| 1. |
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| 2. |
- Hellgren, Rikard, 1988-
(författare)
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Condition assessment of concrete dams in cold climate
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dams in many countries are approaching their expected service life. Proper assessment of the aging dams structural health increase the knowledge of the current safety, and allow for better planning of renovation and rebuilding investments. The behavior of concrete dams is, to a great extent, governed by the ambient variation in temperature and water level. In cold regions, the ice sheet formed in the reservoir may subject a pressure load on the dams. Theoretically, this load has a significant impact on the structural behavior of dams. Despite this, the maximum magnitude, as well as the seasonal variation of the ice load, constitute the most considerable uncertainty in the safety assessment of dams.This thesis presents research that examines how to model the expected behavior of dams in cold climate. The underlying problem is to predict the response of dams due to variation in the external conditions. Since the ice load is such a vital part of the external conditions in cold climate, the understanding and modeling of ice loads have been given extra attention. Models suitable to predict the long-term behavior of dams can be divided between theoretical, data-based, and hybrid. Prediction accuracy is essential to set alert thresholds, and in that regard, the data-based models are generally superior.The major contribution of this thesis is the design and installation of a prototype ice load panel with direct measurement of the ice pressure acting on a dam. The panel is attached on the upstream face of the dam and is large enough so that the whole thickness of the ice sheet is in contact with the panel. The predicted ice load from the best available model that includes loads from both thermal events and water level changes did not correspond to the measured ice loads. As there are no validated models or measurement methods for ice load on the dam, continued research is necessary, both through further measurements to increase knowledge and development of models.
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| 3. |
- Hellgren, Rikard, 1988-, et al.
(författare)
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Measurement of ice pressure on a concrete dam with a prototype ice load panel
- 2020
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Ingår i: Cold Regions Science and Technology. - : Elsevier. - 0165-232X .- 1872-7441. ; 170
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the development and installation of a prototype ice load panel and measurements of ice load from February 2016 to February 2018 at the Rätan hydropower dam in Sweden. The design of the 1 × 3 m2 panel enables direct measurement of ice pressure on the concrete surface is based on previous experience from similar measurements with sea ice. Important features of the design are sufficient height and width to reduce scale effects and to cover the ice thickness and variations in water level. The Rätan dam was chosen based on several criteria so that the ice load is considered to be reasonably idealized against the dam structure.For the three winters 2016, 2016/2017, 2017/2018, the maximum ice load recorded was 161 kN/m, 164 kN/m and 61 kN/m respectively. There were significant daily fluctuations during the cold winter months, and the daily peak ice loads showed a visual correlation with the daily average temperature and with the daily pattern of operation of the power station with its corresponding water level variations.
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| 4. |
- Hellgren, Rikard, 1988-, et al.
(författare)
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Measurement of ice pressure on a concrete dam with aprototype ice load panel
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents the development and installation of a prototype ice load panel andmeasurements of ice load from February 2016 to February 2018 at the Rätan hydropower damin Sweden. The design of the 1x3 m2 panel enabling direct measurement of ice pressure on theconcrete surface is based on previous experience from similar measurements with sea ice.Important features of the design are sufficient height and width to reduce scale effects and tocover the ice thickness and variations in water level. The Rätan dam was chosen based onseveral criteria so that the ice load is considered to be reasonably idealized against the damstructure.For the three winters 2016, 2016/2017, 2017/2018, the maximum ice load recorded was 161kN/m, 164 kN/m and 61 kN/m respectively. There were significant daily fluctuations duringthe cold winter months, and the daily peak ice loads showed a visual correlation with the dailyaverage temperature and with the daily pattern of operation of the power station with itscorresponding water level variations
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| 5. |
- Hellgren, Rikard, 1988-, et al.
(författare)
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Utvärdering av modeller för prediktion av islast mot dammar : RAPPORT 2019:631
- 2019
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- De kalla svenska vintrarna innebär att betongdammarna riskerar att utsättas för istryck. I Kraftföretagens riktlinjer för dammsäkerhet (RIDAS) anges ett dimensioneringsvärde som varierar mellan 50 – 200 kN/m från söder till norr. Särskilt vid låga dammar i norra Sverige så kan islasten utgöra en stor andel av den horisontella kraften som verkar på dammen. Trots islastens stora påverkan, är kunskapen om den faktiska islasten som verkar direkt mot dammen relativt låg.Inom det tidigare SVC projektet ”Lastförutsättningar avseende istryck” har en prototyp av en lastpanel utvecklats, vilken är avsedd för att mäta islaster som verkar på betongdammar. Panelen sitter sedan 2016 installerad på Rätans betongdamm och har sedan dess mätt islasten mot dammen.I denna rapport presenteras en utvärdering av olika prediktionsmodeller avsedda för att förutsäga istryck, där dessa prediktioner jämförs mot de uppmätta istrycken vid Rätan. För detta har två befintliga modeller använts:• En halvempirisk modell föreslagen av Comfort, et al. (2003). • En mekanisk modell där istryck modelleras utifrån spänningar i isen, där isen modelleras med Norton-krypning enligt Petrich, et al. (2015).Beräkningarna visar att de modeller som tillämpas i denna rapport ger signifikanta avvikelser jämfört med de islastmätningar som genomförts vid Rätan. Detta gäller både den maximala islasten och den relativa skillnaden mellan de dagliga islasttopparna som sker under säsongen. Det är utifrån dagens kunskapsläge inte möjligt att avgöra om orsaken till denna avvikelse mellan modeller och mätningarna beror på felaktigheter i modellerna eller mätningarna, eller om det är en kombination av båda. Det framgår dock tydligt vid utvärderingen att avvikelsen mellan dessa främst beror på den del av islasten som uppkommer till följd av vattenståndsvariationer.Tidigare studier visar att islaster orsakade av vattennivåvariationer är den mekanism som resulterar i de största islasterna mot dammar. Trots detta, så är denna effekt exkluderad i majoriteten av alla modeller för islaster. Det finns således ett tydligt gap mellan de mekanismer som observerats i mätningar och befintliga prediktionsmodeller. Modellen som utvecklats av Comfort beaktar inverkan från vattenståndsvariationer, men resultatet visar att just denna del av modellen
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| 6. |
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| 7. |
- Abbasiverki, Roghayeh, et al.
