The Zeelandbrug serves as an important connection between Noord-Beveland and Schouwen-Duiveland. The bridge completed construction in 1965 using the balanced cantilever method and has been in operation for nearly 60 years. After the passage of these years, the current status of the bridge’s bearing capacity is unknown. Monitoring devices can be applied to obtain more information about the bridge’s condition and to help extend its lifespan. Hence, this study aims to investigate the application of monitoring strategies for the Zeelandbrug. As part of the process of obtaining the monitoring plan, this research also aims to gain a better understanding of the bridge’s structural behaviour. To achieve this, a linear Finite Element Model using one-dimensional beam elements was developed for the Zeelandbrug. This model was used to evaluate how the settlement of a support affects the magnitude of shear forces within the bridge’s superstructure. A structural assessment on the bridge’s superstructure was conducted, with in longitudinal direction incorporating the additional shear forces resulting from a support settlement. Through this assessment, failure modes were identified together with their associated physical parameters. Lastly, a brief investigation was performed on monitoring strategies that can be applied to monitor these parameters.

Transitioning towards a more circular construction industry is crucial for achieving climate-neutral and fully circular operations by 2030. One significant contribution to this goal is fully utilising the reuse potential of prefabricated inverted T-girders. The primary challenge lies in adapting the inverted T-girder system to enhance the circularity of the girders. To address this, a comprehensive literature review is conducted to understand the structural response and disassembly challenges associated with the system, providing valuable insights into potential improvements. This study employs a numerical model based on the Finite Element Method using DIANA FEA to assess the impact of various design adjustments, specifically the removal of end transverse diaphragms and increasing deck slab thickness. The numerical model uses horizontal and vertical curved shell elements and is subjected to several critical load cases to determine the maximum stresses for different design parameters. The results indicate that eliminating the end transverse diaphragms has minimal impact on the distribution of longitudinal bending moments and shear forces. However, it causes a localised increase in support forces by 17\%. Additionally, significant differences are observed in the deck slab, necessitating an additional capacity of 16\% for transverse bending moments and 17\% for twisting moments. The study also examines the effect of skew angles to evaluate the applicability of orthogonal system findings to skewed systems. As the skew angle decreases, a greater proportion of forces and moments is absorbed by the deck slab rather than the composite girders. However, the mutual differences between systems with and without end transverse diaphragms generally vary by up to 2.6\%, indicating that the findings for the orthogonal system are broadly applicable to skewed systems as well.

This thesis focuses on the structural behaviour of prestressed concrete girder beams with a particular emphasis on stiffness degradation near intermediate supports due to cracking, in inverted-T girders. Despite advancements in bridge engineering, there is still a lack of understanding regarding crack development along the length of the beam and its impact on stiffness, including the redistribution of forces. The thesis aims to analyse the implications of these structural changes by utilizing methodologies such as the moment-curvature diagram and Finite Element Method (FEM) models. The unique properties of the moment-curvature diagram are leveraged to accurately reproduce changes in stiffness caused by cracking. Two case studies are conducted to achieve these objectives. Case Study 1 involves comparing actual tests with FEM models, it revealed a disparity between the FEM models and actual test results. Particularly in load-deformation behaviour and stiffness variation due to crack development, this case study highlights the necessity to accurately calculate the moment regions, especially the cracking moment, which defines the moment-curvature diagram. While Case Study 2 evaluates the performance of FEM models to realistic loading conditions, including traffic loads, with some models surpassing expected performance while others failed to withstand traffic loads. Overall, the study underscores the importance of a proper methodology to accurately translate the material parameters and complex behaviours into a FEM environment.

In the Netherlands, numerous bridges face reassessment. During this assessment, it is observed that in some cases, the applied shear reinforcement (stirrups) does not meet the detailing requirement given in the national annex of the NEN-EN 1992-2. This requirement, which states that the stirrups must enclose the longitudinal reinforcement to ensure adequate anchorage, is referred by the RBK. This guideline calculates the shear capacity by combining the concrete and the stirrup contribution. However, the contribution of the stirrups can only be included when the detailing requirement is satisfied. In this research, a case study is used in which stirrups are applied that do not meet this requirement. These stirrups are expected to still contribute to the total shear capacity. Therefore, the main aim of this research is to develop a model that can predict the shear capacity by including the anchorage influence of these non conforming stirrups.

In this research, a layered approach is modeled to determine the shear capacity. This approach divides the cross section into several layers, and each of these layers is individually analyzed with the Modified Compression Field Theory (MCFT). The next step in the development of the model is to implement the anchorage behavior. There are two rebar anchorages included in this research; the straight and hooked rebar anchorage. Separate approaches are used to determine the anchorage capacities, which are based on existing experimental research. In both approaches, the axial stress in the applied shear reinforcement could be limited to these anchorage capacities.

Due to the limited availability of experimental research on reinforced concrete beams with non conforming stirrups, this research includes a constrained validation of the model. Subsequently, the shear capacity of the bridge within the case study is predicted. The first cross section in the span region, where the hooked rebar anchorage is governing. As a result of the high anchorage capacity, little influence is observed in the shear capacity of this cross section. The straight rebar anchorage of the stirrup is governing in the support region. This type of anchorage has a greater influence due to the lower anchorage capacity compared to the anchorage capacity of the hooked rebar. However, in both cases, the predicted shear capacity of the model exceeds the concrete shear capacity based on the RBK. Therefore, based on these results, it can be concluded that there is still a contribution of the non conforming stirrups to the total shear capacity.

