| 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. |
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| 4. |
- Abbasiverki, Roghayeh, et al.
(författare)
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Analysis of shallowly buried reinforced concrete pipelines subjected to earthquake loads
- 2014
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Ingår i: Nordic Concrete Research. - 0800-6377. ; :51, s. 111-130
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Tidskriftsartikel (refereegranskat)abstract
- Buried reinforced concrete pipelines are widelyused in e.g. water and wastewater systems. Failure of these infrastructures mayresult in drastic effects and recently they have been brought into focus asvital components in safety systems for nuclear power installations. The highlevel of safety has here lead to a demand for reliable earthquake risk analyses.In this paper, methods are compared and the use of seismic design loadsdemonstrated. FE analysis in 2D of soil-pipe interaction under seismic wavepropagation is performed. The performance of concrete pipes subjected toseismic waves with different frequency content is evaluated with respect todifferent soil condition but also water mass effect.
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| 5. |
- Abbasiverki, Roghayeh
(författare)
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Analysis of underground concrete pipelines subjected to seismic high-frequency loads
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Buried pipelines are tubular structures that are used for transportation of important liquid materials and gas in order to provide safety for human life. During an earthquake, imposed loads from soil deformations on concrete pipelines may cause severe damages, possibly causing disturbance in vital systems, such as cooling of nuclear power facilities. The high level of safety has caused a demand for reliable seismic analyses, also for structures built in the regions that have not traditionally been considered as highly seismically active. The focus in this study is on areas with seismic and geological conditions corresponding to those in Sweden and Northern Europe. Earthquakes in Sweden for regions with hard rock dominated by high-frequency ground vibrations, Propagation of such high-frequency waves through the rock mass and soil medium affect underground structures such as pipelines.The aim of this project is investigating parameters that affect response of buried pipelines due to high-frequency seismic excitations. The main focus of the study is on reinforced concrete pipelines. Steel pipelines are also studied for comparison purposes. The effects of water mass, burial depth, soil layer thickness and non-uniform ground thickness caused by inclined bedrock are studied. The results are compared to those obtained for low-frequency earthquakes and the relationship between strong ground motion parameters and pipelines response is investigated. It is shown that, especially for high frequency earthquake excitations, non-uniform ground thickness due to inclined bedrock significantly increase stresses in the pipelines. For the conditions studied, it is clear that high-frequency seismic excitation is less likely to cause damage to buried concrete pipelines. However, the main conclusion is that seismic analysis is motivated also for pipelines in high-frequency earthquake areas since local variation in the ground conditions can have a significant effect on the safety.
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| 6. |
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| 7. |
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| 8. |
- 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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| 9. |
- Abbasiverki, Roghayeh
(författare)
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Numerical modelling considerations for analysis of concrete hydraulic structures subjected to high-frequency seismic loads
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Concrete hydraulic structures are of great importance in today's society. When situated in areas with hard bedrock, these structures may become extra vulnerable to seismic excitations as these here are dominated by high-frequency vibrations which can have disastrous consequences for slender structures. The aim of this thesis was to investigate special considerations that must be made when conducting analyses of such hydraulic structures during high-frequency excitations. Underground and on the ground structures were investigated separately. Underground concrete pipelines and concrete buttress dams were selected for the study because their behaviour when exposed to seismic excitations is dominated by their stiffness. The most effective models and modelling methods for the seismic analyses of such structures were implemented and evaluated. Two-dimensional finite element (FE) models were developed for the dynamic analysis of underground concrete pipelines loaded by seismic waves propagating from bedrock through soil. The interaction between the bedrock and the surrounding soil was investigated with respect to rock geometry and soil properties. The surface of dam