(författare)
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Nonlinear Behaviour of Concrete Buttress Dams under High-Frequency Excitations Taking into Account Topographical Amplifications
- 2021
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Ingår i: Shock and Vibration. - : Hindawi Limited. - 1070-9622 .- 1875-9203. ; 2021, s. 1-22
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Tidskriftsartikel (refereegranskat)abstract
- Concrete buttress dams could potentially be susceptible to high-frequency vibrations, especially in the cross-stream direction, due to their slender design. Previous studies have mainly focused on low-frequency vibrations in stream direction using a simplified foundation model with the massless method, which does not consider topographic amplifications. This paper therefore investigates the nonlinear behaviour of concrete buttress dams subjected to high-frequency excitations, considering cross-stream vibrations. For comparison, the effect of low-frequency excitations is also investigated. The influence of the irregular topography of the foundation surface on the amplification of seismic waves at the foundation surface and thus in the dam is considered by a rigorous method based on the domain-reduction method using the direct finite element method. The sensitivity of the calculated response of the dam to the free-field modelling approach is investigated by comparing the result with analyses using an analytical method based on one-dimensional wave propagation theory and a massless approach. Available deconvolution software is based on the one-dimensional shear wave propagation to transform the earthquake motion from the foundation surface to the corresponding input motion at depth. Here, a new deconvolution method for both shear and pressure wave propagation is developed based on an iterative time-domain procedure using a one-dimensional finite element column. The examples presented showed that topographic amplifications of high-frequency excitations have a significant impact on the response of this type of dam. Cross-stream vibrations reduced the safety of the dam due to the opening of the joints and the increasing stresses. The foundation modelling approach had a significant impact on the calculated response of the dam. The massless method produced unreliable results, especially for high-frequency excitations. The free-field modelling with the analytical method led to unreliable joint openings. It is therefore recommended to use an accurate approach for foundation modelling, especially in cases where nonlinearity is considered.
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| 8. |
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| 9. |
- Ansell, Anders, 1967-, et al.
(författare)
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Crack propagation in buttress dams : Application of non-linear models - Part II
- 2010
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The largest and most important concrete dams in Sweden are buttress dams. These consist of a large number of concrete monoliths formed by a front-plate with a supporting buttress. Cracks have been observed in some of these dams which in a long-term perspective may affect their safety. Concrete dams located in cool areas are often subjected to severe environmental impacts. Recent assessments and investigations of a buttress dam built 1954 in northern Sweden points out several types of cracks. The front-plate of the dam was not heat insulated on the downstream side when constructed, which has led to freeze-thawing damages in the plate. However, in 1994 a heat insulation wall that prevents ice-formation and protects the front-plate against frost damage was installed. It is located between two buttresses in the dam, from the rock up to the dam crest. The wall has most probably led to increased mechanical stresses in the pillars as a result of contraction and expansion due to seasonal temperature changes. A finite element model based on non-linear fracture mechanic, plasticity theory and damage mechanics has been utilized to study crack development in a buttress dam. The combined effects of restrained thermal displacements and loads caused by water were studied. The development of cracks due to seasonal temperature variations was simulated, especially with respect to the effect of the insulating wall installed some 40 years after the completion of the dam. The results show that the seasonal temperature variation causes high tensile stresses at different locations on the dam, and that the cracks can be initiated from at least four locations. Thermal stresses in combination with the load caused by water were shown to be the reason for cracking. The results point out that the addition of the insulating wall greatly contributed to the development of cracks in the buttress. A more suitable placement of the insulation wall could have prevented the cracking of the pillars.
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| 10. |
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| 11. |
- Ansell, Anders, 1967-, et al.
(författare)
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Verification of the Cause of the Cracks in a Buttress Dam
- 2009
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Ingår i: ICOLD 23rd CONGRESS PROCEEDINGS.
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Konferensbidrag (refereegranskat)abstract
- Buttress dams located in cool areas are often subjected to severe environmental impacts. Recent assessments and investigations of a buttress dam in northern Sweden points out several types of cracks. The dam was built 1954. The front plate was not heat insulated on the downstream side when constructed, which has led to freeze-thawing damages in the plate. However, in 1994 a heat insulation wall protecting the front plate was installed. It is located between each pillar in the dam, from the rock up to the crest and located approximately in the middle of the pillars. The insulation wall has most probably led to increased mechanical stresses in the pillars as a result of contraction and expansion due to seasonal temperature changes. A finite-element model based on non-linear fracture mechanic has been utilized to study crack development in the buttress dam. The results show that the seasonal temperature variation causes high tensile stresses at different locations on the dam, and that the cracks can be initiated from at least four locations on the dam. All types of cracks can propagate simultaneously. The analysis indicates that the addition of an insulation wall, which did not include the whole body of the pillars, has caused diagonal cracks in the pillars. The insulation wall prevents the ice-formation on the front plate and protects it against frost damage. However, a more suitable placement of the insulation wall could have prevented the cracking of the pillars.
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| 12. |
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| 13. |
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| 14. |
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| 15. |
- Enzell, Jonas, et al.
