| 1. |
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| 2. |
- Bernstone, Christian, et al.
(författare)
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Verifiering av struktur och fuktmekaniska beräkningsverktyg : Tillämpning på reaktorinneslutningars betongkonstruktioner, Vercors Fas 1
- 2017
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Électricité de France har utanför Paris byggt en 1/3 skalmodell av en reaktorinneslutning motsvarande en fransk fullskalereaktor modell P4. Anläggningen möjliggör studier av hur åldringsprocesser påverkar förspända reaktorinneslutningar. Syftet är att efter de 7 år som ett övergripande forskningsprojektet vid på anläggningen pågår, uppnå påverkan motsvarande den åldring som en fullskalereaktor har utsatts för efter 6o års drift.Konstruktionen har försetts med ett omfattande instrumenteringsprogram som gör det möjligt att följa konstruktionens beteende i tidigt skede och över tid. Ett av anläggningens forskningsprojekt omfattar en benchmark av olika beräkningsverktygs möjlighet att kunna förutse konstruktionens beteende i tidigt skede och under själva åldrandet, dess täthet till följd av åldrandet och beteende under svåra haveriförhållanden. Denna benchmark har genomförts via en öppen inbjudning till intresserade experter att delta. I projektet, benämnt VeRCoRs , bidrar deltagande organisationer med egna modelleringsarbeten utifrån de tre olika teman. Arbetet med det första temat startade 2015 (tidigt skede), därefter följer den åldrade konstruktionen (2017) och svåra haverisituationer (2021). Denna rapport redovisar det arbete som har gjorts för det första temat (Tema 1) inom ramen för ett Energiforskuppdrag.Modelleringsarbetet har till största delen genomförts av LTH i nära samarbete med KTH och Vattenfall. Simuleringar har gjorts för den unga betongens beteende under den ett år långa byggfasen, med hänsyn tagen till betongens hydratation (temperaturberäkningar/värmeutveckling), uttorkning (självuttorkning och fukttransport) och deformationer (temperatur-, fukt-, kryp-, och spänningsrelaterade deformationer). Resultatet har jämförts mot data från anläggningens instrumentering. Dessutom så har en genomgång gjorts av resultaten från övriga deltagares arbeten i Tema 1.Sammanfattningsvis så visar resultaten på de fördelar som finns med att genom avancerade beräkningar verifiera designparametrar och prognostisering av reaktorinneslutningars funktion, men också på utmaningar. Med hjälp av FEmodellering så kan det tidiga skedet efter färdigställd konstruktion beskrivas både vad gäller temperatur- och deformationer med rimlig tillförlitlighet. Det finns dock potential till förbättringar. Även om betongens temperatur kan predikteras noggrant så kan det skilja kraftigt i uppskattningen av inducerade påkänningar. En viktig orsak till dessa skillnader beror på osäkerheter i bestämning av krypning och krympning i betongen tidiga skede.
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| 3. |
- Eriksson, Daniel, 1987-, et al.
(författare)
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A Hygro-Thermo-Mechanical Multiphase Model for Long-Term Water Absorption into Air-Entrained Concrete
- 2019
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Ingår i: Transport in Porous Media. - Netherlands : Springer. - 0169-3913 .- 1573-1634. ; 127:1, s. 113-141
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Tidskriftsartikel (refereegranskat)abstract
- Many concrete structures located in cold climates and in contact with free water are cast with air-entrained concrete. The presence of air pores significantly affects the absorption of water into the concrete, and it may take decades before these are fully saturated. This generally improves the long-term performance of such structures and in particular their frost resistance. To study the long-term moisture conditions in air-entrained concrete, a hygro-thermo-mechanical multiphase model is presented, where the rate of filling of air pores with water is described as a separate diffusion process. The driving potential is the concentration of dissolved air, obtained using an averaging procedure with the air pore size distribution as the weighting function. The model is derived using the thermodynamically constrained averaging theory as a starting point. Two examples are presented to demonstrate the capabilities and performance of the proposed model. These show that the model is capable of describing the complete absorption process of water in air-entrained concrete and yields results that comply with laboratory and in situ measurements.