foundations is commonly irregular, resulting in nonuniform motions at the dam-foundation interface. The free-field modelling methods for concrete dam foundations were adapted in order to accurately describe the propagation of earthquake vibrations from the source to the ground surface. The implementation of a threedimensional FE model for concrete buttress dams was investigated. Two different methods for free-field modelling are presented, which can be implemented independently of the software used. The seismic loads are applied as effective earthquake forces at non-reflecting boundaries. In the first method, the free-field motions at the non-reflecting boundaries are determined by the so-called domain reduction method using the direct FE calculation. In the second method, the free-field motions are analytically determined based on the onedimensional wave propagation theory. The results are also compared with the massless foundation modelling approach, in which the topographical amplifications are neglected. It was demonstrated that a two-dimensional model can effectively account for pipeline behaviour. The most important aspect of the models is the ability to capture bending deformations, as segmented structures such as pipelines are vulnerable in this respect. Nonuniform bedrock reduces the safety of concrete pipeline, especially because of bending deformations in the pipe and joints. The massless method gave unreliable results for analyses of dams, especially for high-frequency excitations. The analytical method was also unreliable in estimating the non-linear behaviour of the dams. But, a new time domain deconvolution method was developed to transform the earthquake motion from the foundation surface to the corresponding input motion at depth. It wasfound that free-field modelling of foundations using the direct FE method can accurately capture the topographic amplifications of the seismic excitations. It was shown that a three-dimensional model is required for seismic evaluation of concrete buttress dams. The topographic amplification of high-frequency waves at the surface of canyons had a significant effect on the response of this type of dam.
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| 10. |
- Abbasiverki, Roghayeh, et al.
(författare)
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Seismic response of buried concrete pipelines subjected to highfrequency earthquakes
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Ingår i: Geotechnical and Geological Engineering. - 0960-3182 .- 1573-1529.
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Tidskriftsartikel (refereegranskat)abstract
- Buried pipelines are tubular structures that cross large areas with different geological conditions. During an earthquake, imposed loads from soil deformations on concrete pipelines may cause severe damages. In this study, the use of two-dimensional finite element models of pipelines and surrounding soil for simulation of seismic waves that propagate from the bedrock through the soil are demonstrated. The models describe both longitudinal and transverse cross-sections of pipelines and the soil-pipe interaction is modelled as a nonlinear behaviour. The effects of uniform ground with different burial depths, soil layer thickness, soil stiffness and bedrock geometry on the seismic response of reinforced concrete pipelines is studied. Two earthquakes, with high and low frequency contents, are employed for the dynamic analysis. The results show that there is a much smaller risk of damage from high-frequency earthquakes, but that there is a significant effect on the response due to possible irregular ground with inclined bedrock.
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| 11. |
- Abbasiverki, Roghayeh, et al.
(författare)
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Seismic response of large diameter buried concrete pipelines subjected to high frequency earthquake excitations
- 2020
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Ingår i: Int. J. Structural Engineering. - 1758-7328. ; 10:4, s. 307-329
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Tidskriftsartikel (refereegranskat)abstract
- Buried pipelines are tubular structures that cross large areas with different geological conditions. During an earthquake, imposed loads from soil deformations on pipelines may cause drastic damages. In this study two dimensional finite element models of pipelines and surrounding soils are usedfor simulation of seismic waves that propagate from the bedrock through thesoil. The models describe both longitudinal and transverse cross-sections ofpipelines and the soil-pipe interaction is described as a nonlinear behaviour.The effects of uniform ground with different burial depth and soil layer thickness, soil stiffness and non-uniform ground on the seismic response of reinforced concrete pipelines is studied. Two earthquakes, with high and low frequency contents, are employed for the dynamic analysis. The results show asignificant effect on the response due to non-uniform ground caused by inclined bedrock, especially for high frequency earthquake excitations.
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| 12. |
- Ahmed, Lamis, et al.
