| 1. |
- Eriksson, Daniel, 1987-
(author)
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Multiphase models for freeze-thaw actions and mass transport in concrete hydraulic structures
- 2021
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Doctoral thesis (other academic/artistic)abstract
- A crucial task for civil engineers is to make appropriate designs of new concrete structures and assessments of existing structures to ensure a long service life and sustainable use of the infrastructure. This doctoral thesis aims to increase the understanding of how advanced mathematical models can be used to describe phenomena and processes governing concrete degradation and thereby ultimately contribute to improving tools for design and assessments. The focus is on degradation processes that cause commonly observed concrete damage types in hydraulic structures exposed to cold climates and soft water. During a structure's service life, it is subjected to various deteriorating actions, but for the typical exposure conditions considered in this work, degradation due to freeze-thaw exposure and calcium leaching is of particular concern for the durability. Hence, the work related to improved modelling has been focused on phenomena related to these two degradation processes of concrete and how they may interact to produce damaging synergy effects.All developed models in this doctoral project treat concrete as a multiphase porous medium and use poromechanics to describe the coupled hygro-thermo-mechanical behaviour of the material. Moreover, since the overall aim concerns degradation in hydraulic structures, the model development has focused on obtaining formulations applicable for structural-scale simulations. The models presented in this thesis describe long-term water absorption into air-entrained concrete and the response of partially saturated air-entrained concrete exposed to freeze-thaw conditions. In the latter models, the phase changes and the freeze-thaw hysteresis are explicitly considered in the formulations. The presented simulation examples are performed using the Finite Element Method (FEM), and the capabilities of the models are verified with experimental data from the literature. Additionally, accelerated leaching experiments on air-entrained concrete are presented, where the influence of leaching on the formation and melting of ice inside the pore space due to pore structure alternations are investigated.The main research contribution of this work is the development and evaluation of advanced models applicable for structural-scale simulations that describe essential processes and phenomena related to freeze-thaw exposure of air-entrained concrete. The experimental work shows the significant influence of calcium leaching on the freeze-thaw processes, and the results can also facilitate future development of models considering some of the interactions causing damaging synergy effects. Adopting a multiphase modelling approach has been found suitable for describing the coupled processes and including interactions between different deterioration mechanisms. The theoretical models can also help gain further insights and improve the understanding of the phenomena, and thus, e.g. aid in developing more simplified models suited for daily engineering applications.
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| 2. |
- Hellgren, Rikard, 1988-
(author)
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A systems approach to ice loads on concrete dams
- 2022
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Doctoral thesis (other academic/artistic)abstract
- Dams are mainly used for the storage of water to electricity production and irrigation, or for river regulation. Continuous work to assure a high dam safety level is a prerequisite to minimize the risk for the uncontrolled release of water. An essential part of the safety evaluation of concrete dams is to understand the loads they are exposed to and the expected response of the dam. Under normal conditions, the behaviour of concrete dams is, to a great extent, governed by the ambient variation in temperature and water level. For concrete dams in cold climates, the large variation in ambient temperatures between summer and winter is particularly significant. In addition, these dams may be subjected to a pressure load from the expansion or movement of an ice sheet on the reservoir. The current guidelines for these ice loads are based on the dam's location and state that concrete dams must be designed for a line load of 50-250 kN/m. Thus, the ice load constitutes a significant part of the total load, especially for small dams. Despite its relatively significant impact, the knowledge about ice loads is insufficient, and the magnitude and return period of ice loads constitute one of the greatest uncertainties during stability evaluations of concrete dams. Furthermore, an apparent contradiction is that measurements and models indicate that ice loads are higher than the recommended values. Simultaneously, there are no reported dam failures where the ice-load has been addressed as the reason for the breach.To increase the knowledge about ice loads and the structural behaviour of concrete dams, this thesis applies an approach where the ice and the dam are parts of a structural system. The thesis contains six studies investigating the dam's, the ice's or the system's response to external loads. Studies of the dam are aimed at increasing the understanding of the normal behaviour of concrete dams. Studies of ice loads include measurements, and a major contribution from this project is the development of a 1$\times$3 m$^2$ ice load panel, the installation of the panel on a concrete dam, and subsequent measurements during six winters. In addition, a systematic review and meta-analysis of previous measurements have been performed. The studies of the different parts have been connected through two studies of the entire system. The first is a parameter study where the static interaction between ice and dam is simulated to quantify how geometric variations of the reservoir and ice affect the mechanical ice loads. The second study searches for detectable influence from the ice load in the measured behaviour of concrete dams.The results show that the structural behaviour of concrete dams under normal conditions is primarily governed by the variation in water level and temperature. For the studied dams, these effects are significantly larger than the influence from damage and degradation. Ice load measurements and simulations show that ice loads varies significantly along the dam. This variation makes it difficult to quantify the impact of external factors on the magnitude of ice loads. Despite this difficulty, three independent analyses show that ice thickness, water level change, and the dam's properties have an evident effect on the magnitude of ice loads. If current guidelines are to be updated to consider local conditions at the dam, these three parameters should be included. Ice loads of the magnitudes measured and specified in the current guidelines should have a notable impact on the behaviour of a dam during normal operation. However, such an impact has not been found in the eight dams studied within this project. This result indicates that the ice loads measured locally do not necessarily represent the global ice load that acts on the entire structure.
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| 3. |
- Malm, Richard, 1980-
(author)
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Predicting shear type crack initiation and growth in concrete with non-linear finite element method
- 2009
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Doctoral thesis (other academic/artistic)abstract
- In this thesis, the possibility to numerically describing the behaviour that signifies shear type cracking in concrete is studied. Different means for describing cracking are evaluated where both methods proposed in design codes based on experiments and advanced finite element analyses with a non-linear material description are evaluated. It is shown that there is a large difference in the estimation of the crack width based on the calculation methods in design codes. The large difference occurs due to several of these methods do not account for shear friction in the crack face. The finite element method is an important tool for analysing the non-linear behaviour caused by cracking. It is especially of importance when combined with experimental investigations for evaluating load bearing capacity or establishing the structural health. It is shown that non-linear continuum material models can successfully be used to accurately describe the shear type cracking in concrete. A method based on plasticity and damage theory was shown to provide accurate estimations of the behaviour. The methods based on fracture mechanics with or without inclusion of damage theory, overestimated the stiffness after crack initiation considerably. The rotated crack approach of these methods gave less accurate descriptions of the crack pattern and underestimated the crack widths. After verification of the material model, realistic finite element models based on plasticity and damage theory are developed to analyse the cause for cracking in two large concrete structures. The Storfinnforsen hydropower buttress dam is evaluated where the seasonal temperature variation in combination with the water pressure have resulted in cracking. With the numerical model the cause for cracking can be explained and the crack pattern found in-situ is accurately simulated. The model is verified against measurements of variation in crest displacement and crack width with close agreement. The construction process of a balanced cantilever bridge, Gröndal Bridge, is numerically simulated and a rational explanation of the cause for cracking is presented. It is shown that large stresses and micro-cracks develop in the webs during construction, especially after tensioning the continuing tendons in the bottom flange. Further loads from temperature variation cause cracking in the webs that is in close agreement with the cracking found in-situ. The effect of strengthening performed on this bridge is also evaluated where the vertical Dywidag tendons so far seem to have been successful in stopping further crack propagation.
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