shows potential, but lack of design and calculation methods are still a limitation for the implementation of this technique in the national codes and guidelines. Therefore, the second part of this research was numerical research to investigate the possibilities to match the properties of a numerical finite element model with the experimental results, to be able to verify the strength of printed structures. The tensile bond strength test and the compressive strength test were simulated and both the interface and filament properties were assigned such that the numerical results matched the experimental results in tension. In compression it was possible to match two of the three loading directions. Finally, a hypothetical case study in Mali was studied. An emergency shelter scenario was chosen because the current shelter solutions are mainly built for short term use. 3D printed shelters benefit from the short construction time, but may provide better indoor climate, and a better feeling of safety, which possibly contributes the well-being of humans. The design of the emergency shelter was based on both the construction procedure, the green strength development of printed filaments, and the final strength of the material. In the end, numerical calculations showed that the shelter fulfils the requirements and complies to the code. The 3D concrete printing technique shows potential for new solutions for humanitarian problems, and possibly let people realize that the investment in 3D concrete printed emergency shelters is feasible.