(författare)
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A comparison of models for shotcrete in dynamically loaded rock tunnels
- 2010
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Ingår i: Shotcrete. - : Taylor & Francis Group. ; , s. 1-10
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Konferensbidrag (refereegranskat)abstract
- During blasting in tunnels and mines, the shotcrete-rock interaction is influenced by propagating stress waves. Shotcrete support in hard rock tunnels is here studied through numerical analysis and comparisons with previous numerical results, measurements and observations in situ. The stress response in the shotcrete closest to the rock when exposed to P-waves striking perpendicularly to the shotcrete-rock interface is simulated. The first model tested is an elastic stress wave model, which is onedimensional with the shotcrete assumed linearly elastic. The second is a structural dynamic model that consists of masses and spring elements. The third model is a finite element model implemented using the Abaqus/Explicit program. Two methods are used for the application of incident disturbing stress waves: as boundary conditions and as inertia loads. Results from these three types of models are compared and evaluated as a first step before a future extension to more detailed analyses using 3D models.
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| 13. |
- Ahmed, Lamis, et al.
(författare)
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A comparison of models for shotcrete in dynamically loaded rock tunnels
- 2010
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Ingår i: Shotcrete: Elements of a System. - : Informa UK Limited. ; , s. 11-20
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- During blasting in tunnels and mines, the shotcrete-rock interaction is influenced by propagating stress waves. Shotcrete support in hard rock tunnels is here studied through numerical analysis and comparisons with previous numerical results, measurements and observations in situ. The stress response in the shotcrete closest to the rock when exposed to P-waves striking perpendicularly to the shotcrete-rock interface is simulated. The first model tested is an elastic stress wave model, which is onedimensional with the shotcrete assumed linearly elastic. The second is a structural dynamic model that consists of masses and spring elements. The third model is a finite element model implemented using the Abaqus/Explicit program. Two methods are used for the application of incident disturbing stress waves: as boundary conditions and as inertia loads. Results from these three types of models are compared and evaluated as a first step before a future extension to more detailed analyses using 3D models.
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| 14. |
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| 15. |
- Ahmed, Lamis, et al.
(författare)
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Direct shear strength of high-strength fibre concrete
- 2010
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Ingår i: Magazine of Concrete Research. - : Thomas Telford Ltd.. - 0024-9831 .- 1751-763X. ; 62:5, s. 379-390
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Tidskriftsartikel (refereegranskat)abstract
- An experimental and theoretical study of the shear behaviour of steel-fibre-reinforced concrete is presented. Twenty-seven direct shear push-off tests were carried out on high-strength concrete, with and without steel fibre reinforcement. The test series contained uncracked and precracked specimens for the study of the slipping response and the shear stress that can be transferred across an open crack. The test variables were the fibre content and the reinforcement ratio. The test results were compared with information provided by the available codes and other, previous results. The test results indicated that incorporation of steel fibres and bars in concrete members subjected to shear leads to an improved mechanical behaviour before failure. Based on the presented experimental results, an equation governing the direct shear strength is proposed and verified against test results from other test series.
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| 16. |
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| 17. |
- Ahmed, Lamis, et al.
(författare)
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Evaluation and analysis of laboratory tests of bolts-anchored, steel-fiber-reinforced shotcrete linings
- 2017
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Ingår i: Proceedings of the World Tunnel Congress 2017. - : International Tunnelling Association.
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Konferensbidrag (refereegranskat)abstract
- Results from laboratory tests on statically loaded bolt-anchored, steel-fibre-reinforced shotcrete linings in interaction with rock are here evaluated using a 2D finite element model. Calculations are made to determine the state of stress in the rock-shotcrete interface near the rock joints. Plane-stress elements are used with a non-linear material model, capable of describing cracking and de-bonding during loading. The simulated crack position and force-displacement curves are compared with laboratory test results. Since most construction work in underground hard rock involves the use of explosives for excavation work, dynamic load cases are also analysed and compared to results from previous research on vibration resistance of shotcrete.
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| 18. |
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| 19. |
- Ahmed, Lamis, et al.