(författare)
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Full-scale test of the Dome Plug for KBS-3V deposition tunnels. Project summary and evaluation of the final results
- 2019
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Rapport (refereegranskat)abstract
- This is the final project report that summarizes the dome plug full-scale test (DomPlu) intended to evaluate the design concept of the deposition tunnel end plugs for the KBS-3V reference disposal system. The DomPlu full-scale test, was carried out at the Äspö Hard Rock Laboratory (Äspö HRL) and tested the plug system in a realistic environment with realistic and excessive loads. The current SKB reference design and the DomPlu design for a deposition tunnel end plugs are similar, except for a few experimentally related modifications. The DomPlu therefore represents a more detailed iteration of the basic design rather than a fundamental change to the earlier plug experiments undertaken by SKB. In this project, the behaviour of the plug has been evaluated when subjected to a constant water pressure load of 4 MPa for three years. After this, a gas tightness test was performed to evaluate the gas tightness of the plug. The last test was a strength test intended to test the load carrying capacity of the concrete dome during high pressure loads. Finally, the dome plug was demolished, and the included systems and materials were evaluated. To evaluate the dome plug, monitoring has been performed since construction where about 100 sensors were installed in the different material zones in the dome plug. Besides this, other types of measurements have also been conducted such as; leakage measurements, non-destructive test methods and a large amount of material tests on concrete and bentonite. As a complement to all these measurements, numerical analyses have also been performed to predict and to obtain greater understanding of the results. During the gas leakage test, the filter of the plug was drained to remove the water. After this, the filter was filled with helium and the gas pressure was continually recorded and a sniffer was used to track leakage downstream of the plug. The results from this test showed that the plug can be considered to satisfy the requirements of gas tightness. In the strength test, the water pressure was temporarily increased to 8.1 MPa, resulting in a total pressure about 10 MPa. The results showed that the concrete dome behaved as expected and some nonlinear deformations occurred. The deformations did not, however, result in significant cracking or damages within the concrete dome. The measurements of the leak tightness of the plug showed that the leakage varied between one and two litres per hour (17–33 ml/min). This should be considered as an upper limit of the expected leakage of the DomPlu design, since the natural pressure from the groundwater was lower than the artificial pressure applied in this full-scale test. In addition, the results showed that leakage to a great extent by-passed the bentonite seal through fractures in the rock. Therefore, it is of great importance to choose a plug location with limited fractures. The swelling pressure in the bentonite seal was lower than expected and varied between 0.2 and 1.9 MPa. The results also showed that the water content in the central parts was lower than expected. This has likely been a result of the leakage by-passing the seal in the rock. During demolition, great experience was obtained from material test sampling of concrete and bentonite. A larger waterfilled cavity was also detected in the top of the concrete dome, which had not been seen with the non-destructive test methods. Overall, the DomPlu full-scale test has showed that the plug design is robust and could sustain high water pressures and obtain quite low rates of leakage despite the fractures in the rock and the cavity in the top of the concrete dome. The full-scale test has also showed that it is possible to build and (if necessary) breach and demolish a plug.
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| 16. |
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| 17. |
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| 18. |
- Enzell, J., et al.
(författare)
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Non-linear Behavior of a Concrete Gravity Dam During Seismic Excitation : A Case Study of the Pine Flat Dam
- 2021
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Ingår i: Numerical Analysis of Dams. - Cham : Springer Nature. ; , s. 99-112
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Konferensbidrag (refereegranskat)abstract
- In this paper, seismic analyses of Pine Flat Concrete dam performed as part of theme A in the 15th benchmark workshop are presented. The results presented focuses on differences between mass and massless foundation and the influence from non-linear material behavior. The analyses performed with mass foundation using analytical free field input records and infinite boundary elements corresponded with the expected free surface results, for lower frequencies. For higher frequencies some discrepancies caused by the influence from the dam and the reservoir as expected. The corresponding analyses with massless foundation showed significantly higher accelerations but good agreement with the expected free surface displacement at the dam toe. To consider the influence from nonlinear material behavior, a dynamic push-over analysis (endurance time acceleration function, ETAF) was performed. It was possible to perform the analysis for the full duration of the record, despite significant non-linear material behavior. The initial damage occurred at the upstream toe and then showed significant induced damage as the level of excitation successively increased. In the end of the analysis, the top of the dam is cracked through which would cause an instability failure of the top of the dam.
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| 19. |
- Enzell, Jonas, et al.
(författare)
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Physical Model Tests of Concrete Buttress Dams with Failure Imposed by Hydrostatic Water Pressure
- 2023
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Ingår i: Water. - : MDPI. - 2073-4441. ; 15:20
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Tidskriftsartikel (refereegranskat)abstract
- Although the failure of a concrete dam is a complex and highly dynamic process, the current safety assessments of concrete gravity and buttress dams rely on a simplified 2D stability analysis, which neglects the load redistribution due to 3D monolith interactions and the valley shape. In addition, the estimation of breach parameters in concrete dams is based on assumptions rather than analyses, and better prediction methods are needed. Model tests have been conducted to increase the understanding of the failure behavior of concrete dams. A scale model buttress dam, with a scale of 1:15, consisting of 5 monoliths that were 1.2 m in height and 4 m in width, was constructed and loaded to failure using water pressure. The model dam had detachable abutment supports and shear keys to permit variations in the 3D behavior. The results showed that the shear transfer was large between the monoliths and that the failure of a single dam monolith is unlikely. A greater lateral restraint gives not only a higher failure load but also a better indication of impending failure. These findings suggest that the entire dam, including its boundary conditions, should be considered during a stability assessment. The results also suggest that the common assumption in dam safety codes that a single monolith fails during flooding analysis is not conservative. The dataset obtained provides a foundation for the future development of dam-monitoring alarm limits and for predictive models of dam-breaching processes.
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| 20. |
- Enzell, Jonas, et al.