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| 4. |
- Eriksson, Daniel, 1987-, et al.
(författare)
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Comparison of mechanistic and phenomenological approaches to model drying shrinkage of concrete
- 2017
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Ingår i: Nordic Concrete Research. ; , s. 287-290
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Konferensbidrag (refereegranskat)abstract
- Drying shrinkage of concrete is often estimated using a phenomenological approach where it is assumed that shrinkage is proportional to the change in internal humidity. In this study, a mechanistic approach was used which instead aims to mathematically describe the physical processes of drying shrinkage. Simulations of two laboratory tests were made and compared to results from two models based on the phenomenological approach. The results show that the developed model can describe drying shrinkage of concrete equally well as the phenomenological models but without the need to specify the final drying shrinkage strain.
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| 5. |
- Eriksson, Daniel, 1987-, et al.
(författare)
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Freezing of partially saturated air-entrained concrete: A multiphase description of the hygro-thermo-mechanical behaviour
- 2018
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 152-153, s. 294-304
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Tidskriftsartikel (refereegranskat)abstract
- Even though air-entrained concrete is usually used for concrete structures located in cold climates that are exposed to wet environments, frost damage is frequently detected during inspections. However, it is often hard to assess the extent and severity of the damage and, thus, there is a need for better tools and aids that can complement already established assessment methods. Several studies have successfully shown that models based on poromechanics and a multiphase approach can be used to describe the freezing behaviour of air-entrained concrete. However, these models are often limited to the scale of the air pore system and, hence, hard to use in applications involving real structures. This study proposes a hygro-thermo-mechanical multiphase model which describes the freezing behaviour of partially saturated air-entrained concrete on the structural scale. The model is implemented in a general FE-code and two numerical examples are presented to validate and show the capabilities of the model. The first concerns a series of experimental tests of air-entrained cement pastes, whereas the second aims to show the capability of the model to account for an initial non-uniform distribution of moisture. While the model predictions underestimate the magnitude of the measured strains, the results still show that the model can capture the general freezing behaviour observed in the experimental tests on the structural scale. Furthermore, the results demonstrate that the model is capable of describing freezing induced deformations caused by non-uniform moisture distributions.
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| 6. |
- Eriksson, Daniel, 1987-, et al.
(författare)
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Influence of air voids in multiphase modelling for service life prediction of partially saturated concrete
- 2018
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Ingår i: Computational Modelling of Concrete Structures. - London, UK : CRC Press. - 9781138741171 - 9781315182964 ; , s. 317-326
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Konferensbidrag (refereegranskat)abstract
- The purpose of this study is to show the influence and significance of including water filling of air pores when studying moisture conditions in concrete structures cast with air-entrained concrete and in contact with free water. Especially if the aim is to assess the risk for frost damages in different regions of the structure, based on a critical degree of saturation, in order to ultimately perform a service life prediction. A hygro-thermo-mechanical multiphase model that includes the effect of water filling in air pores, recently presented by the authors, is briefly described and applied in two numerical examples. The results show moisture distributions that would not be possible to capture without the air pore filling included in the model. More importantly, the general shape of these distributions complies well with measured distributions in real concrete structures as well as with distributions obtained in laboratory measurements.
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| 7. |
- Gasch, Tobias, et al.
(författare)
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A coupled hygro-thermo-mechanical model for concrete subjected to variable environmental conditions
- 2016
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Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 91, s. 143-156
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Tidskriftsartikel (refereegranskat)abstract
- It is necessary to consider coupled analysis methods for a simulation to accurately predict the long-term deformations of concrete structures. Among other physical fields that can be considered, both temperature and moisture have a significant influence on the deformations. Variations of these fields must therefore be included implicitly in an analysis. This paper presents a coupled hygro-thermo-mechanical model for hardened concrete based on the framework of the Microprestress-Solidification theory. The model accounts for important features of concrete such as ageing, creep, shrinkage, thermal dilation and cracking; all of these under variable temperatures and moisture conditions. It is discussed how to implement the proposed model in a flexible numerical framework that is especially suitable for multi-physics analyses. The capabilities of the model are shown through the analysis of three experimental data sets from the literature, with focus on creep and shrinkage. Overall, the agreement between the analysis and experimental results is good. Finally, a numerical example of a concrete gravity dam with dimensions and loads typical to northern Sweden is analysed to show the capabilities of the model on a structural scale.
