Concrete structures are responsible for a substantial share of the total global CO2 emissions. This has led to the establishment of the Betonakkoord in the Netherlands, which aims to significantly reduce the emissions from concrete structures.

Reinforcement bars made from Basalt Fibre Reinforced Polymer (BFRP) offer a potential solution for making concrete structures more sustainable. Firstly, the Environmental Cost Indicator (ECI) of BFRP rebar is 43% lower than that of steel rebar. Additionally, BFRP does not corrode, which eliminates the requirement for a thick concrete cover to meet environmental class standards. Consequently, the use of BFRP reinforcement instead of steel could potentially reduce the amount of concrete needed, further enhancing the sustainability of concrete structures.

This study investigates the feasibility of enhancing the sustainability of a bridge deck in an inverted T-girder bridge by using BFRP rebars. BFRP differs from steel in several material properties. Although its strength, at approximately 1200 N/mm², is significantly higher than that of B500B steel, the much lower E-modulus of BFRP presents challenges. Additionally, BFRP behaves in a fully linearly elastic manner until failure in the absence of a yield plateau. The lower stiffness results in higher deformations and crack widths. The hypothesis is that this could potentially be problematic for shear capacity, as the concrete compression zone is reduced, aggregate interlock decreases, and dowel action is less effective due to the low transverse strength of the rebar.

In this study, various design variants for a bridge deck in an inverted T-girder bridge were modeled to assess the impact of different design parameters. A reference design variant with steel reinforcement was used as a baseline and compared with several variants incorporating BFRP rebars. The BFRP design variants differed in terms of reinforcement quantity, concrete cover, and effective depth.

A quasi-linear model of an inverted T-girder bridge was developed using the numerical software SCIA Engineer. The bridge deck design alternatives were modeled as an orthotropic plate with centroidal ribs. The numerical model clearly demonstrated that, due to the less stiff nature of the BFRP-reinforced bridge decks, there is less distribution of traffic loads compared to relatively stiff steel-reinforced bridge deck. Ultimately, the model showed that shear force is indeed the critical failure mechanism for BFRP-reinforced bridge decks. Reducing the effective depth of the BFRP reinforced bridge deck variants, decreases the shear force distribution in transverse direction by 5% and further decreases the shear capacity by 23% to 30%, depending on the adjusted parameters for each design variant.
The application of BFRP rebar in a bridge deck and the reduction of concrete cover to 25 mm, instead of the usual 50 mm used with steel reinforcement, is shown to be feasible in this study. An optimization was conducted to develop a bridge deck design, that meets the structural performance criteria of shear force while minimizing concrete usage. This resulted in an optimized design with a bridge deck height of 215 mm, compared to the conventional 250 mm.

A sustainability study based on the LCA cradle-to-gate life cycle phases (A1-A3) has demonstrated that BFRP reinforcement can significantly enhance the sustainability of a bridge deck. For the optimized design variant that meets all the structural requirements, the ECI reductions range from 27% to as much as 32%, depending on the cement type used in the concrete mixture. This study has shown that the application of BFRP rebars in a concrete bridge deck is certainly feasible and results in significant sustainability improvements.