(författare)
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Post-peak behavior of concrete dams based on nonlinear finite element analyses
- 2021
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Ingår i: Engineering Failure Analysis. - : Elsevier. - 1350-6307 .- 1873-1961. ; 130
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Tidskriftsartikel (refereegranskat)abstract
- Dam failures are catastrophic events and in order to improve safety, engineers must have good tools for analysis and an understanding of the failure process. Since there are few cases of real failures in concrete dams, which can work as validation, physical model tests are a good way of improving numerical models and the understanding of the failure process. In this article, a physical model test of the buttress from a concrete Ambursen type dam is used as a benchmark for calibrating a FE-model. The dam failure is thereafter simulated using the concept of safety commonly used in the design codes. The advantages and drawbacks of performing load- and displacement-controlled simulations are compared. A new method for performing displacement-controlled simulations, using nonlinear springs to introduce the hydrostatic pressure and ice load is thereafter suggested and tested. The proposed method gives results which corresponds to the classical methods of analysis but has some advantages. Primarily, the new method is stable and does not suffer from convergence issues as was the case with the other methods. It is also simple to introduce in most commercial software compared to classical displacement-controlled simulations.
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| 21. |
- Enzell, Jonas, et al.
(författare)
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Predicting the Influence of Seasonal Thermally Induced Cracking on a Reinforced Concrete Arch Dam
- 2022
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Ingår i: KSCE Journal of Civil Engineering. - : Springer Nature. - 1226-7988 .- 1976-3808.
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Tidskriftsartikel (refereegranskat)abstract
- Many of the world’s concrete dams are approaching the end of their expected service life and many of these dams are cracked due to environmental effects. It is thus desirable to be able to extend the service life of many of these dams. It is therefore important to develop reliable methods of analysis to assess the current status of these dams and to determine the safety of these, partly damaged structures. A slender reinforced concrete arch dam is used as a case study with the aim of predicting the structural response, the cracks found in-situ and of assessing the safety of the dam. This dam has cracked extensively along the downstream face primarily due to seasonal temperature variations. A detailed finite element model has been developed to simulate the history of the dam with the variations in ambient conditions that have occurred over its lifetime. The results show good agreement regarding both the crack pattern and displacement of the dam. A procedure to simulate a progressive dam failure, starting from the current state, is utilized to assess the current level of safety. The results show that the cracking has a limited effect on the safety factor of the dam.
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| 22. |
- Eriksson, Daniel, 1987-, et al.
(författare)
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Assessment of frost damage in hydraulic structures using a hygro-thermo-mechanical multiphase model
- 2019
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Ingår i: Sustainable and Safe Dams Around the World. ; , s. 332-346
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Konferensbidrag (refereegranskat)abstract
- This paper presents an extension of a novel hygro-thermo-mechanical multiphase model for simulation of freezing of partially saturated air-entrained concrete on the structural scale to account for the effect of damage in the material. The model is applied in an example which investigates the extent and severity of frost damage caused by extremely cold climate conditions in a typical concrete wall in a waterway constructed with air-entrained concrete. The results were concluded to comply with observations made in experimental work and testing of freezing air-entrained concrete under exposure conditions similar to those in hydraulic structures. Furthermore, the results indicate that the effect of short periods of time with high rates of freezing was rather small on the obtained damage. Additionally, increasing the depth of the boundary region with an initially high degree of water saturation on the upstream side had also a rather small effect on the damaged zone.
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| 23. |
- 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
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Even though air-entrained concrete is usually used for concrete structures located in cold climates that are exposed to wet environments, frost damages are frequently detected during inspections. However, it is often hard to assess the extent and severity of these damages 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. The results show that the model can reproduce the freezing behaviour observed in the experimental tests on a structural scale as well as being capable of describing freezing induced deformations caused by non-uniform moisture distributions.
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| 24. |
- 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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| 25. |
- Eriksson, Daniel, 1987-, et al.
(författare)
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Hygro-thermo-mechanical modeling of partially saturated air-entrained concrete containing dissolved salt and exposed to freeze-thaw cycles
- 2021
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Ingår i: Cement and Concrete Research. - : Elsevier BV. - 0008-8846 .- 1873-3948. ; 141
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Tidskriftsartikel (refereegranskat)abstract
- In cold regions, understanding the freeze-thaw behavior of air-entrained concrete is important for designing durable structures and assessing the remaining service life of existing structures. This study presents a hygro-thermo-mechanical multiphase model that describes the cyclic freeze-thaw behavior of partially saturated air-entrained concrete containing dissolved salt. An equilibrium and a non-equilibrium approach are adopted to model the ice formation, including the freeze-thaw hysteresis, inside the porous network. The model also considers the diffusive and convective transport of the dissolved salt coupled to the freeze-thaw processes. Two examples are presented to verify and highlight the capabilities of the model. The first example shows that the model is capable of reproducing the experimentally observed mechanical response of specimens containing NaC1-solutions of different concentrations. In the second example, a larger absorption of liquid from an external reservoir is obtained with an increasing salt concentration in the reservoir, which is consistent with experimental observations.
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| 26. |
- Eriksson, Daniel, 1987-, et al.
(författare)
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Influence of calcium leaching on ice formation in air-entrained concrete: Accelerated experiments and hygro-thermo-mechanical modelling
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Concrete structures in cold regions exposed to long-term contact with soft water often suffer from damage caused by combined calcium leaching and frost actions. This paper experimentally investigates the influence of leaching on the formation and melting of ice inside the pore space of air-entrained concrete. For reference, a non-air-entrained concrete is also tested. An electrochemical accelerated leaching method is adopted to degrade the concrete, and low-temperature calorimetry is used to measure the ice formation. Furthermore, utilising the experimental data, the mechanical response of leached specimens during freeze-thaw exposure is theoretically studied using a hygro-thermo-mechanical multiphase model to explore their coupled effect. The results show a substantial increase in freezable water content that phase changes at relatively small temperature depressions, where the largest increase occurs in air-entrained concrete. The simulation results indicate that the critical degree of saturation becomes lower in leached concrete, hence increasing its susceptibility to frost damage.