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| 8. |
- Gasch, Tobias, et al.
(författare)
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A coupled multi-physics model for creep, shrinkage and fracture of early-age concrete
- 2016
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Ingår i: 9th International Conference on Fracture Mechanics of Concrete and Concrete Structures.
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Konferensbidrag (refereegranskat)abstract
- The behaviour of concrete at early-age is complex and involves several physical fieldssuch as temperature, moisture and deformations. In this paper a hygro-thermo-chemo-mechanicalmodel for the analysis of early-age concrete based on a combination of models from the literature ispresented. The chemical model is based on the reaction degree concept, also used to define internal actions such as self-desiccation and ageing of mechanical properties. A mechanical model based on the Microprestress-Solidification theory for concrete creep is used, that in a simplified manner alsoconsiders concrete fracture. The model has been implemented in a numerical framework suitable for coupled multi-physics problems. It is here applied to a case study of an un-reinforced concrete tunnel plug made of a low-pH self-compacting concrete. Good agreement is generally obtained with measurements and hypotheses previously made on the behaviour of the plug are verified.
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| 9. |
- Gasch, Tobias
(författare)
-
Concrete as a multi-physical material with applications to hydro power facilities
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During its lifetime, a concrete structure is subjected to many different actions, ranging from mechanical loads to environmental actions. To accurately predict its integrity from casting and throughout its service life, a modelling strategy is required that considers mechanical loading but also implicitly accounts for physical effects such as temperature and moisture variations. This is especially true for large concrete structures found in many infrastructure applications such as bridges, nuclear power plants and dams. Modelling concrete as a multi-physical material is becoming an increasingly used approach for which large research efforts are being made, including the development of more refined mathematical and numerical methods as well as considering more physical and chemical variables in the coupled model.The research project, of which this licentiate thesis is the first phase, aims at investigating aging concrete structures at hydro power facilities, with focus on the internal structures of the power plants. This thesis presents a review of advanced mathematical methods and concepts for modelling aging concrete found in the literature which can later be applied to study such structures. The focus is on models that describe the deformational behaviour of concrete where aspects such as aging, cracking, creep and shrinkage are investigated. However, in order to accurately describe such phenomena, a multi-physical approach is adopted where moisture and temperature variations in the concrete are studied. Also, models that describe the chemical behaviour related to hydration and thus in extension aging, are also reviewed and introduced in the multi-physical framework. The use of such models are discussed in the context of the finite element method (FEM), in which coupled models are implemented, verified and applied in the appended papers using two different FE codes.Several verification examples are presented covering different aspects of the implemented models, both in isolation and coupled in a multi-physical setting. By comparing the numerical results with experimental data from the literature it can be shown that it is possible to predict most aspects of aging concrete that have been of interest here. While these examples are all on a laboratory scale, numerical examples and case studies are also provided that exemplify how the models can be applied on a structural scale. By using the developed analysis tools, valuable information and insights can be gained on aging concrete structures and these tools will in the next phase of the research project be applied to large concrete structures at hydro power facilities.
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| 10. |
- Gasch, Tobias, et al.
(författare)
-
Cracking in Quasi-Brittle Materials Using Isotropic Damage Mechanics
- 2016
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Konferensbidrag (refereegranskat)abstract
- An extension of the Solid mechanics interface in Comsol Multiphysics is presented to analyse localized deformations of quasi-brittle materials, for example cracking in concrete. This is achieved by implementing an isotropic damage mechanics constitutive law, which is combined with both a local and a non-local regularization technique to ensure mesh objectivity. The implementation is made using equation based modelling and modification of built-in features. To facilitate easier use of the model, a custom user interface is constructed using the Physics Builder. Lastly, the implemented model is validated using examples of both plain and reinforced concrete, with two examples taken form the literature. Overall the simulation results are in good agreement with the experimental observations.
