Cracking in reinforced concrete structures can occur at any stage during their lifetime, and is anticipated for during its design. It is crucial to control crack widths by applying sufficient reinforcement to prevent impairments to the structure’s functionality. Cracks may develop due to external loads applied to the structure or due to imposed deformation. Imposed deformation occurs when movement is restrained, for example due to adjoining structures or internal thermal strain gradients in thick concrete sections. With the increased use of high-strength concrete since the late 20th century, autogenous shrinkage has emerged as a significant source of early-age cracking as well.
In practice, observed crack widths due to a combination of imposed deformation and mechanical loading during the early age of concrete are often smaller than predicted by structural design codes. Thissuggests that less reinforcement could be used in concrete structures, offering potential durability benefits. However, literature lacks sufficient validation of this observation, as well as an understanding of its origin and how it can be incorporated into design practices.

Liang (2024) developed a mini-TSTM specifically for plain mortar and validated its setup using results
from the regular TSTM, which has been extensively documented in the literature. This setup allows for precise control over both deformation and temperature development and is integrated with an Instron loading machine. By restraining the mini-TSTM specimen during its early age, stresses due to autogenous shrinkage and thermal deformation are induced.

The following research question was explored: How do crack widths develop in reinforced concrete
under the combination of imposed deformation and mechanical loading?
The first method used to answer this research question was to compare experimentally observed crack widths due to the combination of imposed deformation and mechanical loading to predictions from five structural design codes and guidelines.
Secondly, the impact of increasing the fraction of imposed deformation relative to mechanical loading on crack width in reinforced concrete was investigated.
Initially, the experimental setup and procedure, including reinforcement, were developed, followed by three experiments conducted at different times after the initial setting time and at various levels of imposed deformation. Using data from the Instron and a DIC analysis, experimental crack widths were obtained and compared to predictions from five structural design codes and guidelines.The optical microscope analysis provided further insights into crack locations.
Model Code 2010 proved to be the most accurate and consistent during the crack formation stage, while Van Breugel’s method was most accurate and consistent during the stabilized cracking stage.
Crack widths due to the combination of loading is generally overestimated by the design codes and guidelines, highlighting the importance of understanding their underlying principles and theories for accurate predictions.
There appears to be a trend indicating that an increase in the fraction of imposed deformation relative to mechanical loading results in a decrease in the final crack width in reinforced concrete. However, the varying weighted maturities across experiments also influence the predicted crack widths, making it difficult to confirm this hypothesis with certainty.

The development of the reinforced mini-TSTM experimental setup opens up new research opportunities. It significantly reduces the required workforce compared to the regular TSTM and, with the addition of the Instron, facilitates mechanical tensile testing of reinforced mortar.

Cement production for concrete is responsible for 5-8% of the global anthropogenic emissions, making it essential to search for alternative binders. This study focuses on alkali-activated materials (AAMs) as a potential replacement for Portland Cement (PC) to reduce carbon emissions. The practical implementation of AAMs in the construction industry faces challenges, particularly related to volume instability caused by drying shrinkage. Drying shrinkage is linked to both water loss through evaporation and the hydration process. The underlying mechanism of drying shrinkage in AAMs is not well understood yet, hindering their widespread application.

The aim of this study is to advance the understanding of the drying shrinkage mechanism in AAMs, by considering the contribution of the pore size distribution and gel characteristics of AAMs. A detailed analysis is performed to identify the governing parameter related to the drying shrinkage mechanism. This is done by controlling the pore size distribution and total porosity at moment of exposure, while differences in the gel composition were obtained. Moreover, the impact of different mix design parameters of blended AAMs on drying shrinkage behaviour, weight loss, microstructural development, flexural and compressive strength are investigated. The selected mix design parameters in this study include slag-to-fly-ash ratio (1, 0.7 and 0.5), curing time (3, 7, 14 and 28 days) and Na₂O wt.% content (4 and 5 wt.%).

Results from the study indicate that drying shrinkage in AAMs is influenced by factors beyond water loss, diverging from the observed correlation in PC. The study highlights important findings related to mix design parameters. In terms of the slag-to-fly-ash ratio, an increase in ground granulated blast furnace slag (GGBFS) content correlates with reduced drying shrinkage, weight loss, refinement of the pore structure and total porosity. Regarding curing time, prolonged curing durations lead to decreased drying shrinkage and weight loss, coupled with improved flexural and compressive strength. As for activator content, an increased amount of activator refines the pore structure, resulting in reduced total porosity, weight loss, and increased compressive strength. However, drying shrinkage remains relatively constant over the 56 days exposure.