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| 27. |
- Eriksson, Daniel, 1987-
(författare)
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Multiphase models for freeze-thaw actions and mass transport in concrete hydraulic structures
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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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| 28. |
- Eriksson, Daniel, 1987-
(författare)
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Numerical models for degradation of concrete in hydraulic structures due to long-term contact with water
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The durability of concrete is of major concern in all types of concrete structures where the combined effect of exposure conditions and the type and quality of the concrete material usually determines the rate of degradation. Furthermore, there are synergy effects between different deterioration mechanisms, which means that the combined rate of degradation is higher than the sum of the individual rates of each mechanism. Therefore, to accurately predict the residual service life of existing structures or when designing new structures, it is essential to consider all these aspects. This means that various chemical and physical processes, as well as how these interact, must be taken into account in models aiming to be used for service life predictions.This thesis presents the first part of a research project with the aim to investigate common deterioration mechanisms of concrete in hydraulic structures, and to improve the knowledge how these and other related phenomena can be described using mathematical models. The objective is also to study how different mechanisms interact and to find suitable approaches to account for these interactions in the models. To this end, a literature survey on commonly detected damage in hydraulic structures is presented. In addition, it also addresses in what types of and where in hydraulic structures the various damage types are usually observed. The mathematical models presented in this part of the project are focused on long-term water absorption in air-entrained concrete as well as on freezing of partially saturated air-entrained concrete. Both models are based on a multiphase description of concrete and poromechanics to describe the coupled hygro-thermo-mechanical behaviour. The thesis also presents some of the basic concepts of multiphase modelling of porous media, including discretization of the models using the finite element method (FEM). Furthermore, it covers the simplifications that are usually introduced in the general macroscopic balance equations for mass, energy and linear momentum when modelling cement-based materials.To verify the developed models and to show their capabilities, simulation results are compared with experimental data, in situ measurements and other simulations from the literature. The results indicate that both models perform well and can be used to predict long-term moisture conditions in hydraulic structures as well as freezing-induced strains in partially saturated air-entrained concrete, respectively. Even though no interactions with other deterioration mechanisms are included in the models, the development and use of these have given insights to which parameters that are important to consider in such extensions. Furthermore, based on the insights gained, the complexity of describing the full interactions between several mechanisms in mathematical models is also discussed. It is concluded that models aiming to be used for service life predictions of hydraulic structures in day-to-day engineering work need to be simplified. However, the type of advanced models presented in this thesis can serve as a basis to study which aspects and parameters that are essential to consider in simplified prediction models.
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| 29. |
- Eriksson, Daniel, 1987-, et al.
(författare)
-
Simulering av frostsprängning i betong
- 2018
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Ingår i: Bygg och Teknik. - Stockholm. - 0281-658X .- 2002-8350. ; :7, s. 19-22
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Betongkonstruktioner belägna i kalla och fuktiga miljöer riskerar att skadas av frostsprängning under de kyliga vintermånaderna på grundav vattnets nioprocentiga volymsexpansion då det fryser inuti betongen. För att begränsa denna problematik är det vanligt att till exempel dammar, broar och betongvägar byggs med lufttillsatt betong i både Sverige och andra delar av världen. Trots detta observerasfrostskador frekvent i sådana konstruktioner. Det är därför viktigtatt kunna uppskatta när i tiden frostskador förväntas uppkomma i en konstruktion samt kunna bedöma omfattningen av en redan uppkommen skada. För att göra detta, utvecklades numeriska modeller i ett pågående doktorandprojekt vid KTH, finansierat av Svenskt Vattenkraftscentrum (SVC). Doktorandprojektet har som mål att bidra till en förbättring av dessa typer av bedömningar genom att beskriva de komplexa processer som är förknippade med frysning i lufttillsatt betong.
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| 30. |
- Gasch, Tobias, et al.
(författare)
-
Instrumentation and Modelling of a Reactor Containment Building
- 2018
-
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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| 31. |
- Gasch, Tobias
(författare)
-
Multiphysical analysis methods to predict the ageing and durability of concrete
- 2019
-
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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| 32. |
- Gasch, Tobias, et al.
(författare)
-
Three-dimensional simulations of ageing concrete structures using a multiphase model formulation
-
Annan publikation (övrigt vetenskapligt/konstnärligt)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. (2019) 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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| 33. |
- Gasch, Tobias, et al.
(författare)
-
Three-dimensional simulations of ageing concrete structures using a multiphase model formulation
- 2019
-
Ingår i: Materials and Structures. - : Springer Netherlands. - 1359-5997 .- 1871-6873. ; 52:4
-
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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| 34. |
- Goldgruber, Markus, et al.
(författare)
-
Nonlinear Seismic Simulation of an Arch Dam using XFEM
- 2014
-
Konferensbidrag (refereegranskat)abstract
- The “Committee on Computational Aspects of Analysis and Design of Dams” in International Commission on Large Dams (ICOLD) is responsible for organizing benchmark workshops every second year, where the 12th benchmark workshop was held in the city of Graz back in October 2013. One of the three topics in this benchmark workshop was titled “Fluid Structure Interaction, Arch Dam - Reservoir at Seismic Loading”, formulated by the Institute of Hydraulic Engineering and Water Resources Management from Graz University of Technology. The focus of this topic was on the interaction between the reservoir and the dam, and thereby studies the influences of different approaches to simulate the reservoir. To keep the results comparable between the 13 participants the simulations had to be performed just linear. However, structures like arch dams undergo nonlinear behavior. Due to the massive amount of concrete, such structures are divided into almost independent vertical blocks acting like cantilevers. Furthermore, the hydrostatic water pressure can lead to openings in the contact plane between dam and the foundation. A cooperation between Graz University of Technology and KTH Royal Institute of Technology led to the idea to investigate the structures behavior by taking into account the contacts ( block joints, abutment) and cracking (tensile failure), due to the seismic acceleration by using XFEM. The results are pointing out the possibilities and borders of such complex nonlinear simulations.