(författare)
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Finite element simulation of shotcrete exposed to underground explosions
- 2012
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Ingår i: Nordic Concrete Research. - : Nordic Concrete Federation. - 0800-6377. ; :45, s. 59-74
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Tidskriftsartikel (refereegranskat)abstract
- An elastic finite element model is used tosimulate theinducedstress waves from blasting, propagating in rock towards shotcrete on a tunnel wall. Due to the inhomogeneous nature of the rock, the stress wavesattenuate onitsway from the point of explosiontowardsthe shotcrete on the rock surface. Material damping for the rock-mass is estimated from in-situ measurements. The vibration resistance of the shotcrete-rock support system depends on the material properties of the shotcrete. Age-dependent material properties are varied to investigate the behaviour of young shotcrete subjected to blast loading. Finally, finite element analysis results are presented and verified through comparison with other numerical models, measurements and observations.
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| 20. |
- Ahmed, Lamis, et al.
(författare)
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Laboratory investigation of stress waves in young shotcrete on rock
- 2012
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Ingår i: Magazine of Concrete Research. - : Thomas Telford Ltd.. - 0024-9831 .- 1751-763X. ; 64:10, s. 899-908
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Tidskriftsartikel (refereegranskat)abstract
- To study the behaviour of shotcrete under dynamic load, a non-destructive laboratory experiment was set up with P-wave propagation along a concrete bar, with properties similar to rock. Cement-based mortar with properties that resemble shotcrete was applied to one end of the bar with a hammer impacting the other. The shape of the stress waves travelling towards the shotcrete was registered using accelerometers positioned along the bar. Finite-element modelling was used to verify the test results, which showed that the laboratory model with an impacting hammer could be used to initiate the same type of stress waves that result from blasting in good-quality rock. Previously recommended maximum allowed peak particle vibration velocities were verified.
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| 21. |
- Ahmed, Lamis
(författare)
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Models for analysis of shotcrete on rock exposed to blasting
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In underground construction and tunnelling, the strive for a more time-efficient construction process naturally focuses on the possibilities of reducing the times of waiting between stages of construction. The ability to project shotcrete (sprayed concrete) on a rock surface at an early stage after blasting is vital to the safety during construction and function of e.g. a tunnel. A complication arises when the need for further blasting affects the hardening of newly applied shotcrete. If concrete, cast or sprayed, is exposed to vibrations at an early age while still in the process of hardening, damage that threatens the function of the hard concrete may occur. There is little, or no, established knowledge on the subject and there are no guidelines for practical use. It is concluded from previous investigations that shotcrete can withstand high particle velocity vibrations without being seriously damaged. Shotcrete without reinforcement can survive vibration levels as high as 0.5−1 m/s while sections with loss of bond and ejected rock will occur for vibration velocities higher than 1 m/s. The performance of young and hardened shotcrete exposed to high magnitudes of vibration is here investigated to identify safe distances and shotcrete ages for underground and tunnelling construction, using numerical analyses and comparison with measurements and observations. The work focuses on finding correlations between numerical results, measurement results and observations obtained during tunnelling. The outcome will be guidelines for practical use. The project involves development of sophisticated dynamic finite element models for which the collected information and data will be used as input, accomplished by using the finite element program Abaqus. The models were evaluated and refined through comparisons between calculated and measured data. First, existing simple engineering models were compared and evaluated through calculations and comparisons with existing data. The first model tested is a structural dynamic model that consists of masses and spring elements. The second is a model built up with finite beam elements interconnected with springs. The third is a one-dimensional elastic stress wave model. The stress response in the shotcrete closest to the rock when exposed to P-waves striking perpendicularly to the shotcrete-rock interface was simulated. Results from a non-destructive laboratory experiment were also used to provide test data for the models. The experiment studied P-wave propagation along a concrete bar, with properties similar to rock. Cement based mortar with properties that resembles shotcrete was applied on one end of the bar with a hammer impacting the other. The shape of the stress waves travelling towards the shotcrete was registered using accelerometers positioned along the bar. Due to the inhomogeneous nature of the rock, the stress waves from the blasting attenuate on the way from the point of explosion towards the shotcrete on the rock surface. Material damping for the rock mass is therefore accounted for, estimated from previous in-situ measurements. The vibration resistance of the shotcrete-rock support system depends on the material properties of the shotcrete and here were age-dependent properties varied to investigate the behaviour of young shotcrete subjected to blast loading. The numerical simulations require insertion of realistic material data for shotcrete and rock, such as density and modulus of elasticity. The calculated results were in good correspondence with observations and measurements in-situ, and with the previous numerical modelling results. Compared to the engineering models, using a sophisticated finite element program facilitate modelling of more complex geometries and also provide more detailed results. It was demonstrated that wave propagation through rock towards shotcrete can be modelled using two dimensional elastic finite elements in a dynamic analysis. The models must include the properties of the rock and the accuracy of the material parameters used will greatly affect the results. It will be possible to describe the propagation of the waves through the rock mass, from the centre of the explosion to the reflection at the shotcrete-rock interface. It is acceptable to use elastic material formulations until the material strengths are exceeded, i.e. until the strains are outside the elastic range, which thus indicates material failure. The higher complexity of this type of model, compared to the engineering models, will make it possible to model more sophisticated geometries. Examples of preliminary recommendations for practical use are given and it is demonstrated how the developed models and suggested analytical technique can be used to obtain further detailed limit values.