The proposed model within this research could be used to predict the shear capacity of reinforced concrete beams with non-conforming stirrups. However, for more accurate results, it is recommended to further develop this model to overcome its current limitations. Additionally, it is recommended to conduct more experimental research on these types of beams, due to the limited amount found in literature. Finally, it should be taken into account that the model in this research uses a conservative assumption that the crack is perfectly aligned with the non-conforming stirrup.

Bridges are instrumented with joints to facilitate free thermal expansion of separate structural elements and prevent development of internal stresses due to differential settlements of the supports. In the past, mostly between the years 1960 and 1970, joints were frequently designed as half-joints, which were easy to construct and automatically maintained a level-running surface on the bridge. Additionally, half-joints can be implemented outside of the supports, which minimises the magnitude of the sagging bending moment caused by traffic loads. However, half-joints quickly started displaying signs of degradation, caused by development of a crack in one of the re-entrant corners in combination with water leakage into the joint. This crack, often buried deep inside the joint, is difficult to inspect, causing the exact state of the half-joints to often remain unknown. Another way of assessing the structural integrity of the half-joints of a bridge, is to monitor it using a structural health monitoring (SHM) system.

In this research, the measurement data of the SHM system on the Naardertrekvaart bridge is used to to evaluate its current state and serve as an early warning system for detecting damage. The SHM system, which has been collecting measurement data since 2022, includes inclinometers, displacement sensors, and temperature sensors. The research consists of an extensive data analysis procedure on two datasets of the SHM system. The first dataset contains two years of measurement data, obtained at a low measurement frequency. The second dataset contains one day of high-frequency measurement data. Next, the structural integrity of the half-joints is inferred from the measurement data using multiple custom-built FEM models in combination with manual calculations. Based on the outcome of this research, recommendations are provided on SHM systems on other half-joint bridges and improvements of the SHM system of the Naardertrekvaart bridge are proposed.

Analysis of the deformation of the bridge revealed a distinct dependence on seasonal temperature changes, presumably caused by hindered thermal contraction of the half-joints. Analysis of high-frequency measurement data showed that traffic loads significantly affect bridge deformation, with a substantial portion of rotations occurring from the support platforms' movement. Differences in rotational behaviour can be observed along the width of the bridge and a stiffness parameter is used to identify potential damage. A significant variation in stiffness can be observed at specific support locations, particularly on the east and west sides of support 6. The study highlights difficulties in using the SHM system to determine the bridge's state, suggesting improvements such as understanding traffic load magnitudes, modelling damage effects, and increasing measurement frequency. These adaptations may require cloud storage solutions. Oscillatory measurement approaches on half-joint bridges can reduce thermal influence sensitivity, mitigate the need for development of a digital twin, and enable broader monitoring with fewer sensors.

This master thesis investigates the influence of flanges on the shear capacity of reinforced non-rectangular members without shear reinforcement. The study adapts the evaluation procedure developed by Yang (2014) for rectangular cross-sections to analyze plates with holes, I-beams and T-beams.

To stop the depletion of natural resources, reduce climate change and fight biodiversity loss a circular economy in 2050 is pursued. In the upcoming years a challenging opportunity arises. Many existing bridges and viaducts with bridge decks consisting of prefabricated concrete girders have to be replaced. These girders although not designed according to circularity concepts have potential to be reused in a new structure, which is in line with the highest achievable level of circularity. However, the construction market is not ready for this innovation. So, while current research still aims at the feasibility and suitability of the girders for reuse in new structures, this research aims at the next step of preparing the construction market. This is of high relevance because it speeds up the introduction process of the innovation and thereby safes girders from demolition.

In this research the focus is on the adaptions and modifications needed in the traditional design process to ensure a more frequent implementation of reuse of existing bridge girders in new designs. After a literature review into the type of bridge girders in the Netherlands, the structural feasibility, obstacles for reuse identified by the industry, the design process and environmental impacts a design approach is developed. Simultaneously to the development of this design approach a case study is performed to give a more practical view to design aspects. In this way the approach could be verified, adapted and modified.

The design approach consists of roadmaps, possible procedures and recommendations that guides project teams through each step of the system and preliminary design. In the case study a bridge deck for a 107 [m] long bridge, divided over 5 spans with reuse of existing girders is designed. The design approach is suspectable to changes due to experiences, gained knowledge and developments in the construction industry. Therefore, it needs review over time. The design approach concentrates on inverted T-girders but can be extended and applied to other girder types as well.

In conclusion, this research provides the foundations for a changed design approach that is needed to prepare the construction market for reusing existing girders. By giving guidance to project teams, the view shifts from using new girders to reusing existing girders. This is valuable to reach the environmental objective of a circular economy in 2050.