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| 11. |
- Gasch, Tobias, et al.
(författare)
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Influence of varying ambient conditions on time-dependent deformations inconcrete using multi-field modelling
- 2017
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Ingår i: XXIII Nordic Concrete Research Meeting. - : Nordic Concrete Research Publications.
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Konferensbidrag (refereegranskat)abstract
- Time-dependent deformations, such as creep and shrinkage, are important when dealing with durability aspects of concrete. In the current study, a multi-field analysis method is described, verified and used in a numerical study to investigate the influence of short and long term variations in temperature and relative humidity. It is found that especially the creep behaviour is significantly influenced by the seasonal variations in climate conditions and also to a lesser extent the daily variations.
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| 12. |
- Gasch, Tobias, et al.
(författare)
-
Instrumentation and Modelling of a Reactor Containment Building
- 2018
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Nuclear concrete containment buildings typically consist of pre-stressed concrete. The pre-stressing tendons are utilized to enforce a compressive state of stress to ensure that cracks do not occur in the containment structure. The tendons are thereby an important part of the containment building and important for its structural integrity. In many cases, these tendons are grouted with cement grout to prevent corrosion. This results however in that it is not possible to directly assess the tendons or re-tension these if significant long term losses occurs. The drawback with cement grouted tendons is, thereby, that it is not possible to directly measure the current tendon force. One conventional method to assess the status of the containment building, and thereby indirectly the tendons, is to perform pressure tests. The pressure tests are performed where the pressure in the containment building is increased. The response of the containment can after this be determined based on measurements of displacements and strains. The purpose of this project is to perform simulations of a pressure test of a Boiling Water Reactor (BWR) that is common in Sweden and Finland. Based on these simulations, the response of the containment building is determined and suggestions are made regarding suitable placement of measuring sensors. The suggested instrumentation has been divided into different types of sensors defined as detectors and support sensors. The detectors are needed to monitor the structural response of the containment while the support sensors are needed to give sufficient input to numerical analyses. It is suggested that detector sensors are placed at four vertical positions and at three points along the perimeter. At these locations, it is recommended that displacement sensors, strain gauges and temperature sensors are installed. In addition, it is also recommended that the relative radial displacement between the intermediate slab and the cylinder wall is monitored. As support sensors, it is recommended that the ambient temperature and relative humidity is measured since these constitute important boundary conditions for numerical analyses and thereby prediction of the structural behaviour.
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| 13. |
- Gasch, Tobias
(författare)
-
Multiphysical analysis methods to predict the ageing and durability of concrete
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the societal demand for sustainability and the increasing age of infrastructure, a crucial task for the civil engineering community is to improve the durability of concrete structures. This thesis aims to contribute to such development through theoretical studies using mathematical modelling and numerical simulations. During its service life, a concrete structure is subjected to many different actions, ranging from mechanical loads to chemical and physical processes. Hence, a sound modelling strategy requires multiphysics and the inclusion of coupled chemical and physical fields (e.g. temperature, moisture and cement hydration) in addition to methods that describe mechanical integrity of the material. Conditions and phenomena critical for concrete structures at hydropower facilities have been of particular interest to study.The thesis presents several mathematical models of various complexity to describe the multiphysical behaviour of concrete using a material point description. A significant focus is on models that describe the mechanical behaviour of concrete where aspects such as ageing, cracking, creep and shrinkage are investigated. For the creep behaviour, a state-of-the-art model based on the Microprestress–Solidification (MPS) theory is reviewed and further developed. The appended papers (III to IV) presents a mathematical framework for the modelling of durability aspects of concrete based on multiphase porous media theory. The governing equations are derived with the Thermodynamically Constrained Averaging Theory (TCAT) as a starting point. It is demonstrated how this framework can be applied to a broad variety of phenomena related to durability; from the casting and hardening of concrete to the long-term absorption of water into air-entrained concrete. The Finite Element Methods (FEM) is used to solve the proposed mathematical models, and their capabilities are verified using experimental data from the literature.The main research contribution is the development and evaluation of theoretical models that advance the understanding and improve knowledge of the ageing and durability of concrete and concrete structures. More precisely, it is shown how multiphysical models and the developed multiphase framework can be used to gain insights on the material behaviour of concrete at smaller scales while they are also applicable to structural-scale simulations. During all model development, the efficient solution of structural problems has been fundamental. Through case studies and several examples from the literature, it is exemplified how these models can be used to enhance the performance and thereby increase the durability of concrete structures.