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| 22. |
- Ahmed, Lamis
(författare)
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Models for analysis of young cast and sprayed concrete subjected to impact-type loads
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The strive for a time-efficient construction process naturally put focus on the possibility of reducing the time of waiting between stages of construction, thereby minimizing the construction cost. If recently placed concrete, cast or sprayed, is exposed to impact vibrations at an early age while still in the process of hardening, damage that threatens the function of the hard concrete may occur. A waiting time when the concrete remains undisturbed, or a safe distance to the vibration source, is therefore needed. However, there is little, or no, fully proven knowledge of the length of this distance or time and there are no established guidelines for practical use. Therefore, conservative vibration limits are used for young and hardening concrete exposed to vibrations from e.g. blasting.As a first step in the dynamic analysis of a structure, the dynamic loads should always be identified and characterized. Here it is concluded that impact-type loads are the most dangerous of possible dynamic loads on young and hardening concrete. Shotcrete (sprayed concrete) on hard rock exposed to blasting and cast laboratory specimens subjected to direct mechanical impact loads have been investigated using finite element models based on the same analysis principles. Stress wave propagation is described in the same way whether it is through hard rock towards a shotcrete lining or through an element of young concrete. However, the failure modes differ for the two cases where shotcrete usually is damaged through loss of bond, partly or over larger sections that may result in shotcrete downfall. Cracking in shotcrete due to vibrations only is unusual and has not been observed during previous in situ tests. The study of shotcrete is included to demonstrate the need of specialized guidelines for cases other than for mass concrete, i.e. structural elements or concrete volumes with large dimensions in all directions.Within this project, work on evaluating and proposing analytical models are made in several steps, first with a focus on describing the behaviour of shotcrete on hard rock. It is demonstrated that wave propagation through rock towards shotcrete can be described using two-dimensional elastic finite element models in a dynamic analysis. The models must include the material properties of the rock and the accuracy of these parameters will greatly affect the results. It is possible to follow the propagation of stress waves through the rock mass, from the centre of blasting to the reflection at the shotcrete-rock interface. It is acceptable to use elastic material formulations until the strains are outside the elastic range, which thus indicates imminent material failure. The higher complexity of this type of model, compared with mechanical models using mass and spring elements, makes it possible to analyse more sophisticated geometries. Comparisons are made between numerical results and measurements from experiments in mining tunnels with ejected rock mass and shotcrete bond failure, and with measurements made during blasting for tunnel construction where rock and shotcrete remained intact. The calculated results are in good correspondence with the in situ observations and measurements, and with previous numerical modelling results. Examples of preliminary recommendations for practical use are given and it is demonstrated how the developed models and suggested analytical technique can be used for further detailed investigations.The modelling concept has also been used for analysis of impact loaded beams and concrete prisms modelled with 3D solid elements. As a first analysis step, an elastic material model was used to validate laboratory experiments with hammer-loaded concrete beams. The laboratory beam remained un-cracked during the experiments, and thus it was possible to achieve a good agreement using a linear elastic material model for fully hardened concrete. The model was further developed to enable modelling of cracked specimens. For verification of the numerical results, earlier laboratory experiments with hammer impacted smaller prisms of young concrete were chosen. A comparison between results showed that the laboratory tests can be reproduced numerically and those free vibration modes and natural frequencies of the test prisms contributed to the strain concentrations that gave cracking at high loads. Furthermore, it