There are about 70 prestressed concrete T-beam girder bridges built between 1953 and 1970 in the Netherlands. Ensuring the safety of these existing bridges under current traffic conditions is
imperative. Upon initial assessment of these existing prestressed concrete T-girder bridges, half of them didn’t meet the safety requirement specified by the design code even though they didn’t show any sign of distress during inspection. This is because the system behaviour of the T-girder bridges (i.e.) load transfer mechanisms such as CMA (compressive Membrane Action) and load redistribution were not considered, which could potentially increase the calculated strength capacity of these existing bridges. Therefore, a computationally efficient method for evaluating these bridges is needed.
This research addresses the challenge of accurately predicting the strength capacity of prestressed concrete T-girder bridges using a computationally efficient approach. The study involves modelling the 2D bridge deck in the horizontal plane using orthotropic plate elements and 2D individual girders in the vertical plane with non-linear material properties. The 2D bridge model was compared with a 3D linear bridge deck model, showing a variation in bending moment between 10% to 13%, sufficient for studying load effects. The 2D individual girder model built was validated using experimental data of the disconnected T-beam test of the Vecht Bridge, incorporating a quasi-Newton solution method with the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm. The predicted load versus deflection curve from the 2D non-linear individual girder model closely followed the load versus deflection curve obtained from the experimental test results. To combine the 2D bridge deck model with the 2D non-linear individual girder model, an equivalent loading technique was developed by numerically solving the shear force distribution of the critical girder in the 2D bridge deck model. The staggered 2D Non-Linear Finite Element Approach developed utilising the equivalent loading technique accurately predicted the ultimate failure load of the connected T-beams (system behavior) within 10% of experimental values, despite neglecting the effect of end crossbeams. The 2D bridge deck model without considering crossbeam effects showed conservative stiffness estimates. Crossbeam inclusion in the model indicated significant improvements in stiffness, load distribution and redistribution. The 3D non-linear finite element model predicted 87% to 95% of the failure load of the connected T-beam tests [10]. The 2D non-linear individual girder model using the staggered non-linear approach predicted up to 96.5% of the ultimate failure load of the connected T-beam test, achieving this with a run time of approximately 18 to 21 minutes. Overall, the staggered 2D Non-Linear Finite Element Approach developed shows promise for preliminary bridge safety assessments offering a balance between computational efficiency and accurate prediction of strength capacity.

Across the Netherlands close to 70 prestressed concrete T-beam bridges with cast-in-between slabs and transverse prestressing built in the 60's and are still in service. The current code NEN-EN 1992-1-1+C2:2011 assesses the shear capacity more conservatively. In addition, the NEN-EN 1991-2+C1:2015 prescribes an increased traffic load. This results in this type of bridge not complying with the current codes and the shear capacities are considered insufficient. However, upon inspection these bridges do not show signs of distress. This suggests the presence of additional load-carrying capacity, which is not considered in the current Eurocode.

Non-linear finite element analysis (NLFEA) can be used to accurately approximate the structural behaviour. This includes yielding of steel, cracking and crushing of concrete, the development of alternative load paths as well as snap-back and snap-through behaviour. However, performing each load step requires a great amount of computational effort depending on the amount of degrees of freedom of the model. In FEA a continuous shape is divided into discrete elements which together form a mesh. These meshes can be volumes, surfaces or lines.

To describe the geometry of the prestressed concrete T-beam bridges with cast-in-between slabs either volume elements or multiple surface meshes in different planes are required. Using a mesh of solids would result in system with such a high number of degrees of freedom which might even exceed the available computational capabilities or result in a very long duration of the analysis at best. Using shell elements to construct the mesh reduces the number of degrees of freedom by at least two thirds.

In this thesis I investigate to which extent we can simulate the structural behaviour of a prestressed T-beam slab bridge deck using a non-linear finite element model with a 3D non-planar mesh of shell elements. The Vechtbrug bridge near Muiden was a bridge of this type. A team of researchers from TU Delft have performed several collapse tests on this bridge. This includes extensive measurements of all the experiments as well as material testing on concrete and steel samples. For my own research, a single case study is conducted by recreating collapse tests performed on the Vechtbrug in which both isolated beams and unmodified spans have been loaded past failure. The results of the material tests provide accurate material properties as input for my numerical models. The results of the collapse tests allow for verification and validation of the outcome of the performed finite element analyses.\\

The results of the numerical analyses show a close approximation of the true collapse load with an overestimation of 15\% for the isolated beam model and 12\% for the cooperative beams model. The deflection again is overestimated with 18 and 56\%. The deflection of the adjacent beams relative to the loaded beam is too low. The numerical model is underestimating the transverse load distribution by $\pm$ 25\% for the adjacent beams and $\pm$ 34\% for the beams adjacent to those. The Guyon Massonnet method was applied to estimate the transverse load distribution with the supplied material properties and including the two cross beams. By contrast, the results were an overestimation of approximately 70% for the immediate adjacent beams. In the third numerical analysis the complete bridge deck and ultimate limit state verification is performed by applying the prescribed traffic load with all safety factors applied. The bridge can withstand 234\% of the prescribed load which agrees with the lack of damage present on the Vechtbrug after experiencing over 50 years of traffic load.