Based on the starting point that the pore size distribution of selected samples at the moment of exposure to drying was controlled, it is suggested that gel characteristics, i.e. reaction products, rather than pore size distribution, govern the drying shrinkage phenomenon in AAMs. Comparative analyses underscore the influence of homogeneous reaction products, a higher atomic Ca/Si ratio and the availability of sodium and silicate from the activator. At the moment of exposure, the type of gel is more crucial than the quantity, as demonstrated by the sample with more reaction products exhibiting greater drying shrinkage in the analysis.

The drying shrinkage mechanism in AAMs is strongly correlated with microstructure and the nature of reaction products. The comprehensive results of this study suggest that the gel characteristics have an crucial role as a driving force in the mechanism of drying shrinkage. The study underscores the substantial influence of mix design parameters on drying shrinkage, offering valuable insights for the practical implementation of AAMs in construction. This research marks a significant step forward in enhancing our understanding of this complex phenomenon for AAMs.

The study provides several recommendations for future research, including extending the range of mix design parameters and curing times to evaluate the findings of this study, assessing the rate of reaction of mixtures and considering the impact of drying on the exposed pore structure. Furthermore, the application of N₂ adsorption to detect the smaller range of pores in AAMs, the determination of the influence of internal relative humidity on drying shrinkage, and the investigation of cracking potential of AAMs related to drying shrinkage are also suggested.

This graduation project investigates the integration of green concrete in construction projects for sustainable development. Green concrete, characterized by the use of recycled materials, reduced cement content, and environment-friendly processes, is crucial in minimizing the construction industry's environmental impact. The study addresses the central question of how green concrete can be effectively implemented in construction projects, considering industry-specific challenges.
A mixed-methods approach, incorporating literature review, an online survey, and expert interviews, was utilized. The study extends the definition of green concrete to encompass lower energy and water consumption, a longer lifespan, and contributions to environmental, social, and economic sustainability.
The results indicated moderate awareness of green concrete among industry professionals, highlighting the need for further education. Despite this, the relevance and importance of green concrete were acknowledged. Environmental considerations were found to be the most influential factors in decision-making, followed by social and economic aspects. Seven critical factors affecting the decision-making process were identified, including industry support, personal attitudes, resources, market conditions, leadership, collaboration, and communication barriers. The most significant barrier to innovation was short-term thinking.
Expert interviews reaffirmed the significance of green concrete, though they also emphasized challenges such as high costs, performance concerns, and the lack of established norms.
In response to these findings, the study introduces the Green Concrete Integration Model (GCIM), an adaptive, six-step iterative framework to streamline the incorporation of green concrete into construction projects. The GCIM focuses on adaptability and continuous improvement, addressing technical, economic, socio-cultural, and regulatory challenges through identification, estimation, planning, testing, refinement, scaling up, and monitoring. The model also stresses the importance of stakeholder management and continuous assessment of environmental, social, and economic impacts.
In conclusion, the study offers a pioneering framework in the form of GCIM to facilitate the effective implementation of green concrete, promoting sustainable advancements in the construction industry.

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

Because of the sustainability transition in the asphalt industry, it is increasingly popular to apply reclaimed asphalt pavement (RAP) instead of new asphalt. Then, less new material is needed and the amount of waste products is reduced. However, there are questions regarding the performance of asphalt mixtures with high RAP percentages, particularly in terms of potentially excessive high stiffness and a decrease in workability. In the Netherlands, a RAP content of 60% in combination with a softer binder is common practice, but an increase in RAP content means that rejuvenators are often required.

The research objective was to study the multi-scale material properties of hot mix asphalt mixtures with high reclaimed asphalt pavement percentages using micromechanics. At binder level, the DSR test was done and it was also used to select and reject the rejuvenators which are applied at mixture level. The chosen rejuvenators, Neomex HR and Cecabase RWI, were applied to the mixtures with 80% RAP. Besides that, a reference mixture was tested with 65% RAP and a reference mixture of 80% RAP. Further, the results were used to fit a modified micromechanical Hirsch model for RAP mixtures with rejuvenators.