In Amsterdam, more than 30 steel-concrete composite bridges were constructed from 1880-1960 without mechanical connectors and transverse reinforcement. Currently, there are no simplified analytical methods to determine the bearing capacity of these bridges. Thus, the bearing capacity is verified using NLFEM or oversimplified analytical calculations. This research proposes an analytical method to determine the bearing capacity of historic steel-concrete-composite bridges without mechanical connectors to avoid time-consuming FEM calculations and offers reasonable results.

An experimental and numerical study is performed on data from in situ and laboratory testing of samples from two different bridge decks from these Amsterdam bridges. The tests are accompanied by a numerical model that has been studied and adjusted to a more generalized loading case. This study determined that the exterior composite girders are critical due to their lower lateral stiffness.

An analytical model is proposed to examine the behaviour of the exterior composite girder. The model considers a 3-point bending load at midspan between the exterior composite and adjacent girder. The force distribution is described through a compatibility-based strut and tie model (C-STM). The concrete in compression is considered elastic compression struts, only limited by the ultimate load of the model. The concrete in tension is interpreted as a tensile tie, which fails when it exceeds the concrete tensile resistance. Following the failure of the tensile tie, it is assumed that a longitudinal crack propagates between the exterior composite girder and the adjacent girder. Additionally, vertical and lateral stiffness components are included in the model. These account for the flexural stiffness of the exterior and interior composite girder. The vertical stiffness is accounted for as elastic springs, and the lateral stiffness as spring beams. The interior lateral spring beam summarises all the interior composite girders' stiffness, whereas the exterior lateral spring beam only considers the exterior composite girder. Therefore, the configuration assumes that the interior spring beam is significantly stiffer than the exterior. Moreover, the stiffness of the exterior spring beam reduces when the longitudinal cracking occurs, assuming a part of the concrete fails. The C-STM is linked to the cross-section verification of longitudinal shear, biaxial bending and vertical shear resistance in two stages. Stage 1, at the load at longitudinal cracking, determines if the specimen fails at this moment, indicating that there possibly is a brittle failure. Stage 2 is after longitudinal cracking, where the steel-concrete contact perimeters have reduced, and the corresponding resistances accordingly reduce.

The failure modes obtained by the analytical model are comparable to the ones observed during the experimental testing. The analytical model showed that the bridges failed due to biaxial bending limited by partial shear interaction. One of the specimens from the testing yielded due to bending but with limited ductility. The other specimen also yielded due to bending with concrete crushing at the top concrete fibre. Further, the bearing capacities obtained from the analytical model are comparable to the failure loads from the experimental and numerical results.

The model predicts the failure modes and the bearing capacity and can therefore contribute to the assessment of the historic Amsterdam bridges, helping to reduce the assessment time of the bridges and understand their load-bearing behaviour better. Future work should focus on verifying the method by examining more bridges using FEM.

Geopolymer concrete, a relatively new building material has been suggested as an alternative to Ordinary Portland Cement (OPC) based concretes for within the building sector, due to it having a relatively low CO2 emission during production. An issue with geopolymer concrete however, is that high amounts of early age creep, shrinkage and decrease in elastic modulus over time has been observed which in conjunction with the existing research on geopolymer concrete having been predominantly conducted in laboratory settings, leads to questions about the applicability of these findings and thus the use of geopolymer concrete as a replacement for OPC based concretes in concrete structures. To address this issue, this study makes use of Fiber Bragg Grating (FBG) fiber optic sensors, placing them within reinforced steel which in turn are placed inside geopolymer concrete to obtain data that is more applicable to real world settings compared to prior findings observed within laboratory settings.
Four precast prestressed geopolymer concrete girders are produced and rebars containing FBG fiber optic sensors are placed prior to casting. Measurements before and after the release of prestressing are analysed to determine the apparent transmission length of the prestressing strands and compared to methods defined by Eurocode EN 1992-1-1. The applicability of these guidelines is evaluated, as these are intended for use with conventional concretes. The changes in strain over time, and elastic modulus changes during curing of the precast girder is monitored and analysed, focusing on the effect of time dependent changes of the concrete to the effectiveness of the prestressing forces. The girders are combined with a geopolymer concrete topping to form two beams and one slab. The beams are subject to a flexural test and a shear test respectively. The slab undergoes a cyclic loading test before being tested in two critical point load locations in two separate tests. The FBG measurements gained during the tests are evaluated using analytical methods.
The results show that the FBG fiber optic sensors integrated into steel rebar can deliver strain measurements in locations conventional strain sensors cannot be applied. The FBG data suggests that conventional concrete transmission length guidelines are applicable to this geopolymer concrete mixture. As the FBG-integrated rebars were all placed in the same location in the precast girder cross section, the strains measured by these FBGs are only representative of concrete near the core of the girder, where the concrete is not exposed to drying conditions. And because this geopolymer concrete is susceptible to drying, the difference between measurements and expected values such as the elastic modulus and the drying shrinkage can be explained by the low amount of drying that the concrete in the core of the girder experiences. Due to the positioning of rebar in the cross section of the precast girder in this study, crack initiation during the flexural test was not detected. The occurrence of cracks is detected in the measured strains as it diverged from the linear elastic model during the crack propagation phase. This shows that the cross sectional position of the sensors is important for the desired function and role of concrete in-situ FBG monitoring.
Overall, the data that the (rebar-integrated) FBG strain sensors can provide can be of great use towards the larger task of bringing geopolymer concrete structures to a real life application. In-situ FBG strain data of the precast prestressed girders provide additional information into the application of this SCGC mixture in larger scale structural elements. This data is a useful addition to the data gained through conventional methods, and results from lab testing. FBG monitoring can provide strain data from the core of structural elements, giving an insight of the behaviour during curing and (destructive) testing that is otherwise difficult to obtain.

