This thesis focusses on the application of vertical profiled mortar connections in precast concrete shear walls that are built up with a stacked element configuration. The goal of the project is to develop a practical modelling approach in order to be able to apply the connections in a FEM model for structural analyses. For this purpose several research aspects concerning the behaviour of the connection were analysed. ... Read more

A lot of concrete structuressufferingfrom ASR show a decrease in material properties. However, it is not clear what might be the influence of ASR on structural behavior. Influence of restraint is often ignored in material property measurement, while reinforcement is present as a restraint of ASR expansion in most structures.Thisresults in so called chemical prestresseffect, whichshould not be neglected. In the presented study,the result indicatesthat neglecting the chemical prestresswould miss the failure mechanism transition and lead to a lowerbeam stiffness. Therefore, a newway to model ASR-affected structures is proposed, called the ASR-layered model.The numerical analysis is done with ATENA.Depending on the reinforcement arrangementand amount, with longitudinal reinforcement acting as a restraint, expansion to the greatest extend occurs in the direction of least confinement and the cracks become parallel to reinforcement. Considering the potential to connect with each other and form long horizontal cracks, the ASR crackingin restrained structuresare modelled as horizontal layers with reduced material properties. The propertyreduction is concentrated in ASR layers only, this local reduction is more realistic than the traditional method done by (Ferche, Sheikh, & Vecchio, 2017)in which a global reductionon material propertiesis applied. For chemical prestress, it is difficult to separate the influence of prestressin experiments,so experimentalresult is always affected both by the reduction of properties and prestress and probably some other phenomena that exist. The chemical prestress effect is modelledherein the way of physical prestress, but the expansion of ASR gel in chemical prestress would possibly influence the bond strength between concrete and reinforcement, so bond model is also considered.Some conclusions can be drawn on this study. The material reduction caused by ASR would only result in a decrease of load capacity. While in combination with prestress, the load capacity of ASR affected beam may be increased to be even higher than the unaffected beam. The application of prestress could also lead to the possible change of failure mechanism from shear to bending failure, whichis also observed in experiments. Including the bond-slip effect in prestressed ASR-layered model provides a better fitting with experimental results,but the influence of ASR on bond strength remains unclear. With parameter study, no obvious influence on ASR-affected structures could be linked to a/d ratio and reinforcement ratio. 1 or 2 ASR layers will only influence slightly on the ultimate load. The limitations of this model exist, the expansion caused by ASR is not directly modelled. Information on ASR layer property is quite limited, it would be difficult to measure the cracking part of ASR affected structures alone. In addition, the chemical prestress and possible cracking depend on the amount of ASR expansion as well as the reinforcement arrangement and amount, but they are not considered to correlate with each other in this model. ... Read more

The shear capacity of concrete members is a major challenge of structural concrete research through the years. Many theoretical models have been developed and various experiments have been performed, focusing on the accurate prediction of the shear behavior. Many of the available theoretical models assume that a large contribution of the shear capacity is transferred through cracks by a mechanism often recognized as aggregate interlock. At the same time, due to the increase in the complexity of the structures and the development of the concrete technology, the mechanical properties of concrete have improved significantly. This fact leads to the need for modification of the existing models or the development of new ones, accommodated to the improved materials. Since, the aggregate interlock plays a significant role in the development of the shear capacity, the present research proposes a new numerical methodology for the calculation of the aggregate interlock in high strength concrete in which aggregates break, based on the widely recognized model proposed by Walraven and the results of direct surface roughness measurements. The crack surfaces of concrete cylindrical specimens drilled from a 70 years old existing concrete bridge and newly casted cubic specimens generated by splitting tensile tests were measured by a laser scanner. Moreover, the surface of a reinforced deep beam after flexural shear failure was measured as well. The measured crack surfaces were used to implement the plasticity based aggregate interlock model proposed by Walraven with an algorithm which was validated with Walraven’s theoretical model, using a so-called mesostructural model. The output of the analysis gave suggestions on the adjustment of the available aggregate interlock model for high strength concrete. The proposed model is then implemented into a shear test on a 1.2 m concrete beam, which has a concrete strength larger than 70 MPa and the aggregate interlock seems to influence significantly the shear resistance of a cracked section. Based on the observation of the surface roughness of a crack, the thesis further proposed that with a sufficiently large crack face, the localized variation in the crack surface is averaged out. Thus, the surface can be used to develop a master curve for the given concrete type. In the last part of the study, two improvement suggestions are given regarding the Critical Shear Displacement Theory. The one point is relevant to the simplification of the crack profile that can be changed from a straight line into a more inclined and the second point is related to the correction factor considering the fracture of the aggregates, that should be dependent on the crack width. ... Read more