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| 35. |
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| 36. |
- Hansson, Håkan, 1969-, et al.
(författare)
-
Initial study of oblique hard target projectile impact of normal and high strength concrete targets
- 2011
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Ingår i: Nordic concrete research: Research projects 2011. - 9788282080255 ; , s. 63-66
-
Konferensbidrag (refereegranskat)abstract
- The ability to predict penetration resistance in concrete is necessary to evaluate the vulnerability of protective designs for impacts by penetrating weapons, or deformable projectiles. The paper presents experimental work regarding oblique projectile impact of both normal strength and high performance concrete targets with modern type of hard target penetrators. Furthermore, finite element (FE) analyses of non-normal projectile impacts of the normal strength concrete targets are presented, and its limitations discussed.
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| 37. |
- Hansson, Håkan, 1969-, et al.
(författare)
-
Non-linear Finite Element Analysis of Deep Penetration in Unreinforced and Reinforced Concrete
- 2011
-
Ingår i: Nordic Concrete Research. - 0800-6377. ; 44, s. 87-107
-
Tidskriftsartikel (refereegranskat)abstract
- Penetration and perforation of concrete targets are studied by the use of numerical simulations to enhance the understanding of the penetration phenomenon. Comparisons were made with test results obtained for both reinforced and unreinforced 48.0 MPa normal strength concrete. The studied projectiles were made as generic models of penetrators for buried hardened target defeat. Varying impact velocities and angles for the penetrators were investigated. The simulations gave reasonable results for the different simulation cases, with the best results were obtained for reinforced concrete targets.
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| 38. |
- Hassanzadeh, Manouchehr, et al.
(författare)
-
Analysis of Displacements and Crack Formations in Foundations for Hydropower Generators
- 2012
-
Konferensbidrag (refereegranskat)abstract
- A rather extensive program for improvement of the Swedish hydropower plants is ongoing. The aims are to secure future production and to maintain and further develop an already high dam safety. In connection with earlier work, which dealt with assessment of an existing buttress dam where a non-linear finite element model was applied to determine the cause of the observed cracks. The results showed that the non-linear finite element method is a powerful tool to determine the structural behaviour of large concrete structures. The study in this paper is a continuation of the previous project, aiming at applying the method to other parts of dam structure such as foundation supporting the generator (stator and rotor), rotor spider, turbine shaft, spiral casing, turbine and draft tube.The hydropower plant, which is studied, was constructed in the early forties. During the inspection, structural damages (cracks) were discovered around some of the stator and rotor spider supports. The cracks were believed to be related to the function of the stator supports and to new patterns of generator operation. In earlier times, the generators ran continuously, while nowadays there are many stops and starts, some times even several times during one day. The purpose of this study is to illuminate the complex stress conditions in the generator foundations of a hydropower plant and to reveal the causes of the stresses and to verify their role in formation of the cracks.The structural behaviour of a foundation has been analysed taking into account the transient thermal gradients in combination with dead loads and some of the operational loads imposed to the foundation. A three dimensional non-linear finite element model has been applied in order to analyse formation and propagation of the cracks. The analyses showed that based on the assumption made, the concrete foundation cracks mainly on the outside but also near some of the stator supports due to the combination of mechanical and thermal loads. However, the studied loads cannot explain all of the types of damages that can be found in-situ. It is likely that especially the drying shrinkage may be the one of the reasons for the cracks that has been found near the stator supports and especially the rotor spider supports.It is important from a dam safety perspective to determine the causes of the structural cracks that have been found in-situ and also to evaluate the effect of the reduced stiffness due to cracking, since a reduced structural stiffness can result in larger loads imposed on the structure from the magnetic eccentricity and turbine imperfections or alternatively lead to a fatigue failure of for instance the reinforcement.
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| 39. |
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| 40. |
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| 41. |
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| 42. |
- Hellgren, Rikard, 1988-, et al.
(författare)
-
A parametric numerical study of factors influencing the thermal icepressure along a dam
- 2020
-
Ingår i: Proceedings of the 25th IAHR International Symposium on Ice (Trondheim, 2020). - Trondheim.
-
Konferensbidrag (refereegranskat)abstract
- The uncertainty regarding size and variation of the ice pressure constitute one of the mainsignificant sources of concern for condition assessment of concrete dams in cold regions. Inmost current dam safety guidelines, the design ice load is determined solely from thegeographic location of the dam. However, factors such as variation in temperature and waterlevel, the slope of the banks, wind etc. may also influence the size of the load. Previousmeasurements indicate that the pressure on a dam varies along the dam line during the sametime and that the average ice pressure decreases as the area of the ice-structure interface isincreased. This paper presents numerical studies on how the ice pressure varies along the damwall. A finite element model that includes several dam monoliths, the ice and the beaches ofthe reservoirs are used to simulate thermal ice loads. The pure elastic load caused by therestrained expansion of an ice sheet subjected to a change in thermal gradient are greater thanthe design ice load in current dam safety guidelines and the loads measured. The simulationsshow that the ice load vary significantly along the dam, and the variation in cross-sectionstiffness along the dam greatly influence the magnitude of the total ice pressure. Theparameter study shows that the elastic thermal ice load increases with an increased slope ofthe banks, reservoir length, and ice thickness and decreases as the angle of the connectionbetween the dam and bank increases. However, the difference in ice load between theindividual monoliths with the same geometry and temperature change are in several cases ofthe same magnitude as the variation in the external factors.