The results show that a non-planar shell mesh can generate a realistic structural response considering the collapse load approximates the actual one found in the collapse tests. However, this is somewhat limited for decks consisting of multiple beams since the implementation of the transverse load distribution in the numerical model was inaccurate. The structural response of the structure was too ductile in the numerical analysis with the deflection being overestimated and the strain under the loading plate double the value of the collapse test. Both Mustafa and Ensink have performed a numerical analysis of the isolated beam model using a mesh of solid elements prior to my thesis work. The results of the isolated beam model match closely in both the results of the NLFEA performed by Mustafa and Ensink. The solid mesh does yield more realistic cracking patterns. The isolated beam model showed the required evidence to demonstrate the activation of arching action: An increase in the horizontal reaction force required to lateral restrain the beam with the bending crack occurring under the loading plate so the arch action phenomenon could be activated. In the complete deck model evidence of compressive membrane action in the transverse direction was detected. In both the complete deck models, evidence of lateral confinement was demonstrated, increasing the maximum compressive stress of the concrete.

Finally we can conclude that a NLFEA with a 3D non-planar mesh of shell elements yields accurate results when considering a single strip of the bridge deck. However, the model with a the mesh representing the complete bridge deck, the capacity of the transverse load distribution is underestimated and the structure shows overly ductile behaviour. The model is capable of including the load-carrying mechanisms arch-action, compressive membrane action and fixed boundary action as well as the effect of lateral confinement.

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

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.

With the rising demand for renewable energy sources the amount of wind turbines which are built on land are growing rapidly. Because of the repetitive nature of the foundation small design improvements can add up to large savings in material usage and building cost. The most critical part of the foundation is the connection between the metal mast and the concrete foundation. This connection is made by implementing an anchor cage. The application of an anchor cage results in complex multiaxial stresses in the surrounding area. Confinement plays a large role in the compressive strength of the concrete subjected to the partially loaded area created by the anchor flange. The main goal of this thesis is investigating the best way of modelling the concrete surrounding the anchor cage at confinement levels being present in the wind turbine foundation. This is done by investigating the theory regarding confinement and the effects of it. But also the way this is captured by analytical models. To make a comparison between DIANA and the analytical models the present stress situation in the wind turbine foundation is analysed. Looking at the stress distribution and the confinement levels. Next a comparison of the ideal case of confinement is made. This is later expanded to a larger scale model, to compare the effects of confinement. The confinement present will enhance the concrete strength and strain properties. The concrete specimen can resist larger loads and reacts more ductile. These properties are captured in analytical confinement models, these are compared to the way DIANA approaches confinement. The anchor rods between the anchor flanges are prestressed, this prestressing results in a permanent stress situation. Within this stress situation there is a small part beneath the anchor flange subjected to confinement. With increasing pressure, originating from the moment load which results from the wind on the tower and it’s blades, the confined area increases to the top half of the foundation. With significant confinement in the pedestal. Comparing the DIANA compressive behaviour models with the analytical confinement models and experimental data within a one element model. It is seen that compressive behaviour models do react on the confinement. Although they show a lower peak strength than the analytical models. A larger difference is noted in the underestimation of the peak strain. This difference in outcomes is the result of the different approaches between DIANA and the analytical models. As well as some assumptions that DIANA makes concerning the strength and strain increase factor. The Parabolic compressive curve is most suitable for modelling the confinement in this ideal case. A case study is conducted to further investigate how the confinement is represented in a total non-linear model. Modelling the top part of the foundation and checking the effect that confinement has on this model. For this model the prestressing of the anchor rods is increased until failure. Without confinement the concrete elements directly beneath the anchor flange fail in compression. Running the model with confinement it is noticed that the underlying weaker concrete is failing in compression first. This is the result of the confinement being present surrounding the anchor flange. The model with confinement is also able to resist a 30% larger load than the model without confinement. The parabolic compression curve is the most suitable curve currently available inside the strain based crack model for modelling the effects of confinement. Overall DIANA is able to capture the effects of confinement well enough to see the positive effects in the case study. Due to limitations in the analyses performed and in their interpretations, it is not possible to give an unequivocal answer on the effect of confinement within the foundation of the wind turbine.

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.