The use of precast Reinforced Concrete has been the norm for the construction of tunnel linings for decades. However, the use of Steel Fiber Reinforced Concrete is gaining popularity due to its beneficial mechanical properties, improved sustainability, improved durability, and lowered costs compared to traditional Reinforced Concrete (RC). The amount of reinforcing steel can be reduced significantly, lowering the costs and carbon footprint of a project making it an appealing alternative. One application of SFRC is found in tunnels making use of precasted concrete segments, typically built with a Tunnel Boring Machine (TBM). This type of tunnel is found in Amsterdam, namely the
Noord/Zuidlijn: a metro line linking the north and south of the city. This project is used as a reference to perform a feasibility study on the use of SFRC and investigate its benefits.

The research involves a structural analysis of an SFRC design considering the loads and boundary conditions of the original Noord/Zuidlijn. The loads occur at different phases in the realization of the tunnel: transient, construction, and service phase. These loads can lead to various failure mechanisms, with the primary concern being tensile splitting of the concrete. Each phase is defined by a dominant component that governs its behaviour. During the transient stage, involving demoulding, stacking, and handling of the segments, the bending stresses inside the segment need to be checked. During the construction stage, the ring joints of the segments presents a weakness and need to be checked for
spalling and/or splitting of the concrete. During the service stage, the longitudinal joint needs to be checked for splitting of the concrete and the global cross sectional stresses of the lining need to be examined.

Numerical models are created to assess SFRC’s performance in the governing parts of the tunnel during the three phases, with the addition of identical RC models as a basis for comparison. The general bending and shear stresses, as well as localised splitting and spalling stresses, are investigated in both the Serviceability Limit State and Ultimate Limit State. The structural behaviour of SFRC corresponded with the characteristics found in literature, with a higher initial cracking load than RC and a more stable crack propagation due to its residual tensile strength. The peak strength of SFRC is found to be lower, but still passes all the checks. The results show that an SFRC design with the
minimum fiber content of 30 kg/m3 is sufficient for the transient and construction phases, but 40 kg/m3 is needed for the service phase. The benefits of implementing SFRC in the Noord/Zuidlijn can be quantified in a steel reduction of 60%, which would mean a decrease in steel consumption of 3050
tons. The CO2 emissions would decrease with 5500 tons, equivalent to the amount absorbed by 220.000 trees over the course of one year.

The concluding results can be used as guidance when opting for SFRC in a new bored tunnel project. The performed design checks show a governing load situation in both the construction phase and the service phase. The sufficiency of SFRC for the ring joint check during construction is governed by the
splitting force between the loading shoes of the TBM. The magnitude of this splitting force depends on the size of the TBM, the characteristics of the soil, and the depth of the tunnel. A large diameter tunnel, high-friction soil, or deep tunnel will decrease the likelihood of a design with solely fibers. The same unfavorable conditions cause large internal bending moments which could pose problems for the longitudinal joint and global cross section check. A small diameter tunnel and a tunnel constructed in stiffer soil will increase the feasibility of a design making use of solely fibers

This thesis addresses the topic of ongoing (excessive) deformations observed in balanced cantilever prestressed concrete bridges all over the world. Many authors attribute this behavior to the time-dependent phenomena of creep and shrinkage. Balanced cantilever bridges are classified as creep-sensitive structures, and for that reason, a detailed analysis of the long-term structural behavior, such as deformations and prestress losses, is recommended. However, during the design of these bridges, commonly used code-based models generally tend to underestimate the long-term creep and shrinkage effects. Additionally, various (simplifying) assumptions are made when modeling these bridges, making their actual creep and shrinkage behavior unclear.

This work aims to investigate whether the long-term effects of creep and shrinkage are indeed a plausible explanation for the excessive and ongoing deflections detected in a specific balanced cantilever bridge in the Netherlands: the Rooyensteinse Brug. This bridge, inaugurated in 1977 in Zoelen, currently exhibits a deflection at midspan of 0.43 m, more than two times what was anticipated in bridge design. To investigate this behavior, a detailed two-and-a-half-dimensional finite element model was developed, incorporating a time-dependent phased analysis accounting for the construction phases. Creep and shrinkage effects were incorporated into the concrete material model through creep compliance and shrinkage strain curves.