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| 14. |
- Gasch, Tobias, et al.
(författare)
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Non-linear analyses of cracks in aging concrete hydro power structures
- 2016
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Ingår i: Dam Engineering. - : International Water Power & Dam Construction. - 0958-9341.
-
Annan publikation (populärvet., debatt m.m.)abstract
- The concrete structures at Swedish hydro power facilities were built during the early to mid-20th century and many of them are starting to exhibit age related wear and deterioration. It isimportant to ensure the integrity of these concrete structures from a dam safety perspectiveand also to secure a safe operation of the power facility in the future. With the latter in mind,this paper aims to study the concrete structures that house the power generating machinery ofthe facility, especially the parts close to the generator where the loads from the power unit aresupported. Cracks observed in these structures will reduce its stiffness, which affects theoperation of the rotating machinery. This paper presents and discusses some generalconsiderations and loads that are of importance for this type of structures and highlights sometypical cracks that have been observed in Swedish hydro power facilities. To complement thisdiscussion, a case study is presented of a hydro power facility where cracks have been foundin the concrete support structure of the power unit, especially at the interconnections betweenthe unit and the concrete. The most likely cause of these cracks are investigated through nonlinearfinite element analysis considering mechanical loads as well as physical loads such asdrying shrinkage and temperature variations. It is concluded that the long-term physicalloading is the most probable cause of the observed cracks. However, the operation of thepower unit and changes in its operational pattern can cause further propagation of thesecracks. Finally, suggestions on possible enhancement of the analysis methods used in the casestudy are proposed and discussed for further studies of this type of concrete structures.
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| 15. |
- Gasch, Tobias, et al.
(författare)
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On the behaviour of con-crete at early-ages: A multiphase description of hygro-thermo-chemo-mechanical properties
- 2019
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Ingår i: Cement and Concrete Research. - : Elsevier. - 0008-8846 .- 1873-3948. ; 116, s. 202-216
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the early-age behaviour of concrete is of importance for designing durable concrete structures. To contribute to the improvement of this, a hygro-thermo-chemo-mechanical model is presented that accounts for phenomena such as hydration, external and internal drying, self-heating, creep, shrinkage and fracture. The model is based on a multiphase porous media framework, using the Thermodynamically Constrained Averaging Theory (TCAT) as starting point to derive the governing equations of the system. This allows for a systematic treatment of the multiscale properties of concrete and how these develop during hydration, e.g. chemical and physical fixation of water. The proposed mathematical model is implemented within the context of the Finite Element Method (FEM), where all physical fields are solved in a fully-coupled manner. Chosen properties of the model are demonstrated and validated using three experimental results from the literature. Generally, the simulated results are in good agreement with the measurements.
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| 16. |
- Gasch, Tobias, et al.
(författare)
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Thermally-induced cracking of a concrete arch dam using COMSOL Multiphysics
- 2017
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Konferensbidrag (refereegranskat)abstract
- The design and maintenance of concrete dams in cold regions is a challenging task, in large due to the temperature difference between summer and winter. In order to enhance the knowledge of this, theme A of the 14th International Benchmark Workshop on Numerical Analysis of Dams is dedicated to the prediction of the extent of cracking in a concrete arch dam due to temperature variations. The current study proposes a solution to this using the finite element software COMSOL Multiphysics. A global model is set up to analyze the transient temperature variations as well as the displacements given the assumption of a linear material behavior. To predict the extent of cracking, a rate-dependent isotropic damage model is implemented as an extension of the built-in functionality of COMSOL Multiphysics. Furthermore, a submodel is created to allow for a higher mesh resolution in the non-linear analysis. The results indicate that the considered arch dam suffers a large risk of cracking due to temperature variations, especially on the downstream side. Most cracks propagate during the winter, although some cracks appear already when static loads are applied.