Much of the Dutch infrastructure was built in the 1960s and 1970s and includes an estimated 55.000 bridges and viaducts. These aging structures have reached the end of their designed service life or will reach it in the short term. It is estimated that 15% of these structures have a box-shaped cross-section as being reinforced concrete underpasses and culverts. Replacing all these structures in the short term is not practical and financially feasible. In order to preserve these structures, it must be demonstrated based on a structural assessment that the structural safety is guaranteed. So many assessments will be performed by different engineers which is a time-consuming process and results in a non-consistent assessment where unnecessary disapproval occurs which in turn leads to unnecessary costs and loss of time. The aim of this thesis is to set up a structured consistent automated method with which unnecessary disapproval and unjustified approval is avoided, a necessary efficiency improvement is achieved and structures where structural safety is at risk can be identified. Quantitative research has been conducted in which numerical data from parameter studies is collected and analysed to investigate relationships between the studied variables, find patterns and generalize results. First a data analysis of existing underpasses and culverts in the Netherlands was carried out, after which the modeling of the load effect and the capacity of existing reinforced concrete structures was investigated. An automated analytical model was created for the first step to determine the load effect, including the method of Guyon-Massonnet for traffic load distribution for the determination of bending moments and an application of method 7.3-7 from CEB-fib model code 2010 to determine the shear force as a result of the local tandem system loads. The determination of the capacity of existing concrete structures was investigated arithmetically. As a second step to determine the load effect, a model refinement was performed by automating a 2,5D FEM plate model. Structures from the database were assessed with both the analytical model and the 2,5D FEM model and a sensitivity analysis was performed based on theoretical structures within the boundaries derived from the data analysis. It is concluded that it is the most efficient method to first assess the large batches with the analytical model and that the 2,5D FEM plate model for structures with an intersection angle smaller than 80gon and structures that are disapproved with the analytical model offers added value in terms of a lower UC. From the comparison with the 2,5D FEM plate model it is concluded that the method as an extension of method 7.3-7 from (CEB-fib, 2010) results in too optimistic values for the occurring shear force as a result of the tandem system load due to an overestimation of the vertical load distribution. This research shows that an automated structural assessment can be of added value on the classic approach of assessing existing concrete underpasses and culverts by significantly improving efficiency and therefore to realize a cost-efficient method.

Piles and pile caps are commonly used in the Netherlands due to the soft shallow subsurface soil that is predominant in the country which does not have sufficient bearing capacity to support heavy structures. Pile caps are currently designed analytically using the strut and tie model (STM). This is believed to be conservative and results in an over-reinforced structure with higher cost and unsustainable design due to inefficient use of materials. The main objective of this thesis is to investigate the application of Non Linear Finite Element Analysis (NLFEA) to design pile caps. Five experiments were selected from literature and modelled in DIANA. These pile caps had flexural, corner shear, flexure-induced punching and combined flexure and corner shear failure modes. Quarter of the pile caps were modelled using Finite Element