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| 43. |
- Hellgren, Rikard, 1988-
(författare)
-
A systems approach to ice loads on concrete dams
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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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| 44. |
- Hellgren, Rikard, 1988-, et al.
(författare)
-
Estimating the effect of ice load on a concrete dams displacement with regression models
- 2017
-
Ingår i: <em>Proceedings ICOLD 2017 International Symposium</em>. - Prag.
-
Konferensbidrag (refereegranskat)abstract
- In colder climates, the ice load is an important factor to consider when assessing the dam safety. The size of the ice load is associated with uncertainties, regarding both its maximum size and seasonal variation. There is today no reliable method to determine the ice load acting on a dam, since available measurements and theories results in a large scatter regarding the maximum ice loads.In this project, the possibility to determine the ice load based on traditional measurements of a dam behavior is investigated. It’s evaluated if the ice thickness is a significant predictor for the displacement of a Swedish arch dam. A case study on inverted pendulums was performed where dynamic regression models were used to calculate the displacement of the dam.The model showed significant results which could explain the displacements. The ice thickness shows an effect on a 5 % significance level. The results indicate that the seasonal ice load is large enough to be traceable, and could be determined from conventional pendulum measurements. Since pendulums are installed on a large number of dams, a method that estimates the ice load from those measurements could vastly reduce uncertainties regarding ice loads on dams.
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| 45. |
- Hellgren, Rikard, 1988-, et al.
(författare)
-
Estimating the ice loads on concrete dams based on their structural response
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In the assessment of concrete dams in cold climate, it is common that the theoretical stability becomes insufficient for load cases that includes ice loads. However, the magnitude and return period of these ice loads have a high degree of uncertainty. This study estimates the magnitude of ice loads on eight concrete dam monoliths using measurements of their displacement from 29 winters. In the displacement signals, events are identified and assumed to be caused solely by ice loads. The observed displacement during an event is interpreted as an ice load using a load-displacement relationship derived from FE simulations of each dam. These simulations show that ice loads of the magnitudes given in design guidelines and recorded in previous measurements would significantly affect the structural response of the studied dams. However, only small traces of ice loads can be found in the observed responses of the studied dams. The estimated ice loads are significantly lower than the ice loads recorded in traditional ice load measurements. These results indicate that the average magnitude of ice load on an entire monolith is significantly lower than the measured local pressures. This would imply that ice loads may be a smaller concern regarding the dam safety than previously believed.
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| 46. |
- Hellgren, Rikard, 1988-, et al.
(författare)
-
Estimating the Ice Loads on Concrete Dams Based on Their Structural Response
- 2022
-
Ingår i: Water. - : MDPI AG. - 2073-4441. ; 14:4
-
Tidskriftsartikel (refereegranskat)abstract
- In the assessment of concrete dams in cold climate, it is common that the theoretical stability becomes insufficient for load cases that include ice loads. However, the magnitude and return period of these ice loads have a high degree of uncertainty. This study estimates the magnitude of ice loads on eight concrete dam monoliths using measurements of their displacement from 29 winters. In the displacement signals, events are identified and assumed to be caused solely by ice loads. The observed displacement during an event is interpreted as an ice load using a load-displacement relationship derived from FE simulations of each dam. These simulations show that ice loads of the magnitudes given in design guidelines and recorded in previous measurements would significantly affect the structural response of the studied dams. However, only small traces of ice loads can be found in the observed responses of the studied dams. The estimated ice loads are significantly lower than the ice loads recorded in traditional ice load measurements. These results indicate that the average magnitude of ice load on an entire monolith is significantly lower than the measured local pressures. This would imply that ice loads may be a smaller concern regarding dam safety than previously believed.
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| 47. |
- Hellgren, Rikard, 1988-, et al.
(författare)
-
Ice load measurements on Rätan concrete dam using different sensor types
- 2021
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Ingår i: Cold Regions Science and Technology. - : Elsevier BV. - 0165-232X .- 1872-7441. ; , s. 103425-103425
-
Tidskriftsartikel (refereegranskat)abstract
- Concrete dams in cold regions are designed to withstand loads from the ice sheet on top of the reservoir. However, the ice load's magnitude and return period are among the most considerable uncertainties in safety assessments of concrete dams. In a previous study, the development and installation of a 1 × 3 m2 prototype ice load panel attached at the upstream face of a concrete dam was presented. The panel is large enough for the ice sheet's cross-section to remain in contact with the panel as the water level varies, and it measures the total ice load without interpolation. This paper presents measurement results from the load panel from winters 2018–19 and 2019–20, an update to the measurement design, and additional ice pressure measurements with traditional stress cells. The panel measured seasonal maximum ice loads of 100 and 200 kN/m for the two winters, respectively. Winter 2019–20, when the panel measured the largest loads, was mild for the location, with great ice thickness near the dam face (1.2 m) and an almost snow-free ice sheet throughout the winter. Two 2.75 × 1.75 m2 dummy panels were installed adjacent to the load panel prior to the winter 2019–20 to minimize the load panel's protruding effect. These panels significantly reduced the local impact, as evident by the crack-pattern of the ice sheet near the load panel. The load panel recorded large ice loads (>75 kN/m) for all combinations with increasing/decreasing air temperature and/or water level. Identification of temperature change events and water level change events during the winters, shows that a change in air temperature, water level, or any combination of these, is not sufficient alone to explain large ice loads at Rätan dam. These findings suggest that other conditions must be satisfied before a water level or temperature change results in large ice loads. In February 2020, three panels consisting of a steel frame with four stress cells on each were placed on the dummy panels’ upstream face, and one single stress cell was placed 6 m out in the reservoir in front of the load panel. The majority of the stress cells recorded ice pressure larger than their measurement range. At the end of the ice season, only two of the panels’ twelve stress cells were still functional, and the ice vastly deformed the steel frames. From the period before the frames were damaged and unrelated to the choice of interpolation method, the recordings by the three stress cell panels at the dam are among the historically largest inferred ice loads on dams.
