The Interfacial Transition Zone (ITZ) in Fiber Reinforced Concrete (FRC) fibre-reinforced concrete is the microscopic zone between the fibres and the cement matrix, which is essential to the overall performance of the composites. To explore the micro-mechanical properties of the ITZ, a combined experimental and numerical study was carried out targeting this microscopic area. Specifically, an indentation splitting test was developed and the single fibre pullout test was conducted. Those two test methods were then simulated to give a comprehensive understanding of the mechanical behaviour and fracture patterns of the interface.

For the indentation splitting test, micro-cubes (300 μm long) containing a segment of vertically aligned microfibre were prepared using CEM I, CEM III and CEM III mixed with limestone powder at varying water-to-cement (w/c) ratios from 0.3 to 0.5. These specimens were then subjected to a splitting test under a nano-indenter equipped with a wedge tip, and the results were compared to those of microcubes without fibres. Mechanical properties including load capacity, deformation at peak, stiffness and fracture energy were analysed. Following the experiment, lattice models with simplified and realistic microstructure were built. For the lattice model with simplified microstructure, the mechanical properties of local phases were calibrated with the experimental results. The modelling result shows that the material properties of the ITZ are approximately equivalent to 20% of the paste properties regardless of the w/c ratio. The lattice models with realistic microstructure were built based on 3D CT scans of the micro prisms with. For the 3D reconstructed digitallattice model with real microstructure, two material assignment methods, thresholding and greyscale mapping, were compared. The simulation results showed significant differences between the two methods, which may be attributed to the empirical parameters used to calculate tensile strength.

Furthermore, the single fibre pullout test was conducted to examine the interphase properties and compared with the experimental result of the indentation splitting tensile test. Results show that CEM I demonstrates the highest chemical bond energy (Gd) among the tested materials, decreasing significantly with an increase in the w/c ratio. In contrast, CEM III+L displays an inconsistent trend, with the highest Gd observed at a w/c ratio of 0.4. However, the experimental results for frictional bond strength (τ0) and slip-hardening coefficient (β) do not exhibit significant trends or differences. The simulation of single fibre pullout accurately captures the initial load phases up to full debonding.

The developed indentation splitting test offers a valuable method for assessing the tensile properties of the ITZ. The modelling results indicate that the mechanical properties of the ITZ are approximately 20% of those of the bulk paste. This finding can be integrated into larger-scale simulations by incorporating the effects of the ITZ. Coupled with the lattice model, this approach provides an effective way to enhance the fibre-matrix ITZ of FRC. Additionally, the insights from this study can be leveraged to optimize the design and performance of FRC, potentially leading to cost savings and extended durability in construction projects.

The emergence of innovative construction materials is dawning a new era of ambition within the civil engineering community. Among these innovative materials, Basalt Fibre Reinforced Polymer (BFRP) has recently surfaced with promising potential as a reinforcing material in concrete. Currently, in the Dutch concrete construction industry, the choice for reinforcement steel has remained unchanged for the past decades. However, the increasing availability of innovative alternatives could help the transition to a more sustainable concrete industry. Although BFRP has promising potential for application in concrete structures, the global application has not been established yet. One of the reasons for this limited research into the structural behaviour of concrete structures reinforced with BFRP-bars. Furthermore, the limited development of codes specifically designed for concrete reinforced with BFRP-bars and the modest availability compared to reinforcement steel also play into the unknowns about the material.

BFRP-bars contain certain qualities that reinforcement steel does not. One of the most prominent is resistance against corrosion due to environmental influences on concrete structures. This eliminates the requirement for the concrete cover to protect the reinforcement from corrosion. Hence, the concrete cover only serves its purpose to ensure effective bond action between the reinforcement bars and the concrete. This inherent quality of BFRP-bars eases the crack width control requirements in the codes for the design of structures reinforced with BFRP-bars to a range of 0.5 mm to 0.7 mm. Although this is a significant increase in comparison to the Eurocode for concrete structures (0.2 mm to 0.4 mm), the properties of BFRP-bars cause larger crack width development.