A sensitivity study is conducted to analyze the impact of: (i) different code-based models for creep and shrinkage, (ii) accounting for the large creep and shrinkage model uncertainties, (iii) different maturities on the creep compliance curve, and (iv) considering cross-sectional variability in the drying characteristics. The main findings showed that commonly used code-based models, including the current standard Eurocode 2, significantly underestimate the long-term (multi-decade) deflections observed in the Rooyensteinse Brug by 30%. Notably, only the RILEM B4 model (B4), considered the most theoretically grounded creep and shrinkage model, was able to capture the long-term deflection trend reasonably well. Further, acknowledging the inherent uncertainties in B4 significantly widened the range of potential deflections and prestress losses. Using the bounds of the 90% credible interval, the midspan deflections after 60 years range between 0.514 m and 2.91 m, compared to a mean of 0.895 m. The prestress losses range between 18% and 30% in the shear-critical zone, against a mean of 23%. Additionally, accounting for the cross-sectional variability in drying characteristics led to an improvement in both the short-term (first 10 years of service life) and long-term (multi-decade) deflection prediction when comparing the results to in-situ measurements, with differences of 14% and 6.1%, respectively. Based on these predictions, it is expected that the current deflection trend of the Rooyensteinse Brug will continue to decrease linearly (log-scale) for the next two decades.

This study demonstrates that a detailed finite element model incorporating time-dependent phased analysis, in combination with the RILEM B4 model and accounting for cross-sectional variability, can explain the observed behavior in the Rooyensteinse Brug. The results support the hypothesis that the long-term effects of creep and shrinkage are the cause of the ongoing trend of excessive deformations in this balanced cantilever prestressed concrete bridge.

With the continuous increase in transportation volume, bridges face the challenge of carrying higher traffic loads. As a result, bridges undergo fatigue due to repeated and increasing loads, leading to progressive deterioration of their structural reliability. While existing codes already account for several variables affecting the concrete's ability to withstand compression fatigue, some critical factors are still not considered. Furthermore, the modelling aspects of Finite Element Analysis (FEA) are often simplified or disregarded in practical applications, leading to an inaccurate estimation of fatigue life for concrete. As a result, a considerable number of bridges may be rejected for fatigue despite the possibility that they are not susceptible to this issue.
To address these issues, this thesis presents a parametric study that explores and quantifies the influence of various parameters on the fatigue life of inverted T-girder bridges, focusing on modelling aspects and material degradation. For this reason, a special case study was specifically designed to fail in fatigue within its lifespan. The fatigue analysis process employed both analytical and numerical methods, with the fatigue failure of the beams determined based on the bending failure at the beam's midspan criterion. A basic case was established as a basis for analysis, and the effect of each parameter was evaluated by incorporating it into the basic case and calculating the fatigue life of the bridge.
The analysis results demonstrate that stress distribution affecting parameters are significantly important in determining the fatigue performance of a bridge. The analysis identified that accounting for the gradual development of prestress losses, rather than instantaneous losses, and utilising area loads for vehicle modelling are critical aspects that substantially prolong the bridge's fatigue life. Furthermore, performing a historical lane configuration analysis is crucial for accurately assessing fatigue, as it significantly impacts the fatigue life of the bridge and identifies its critical components. The inclusion of material degradation caused by cyclic loading in the fatigue analysis is highly advantageous. It can even result in the bridge no longer being susceptible to fatigue. It is worth noting that accounting for the cracking of the slab in the lateral has a major negative effect on the fatigue life of the bridge. However, it is necessary to include it in the analysis to avoid an inaccurate and overly optimistic fatigue assessment. The study also provides specific equations to calculate the effect of time-dependent traffic volume on fatigue life. Lastly, the research investigated other parameters, such as the composite action of the structural components, which had a minor effect on the fatigue life of the bridge.
It is important to note that the influence percentage of most parameters cannot be generalised to all bridges, as they are contingent on specific factors unique to each case. Nevertheless, this research provides insights into the magnitude and contribution of the investigated parameters' effect on the fatigue life of concrete bridges, outlining those that must be necessarily included in the fatigue assessment.

This research aims to improve the structural safety of wide floor slabs with openings due to embedded non-structural elements. The multidisciplinary design of these slabs has resulted in various safety issues, including errors in the design due to the absence of a calculation method and insufficient integration between designs of different disciplines. The study assesses the influence of non-structural elements on the bearing capacity of the slab, maps possible errors in the integration process, and proposes an integrated design strategy to resolve safety errors.

The structural analysis focuses on four parameters of the openings (shape, size, longitudinal placement, height placement) and their effects on three failure modes (bending moment failure, shear failure, and interface failure). A workflow of calculation methods is formed to calculate the bearing capacity of a slab with an opening and the influence of the openings are calculated with a case study.

The management analysis assesses the integration process by examining the project process used in the building industry and comparing it to a theoretical process. It was found that the integration of the designs is heavily affected by a phase shift between the structural and mechanical disciplines. This phase shift is caused by multiple disrupting events.

Using the influence of the openings and the disrupting events, an integrated design strategy is proposed, which includes design rules and an adjusted design process. The feasibility and effectiveness of this strategy were assessed by an expert panel, who found it to be feasible if applied to all structural elements, but concluded that it could only mitigate the problems caused by the design shift. Other measures, such as changing the DNR and adapting another project delivery model, may be necessary to fully solve the safety issues.

Sustained economic growth based on a linear production model is not feasible on a planet with finite resources and a limited capacity to absorb wastes. By 2050, the world's population has grown exponentially to 9.7 billion and a tripling of the global use of materials can be realistically expected. Therefore, the transition towards a circular economy has become one of the top priorities of the European Union and its national governments. The Dutch government has stated the ambition to achieve a fully circular economy by 2050 at the latest.