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| 48. |
- Hellgren, Rikard, 1988-, et al.
(författare)
-
Modeller för övervakning av betongdammar : Energiforskrapport 2019:580
- 2019
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Inom dammövervakning används begreppen larm och varning för meddelanden om upptäckten av en potentiell dammsäkerhetsrisk. Larm är kopplade till ett akut farligt beteende och en varning till ett oförväntat beteende. Dammövervakning är således ett klassificeringsproblem där varje nytt uppmätt beteende ska klassificeras som säkert eller osäkert, alternativt som förväntat eller oförväntat. Ett farligt beteende innebär att dammsäkerheten är hotad akut medan ett oförväntat värde uppstår när dammen inte beter sig enligt förväntan utifrån rådande yttre förhållanden. En sådan avvikelse betyder inte per automatik att något är fel eller att säkerheten är hotad men en oskadad damm bör bete sig enligt ett förväntat mönster och en avvikelse från det förväntade beteende kan indikera en initiering av en skada hos dammen.För att bestämma det förväntade mätvärdet behövs någon typ av prediktionsmodell som förutsäger dammens beteende utifrån de yttre förhållandena. I denna rapport delas prediktionsmodellerna upp i teoribaserade och databaserade modeller. De teoribaserade modellerna bygger på en fysikalisk koppling mellan yttre förhållanden så som vattennivå och temperaturer och dammens beteende. I de databaserade modellerna är denna koppling istället empirisk och utan fysikalisk koppling.Det finns flertalet olika typer av instrument och givare för dammövervakning. I denna rapport sammanfattas vanliga typer avsedda för dammätningar, inklusive dess mätnoggrannhet. Dessutom diskuteras användningen av olika givare baserat på om de lämpar sig som detektorer vid ett potentiellt dammbrott eller om givarna främst är att anse som stödinstrumentering vars syfte är att bidra mer information om dammens beteende eller yttre förhållanden.Varje modelltyp har unika egenskaper med för- och nackdelar. Det är därför viktigt att välja modelltyp utifrån behov och syfte. En teoribaserad modell, t.ex. FE-modell, ger möjligheten att tolka resultaten och hitta fysikaliska samband, men är sämre på att förutsäga exakt beteende. De mest avancerade databaserade modellerna är utan fysikalisk mening, men kan förutsäga framtida beteenden väldigt bra. Att välja den ena modelltypen framför den andra innebär, något tillspetsat, ett val mellan förståelse och prestation. Lyckligtvis utesluter inte användandet av en modelltyp en annan. De fysikbaserade modellerna som hjälper oss att förstå och tolka en damms beteende kommer alltid ha en plats i dammsäkerhetsarbetet. Det är dessutom den modelltyp som bäst kan användas för att studera dammbrottsscenarion och därmed användas för definition av eventuella larmnivåer. Som ett komplement till dessa kan de databaserade modellerna användas för att ge tidiga indikationer när dammens beteende skiljer sig från det förväntade och lämpar sig därmed väl för val av varningsnivåer. De databaserade metoderna ger tidigare indikationer på eventuella avvikelser eftersom de generellt presterar bättre med mindre spridningsmått än motsvarande teoribaserade modeller.
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| 49. |
- Hellgren, Rikard, 1988-, et al.
(författare)
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Modelling of the ice load on a Swedish concrete dam using semi-empirical models based on Canadian ice load measurements
- 2019
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Ingår i: Sustainable and Safe Dams Around the World. ; , s. 3068-3080
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Konferensbidrag (refereegranskat)abstract
- In cold regions where the water surface of a river or lake freezes during the winter, concrete dams may be subjected to a pressure load from the ice sheet. This pressure load may constitute a large portion of the total horizontal load acting on a small dam. From a dam safety perspective, it is important to determine the design value of the ice load. In February 2016, a prototype of an ice load panel was installed on a Swedish concrete dam. The 1x3m2 panel measures the ice pressure with three load cells. In this paper, the ice load measured on the Swedish dam is predicted using a Canadian empirical model, previously developed from a 9-year field program to estimate the ice loads caused by thermal effects and variation in water level. The predictions from the model could not accurately predict the measured ice loads. Since the current understanding of ice load is limited, it is not possible to determine whether the measurement, the model or both are inaccurate.
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| 50. |
- Hellgren, Rikard, 1988-, et al.
(författare)
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Performance of data-based models for early detection of damage in concrete dams
- 2020
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Ingår i: Structure and Infrastructure Engineering. - : Taylor and Francis Ltd.. - 1573-2479 .- 1744-8980.
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Tidskriftsartikel (refereegranskat)abstract
- A failure of a massive concrete dam could cause catastrophic consequences. The purpose of monitoring is to detect anomalies and damage at an early stage to prevent failure. Data-based models for anomaly detection are based on the hypothesis that the behaviour of an undamaged dam will follow an expected pattern, and deviation from this pattern is an indication of damage. In this study, simulations were used to create time series for an undamaged dam and three different damage scenarios at three different locations in the dam body. Three common data-based models were used to predict a dams crest displacements, both on the generated artificial data and the corresponding measurements from the dam. Prediction bands for future measurements were created, and the ten time-series were used to test the ability to detect damage. All models could detect instantaneous damage but struggle to detect progressive damage; the Neural network outperforms the two regression models. The choice of the mathematically optimal threshold limit leads to a large number of false alerts. Requiring three consecutive values outside the threshold before an alert is issued, increases the possibility to receive an early alert compared to the standard approach where observations are classified individually.
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