Model (FEM) as it saves computational time and cost by making use of symmetry while still predicting the failure mechanism and failure load within 99% of the full model. The reinforcement was modelled using both embedded and Shima bond-slip. The FEM results were subsequently compared with the experiment to gain insight into how accurately FEM can capture the structural response of pile caps. The comparison shows that failure mechanism and crack pattern can be accurately predicted for all pile caps. However, the accuracy of the failure load depends on the failure modes of the pile cap as ductile failures are captured more accurately than those with brittle failure. The difference between the peak load in the FEM and the experiment is observed to be 5 - 7% for ductile failures while it varies between 25 - 42% for brittle failures. These differences are liberal estimates. Moreover, three pile caps that were designed using STM were modelled numerically to obtain the design resistance and compare the results. The comparison show that STM overestimates the stresses in the concrete by 40% – 70% as well as the crack width by 60 – 65%. This is because the effect flank reinforcement and post cracking contribution of concrete are not accounted in the STM. Numerical model results are also closer to the experimental results than analytical calculations by 50% on average. The comparison between STM and numerical model revealed that optimization of pile caps is possible. Subsequently, four parameters: pile cap geometry, bottom rebar percentage, number of flank rebar and concrete quality were reduced to evaluate the effect on the structural response of pile cap. These parameters were selected based on the interview with experts and results of the comparison between the FEM and experimental results. The parametric study was performed on a pile cap with punching failure. It was found that reducing the pile cap depth by 0.1m increases the rebar stress by 25 - 35% and reduces the failure load by 2 - 8%. Reduction of the bottom rebar percentage by 10% increases the crack width by 15 - 30% and lowers the failure load by 2 - 8%. A 50% decrease in the number of flanks is found to increase the stress in the bottom reinforcement by 20 - 25% but not affect the failure load significantly. Change in these three parameters does not change the failure mode and the failure load remained greater than the design load. However, decreasing concrete quality accelerates the onset of crack which decreases failure load and changes the failure mechanism from punching to corner shear. Cost analysis and environmental impact assessment also show that geometry optimization has more environmental and cost advantage than reducing the reinforcement. For every 0.1 meter reduction in depth, there is a 6% reduction in cost per pile cap and a 70 - 200 kg reduction in the CO2 footprint. Two sets of experiments were designed to validate the key findings of this thesis. The first set was designed to investigate if punching failure can be accurately predicted by FEM. This will be conducted on a scaled down pile cap with expected punching failure. A second set of experiment was designed to explore if the optimization observed in the numerical models can be achieved in reality. Two pile caps, with brittle and ductile failure were selected. Each will have a variable geometry, bottom rebar percentage, flank reinforcement and concrete quality. The current STM approach does not capture all the failure modes of pile caps since the unity check does not distinguish between certain failures such as concrete crushing and punching. It also does not account for the contribution of flank reinforcement and concrete contribution to the tensile strength post-cracking. Therefore, future designs of pile caps should take these parameters into account to obtain a safe design without underestimating the capacity of the pile cap. This would result in a more efficient design with lesser material and lower cost.