Based on the material tests and comparing it with the reference (65% RAP mixture), it turned out that there is potential to implement 80% RAP mixtures. Both moisture sensitivity and the rutting resistance are improved when a higher RAP content is applied, while the total fracture toughness has remained the same. The stiffness of the 80% RAP mixture is substantially higher, but it is still applicable. The biggest concern is the fatigue resistance, because the fatigue resistance of the 80% RAP mixture without rejuvenator is worse for higher strains.

The application of the rejuvenating additives led to the properties of the 80% mixtures being more similar to the 65% RAP mixture and the workability is improved. However, it did appear that the dosage of the additives is on the high side. The stiffness is lower than the reference mixture, especially the mixture with Neomex HR. Also, the rutting resistance is lower than the reference mixture, especially the mixture with Cecabase RWI. Future research may optimize the dosage to improve results.

The micromechanical Hirsch model provided stiffness results with a reasonably accuracy, although this model used general model coefficients. The simplified Hirsch model with inclusion of the factor Pa could not be simply adopted when it is fit for other types of mixtures. The predicted stiffness for all three mixtures was much higher than it actually is. The adjustment of the fitting parameters, according to the laboratory results of the 80% RAP, led to accurate results making the simplified Hirsch model with inclusion of the factor Pa applicable to the mixtures with 80% RAP and a rejuvenating additive.

Structural elements under compression can fail due to instability. Such type of failure may present itself as flexural buckling, which is the sudden change of configuration of one state to another at a critical compressive load. Actual structure characteristics, such as cracks, real joints, sudden changes of flexural stiffness, bracing, and actual boundary conditions, make current analytical buckling methods inefficient. For this reason, most structures are evaluated using Finite Element (FE) software based on the Finite Element Method (FEM). However, such a numerical method may have a high computational cost when parametric studies need to be performed. The motivation for this thesis was to develop an analytical alternative for the buckling analysis of columns and beam-columns.
The thesis analyses jointed Euler-Bernoulli columns and beam-columns with N discontinuities due to step-changes of flexural stiffness, and influence of open edge cracks, real joints, bracing, and actual support conditions. The approach considered utilizes the Heaviside function to obtain a single piecewise expression for the deflection, slope, moment, and shear of columns and beam-columns. The use of the Heaviside function, along with proposed closed-form expressions, reduce the number of unknown integration constants to only four.
Main findings are listed as following: (i) Linear buckling analysis of a column only depends on solving the determinant of the 4x4 matrix of unknown coefficients, obtained by imposing four boundary conditions. (ii) Solving the four unknown constants of integration results in performing a geometrical non-linear static analysis of the beam-column, which perfectly agrees with results obtained through FE software. (iii) Closed-form expressions reduce the number of unknown constants of integrations to only four, regardless of the number of sections composing a column or beam-column.
Concluding remarks and recommendations include: (i) Closed-form expressions accurately provide linear buckling loads for columns and geometrical non-linear expressions for beam-columns. (ii) Equations solved algebraically can be stored and used for future analysis, saving the computation time of rerunning the analysis for specific columns or beam-columns. Having the equations expressed algebraically allows for the ease of performing parametric analysis and exploring the performance of new designs. (iii) Computational cost (time needed for the computer to obtain the results) may depend on the mathematical software used for the calculations. Other software may result in having lower computational time for the same calculations.

In the Netherlands, yearly 33 Mton new concrete (roughly 15 billion m3) is producewhile 12 Mton concrete waste becomes available. Recycling of the yearly 12 Mton concrete waste results in roughly 7 Mton coarse recycled concrete aggregate (CRCA) and 5 Mton fine recycled concrete aggregates (fRCA). Currently, fRCA is not used in structural concrete, but only for low grade applications. This research aims to investigate the effect of recycled concrete aggregates (RCA) incorporation on the performance of structural concrete mixtures. Three streams of fRCA were considered in this research and two deliveries of each source were examined. River sand was replaced by 25% fRCA, which was found to be the optimal amount that does not require cement increase. Both deliveries were used in order to identify potential differences. Additional concrete mixtures with simultaneous replacement of river sand by 25% fRCA and river gravel by 30% CRCA were evaluated. Parallel work was conducted during an 8months internship at TNO, where optimization of mortar mixtures with full replacement of river sand by fRCA was investigated. This was performed by considering the water demand of solid constituents (binder and fRCA) and packing of fRCA. Optical microscopy was used for closer inspection of the microstructure of fRCA. Fresh properties of studied mixtures were measured. Assessment of the mechanical performance of concrete mixtures was expressed by means of compressive strength and elastic modulus. Long term behavior was also investigated by shrinkage deformation measurements up to 95 days after casting. Results show that substantial rise in cement content is necessary for obtaining the desirable flow and high levels of compressive strength, when full replacement of fRCA is performed. Regarding concrete mixtures with 25% fRCA, increase in air content was measured. Entrapped air is strongly dependent on the porosity of adhered cement paste on RCA, content of 00.25 mm fraction as well as agglomeration and presence of impurities in as received fRCA. This affects negatively their mechanical performance. Additional porosity resulting from CRCA incorporation was responsible for further reduction in mechanical properties of concrete mixtures. Mixtures with RCA showed three times higher mass loss at the age of 95 days.