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| 17. |
- Gasch, Tobias, et al.
(författare)
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Three-dimensional simulations of ageing concrete structures using a multiphase model formulation
- 2019
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Ingår i: Materials and Structures. - : Springer Netherlands. - 1359-5997 .- 1871-6873. ; 52:4
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Tidskriftsartikel (refereegranskat)abstract
- The durability of concrete structures is in no small degree determined by the quality and integrity of the concrete, where structural damages such as cracks negatively affect many of the functions of the structure. Often cracks are formed due to restrained thermal and hygral deformations, where the risk is exceptionally high during the early stages after casting. This study presents a hygro-thermo-chemo-mechanical model that accounts for phenomena such as hydration, external and internal drying, self-heating, creep, shrinkage and fracture. The model is derived as a modified version of a fully-coupled multiphase model recently proposed by Gasch et al. (Cem Concrete Res 116:202–216, 2019. https://doi.org/10.1016/j.cemconres.2018.09.009) and implemented in the Finite Element Method. Here the governing equations are simplified, and a more efficient solution method is proposed. These modifications are made with the intention to obtain a model more suitable for structural scale simulations. To validate the model, one of the end-restrained beams tested within the French research project CEOS.fr is analyzed. Laboratory data on the concrete is used to calibrate to model and recordings of ambient conditions makes it possible to define accurate boundary conditions. Results from the simulation are compared to measured temperatures and deformations from the first 60 days after casting and are found to generally be in good agreement. Compared to the fully-coupled model by Gasch et al. (2019), the modifications proposed in this study reduce the computational cost by a factor five; without any noticeable differences to the structural results.
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| 18. |
- Hellgren, Rikard, 1988-, et al.
(författare)
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Fluid structure interaction
- 2015
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Ingår i: International Water Power and Dam Construction. - 0306-400X. ; :8, s. 40-45
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study is to investigate how Fluid-Structure interaction may beincluded in numerical earthquake analyses of dams. The basis for this project is theme Afrom ICOLDs 12th international benchmark workshop on numerical analysis of dams,which was held in October 2013. The focus of theme A was on how to account for fluidstructure interaction in numerical earthquake analyses of dams. In this study, parametricnumerical analyses have been performed where the purpose was to isolate someimportant parameters and investigate how these influence the results in seismic analysesof dams. These analyses were performed through the use of the finite element methodare the choice of Rayleigh damping parameters, reservoir boundaries and waveabsorption in the foundation-reservoir interface. The use of acoustic elements has proven to be a powerful approach for FSI analyses of adam-reservoir-computation time, while allowing for more advanced features such as bottom absorptionand non-be a challenging task, where it has a significant impact on the results. The methodproposed by Spears and Jensen has a physical meaning in the sense that this methodexcites the same e ective mass for the Rayleigh damped case as for the modal dampedcase. If a constant modal damping is desired or prescribed in a standard, this methodprovides a reasonable and sound method to choose the Rayleigh damping parameters fora complex structure. A more straightforward method is to choose the two frequencies insuch a way that the span between the frequencies covers about 80% of the e ective mass.The choice of reservoir boundary conditionsparameter showed to be the one that least a ected the results in the time-history analysis.results and this coe cient should be used carefully.
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| 19. |
- Hellgren, Rikard, 1988-, et al.