With the arrival of the Eurocode, the calculations regarding shear resistance, have become increasingly conservative compared to former concrete standards. For example in Voorschriften Beton 1974 (VB 74), a former concrete standard, a shear resistance combination of concrete, stirrups and prestress was allowed. Whereas the Eurocode assumes the shear resistance of the concrete is zero if it’s standalone contribution is insufficient. Meaning that once stirrups are required, the total applied shear stress is controlled by the stirrups.
Prorail is the party in the Netherlands which is responsible for the construction and maintenance of the railway infrastructure. The change in regulations results in concerns for parties like Prorail and in particular to the shear resistance of concrete railway bridges constructed according to the VB 74.

A through railway bridge consists of a relatively thin floor and two prestressed girders. Whenever a train drives over the floor, a great deal of the loading is spread in transverse direction, causing large shear forces and torsion in the two prestressed girders. Because this combination can be critical for the shear resistance, two prestressed through railway bridges, constructed according to the VB 74, are investigated in this master thesis.
It is verified with hand calculations whether or not these existing structures can guarantee structural safety by considering three shear resistance checks; the risk of shear tension failure, capacity of the stirrups and resistance against fatigue. Ultimately it is concluded, that the largest unity check is 1,01 and that both bridges can guarantee structural safety regarding shear resistance.

The reassessment of an through bridge, is an typical assignment for engineering firm such as Witteveen+Bos. But because hand calculations are too time consuming, the design loads are determined with SCIA Engineer (FEA program).
However in the earlier days FEA programs were not available and torsion in the girders was derived from a set of differential equations (analytical solution). Because structural engineers from today completely rely on programs like SCIA, a comparison is drawn up between SCIA and the analytical solution for torsion in the girder.

The plate, beam model 1A and 1B are the three types of models available in SCIA to model a through bridge. The plate model consists of a 2D-floor and 2D-girder, where the beam models form a combination of a 2D-floor and 1D-girder. But the plate and beam model 1B have in common that rigid connections are applied every ¼ meter between the girder and floor.
The analytical solution is derived with the assumption that the bridge is divided into strips with a length of 1,0 meter, which is implemented in the plate and beam models by reducing the E-modulus of the floor to roughly a third (cracked floor). For the governing load combination, this leads to values for torsion, which remain 10-15%, 40% and 10% behind the analytical solution for respectively the plate and beam model 1A and 1B. The large deviation of beam model 1A is remarkable and can be explained from the fact that no rigid connections between the girder and floor are applied, resulting in a loss of bending and torsional stiffness of the girder.

To conclude, the analytical solution is based on a number of assumptions, like no load distribution of the floor in longitudinal direction. In reality loads will be as well distributed in longitudinal as transverse direction and the analytical solution therefore needs to be considered as an safe upper limit.
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A series of accelerated corrosion tests were done by SGS INTRON (commenced by Combinatie Aanpak Maastunnel, constructor of the Maastunnel project) to investigate for a proper repair material for the deteriorated concrete floor in the Maastunnel at Rotterdam. Five types of fiber reinforced mixtures with different tensile behavior ranging from SHCC (strain-hardening cementitious composite) to ON06 (similar tensile behavior as normal concrete) were designed. Different cracking behavior was observed due to the different tensile behavior in the repair mortar. It is of utmost importance to investigate if the cracking behavior can be simulated by numerical modeling and if in the future, the parametric analysis might be performed without large experimental series.

Two types of numerical models are implemented in this master thesis, namely the lattice model and the continuum model. The lattice model can simulate the crack pattern of different materials in the accelerated corrosion test. The continuum model cannot show the behavior of decreasing number of cracks with decreasing fracture energy in the strain-softening materials due to the bifurcation problem brought by its incremental solution method. However, the lattice model shows the trend to underestimate the crack width of the strain-softening material. The continuum model has better performance in predicting the crack width. Also, the lattice model can predict the influence of the repair mortar-substrate bond strength on the crack pattern. Meanwhile, the continuum model always shows a complete failure in the repair mortar-substrate interface.
The boundary conditions at the bottom edge are observed to influence the direction of the bottom cracks. The edges of the specimen are kept free in the experiment. However, the repaired area is constrained by the surrounding concrete in reality. This indicates that the laboratory test may not represent the cracking behavior of the concrete floor in the tunnel accurately. SHCC is observed to be more sensitive to the repair mortar-substrate bond strength than material with lower stain capacity. Extra caution on the bond quality is advised while applying SHCC in a concrete repair system.