The Helperzoom bridge is one of few existing prestressed slab-between-girder bridges that is being managed by Rijkswaterstaat. These bridges are built before 1980 and do not comply with the current standards. To evaluate the structural safety and capacity these bridges are being assessed. A total of four girders were taken from the Helperzoom bridge, named HPZ1 through HPZ4, and have been destructively tested at TU Delft to determine the shear capacity and failure mode. The first two experiments, HPZ1 and HPZ2, have been considered in this research. Shear-tension failure is one of the failure modes that was being looked for and where the experiments were based on. The most important results of HPZ1 and HPZ2 have been combined and summarized in the experimental analysis, such as the failure load, the failure mechanism, the load at which the first inclined crack occurred, the cracking angles of the observed cracks, the calculated inclinations of the compression field from the LVDT measurements, the contribution of the shear reinforcement and the contribution of the prestressing cables to the shear capacity. Remarkably, the outcome of the test resulted in shear-compression/flexure-shear failure instead of the predicted shear-tension failure. The purpose of this thesis is to check whether the observed shear capacity and failure mode from HPZ1 and HPZ2 can be validated with existing models that are currently used by (inter)national standards and numerical programs. The Euler-Bernoulli beam theory and five (inter)national codes: ACI 318-14 (American Concrete Institute), AASHTO 8th Edition (American Association of State Highway and Transportation Officials), NEN-EN 1992-1-1:2011 (Eurocode 2), RBK 1.1:2013 (Richtlijnen Bestaande Kunstwerken) and EC2 draft 2018 (prEN 1992-1-1:2018 D3), have been used to calculate the shear capacity and if possible the failure mode of the Helperzoom girders. Next to the nominal shear resistance, failure of the compression field is considered for three codes: AASHTO, EC2 and EC2 draft 2018. For the two Eurocodes the value of the inclination of the compression field can be chosen by the user: the EC2 has a lower limit of theta = 21.8°, the lower limit of the EC2 draft 2018 (based on state of strains) is reached when theta is chosen such that failure of the compression field occurs simultaneously with yielding of the stirrups. For the EC2 draft 2018 different ways have been used to determine the longitudinal strain: with the expression given in the code, analytically determined with a sectional analysis and with the results of the LVDTs in the two experiments. In addition to the analytical calculations, a non-linear finite element program Response-2010 has been used, with which a cross-sectional analysis was made with two values for the prestressing force. With this program it is easy to determine the maximum shear capacity, the failure mechanism and many other results per loading step, such as the inclination theta, acting and maximum compressive- and tensile stresses, strains, etc. The maximum shear capacity is divided into components: the contribution of the uncracked concrete, cracked concrete, transverse reinforcement and a vertical prestressing force. Four different shear capacities have been calculated: the inclined cracking load, the nominal shear resistance according to the general procedures of the design codes, failure of the compression field and the moment that failure of the compression field occurs simultaneously with yielding of the stirrups. The Euler-Bernoulli beam theory, ACI (flexure-shear) and Response-2010 show that the load for the first inclined crack are close to the results of HPZ1 and HPZ2. For the nominal shear capacity, none of the codes nor Response-2010 are in line with the results of the two tests: all shear capacities are underestimated. The AASHTO overestimates the shear capacity for failure of the compression field,
because the inclination theta is not included in the expression. The EC2 overestimates failure of the compression field due to limitation of theta = 21.8°. The EC2 draft 2018 allows for lower values for theta, when the longitudinal strains are based on the state of strains with theta lower than 19.08°. The results for the shear capacity and inclination of the compression field of the latter design code are much more in line with the observed failure loads and inclinations calculated from the LVDTs. The lower limit for the shear capacity is when failure of the compression field occurs simultaneously with yielding of the stirrups. With this limit, the shear capacity is underestimated and the inclination of the compression field is lower than the inclination calculated from the LVDTs.
The results between the three analyses have been compared with each other and it was concluded that there are major differences between the analyses for the calculated shear capacities, internal lever arms, critical positions and the inclinations of the compression field. None of the results from the codes match the observed failure load in the experiments. Considering the expressions given for failure of the compression field, the EC draft 2018 based on the state of strains were closest to the test results for both the shear capacity and the calculated inclinations of the compression field from the LVDTs. The moment that yielding of the stirrups occurs simultaneously with failure of the compression field, the EC2 draft 2018 gave too low values for the shear capacity. For the numerical results, Response-2010 gave a representative shear force for the first inclined crack and failure mechanism, but not for the ultimate shear capacity. After the onset of the first inclined crack, the girder did not build up extra capacity as was observed in the experiments. The critical position was also not in line with the results from the test. It can be concluded that the results should always be looked at from a critical point of view and that no clear answer can be given as to which analysis is the "best" to determine the shear capacity of (prestressed) concrete T-girders. For the Helperzoom girders the EC2 draft 2018 based on the state of strains with theta lower than 19.08° gave the best results for the shear capacity.