Since 2006, the Delft University of Technology has been working on the development of bacterial self-healing concrete (BSHC). The self-healing ability of this concrete is based on a biological mechanism, in which mineral producing bacteria are added to repair cracks autonomously. This not only improves the durability of concrete structures, but could also reduce the need for crack-limiting reinforcement. So far, bacterial self-healing technology has been used in variety of applications, especially watertight structures, but always only as an additional safety measure. Hence, the full crack-sealing capacity, related to the reinforcement reduction potential, could not be proven. This will have to be resolved in a dedicated full-scale demonstrator project. For this demonstrator project, a rectangular water reservoir has to be designed, which will essentially serve as a test case for the newly developed bacterial self-healing technology. This thesis aims to devise the concrete mixture and reinforcement layout for the side walls of this reservoir in such a way that imposed deformations induce a given degree of cracking at an early-age, which allows to demonstrate the crack-sealing capacity and reinforcement reduction potential of BSHC. The design of a concrete mixture intended for the side walls of the reservoir led to four different mixtures, all of which have a cement content of 418 kg/m³ (26% of CEM I 52.5 R and 74% CEM III/B 42.5 N), but vary in the addition of filler in the form of limestone powder and healing agent; a mixture of bacteria and calcium lactate encapsulated in PLA strings. The mixtures with healing agent, which represent BSHC, are meant for side wall A, whereas the mixtures without healing agent representing ordinary concrete can be used for side wall B, so it can serve as a reference. To investigate the effect of these additions, as well as to verify the designs and quantify the relevant physical and mechanical properties of the concrete mixtures, several tests were conducted. These revealed that the mixtures with filler exhibit a higher consistency, improved cohesiveness and more prosperous strength development compared to the mixtures without filler. Both the fresh properties and strength development were not affected by the addition of healing agent. Furthermore, it was found that the addition of filler causes autogenous shrinkage to increase by about 15%, whereas the addition of healing agent causes autogenous shrinkage to decrease by about 20%. From the cracking calculations it is concluded that it is very likely that the early-age cracking of the side walls of the reservoir occurs as a consequence of imposed deformations. Cement hydration causes a large temperature rise, resulting in significant thermal shrinkage due to the subsequent temperature drop. The imposed deformations are restrained by the floor of the reservoir, as well as the foundation material underneath. This results in a maximum probability of cracking of 65% and 93% according to two different methods. In order to demonstrate the true crack-sealing capacity of BSHC, if is preferred to obtain various crack widths along the length of the side walls and the reinforcement layout must be configured to allow this to happen. Therefore, several methods that deal with the prediction of crack widths are implemented in this case. Based on the average of all prediction methods it is found that the following crack widths occur given the longitudinal reinforcement distribution in brackets: 0.09 (Ø20-100), 0.20 (Ø20-160), 0.31 (Ø20-210) and 0.43 mm (Ø20-250). However, it was also discovered that the mutual differences between the prediction methods are very large, in particular at lower reinforcement ratios. But then again, a reinforcement layout divided into multiple sections, consisting of the aforementioned longitudinal reinforcement distributions, offers the best chance of demonstrating self-healing ability.