From the material point of view, with increasing fracture energy and strain capacity, more but thinner cracks can be performed in the accelerated corrosion test. SHCC material can perform the distributed crack pattern with a maximum crack width of 0.1mm which is ten times smaller than normal concrete. This behavior of SHCC is very suitable for being applied to a concrete repair system. The distributed cracks with smaller crack width can effectively limit the possibility of further corrosion. Besides SHCC, some strain-softening (under direct tension) materials can also show the deflection-hardening behavior in the bending test and produce the distributed crack pattern. SHCC and fiber reinforced concrete with deflection-hardening behavior in the bending test are suggested to be used in the concrete repair system. ... Read more

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

In literature, several investigation methodologies have been described. However, they often describe only a very general process or on the other hand very specific material related laboratory tests. Descriptions and recommended techniques on how to perform the proposed steps of an investigation are scarce. To a client ordering a damage investigation, the most important parts of the investigation are the conclusion and the recommendations. The client expects that the conclusion is correct. Therefore, it is important to ensure that the used investigation process results in a reliable (consistently good quality and able to be trusted) outcome. Basic knowledge of bias may help to indicate possible threats to the reliability of investigations. Techniques used to increase the validity and reliability of case study research can assist in developing strategies to achieve reliable forensic structural investigations. Therefore, the question of this research is: what is a reliable methodology to perform investigations into the technical cause of damages to concrete structures?

The research question has been answered by studying available literature on structural forensic investigation techniques, interviewing investigators, researching investigation techniques used for aerospace accident investigations and fire cause determination, studying human error theory and case study research and concrete damage mechanisms.

Based on the resulting information from studying all these disciplines the conclusion can be drawn that using a structured investigation process is essential. Therefore, this information has been used to develop an investigation methodology called the ‘investigation process model’. The model recommends the following phases: orientation, data collection, hypothesis generation, hypothesis analysis, conclusion, reporting and follow-up. These phases are split into individual steps that guide the investigator through the process. Each step contains suggestions for specific techniques varying from taking meaningful photographs on location to using a standard layout for reporting in order to execute the step properly. The investigator can select the most appropriate techniques for the project. Special attention has been paid to provide techniques to generate and analyse hypotheses in a reliable way. An example is the tool ‘Concrete Damage Handbook’ to assist the investigator in linking visual damages to possible causes. As a common thread through the model measures have been provided to limit the negative influence of bias (based on the Delft approach).

Validation of the model with 1) a test case based on a real damage file, 2) comparison with real investigation reports and 3) scientific papers proved that the model relates to the daily working practice and is usable, practical and functional within the scope of the defined criteria.
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A new type of finite element model, called the double mesh model is proposed in this thesis. With the level of free expansion as the input, the model is able to simulate the restrained ASR expansion by taking into account the effects of physical restraints. The retrained ASR expansion in reinforced concrete cubes and beams are simulated. Numerical expansion obtained from this new model showed a good agreement with the experiments. Then, the expanded beams are loaded in shear to simulate the shear behaviour of ASR affected concrete beams. In this new model, ASR damage is embedded through a realistic simulation of ASR expansion. Whereas, in the traditional method, ASR damage is taken into account by a direct reduction of the input material properties and the expansions are not simulated. According to the data obtained from experiments, even though the mechanical properties of concrete are reduced due to ASR, but this not necessarily leading to the decrease in the capacity of the beams. In some experiments the change of failure mode is observed where the unaffected beam failed in shear but the ASR affected one failed in bending. This is because in ASR affected beams, the increase in shear load results in the enlargement of the existing ASR cracks instead of generating new diagonal shear cracks, and thus the shear failure is prevented. In the precracking method, the effects of ASR cracks on the capacity and the failure mode are taken into account in the model since the ASR cracks are simulated. Whereas, in the traditional method, where the ASR damage is included through the reduction of the input properties, the effects of ASR cracks are not able to be reflected in the model. ... Read more