The strut-and-tie method is a reliable tool for designing the reinforcement in discontinuity regions in reinforced concrete structures. It provides safe designs, but it is on the other hand also acknowledged to provide conservative designs. As the strut-and-tie model is widely used in the design of structural elements as wall girders and pile caps, it would be very beneficial if its verifications or the strut-and-tie model itself can be optimized. In this thesis the design process of wall girders and pile caps is optimized. This is done by comparing the design verifications and the strut-and-tie models itself with numerical analyses.

In situations with spatial limitations where a bridge is desired, a skewed bridge is a solution. The bridge deck of a skewed bridge has the shape of a parallelogram, in which the angle that remains in the acute corners is defined as the skew angle. This thesis focuses on the design of reinforced concrete skewed slab highway bridges that are simply supported on discrete bearing pads. A parametric tool is created which, based on a set of values for input parameters, generates and analyzes bridge models created in a finite element method (FEM) software program. After analysis, results are summarized, which allows for a parametric study. The goal of this study is to develop knowledge for decision-making in early stages of projects. The influence of main geometric parameters (skew angle, bridge span length, bridge road width) on the load distribution in a skewed bridge is investigated, as well as the influence of FEM mesh element size, applied plate theory, traffic load configuration and support configuration. Another important parameter, being the bridge deck height, is set to be dependent to the span length at first. Skewed slab bridges tend to span from support to support in the shortest direction. This means that loads will concentrate in the obtuse corner. Resultant quantities, mainly bending moments and shear force in the obtuse corner, are studied on their relation with the different parameters. The obtuse corner load concentrations can require the bridge height to become larger than desired. A powerful measure is to add additional triangular sections (ATS) to the side of the bridge, which increases its width. This way, the road skew angle no longer equals the bridge skew angle. Load concentrations in the obtuse corners are reduced, which can result in a lower deck height, less reinforcement, or both. The effect of ATS addition on resultants is studied: great reduction is observed, even for relatively small ATS addition. In order to relate the bridge deck height to bridge geometry, a reinforcement study is conducted. A procedure is described for a certain cross-section, in which the resultants are known from models generated by the parametric tool. Starting point is the longitudinal bending reinforcement based on the crack width criterion, which is usually governing in bridge design. Next, required shear reinforcement is calculated. Following, the ultimate bending moment capacity is checked, which is found to be always sufficient for crack-width-based reinforcement design. Finally, optimization (different bar diameters, add or remove a reinforcement layer, deck height reduction) is investigated and applied if possible. Using the procedure described above, a case study is conducted. A highly skewed bridge (road skew angle of 20◦) is taken. Starting with 0◦, ATS is added in steps of 5◦. For each geometry, a model is created with the tool after which resultants are used for the reinforcement design. For a bridge span of 13.7 m and a road width of 8.3 m, the first bridge (no ATS, skew angle of 20◦) was found to lie beyond the limits of reinforced concrete. Two layers of 40 mm bars required fatigue-sensitive couplers, while detailing and proper anchorage would prove to be unconstructible. Increasing the cross-section also increased governing moments due to concrete self-weight and therefore is not an option. Addition of a 5◦ ATS resulted in a 900 mm high bridge deck, which seems possible to construct. Addition of 10◦ resulted in a 750 mm high deck, 15◦ into 600 mm and further ATS addition reduced deck height even more. Although it should be noted that ATS addition increases deck surface, a significant reduction in deck height can be obtained. ATS addition is shown to be a powerful measure. Depending on project situation, it can provide optimization and cost savings; the parametric tool is proven to be useful in investigating this.

The actual shear capacity of existing reinforced concrete solid slab bridges, constructed before 1960 in the Netherlands, is subject of an extensive research nowadays, since additional sources, increasing the total shear capacity of concrete slabs, have been ascertained to be present. In addition, the increasing occurrence of human-induced earthquakes in the province of Groningen due to gas extraction, arises uncertainties regarding the seismic behaviour and earthquake resistance of existing bridges in this area. The Nieuwklap bridge is a reinforced concrete solid slab bridge located in the Groningen province, combining both characteristics under investigation.
In the course of this research, the first two Levels of Assessment have been applied for the static analysis of the slab part of the deck of the Nieuwklap bridge, which was found to have sufficient shear and bending moment resistance under the application of the traffic loading defined by the current standards. The equivalent proof load tandems, that generate the same shear and bending moment stresses, for each load combination level have been found with both approaches, proving that localized phenomena at the area of load application are more intense when the field experiment takes place. Furthermore, by obtaining the equivalent tandem loading that leads to shear and flexural failure of the concrete slab, it has been concluded that flexural failure due to yielding of the reinforcement will occur first in case of exerting the collapse loading on the flexure-critical positions or on the shear-critical positions next to the end-supports. However, in case of applying the tandem loading on a shear-critical position next to a continuous support it is difficult to define which failure mode will precede.
Regarding the seismic evaluation of the Nieuwklap bridge, the simplified fundamental mode method and a modal response spectrum analysis have been used. The bridge deck has been found to have adequate resistance against the vertical component of the seismic excitation, which cannot be considered critical, generating nearly the half stresses compared to the traffic load (LM1) combination defined by Eurocode 2. The piers and the pendulums supporting the bridge deck have been also evaluated against the two horizontal components of the seismic excitation, for two different earthquake return periods, discovering that they are able to withstand the generated forces without additional measures. Finally, from the performed modal analysis it has been obtained that movement of the bridge on the longitudinal direction can be observed in lower frequencies compared to the vertical and the transverse direction and that more than 10 modes are necessary in order to describe accurately the bridge behaviour.
This Thesis describes the framework that can be used to prepare collapse tests at multiple Levels of Assessment, and for the seismic assessment of existing bridges in regions with recently initiated seismic activity.