The aim of the experiment is to investigate the flexural behaviour of concrete beams reinforced with BFRP-bars as tensile reinforcement. The flexural behaviour of concrete structures reinforced with BFRP-bars is studied both experimentally and numerically. The research program contains 6 beams differing in reinforcement material, concrete covers, reinforcement ratio and bar diameters. To investigate the effects of the concrete cover, 2 beams are designed with concrete covers of 31 mm and 11 mm containing 3 BFRP-bars with a diameter of 8 mm in
the tension zone. To compare the behaviour of these beams, 2 identical beams with reinforcement steel are designed. To determine the effects of the reinforcement bar diameter, 1 beam is designed with 2 bars with a diameter of 10 mm. The reinforcement ratio in beams remains approximately equal, hence the only changing parameter is the bar diameter. The effects reinforcement ratio is investigated by a beam designed with 2 bars with a diameter of 8 mm. By keeping the bar diameter and the concrete cover the same, the reinforcement ratio is the only changing parameter for this beam. By subjecting the beams to a 4-point bending test, a fully developed crack pattern can be established over a certain length. By using digital image correlation (DIC), the flexural behaviour is monitored and analysed. This includes both crack width development and overall pattern forming. The results are verified with linear variable differential transformers (LVDT‘s) and a laser measuring vertical displacements. This procedure is devised to evaluate the stiffness behaviour of the beams as well as the cracking behaviours. In addition, the experimental program includes a series of direct tensile tests with reinforcement bars to determine the stress-strain behaviour of the reinforcement bars themselves...

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

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

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

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

Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) is a promising material for different applications such as bridge deck strengthening due to its improved strength and durability properties. Especially the tensile strength and ductility is heavily dependent on the distribution and orientation of the steel fibers within the element. This fiber behaviour can be unpredictable, causing a large scatter in material properties. This scatter is inconvenient for predicting the material properties. The novelty of the material is the main reason that UHPFRC is not included in the Eurocode. Certain countries like Switzerland and France have developed their own UHPFRC norms.

In order to increase the knowledge and understanding of the fiber distribution and orientation, a non-destructive testing method has been developed. Two types of magnetic probes have been developed: the C-shaped ferrite probe and the single ferrite probe. These probes are based on other probes developed at the University of Milan and Porto. The C-shaped ferrite probe is most suitable for measuring the total fiber content and fiber orientation. The single ferrite probe is able to take fiber content measurements closer to the edge making it more efficient for fiber distribution measurements. This can result in a reliable prediction of the peak-tensile strength. The addition of UHPFRC specific design rules and this non-destructive testing method will make UHPFRC and its applications more attractive.

In this research, 27 different UHPFRC plates were cast with the same dimensions of 10x200x200 mm3. These plates were cast horizontally or vertically with either 1%, 2%, 3% or 4% fiber content. Seven plates were vibrated after casting in order to analyse the influence of vibration. Each fiber content resulted in a relatively homogeneous fiber distribution and random fiber orientation. Horizontal casting of UHPFRC plates usually result in a more homogeneous fiber distribution and more random fiber orientation compared to vertically cast plates. The best distribution and orientation was reached when the elements were not vibrated at all. Vibration can result in severe fiber segregation along the element height and vertical orientation of the fibers. These conclusions hold for longer UHPFRC elements where the element height and the element thickness remains constant.

One of the main pitfalls in this research is the small thickness of the plates. The length of the fibers exceeds the plate thickness, which makes casting very difficult. During casting, fibers will get stuck at the top of the mould, which also influences the results. Moreover, it is important to continuously keep mixing the UHPFRC mixture in order to equally divide the fibers over all plates and to use a mixture that is castable yet not too flowable. High workability mixtures are more prone to fiber segregation.

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.

Repeated crystallisation of salts in the pores of building materials is a common cause of damage in buildings. Sodium chloride (NaCl) is one of the most common salts, responsible for weathering in the built environment. Mortars, especially when used as plasters and renders on the surface of walls, experience fast degradation as they are exposed to conditions perfectly conducive to salt weathering. As a consequence, their service life is often compromised, requiring frequent replacements. Costs associated with replacement interventions have a high economic and a social impact. Over the last two decades, the use of crystallisation inhibitors as an additive to prevent/mitigate salt crystallisation damage in building materials has shown promising results. Sodium ferrocyanide (NaFeCN), a crystallisation inhibitor of NaCl is particularly effective in preventing damage by inhibiting/delaying NaCl nucleation and altering NaCl’s crystal habit. When mixed-in air lime-based mortars, NaFeCN has been shown to considerably improve the salt weathering resistance....@en