Ageing of infrastructure leads to an increasing demand for new technologies and design solutions in current bridge engineering. Over the last decades the traffic intensity has grown tremendously, while design codes and regulations have become stricter. Many current steel bridges realised in The Netherlands suffer from fatigue problems. Replacement of the bridges can be expensive, and thus engineers are asked to provide design solutions that can extend the lifetime of these structures. Rehabilitation through the replacement of steel orthotropic decks with Glass Fiber-Reinforced Polymer (GFRP) sandwich deck panels is considered as one of the few possible solution strategies. FRP's are considered a viable option, due to their versatility, high strength-to-weight ratio, fast installation possibilities, low maintenance costs and their resistance against both corrosion and fatigue. The increasing amount of bridges containing FRP structural elements demonstrates the high potential of the material. However, many engineering consultancies address to face difficult challenges when working with FRP. The main concern in the construction and design field is about the long-term performance of the material. Due to the viscoelastic nature of the polymeric matrix, FRP structural elements show complex creep and recovery behaviour under variable traffic loading. Current standards account for this traffic load by conservatively assuming that 40% of this traffic load should be accounted for in creep calculations, which originates from experience with concrete bridges. However, due to the fast recovery behaviour of FRP compared to concrete, this value might be too conservative. Based on current knowledge and experimental results, a numerical model was created that is able to predict the long-term creep deformation including both creep and recovery behaviour. The model uses experimental results based on UD-plies, which enables the model to calculate the long-term deformation of laminates with a wide variety of layups. The Finite Element Analysis (FEA) software package Abaqus has been used to model a FRP sandwich deck panel and calculate the time-dependent response under traffic loading. Finally, a FRP sandwich deck panel has been analysed by both the numerical model and according to the procedure prescribed by current standards. Results have shown that the amount of traffic load that is currently considered as permanent static load for creep calculations could be reduced. However, it is recommended for future research to improve the numerical model by using more accurate creep prediction models supported with profound experimental work. It is expected that this would lead to an even higher reduction of the amount of traffic load that is currently considered in design recommendations.

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

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

The Strut-and-Tie model (STM) is an efficient technique for the design of concrete structures, but the creation of suitable truss structures gets complicated when these structures become more complex. The topology optimization (TO) is a convenient technique that has been used in recent years for the creation of trusses of complex structures for the STM. This thesis presents a process for the creation of suitable truss structures for the STM using the results obtained with the TO, as well as, an evaluation of them to see which is the most optimal truss structure according to the total amount of tension force present on the full truss, this total amount of tension force is the selected evaluation criterion. Three deep concrete beams were analyzed using two topology optimization (SIMP and BESO approaches) for the generation of stress paths, these approaches are based on the minimization of the strain energy. The procedure starts with the computation of the principal stresses over the results of the topology optimization, then bar elements are placed over the stress paths of these diagrams creating a first (harsh) layout of the trusses. These trusses were not always found stable, but all the trusses were stabilized because, in this way, it is easy to calculate the axial force of in the truss elements, thus satisfy a basic requirement of the STM. To stabilize the truss structures two methods were explored. (i) The addition of new members outside of the stress paths (stabilizers), the essential characteristic of these new elements is that the axial force in them should be zero to not change the stress distribution found during the optimization process. A sensitivity analysis of the stabilizers was performed to track how the axial force changes in these members depending on the position of the nodes connected to them, this process was necessary because when an element outside the stress paths has axial force the stress diagrams have been changed. (ii) The creation of substructures within the stress paths, this process stabilizes the global structure without the addition of members outside the stress paths. Finally, a structural analysis was performed to obtain the axial forces in each member of the truss structure, and through an analysis of these results, the total amount of tension forces in the truss was computed. The truss with the minimum value of total tension force is assumed as the most optimal structure for each case. It is clear through the analysis that the variation of the input parameters does not cause large variations in the results of the topology optimization, but it has an impact in the stabilization process and the performance of the structures according to the evaluation criterion. Furthermore, it has been proved that suitable trusses for the STM can be created using any of the two selected optimization approaches obtaining good results, and a similar performance according to the evaluation criterion.