Worldwide, the construction sector contributes to approximately 13\% of the global economy and employment of over 110 million workers. However, the construction sector is also responsible for 50\% of the total raw material consumption, 40\% of the energy usage and for 39\% of all carbon dioxide emissions. The construction sector therefore shows excellent potential for the transition to a more circular production model. One of the initiatives in the Netherlands to accelerate this transition is called 'het betonakkoord'. This initiative aims to make the concrete supply chain more sustainable by focusing on high quality application of recycled or reused building materials and on circular design.

Another interesting development is ongoing in the Netherlands. Since 2019, the housing shortage is 300.000 units and is continuously increasing to approximately 400.000 units by 2025. Combined with the economic growth of the past decade has resulted in skyrocketing house prices. One of the most effective solutions to overcome this problem is to simply built new houses. However, taken into consideration the transition towards a circular economy, raises a fundamental question of how to design sustainable residential buildings with regard to circular principles. This question is quite extensive and consists of a variety of both structural and non-structural aspects. This thesis project focuses on the circular design of a load-bearing floor system for utilisation as a storey floor.

First, extensive literature review is performed in order to identify circular principles and strategies in the design phase of buildings. This thesis project adopted the framework of Cheshire. The framework consist of six nested circles which represent the hierarchy of the framework. Within the framework, Cheshire proposes five design principles in his book: building in layers, designing-out waste, design for adaptability, design for disassembly and material selection. This thesis project expanded the design principles of Cheshire, by defining and providing hands-on strategies or criteria for each design principle. In total, 42 criteria are proposed for transposition into a comprehensive framework for application in practice.

Next, several recent residential and utility building projects are reviewed to determine the state-of-the-art regarding the implementation of circular design principles in practice. The general tendency is that these projects take the deconstructability and, to a limited extent, the selection of materials of the buildings into account. The common denominator in all recent projects, and especially for residential buildings, is the reduction of environmental impact through the realisation of net zero energy buildings.

Concrete floor systems are still widely used in newly built residential and utility buildings, due to their favourable characteristics. Therefore, the four most commonly used concrete floor systems are reviewed and discussed, These four floor systems are primarily designed in accordance with the demands of users and contractors. As a result of this thesis project, a newly designed floor system is proposed which is fundamentally based on the theoretical design framework developed in this report. Implementation of all five design principles was not considered feasible given the current state-of-the-art and regulatory barriers. Therefore, the newly proposed floor system is based on strategies and criteria related to the design principles of adaptability and disassembly. These design principles form the starting point of the multi-criteria analysis conducted in this report. The multi-criteria analysis is conducted in the first place to evaluate the performances of the four most commonly used concrete floor systems in the context of the considered circular design principles and secondly to compare the newly proposed floor system with the four most commonly used concrete floor systems. Based on the weighted rating scale, a maximum score of one hundred can be obtained. Results from the multi-criteria analysis reveal that three out of four commonly used concrete floor systems score well (scoring between 60 and 80) regarding the circular design principles. The newly proposed floor system scores excellent (scoring between 80 and 100).

The structural resistance of the newly proposed floor system is verified in accordance with codes of practice and state-of-the-art research under normal loading conditions. Horizontal wind loading is taken into consideration as well with respect to the stability of floor systems. Moreover, several stakeholder prerequisites are included in the design. Possibilities for recesses, staircases or vertical shafts for example, and building services are provided and suggested. Buildability aspects, such as the fabrication and assembly process are discussed in the report as well.

To calculate the flexural-shear resistance of prestressed concrete members without shear reinforcement, a method based on Critical Shear Crack Theory (CSCT) has been presented for the new Eurocode (prEN1992). Two alternatives provided in draft 7/2020 (prEN1, prEN2) are the starting point given, and two other alternatives are proposed for analysis by this thesis (prEN3, prEN4). These alternatives and other design codes were evaluated based on the concepts involved, assumptions made, and the results obtained compared with experimental results.

The research started by compiling basic concepts handled by empirical (ACI318-19 or EC2), MCFT-based (AASHTO-LRFD), and CSCT-based (prEN1992) approaches for the calculation of the flexural-shear resistance of members without shear reinforcement. Experimental results of slender prestressed concrete beams without shear reinforcement were collected from the ACI-DAfStb-PC/2015 database, and these tests were classified into subsets according to the relevant criteria used in the different design codes such as type of shear failure or cross-section shape. Subset 1 groups rectangular and I/T shaped beams with flexural-shear failure. Subset 2 retains the rectangular beams from subset 1, and subset 3 filters the tests from subset 2, applying conditions checking for anchorage or flexural failures. In addition, different critical locations were assumed, some according to design code suggestions (x_r=d or x_r=a-d), and others according to an assumption based on test results (x_r=0.65a).

The comparative evaluation results of the flexural-shear strength estimated by the different design codes with the experimental results were made for the 3 subsets and 3 critical locations defined. From this data set, the critical location x_r=a-d was chosen as the appropriate based on a general evaluation of the precision acquired by the approaches and the higher flexural stresses present. Then the data for the 3 subsets at this location are captured and presented in a range from lowest to highest from here on.