Over the last 15 years or so, research has revealed the great self-healing prospects possessed by asphaltic mixtures. Researchers have proposed novel methods to harness this capability, aiming to prolong the service life of asphalt pavement, particularly in porous asphalt. To date, the most promising of the healing methods is the combined capsule-induction system. This thesis aims to ascertain whether such a system would show positive results in stone mastic asphalt (SMA). Following that, an optimisation of the composition of self-healing SMA was proposed by assessing the mechanical and healing properties via laboratory testing. Finally, an evaluation of sustainability from an environmental perspective was done using Life Cycle Analysis (LCA) methodology.

Results of the healing assessment revealed that each combined healing system was able to recover between 58-63\% of its original fracture strength after 8 healing cycles, while the reference mix (without healing) was only able to regain 10\% fracture strength before failure after 2 cycles.

Inclusion of the combined healing system slightly reduced the strength, stiffness and water sensitivity of the SMA mixture compared to the reference. However, improved rutting resistance was observed in each self-healing case. Within the self-healing mixtures, increasing capsule content reduced asphalt density, stiffness and strength and resulted in an increase in asphalt void content.

The LCA results show that the self-healing system had environmental benefits in some facets such as a 14\% reduction in fossil fuel resource depletion and a 21\% reduction in land use. However, the present total known environmental costs of other impacts are approximately 15\% lower in the reference system based on a cradle to gate, and use phase analysis. Almost half of this total cost was attributed to maintenance activities. It was concluded that a 32\% increase in maintenance efficiency would ensure environmental viability of a self-healing mixture over a reference mixture within the constraints of the analysis conducted.

When concrete is subjected to imposed deformations, stresses may develop. If at any point in time this stress exceeds the tensile strength of the material, the concrete will crack. Early-age cracking of concrete structures may lead to problems with durability, serviceability and aesthetics. During hardening of concrete the material properties are still in development. Therefore, to be able to predict the crack width, understanding of the stress- and strength development is required. In addition, concrete is a visco-elastic material which means that stresses are affected by phenomena such as creep or relaxation. If the design codes predict the crack width in concrete structures under imposed deformations accurately remain subject of debate. The CROW report published in 2021 [9] provided the starting point for this research. The aim of this study was to gain more insight on the background of the design codes, and to explain the fundamentals of the crack width prediction in case of imposed deformations. For this purpose, the following research question was formulated:
”What is the applicability of the design codes regarding the crack width prediction of reinforced concrete structures under imposed deformations?” From the literature study it became clear that the main difference between the design codes was related to which boundary conditions and cracking theory were applied. The majority of the design codes applied the well known tension bar model theory where both ends are fully restrained. In addition, there were also codes using the continuous base restraining theory in which the tensile member is continuously restrained along one edge. This made a huge difference in the prediction of the crack width. From the finite element analysis which was performed, it turned out that there is a difference between the steel stress development of imposed loading and imposed deformations. In addition, in case of imposed loading less cracks were developed than under imposed deformations. However, from this specific numerical analysis it turned out that the maximum crack spacing and crack width is smaller in
the imposed loading model in comparison to the imposed deformation model. The only explanation for this is that in case of imposed loading the cracks are more evenly distributed. A case study was used to simulate the hardening process of concrete in combination with autogenous shrinkage as imposed deformation and compared with a finite element analysis. The goal was to determine a set of model properties that are useful for future engineers. The accuracy of the input parameters was verified with the experimental work carried out by M. Sule. Overall, when performing a non-linear finite element analysis a lot of knowledge was required. It turned out that specific choices such as the constitutive model type, the kinematic and equilibrium condition have a major impact on the outcome. Using the modified Bar model of Lokhorst in combination with this numerical approach resulted in good agreement with the experimental findings.