In August of 2013 Elon Musk published his paper ’Hyperloop Alpha’, in which he describes an alternative transportation system for the to be built railway connection between Los Angeles and San Francisco. The Hyperloop system that Musk describes relies on partial vacuum tubes through which vehicles can travel at speeds up to 1000 km/h with great efficiency. An innovative breakthrough in the transportation sector if it can be made into a reality. Therefore his company SpaceX organised a contest to which many teams applied to built a concept vehicle to be used in the tubes. In 2017 a team of students from TU Delft won the overall prize for the best design and afterwards the team leaders founded Hardt to develop their ideas and make Hyperloop into a reality. BAM Infraconsult supports Hardt and together they develop the Hyperloop and its tube infrastructure. Currently the preliminary design for the tubes is made in steel and no alternatives have been considered. This research focusses on an alternative tube design in concrete. The tube has to not only satisfy the strict requirements that apply to the Hyperloop technology in general, but it also has to adhere to the specific design philosophy of the Hardt vehicle concept. A concept in which the vehicle is suspended from tracks on the ceiling of the tube by a series of magnets. The report is based on the following research question: Is a concrete design a feasible alternative for the design of airtight Hyperloop tubes compared to the considered steel tubes? To answer this question two philosophies are considered and researched into depth. Firstly a design in prefabricated normal strength concrete is analysed, making use of prestressing and conventional reinforcement, following the requirements set in the Eurocode. Secondly an innovative ultra high performance steel fibre reinforced concrete (UHPFRC) design is considered. Again prestressing is applied, however conventional reinforcement is not present in this design as it relies on steel fibres and prestressing alone. This design is modelled according to the French UHPFRC guidelines. A requirement for the design is that the inner diameter of the tubes is equal to 3.6 m and that the tubes span 30 m between pylons. The design methodology is based on a variable thickness of the tubes in order to find the optimal concrete structure that satisfies all requirements. Checks are made for multiple failure modes after which the governing minimum thickness is derived. Other than proving to be structurally sound according to the Eurocode, the tube also has to abide to a strict deflection limitation under dynamic loading conditions. In order to research this criterion the tube design is validated using finite element modelling. Here a combination of different parameters related to the concrete material characteristics and the dimensions of the tube is researched by means of a sensitivity analysis. Using the dynamic amplification factor specified by Hardt, the maximum deflections of the tube are checked and the parameters that have the greatest influence on the deflection are discussed. The obtained results are compared to the steel tube. Furthermore the possibilities for the construction of a concrete tube are considered, including a recommendation for the connections between tube sections. Moreover the transportation of the tubes to the building site and final assembly are taken into account. After all factors are considered a cost estimation is made for the design of the concrete tubes and compared to the cost estimation of the initial steel design of the tubes. This cost estimation does not only consider the material and production costs of the tubes, but also takes into account transportation, assembly, maintenance and site costs plus an allowance for risks and profit. It is concluded that a design in normal strength concrete is possible, but not practical. Its dimensions make the production and assembly very difficult. However a design in UHPFRC is feasible, as its required dimensions are considerably better. Furthermore it satisfies all structural requirements without the use of conventional reinforcement and even the strict deflection criterion at dynamic loading conditions can be achieved. Moreover the design in UHPFRC is a competitive alternative to the steel design, as it has a higher score when all criteria are considered.

Steel fibre reinforced concrete (SFRC) is nowadays used as a construction material for pavement and industrial floors in The Netherlands. The common used SFRC mixture, is concrete strength class C30/37 mixed with circa 35 kg/m3 of steel fibre.
The details of the concrete composition mixed with steel fibres are ignored. The mechanical tests result of the experimental program accomplished in this thesis emphasized on the decisive role of the concrete composition and quality on the mechanical properties.

Alkali Silica Reaction (ASR) is one of the degradation mechanisms in concrete that poses a threat to the service life of existing structures. This physio-chemical process is progressive and can affect the the strength, stiffness and stability of concrete structures. A lot of mitigation measures are available to prevent the deleterious effects of ASR in new structures. However, assessment and monitoring of ASR in the existing structures still remain a challenge. Thus the main aim of this thesis is to develop a simplified tool for the assessment of existing structures to assist the asset owners in decision making. This is addressed by a combination of petrographic techniques to quantify the damage
and development of a numerical model to predict the remaining service life. Various existing petrographic tools (Damage rating index and image analysis) are investigated and modified to not only identify the ASR signs but also to quantify the damage. A meso-scale numerical model (ASR expansion model) is developed as an extension to the existing Delft Lattice Model to incorporate the effect of ASR by application of randomly distributed internal expansions. A case study (Afsuitdijk, The Netherlands) is used to apply these methods. The model is compared with the findings from the petrographic analysis using a physical parameter 'crack densities'. This model is able to simulate localised network of cracks typical of ASR and expansion strains as a function of time. These expansions are compared with the permissible expansion thresholds set by
RILEM and CUR to assess the remaining service life. Additionally, the numerical model showcases the importance of different boundary conditions by making a comparative study between free expansions and concrete in confined state. Finally, the limitations and drawbacks of this expansion model are discussed critically.