Based on the results, it was concluded that AASHTO-LRFD and prEN1 are the most precise approaches with COV less than 0.25. However, AASHTO-LRFD tends to obtain very conservative results. ACI318-19M has the worst performance in terms of precision with COV in the range of 0.29 to 0.45, its approximate method tends to get results below the desired level of safety, and its detailed method tends to be more conservative. EC2 achieves a regular precision with COV between 0.28 and 0.31, the linearized alternative for the new Eurocode (prEN2) obtains similar COV values ending up with a regular precision as well, and both approaches have a good level of safety.

To improve prEN2, new alternatives (prEN3 and prEN4) were derived based on the linearization of the main failure criterion of the CSCT. Of the alternatives, prEN4 was the most precise (COV=0.25-0.26) with a good level of safety, recognizing that this alternative correctly applies the concept of prestressing as preload and incorporates the effect of the normal loads on the shear strength with a direct relationship dependent on the ratio d/a_cs, where a_cs=|M_Ed/V_Ed |. Finally, all approaches were tested in terms of usability in design cases for simply supported and continuous slab decks, concluding that prEN4 has a significant advantage in usability since it is an expression similar to the one used in the current EC2, and also obtains results that are on the safe side compared with a more precise approach like prEN1.
Alternative 4 (prEN4) is an approach with good accuracy and safety level, and can incorporate the effect of prestress on shear resistance straightforwardly and consistently, complying with the main assumptions of the CSCT and the assumption of prestressing as a preload indicated for the Eurocodes, thus incorporating only the influence of normal loads applied to the neutral axis to contribute to the shear resistance. prEN4 is also an approach that has been shown to capture the influence of all parameters considered to obtain a reliable estimation of the flexural-shear resistance; therefore, it is recommended to consider it as a potential alternative for a handy and reliable calculation of the flexural-shear resistance of beams without shear reinforcement.

In recent years, the area of structural engineering has found new interesting prospects in the theory of waves. The acoustoelastic properties of materials are utilized in a new promising structural health monitoring (SHM) technique. The propagation of ultrasonic waves through stressed elements gives the ability to find invaluable information on the stress-state of the concerned element. This information is then analyzed with the use of Coda Wave Interferometry (CWI). This technique explores the late arrivals of the wave field, which is referred to as the coda. Due to scattering of the waves through heterogeneous elements, this part of the wave field holds additional information compared to the early arrivals, which makes it very suitable for application on concrete elements.
The use of ultrasonic waves with CWI in concrete elements has been studied enthusiastically by engineers due to its promising prospects. Nevertheless, this technique is still developing both in theory and in practice. The majority of previous similar studies have been conducted in laboratory settings, and thus conditions are very controlled. In this study, the monitoring technique is applied in conjunction with Smart Aggregate (SA) to a concrete structure for practical use. The SA sensors are positioned in groups at key location in the first concrete cast of a cast-in-situ prestressed concrete bridge, which entails two bridge spans of approximately 30 meters length. Measurements are done at different phases during the construction of the bridge, when the stress-state at the positions shifts due to change in loading and boundary conditions. The resulting data is analyzed with CWI and subsequently evaluated whether it coincides with expectations. Deviations from these expectations are rationalized or an attempt thereof is made.
The results show significant differences between field measurement results and expectations based on laboratory tests. These differences are explained by the presence of a greater amount of parameters, which influence the data gathered from field measurements. With the appropriate measures, the various factors acting on the structure are isolated and subsequently validated with laboratory data. The most prominent factors affecting the acoustoelastic properties of the structure are established to be time related. These factors involve concrete shrinkage, creep deformation and the concrete hydration process. With laboratory testing the effect of these factors on the wave propagation velocity are determined. By taking into account the additional factors, a strong correlation between changes in the stress-state and changes in the wave propagation velocity is observed when the data is read in an adjusted manner.
With a proper protocol, the use of smart aggregates in combination with CWI could be very valuable in accessing concrete structures on-site in both its construction stage and service stage. As it stands, prerequisite knowledge of the monitored structure is necessary to make use of the full potential of the SHM involving CWI and SA. The demand for prerequisite knowledge of the structure increases as the monitored structure becomes more complex, because it increases the amount of additional endeavors required to properly asses the acquired data. As such, the discussed monitoring technique in its current stage of development generally has low accessibility for practical use. But with further research and more understanding of monitoring method, the discussed SHM technique might be applicable for general use in the near future.

The main motivation within this thesis is to explore the possibility of increasing the service life of box-beam bridge girders to aid the ambition of the Dutch government to become a circular economy. The construction industry still follows a more or less linear economy system. Many bridges in the Netherlands are approaching the end of their service lives, and efforts are being made to extend the lives of these bridges. This is but a temporary solution, and these bridges will require replacement within 20-30 years. This provides an opportunity to incorporate circularity within new and upcoming structures from the design phase onward rather than only as an end-of-life practice. Therefore this thesis explores the possibilities of incorporating the strategy of lifespan extension, in the form of a cover increase against chloride-induced corrosion. The exact increase was calculated through the use of a relevant service life design model (DuraCrete) and compared with the Eurocodes. The reliability of the model was checked using a Monte Carlo simulation. Subsequently, the circular aspects of future adaptability were also investigated considering a fictitious scenario of increased traffic load. Hence, it was concluded that a marginal increase in concrete volume and prestressing steel per beam would allow for a more durable and adaptable beam for the future.