The parameter study showed that regarding the tension bar models, the bar diameter has the largest influence on the crack width prediction. While with respect to the continuous models there was no onesided answer to the question which parameter had the largest influence on the crack width prediction. In one of the design codes based on the continuous model theory named CIRIA [8] the degree of restraint clearly stands out as the most important parameter. Whereas in the another design code based on this theory, namely the ICE [4], all parameters that were investigated in this thesis have limited effect on the crack width prediction. The design codes considered in this master thesis do not yet fully represent the crack width due to the hardening of concrete or due to autogenous shrinkage. The design codes are applicable for the
crack width prediction of reinforced concrete structures under imposed deformations if conservative assumptions such as a weak bond between concrete and steel reinforcement are taken into account. It is clear that the problem treated in this report has more complexity than what one initially may think.
Opinions on how to determine the crack width of reinforced concrete tensile members under imposed deformations differ. The difference is related to the type of restraint and the cracking theory which are suggested in most used analytical design models. This report contributes to a better understanding of
the crack width development under imposed deformations through non-linear finite element analyses and verification with experiments. The results from this master thesis suggest that an approach to a more consistent crack width prediction under imposed deformations should investigate how bond in the interface between concrete and steel influences cracking. This has lacked attention in the current formulas in the design codes.

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.

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.  

The changing climate and the need for a new purpose for End-of-Life concrete increased the importance and interest of concrete recycling. To decrease the environmental footprint of concrete, in which cement has the highest contribution, it would be of great importance to be able to recycle cement, which would positively affect the environment. The separation efficiency plays an important role in the cement properties. The Smart Crusher is a novel concrete crushing- and separation technology developed to optimize the separation efficiency of the concrete. This study investigates the recycling potential of the cementitious fractions obtained from the Smart Crusher. The aim of this research is to evaluate the reactivity of the cementitious binder retrieved from recycled cement stone to obtain a fully functional binder that can replace primary cement in constructions and structural elements. To gain more insight in the reactivity of cementitious binder retrieved from recycled cement stone, the Smart Crusher cement fractions, which in this study had an unknown origin, are compared with non-hydrated and hydrated reference cements CEM I 52.5 R and CEM III/B 42.5 N. Therefore, the different materials are characterized, using TGA, DSC, MS, XRF and XRD. Analysis of the chemical and mineralogical composition showed the presence of hydration products in the hydrated Smart Crusher, CEM I 52.5 R and CEM III/B 42.5 N samples. Additionally, quartz is found in the cementitious fractions of the Smart Crusher, which indicates the presence of aggregate particles such as sand in these samples. Furthermore, the absence of P2O5 in the cementitious Smart Crusher fractions proved that there is no or a not measurable amount of fly ash present in the cementitious Smart Crusher fractions. Upcycling of the hydrated Smart Crusher, CEM I 52.5 R and CEM III/B 42.5 N materials is done by thermal treatment. To assess the upcycling, again the chemical and mineralogical composition is analysed. Red colouration (500 °C and 800 °C) and melting (1400 °C) of the cementitious Smart Crusher fractions again show the presence of aggregate particles in the material. During thermal treatments of the Smart Crusher, CEM I 52.5 R and CEM III/B 42.5 N materials, the decomposition of hydration products and calcite are monitored, using TGA, DSC and MS. The thermal treatment at 800 °C showed the formation of alite and belite phases in all the samples and was therefore seen as the treatment with the most potential in this research. The only material that shows the formation of alite and belite during other thermal treatments was CEM I. After the thermal treatments the composition and crystalline phases were determined, using XRD and XRF. Insight in the mechanical performance of the different binders was obtained by flexural and compressive strength tests. Mortar prisms were made, containing CEM I and CEM III/B as reference cement and thermally treated CEM I, CEM III/B, 0.0 – 0.063 mm and 0.063 – 0.125 mm cementitious Smart Crusher fractions. The mortar mixtures with the thermally treated binders were observed to be rather dry as a result of an increasing water demand. Compared to new cement, the increase in water demand led to a low degree of compaction, resulting in return to strength properties that were practically unmeasurable. The reference cement showed a lower early strength for CEM III/B compared to CEM I and comparable strength after 28 days due to the higher gain of strength of CEM III/B. Especially the flexural and compressive mechanical tests showed that more research is necessary. To conclude, this study showed that a certain reactivity can be expected in the secondary binders after upcycling, but that the unmeasurable strength properties of the upcycled materials showed the need for more research to make a better estimation of that reactivity.