Shear failure is one of the most critical failure modes of reinforced concrete members, especially for those without shear reinforcement. Despite the extensive research programs in the last decades, there is still no general agreement on a rational theory to assess their shear capacity. In the recent years, researchers have focused on developing mechanical models for shear design that are based on a predefined crack pattern and kinematics assuming that the shear force can be transferred through a critical shear crack by various shear transfer mechanisms. These mechanical models need to be validated by detailed kinematic measurements taken from experiments. This information could not be acquired before, however, in the recent years, new measurement technology has developed quickly, amongst others, Digital Image Correlation (DIC) provides new opportunities to obtain the crack pattern and kinematics through the displacement field of the whole surface of the target specimen. In this research, the possibility of using DIC measurements to apply a detailed analysis on the contribution of the shear transfer mechanisms (uncracked concrete, aggregate interlock, dowel action, and arch action) is explored in order to obtain a better understanding of the shear failure process. The analysis is based on ten representative tests on reinforced concrete beam specimens with a height of 1200 mm. The tests are selected from an experimental program designed to study the shear behaviour of reinforced concrete slab strips without shear reinforcement. A new algorithm is developed to automatically determine the contributions of the different shear transfer mechanisms along a crack from the displacement field obtained by DIC measurements. A comparison between the experimental results and the sum of the calculated contributions yield to a reasonable agreement, with an error of 40% when the shear failure is presented just after the formation of the flexural shear crack. With the help of the new algorithm, new insights on the three different shear failure modes observed in the experiments (flexural shear failure, shear compression, and dowel failure) are discussed. Flexural shear failure is attributed to the loss of aggregate interlock when a sudden increase of the shear displacement of the critical shear crack is observed. However, such decrease occurs earlier before the actual failure occurs. Therefore, the actual shear failure mechanism should be studied at an earlier stage. In a shear compression failure, the arch action turns out to be dominating. For members with large depth and exceptionally low reinforcement ratio (< 0.3%), a different failure mode may occur: dowel failure. Which is defined by the opening of the secondary branch of a major flexural crack along the tensile reinforcement. In the three failure modes, the importance of the crack opening in vertical and longitudinal direction is demonstrated, since the increase of them directly result in the drop of shear force that can be transferred through aggregate interlock. Finally, the results are a valuable input to further improve the Critical Shear Displacement theory. It is suggested that the assumed simplified crack profile is modified to a crack with an angle between 60° to 70° to represent more accurately the results and to allow for larger shear displacements. It is demonstrated that an increase of the shear displacement is critical to triggering the shear flexural failure as proposed by this theory and the actual value of the critical shear displacement is larger than the proposed value based on regression analysis. ... Read more

Ordinary Portland Cement (OPC) consumption has grown nearly exponentially in the last twenty years. OPC has become the highest-volume manufactured product on the planet. Production of OPC is energy-intensive, consumes unrenewable natural resources and is one of the primary contributors to global warming (accounting for at least 5-8% of worldwide anthropogenic CO2 emissions). An alternative for OPC concrete is Alkali-Activated Concrete (AAC), for which Portland cement is completely substituted by an alternative binder. Instead of using OPC and water, precursors (raw materials) like Blast Furnace Slag (BFS) or Fly Ash (FA) are activated with an alkaline activator solution.
Although AAC seems to have promising qualities for structural application in terms of sustainability, worldwide use is not yet established. One of the reasons for this is the fact that there are no available regulations or codes to apply it, the material is relatively new and limited research has been conducted. For OPC concrete, the design codes are based on compressive strength at 28 days (strength at later ages stays either constant or is higher) and most other mechanical properties used in calculations are estimated based on this compressive strength. For AAC it is not yet sure if the same relations and assumptions as for OPC are also valid. First, because mechanical properties that have been reported for AAC in literature vary a lot, depending on mixture composition and curing conditions. Second, the long-term strength development of AAC is scarcely investigated and it is not clear if the compressive strength at 28 days can be used as a safe reference for design. Namely, a few researchers reported a decrease of strength or stiffness over time, for AAC mixtures that contain blast furnace slag. The observed decrease might not be a very desirable phenomenon and should be well-understood prior to wider structural application of AAC. Therefore, the main research question of this thesis is: Can a decrease of stiffness and strength over time, as sometimes reported in literature for AAC, also be found for AAC used at TU Delft and if so, what could be an explanation for this behaviour? Does the amount of BFS in the binder play a role, as a decrease over time has only been reported for AAC containing BFS? And if not, what other cause could lead to a decrease of properties over time? The intention is to make some first steps towards a better understanding of this phenomenon.
The research question is investigated in an experimental manner. Compressive strength, elastic modulus, splitting tensile strength and flexural strength are tested at different ages (28, 56 and 91 days) after being wet-cured (20°C and 95% RH) for 28 days. Two different AAC mixtures are investigated, S100 and S50, characterized by a BFS/FA binder ratio of 100:0 and 50:50 respectively. Furthermore, the flexural behaviour of reinforced beams is investigated by conducting four-point bending tests on both S100 and S50 concrete of two different ages (33/34 days and 69/70 days) and compared to an OPC concrete control beam.
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