The Dutch infrastructure counts many bridges, the majority of which are built in concrete. These bridges have been designed and constructed according to safety codes. A lot of these bridges date from the previous century and have been designed conform outdated safety codes. Therefore, the main problem
of these bridges is the uncertainty with regard to their structural health as well as their performance under the current loading conditions.
The application of ‘smart aggregates’ could potentially solve these issues. Smart aggregates refer to a network of sensors that emit and receive wave signals inside the concrete structure. These sensor are embedded within the concrete and can be implemented in both new and existing structures. The
changes in the medium with regard to the stresses are reflected by the phase changes of the wave signal measured by the smart aggregates. This information allows for the monitoring of the conditions of the bridge during its lifespan. The magnitude of the stress in certain parts of the structure could then indicate the need for maintenance at an early stage, thus preventing unnecessary maintenance while preserving the safety of the bridge. This method, however, requires a thorough understanding of the wave propagation inside a concrete medium subjected to a stress state. This thesis investigates how the relative wave-velocity change of a concrete-like medium is influenced by the stresses to which it is subjected. Throughout the report this relation is referred to as the acoustoelastic effect. The first part of the thesis is centered around the theoretical formulation of the acoustoelastic effect. During this study, the models of Murnaghan and Biot have been studied. Subsequently, their differences with respect to the fundamental assumptions have been indicated. Here, it has been found that the main difference between the two models is demonstrated by the way they regard the second-order deformation terms. Murnaghan assumed that these terms are significant and has included them in the constitutive relation. From the latter, Hughes and Kelly have derived expression for wave velocities of a stressed medium, which have been verified with experimental results. On the other hand, Biot adopted the theory of infinitesimal deformations which omits the second-order deformation terms. In addition he based his theory around the wave propagation of a bending rod and extended this model to a three-dimensional medium subjected to initial stresses. This generalisation of an approximated model has led to analytical expressions for the wave velocity of a stressed solid which are contradicted by experiments. From this comparison, it has been concluded that Murnaghan’s model results in the most accurate representation of the acoustoelastic effect.
The second part of the thesis focuses on the verification of the theoretical acoustoelastic effect through experimental research. For the purpose of verifying the acoustoelastic effect as well as determining the third-order elastic coefficients of a concrete-like medium, four specimens have been tested.
In order to investigate the influence of the inhomogeneity of the material on the changes in the wave velocity, two different material compositions have been investigated. The first type consists of a homogeneous cement paste, whereas the second type represents heterogeneous concrete including
aggregates. During the experiment, the different waveforms have been repeatedly emitted through a specimen subjected to an uniaxial compression. The relative wave-velocity change has then been obtained by post-processing the acquired data, which has been compared with Murnaghan’s model.
The conclusion of this research is that Murnaghan’s theory can be used to accurately predict the relative wave-velocity changes of the cement-paste specimens, and in particular the relative P-wave velocity changes. The results have shown that the radial recordings yield inconsistencies which can be attributed to the small dimensions of the specimens. Furthermore, the influence of the inhomogeneity of the material on the relative wave-velocity changes manifests itself through a discrepancy in the acoustoelasticity.
Here, it is found that the ratio between the aggregate size, the specimen dimensions and the wavelength of the signal determines the sensitivity to the acoustoelastic effect. Therefore, before the data from the smart aggregates embedded in a real structure can be interpreted, the experiments need to be improved and expanded. It is important to investigate the acoustoelasticity of waves with non-orthogonal propagation and particle-oscillation direction, while applying various stress states to the medium. This is because the smart aggregates are arranged in a network, where the signals are emitted signals are propagating through the structure via arbitrary paths between various transducers.

As the average age of bridges in the Netherlands is increasing and loads are rising, more research on the capacity of these bridges is required. One of the methods to determine if a bridge still has sufficient safety is proof load testing. Stop criteria are values of structural responses at which further loading may course irreversibly damage to the structure. Measuring all currently proposed stop criteria requires a lot of single sensors, and it is holding back the application of the method in practice. A method to improve the applicability of proof load testing in practice is to simplify the sensor setup by using semi continuous fiber optical sensors. In this research, a fiber optical measurement system is developed to measure stop criteria for proof load testing. For application of the system to the slab a stiff glued anchorage system is designed, which is able to transfer the concrete strains to the reusable, long gauge fiber optic sensor system. The performance of this anchorage system is checked by preliminary experiments that include the comparison of various glues and investigates time-dependent effects. An installation method for these anchors is developed to improve practical applicability. Stop criteria are developed and checked by a 1:2 scale experiment on a concrete slab bridge. The results from the optical fiber sensors measure the global behavior of the slab bridge such as cracking and deflection well. Internal optical fiber sensors were included within the reinforcement. These sensors provide information on the strains up until the yielding point of the reinforcement bars. In conclusion, the fiber optical measurement system does provide insight on global and local behavior of the slab, and is able to monitor stop criteria. More importantly, the critical location cannot be missed with this sensor setup, as well as that more simplifications in the measurement setup are possible. It is therefore recommended to apply this measurement technique in future applications of proof load testing.