Concrete columns, in almost every shape, can be created by concrete printing. The formwork will be printed and subsequently filled with traditional concrete. Within this research, the influence of concrete printed formwork on the load bearing capacity of a printed column is determined. The research is performed on a small concrete column, from which is concluded that the difference in shrinkage between the two types of concrete has a large role within the occurring stresses in the element. This means that the relative shrinkage determines the load bearing resistance in a small concrete printed column.

In this thesis a parametric finite element model has been developed to predict the printability of concrete structures of arbitrary geometries. The model can be used to effectively explore design and process parameters to optimize printed objects. In addition, the model can be used to improve the reliability of the printing process, reduce the number of failures during production and enhance the production capabilities of complex geometries. A comprehensive experimental programme was designed in which uniaxial compression tests have been performed to determine the development of the mechanical properties of the concrete during the printing process. A novel method was created by which test samples could be extracted directly from printed concrete in order to incorporate the influence of the full printing process on the properties of the concrete. Moreover, from both literature and the valuable experience in printing of Bruil it was known that the temperature of the concrete mix can have a major influence on the mechanical properties of concrete. Therefore, the performance of the concrete under elevated temperatures was included in the research too. On top of that, printing experiments were conducted in which two geometries were printed multiple times to characterize the reliability of the printing process, and to verify the finite element models with.

The growing need for strengthening of concrete structures to improve their structural performance challenges structural engineers to come up with strengthening techniques that is most suitable and effective for a structural system in its deteriorated state. Although there are quite a few techniques in practice for strengthening of concrete structures -reinforced or prestressed, various factors limit their efficiency and performance. Use of novel concrete-based materials such as Ultra High Performance Fibre Reinforced Composite (UHPFRC) can lead to an effective strengthening system due to the exceptional material properties of UHPFRC. In particular, the strain hardening behavior of UHPFRC in tension and its excellent durability. Out of the two main mechanisms of failure in concrete elements namely flexure failure and shear failure, shear failure is the most critical due to the abrupt nature of its mechanisms that leads to complete collapse of the structures with no sign of warning. This makes shear strengthening of prestressed concrete structures, that typically form the supporting elements in large infrastructural units such as bridges, a very essential course of action. To investigate the effectiveness of using UHPFRC in shear strengthening of prestressed concrete elements, the post-tensioned T girders of Helperzoom bridge, Groningen, Netherlands are chosen as reference beams which are strengthened with UHPFRC. The reference beams are subjected to a 3-point bending test to determine their failure mechanism and structural capacity. The reference beam is modelled, and tests are simulated by finite element analysis using ATENA. Since the simulated beam failure in shear compression rather than shear tension by localization of the shear tension crack. In order to understand this behavior and the failure mechanism and to check if the results from the simulations are reasonable, a detailed analysis is performed. The influence of shear reinforcement and prestressing, active in the critical shear region, on the shear capacity and the final failure mode are investigated. The results from the numerical analysis are compared to the results from experimental shear tests performed on existing post-tensioned girders which exhibit a similar mode of failure as observed in the reference T beams. The shear capacity of the reference T beam is also calculated from the Flexural Shear Crack Model proposed by Huber et.al and it is seen that the shear capacity from the numerical analysis from ATENA is in close match to that obtained from the analytical model. The contribution of the arching effect to the shear capacity calculated from the model is between 77% and 87% for the beams tested in this research and the contribution is between 28% and 58% in the beams tested in the experiment. In the last step, the effectiveness of strengthening the reference beam with UHPFRC is determined. The results show that the effect of strengthening with UHPFRC is maximum in the reference beam without stirrups and prestressing both in terms of shear capacity and ductility.

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.

The objective of this research project is to verify whether 3D printing of concrete (3DPC) reduces the environmental impact by increasing the structural efficiency. During the literature study, numerous articles were identified that claimed an increase of structural efficiency by using 3DPC to reduce material use. Some of these articles suggest that an increase in structural efficiency results in a reduction of environmental impact. However, these articles do not support this potential with a research study.