(författare)
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Influence of fluid structure interaction on a concrete dam during seismic excitation
- 2015
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Konferensbidrag (refereegranskat)abstract
- The aim of this study is to investigate how Fluid-Structure interaction may be included in numerical earthquake analyses of dams. The basis for this project is theme A from ICOLDs 12th international benchmark workshop on numerical analysis of dams, which was held in October 2013. The focus of theme A was on how to account for fluid structure interaction in numerical earthquake analyses of dams. In this study, parametric numerical analyses have been performed where the purpose was to isolate some important parameters and investigate how these influence the results in seismic analyses of dams. These analyses were performed through the use of the finite element methodare the choice of Rayleigh damping parameters, reservoir boundaries and wave absorption in the foundation-reservoir interface. The use of acoustic elements has proven to be a powerful approach for FSI analyses of adam-reservoir-computation time, while allowing for more advanced features such as bottom absorption and non- be a challenging task, where it has a significant impact on the results. The method proposed by Spears and Jensen has a physical meaning in the sense that this method excites the same e ective mass for the Rayleigh damped case as for the modal damped case. If a constant modal damping is desired or prescribed in a standard, this method provides a reasonable and sound method to choose the Rayleigh damping parameters for a complex structure. A more straightforward method is to choose the two frequencies in such a way that the span between the frequencies covers about 80% of the e ective mass. The choice of reservoir boundary conditionsparameter showed to be the one that least a ected the results in the time-history analysis. results and this coe cient should be used carefully.
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| 20. |
- Hellgren, Rikard, 1988-, et al.
(författare)
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Influence of fluid structure interaction on a concrete dam during seismic excitation
- 2015
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Ingår i: Dam Engineering. - 0958-9341. ; XXVI:2, s. 1-15
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study is to investigate how fluid-structure interaction (FSI) may be included in numerical earthquake analyses of dams. The basis for this project is Theme A from ICOLD’s 12th International Benchmark Workshop on Numerical Analysis of Dams[5], which was held in October 2013 in Austria. The focus of Theme A was on how to account for fluid-structure interaction in numerical earthquake analyses of dams. In this study, parametric numerical analyses have been performed, where the purpose was to isolate some important parameters and investigate how these influence the results in seismic analyses of dams. These analyses were performed through the use of the finite element method, where the reservoir was modelled with acoustic finite elements. The studied parameters are the choice of Rayleigh damping parameters, reservoir boundaries, and wave absorption in the foundation-reservoir interface. The use of acoustic elements has proven to be a powerful approach for FSI analyses of a dam-reservoir-foundation system. The acoustic finite elements provide reasonable computation time, while allowing for more advanced features such as bottom absorption and non-reflecting boundaries. The choice of Rayleigh damping coefficients has proven to be a challenging task,where it has a significant impact on the results. The method proposed by Spears & Jensen[1] has a physical meaning in the sense that this method excites the same effective mass for the Rayleigh damped case as for the modal damped case. If a constant modal damping is desired, or prescribed in a standard, this method provides a reasonable and sound method to choose the Rayleigh damping parameters for a complex structure. A more straightforward method is to choose the two frequencies in such a way that the span between the frequencies covers about 80% of the effective mass. The choice of reservoir boundary conditions was based on the assumption of an infinite reservoir. A conservative approach is to use a fixed boundary condition, where the pressure waves are reflected at the upstream boundary of the reservoir. However, this parameter proved to be the one that least affected the results in the time-history analysis. The reflection coefficient describing bottom absorption showed to greatly influence the results, and this coefficient should be used carefully.
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| 21. |
- Sjölander, Andreas, 1983-, et al.
(författare)
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Shrinkage cracking of thin irregular shotcrete shells using multiphysics models
- 2016
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Ingår i: 9th International Conference on Fracture Mechanics of Concrete and Concrete Structures.
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Konferensbidrag (refereegranskat)abstract
- Shotcrete (sprayed concrete) is commonly used to support tunnels in good quality hard rock. Including a drainage system often results in end-restrained sections of shotcrete, which have created problems with shrinkage induced cracking. In this paper a multi-physical material model with coupled behaviour between thermal actions, moisture transportation and mechanical strain has been used to model and describe the complex behaviour and effects of shrinkage of such a structure. The model was first calibrated against a free shrinkage test and then used to simulate an experimental set-up for testing of end-restrained shrinkage. The first results lead to a need of tuning of the parameters controlling the drying of the shotcrete to accurately describe the experimental results. This tuning could be an indication that the shrinkage behaviour differs between a restrained and an un-restrained sample. However, further research about possible changes in the pore structure as well as more detailed measurements of the early shrinkage behaviour is needed before any such conclusions can be drawn.
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| 22. |
- Šmilauer, V., et al.
(författare)
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Hygro-mechanical modeling of restrained ring test : COST TU1404 benchmark
- 2019
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Ingår i: Construction and Building Materials. - : Elsevier. - 0950-0618 .- 1879-0526. ; 229
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Tidskriftsartikel (refereegranskat)abstract
- The restrained ring test belongs to a traditional method for estimating cracking tendency of a paste, mortar or concrete mix. The test involves hygro-mechanical interactions with intricate interplay of several phenomena, such as autogenous shrinkage, drying shrinkage, basic and drying creep, together with evolution of tensile strength and fracture energy. The benchmark described in this paper relies on extensive experimental data sets obtained through the extended Round Robin Testing programme (RRT+) of COST Action TU1404. Six teams took part with their simulation models. A series of outputs were produced, starting from mass loss of a prism through its axial deformation up to hoop stress/strain evolution in the ring. Four teams quantified also damage due to drying and strain concentrations. All models showed excellent performance on mass loss while strain validation showed higher scatter and influence of several factors. The benchmark demonstrated high capability of used models and emphasized strong role of calibration with regards to available experimental data.
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| 23. |
- Smilauer, Vit, et al.
(författare)
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Macroscopic hygro-mechanical modeling of restrained ring test - Results from COST TU1404 benchmark
- 2018
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Konferensbidrag (refereegranskat)abstract
- The restrained ring test under constant temperature is used for estimating cracking tendency of pastes, mortar or concrete. This test induces hygro-mechanical interactions, with intricate interplay of several phenomena such as autogenous shrinkage, drying shrinkage, basic and drying creep, as well as evolution of tensile strength and fracture energy. The benchmark described in this paper relies on extensive experimental data sets obtained through the extended Round Robin Testing programme (RRT+) of COST Action TU1404. Five teams took part with their simulation models. A series of outputs were produced, starting from mass loss of a prism through its axial deformation up to stress/strain evolution in the ring. Three teams quantified also damage due to drying and stress concentration around a ring’s notch. All models showed excellent performance on mass loss while strain validation showed higher scatter and influence of several other factors. The benchmark demonstrated high capability of used models and emphasized strong role of calibration with regards to available experimental data.
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| 24. |
- Spross, Johan, 1986-, et al.
(författare)
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Reliability-based alarm thresholds for structures analysed with the finite element method
- 2019
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Ingår i: Structural Safety. - : Elsevier. - 0167-4730 .- 1879-3355. ; 76, s. 174-183
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Tidskriftsartikel (refereegranskat)abstract
- Civil engineering structures are commonly monitored to assess their structural behaviour, using alarm thresholds to indicate when contingency actions are needed to improve safety. However, there is a need for guidelines on how to establish thresholds that ensure sufficient safety. This paper therefore proposes a general computational algorithm for establishment of reliability-based alarm thresholds for civil engineering structures. The algorithm is based on Subset simulation with independent-component Markov chain Monte Carlo simulation and applicable with both analytical structural models and finite element models. The reliability-based alarm thresholds can straightforwardly be used in the monitoring plans that are developed in the design phase of a construction project, in particular for sequentially loaded structures such as staged construction of embankments. With the reliability-based alarm thresholds, contingency actions will only be implemented when they are needed to satisfy the target probability of failure.
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| 25. |
- Åhs, Magnus, et al.
(författare)
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Application of a hygrothermal model to predict temperature and humidity development in the VeRCoRs benchmark case
- 2016
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Konferensbidrag (refereegranskat)abstract
- This study presents a multiphysics model developed to predict the internal temperature and moisture development in a concrete reactor containment at an early age. The work is a part of the benchmark study, VeRCoR, provided by EDF. The model was applied on an experimental reactor containment mock-up erected in the vicinity of Paris. The 3D geometry was provided from VeRCoRs. The model includes concrete hydration, heat release, chemical moisture binding, and a moisture transport model with relative humidity as a driving potential. Results from the simulation was compared with temperature sensors located in the mock-up. The model was found to be able to predict the temperature development at early age. There was no possibility to compare the relative humidity because of lack of humidity sensors.
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