This research project therefore studies the relation between structural efficiency and the environmental impact of a 3DPC bridge. This relation is studied by developing a design model that calculates structural efficiency and determines the environmental impact. The design model is developed in a visual script environment for parametrically controlling visual design software. The parametric script specifies and combines information in an XML file used for structural analysis in finite element software. This analysis is executed on command in the parametric environment and the results are used for verifying the structural safety. The structural safety is expressed in a material utilisation ratio. This ratio indicates the used proportion of strength of the material and can be used as an indicator for structural efficiency. After calculating the structural efficiency, the design model is used to determine the environmental impact of the structure expressed in shadow costs. Shadow costs quantify the environmental impact of materials and processes related to geometric quantities defined in the design model.

Multiple combinations of parameters have been compared to determine the relation between structural efficiency and environmental impact. The results of the comparison show that an increase in structural efficiency results in decreased material use. A decrease in material use reduces the environmental impact because ±59% of the impact is related to 3DPC material. The environmental impact of the 3DPC bridge is then compared to a hollow-core-slab element with a similar functionality to verify the claimed potential. This comparison shows that the environmental impact of 3DPC is almost two times higher than the compared conventional method.

Therefore, using a novel design model and based on the results, following two conclusions can be drawn:
1. Increasing structural efficiency can decrease the environmental impact of a 3DPC bridge, and
2. Considering the state of 3D printing concrete technology at present, applications of this technology for bridges can result in a higher environmental impact than the compared conventional method.

Strain hardening cementitious composite is a new generation material with increased ductility and improved cracking behaviour as compared to traditionally used Normal Strength Concrete (NSC). An important charactersitic of SHCC is the ability of the fibers (2% PVA) in the composite mixture to bridge the cracks and there by preventing localization of cracks, resulting in a narrow network of cracks with width less than 100 micron (Micro cracking). The aim of this research is to investigate the possible benefits of having a layer of SHCC in the tension zone of a reinforced concrete beam under four point bending test. A numerical simulation of the four point bending test is performed using a finite element software called ATENA. Comparison is made between traditionally reinforced concrete beams and beams with steel reinforcement embedded in an SHCC layer. The load capacities and crack patterns might present us with opportunity to understand the influence of an SHCC layer in tension zone of a beam under bending.

In the year 2006, Delft University of Technology started researching ‘Strain hardening Self-healing concrete’ (SSC). The reason for this research was the desire to increase the durability of concrete. This depends mostly on the reinforcement.
To ensure a certain lifespan of concrete structures, a minimum cover depth has to be applied. However, cracks occur in the concrete and cause a significant decrease in durability.
Research by M. Sierra-Beltran in 2014 resulted in SSC that has the capacity to seal these cracks due to the addition of a healing agent. Also, fibers are added to keep the cracks small and speed up the healing process.

In this thesis SSC is compared with traditional concrete (TC). A structural design is made with both materials based on assumed characteristics. Also, tests are performed to check whether those assumptions are right and a short calculation with the results is made.

When making a structural design with the assumed characteristics for TC and SSC, one finds that it is possible to design more slenderly with SSC than with TC. This is caused by the decrease in cover depth and the absence of extra reinforcement to control the crack width. The only exception is the height of the bridge deck; the height of the bridge deck is smaller in a TC design, caused by the low Young’s modulus of SSC.
The positive effect of SSC is expected to be most noticeable on the bridge deck at the locations with negative bending moments. Other locations where the positive effects can be noticed are at the beams and the tensile bars.
The material costs of the bridge designed with SSC are significantly higher than the material costs of the bridge designed with TC. However, it is expected that the maintenance costs of the SSC bridge are much lower.

The results of the tests for the structural characteristics show that SSC mortar has a lower compressive strength and a lower Young’s modulus than TC mortar. Especially, the lower Young’s modulus affects the structural design. The flexural strength of SSC mortar is higher than the flexural strength of TC.

The tests on durability give positive results. The chloride migration shows that, before healing, the SSC and TC have similar migration coefficients. However, after the healing period the migration coefficient of the SSC is lower than the migration coefficient of TC, at the location of the crack as well as the uncracked surface. Although, the difference is not as large as expected, this might change with longer healing periods. Therefore, it is recommended to do more tests with longer healing periods.

When making a short calculation with the characteristics found in the results of the test, one sees that the height of the bridge deck and the beams increases a bit more than assumed due to the lower Young’s modulus. Furthermore, no large changes are found.

Therefore, it is recommended to research the possibility of changing the mixture in a way that the Young’s modulus is increased.