was investigated how a test prism modified with notches at the middle section would behave during laboratory testing. Calculated results showed that all cracking would be concentrated to one crack with a width equal to the sum of the multiple cracks that develop in un-notched prisms. In laboratory testing, the modified prism will provide a more reliable indication of when the critical load level is reached.This project has been interdisciplinary, combining structural dynamics, finite element modelling, concrete material technology, construction technology and rock support technology. It is a continuation from previous investigations of the effect on young shotcrete from blasting vibrations but this perspective has been widened to also include young, cast concrete. The outcome is a recommendation for how dynamic analysis of young concrete, cast and sprayed, can be carried out with an accurate description of the effect from impact-type loads. The type of numerical models presented and evaluated will provide an important tool for the work towards guidelines for practical use in civil engineering and concrete construction work. Some recommendations on safe distances and concrete ages are given, for newly cast concrete elements or mass concrete and for newly sprayed shotcrete on hard rock.
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| 23. |
- Ahmed, Lamis, et al.
(författare)
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Numerical modelling and evaluation of laboratory tests with impact loaded young concrete prisms
- 2016
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Ingår i: Materials and Structures. - : Springer Science and Business Media LLC. - 1359-5997 .- 1871-6873. ; 49:11, s. 4691-4704
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Tidskriftsartikel (refereegranskat)abstract
- Numerical modelling in combination with in situ measurements, observations and laboratory testing will be important to future establishment of reliable guidelines for efficient civil and engineering work involving concrete casting close to e.g. blasting operations. Results from laboratory tests with impact loaded young concrete prisms are here evaluated using a 3D finite element model. Solid elements are used and a non-linear material model implemented, capable of describing cracking during stress wave propagation. The position of cracks and measured particle vibration velocities are calculated and compared with laboratory test results. The damaging effect of impact vibrations is evaluated using crack width and fracture energy as damage criteria. Alternative geometry for the test prisms, with a notched section, is analysed. This will give one wide crack at the centre of the prism instead of two or three cracks distributed over its length which will make future laboratory test more efficient and reliable. Recommended damage limits at concrete ages of 4, 6, 8 and 12 h are given, based on numerical calculations for concrete strength class C25 and C50.
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| 24. |
- Ahmed, Lamis, et al.
(författare)
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Structural dynamic and stress wave models for analysis of shotcrete on rock exposed to blasting
- 2012
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Ingår i: Engineering structures. - : Elsevier BV. - 0141-0296 .- 1873-7323. ; 35:1, s. 11-17
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Tidskriftsartikel (refereegranskat)abstract
- During blasting in tunnels and mines, the interaction between shotcrete (sprayed concrete) and rock is influenced by propagating stress waves. Shotcrete support in hard rock tunnels is studied here through numerical analysis using three different modelling approaches. The stress response in the shotcrete closest to the rock when exposed to P-waves striking perpendicularly to the shotcrete–rock interface is simulated. The first model tested is a structural dynamic model that consists of masses and spring elements. The second is a model built up with finite element beam elements interconnected with springs. The third is a one-dimensional elastic stress wave model. The models give comparable results, although the definition of the dynamic loads is different. The analysis results can be used to estimate whether the shotcrete will fail or not for a prescribed distance to detonating explosives inside the rock.
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| 25. |
- Ahmed, Lamis, et al.
(författare)
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Vibration vulnerability of shotcrete on tunnel walls during construction blasting
- 2014
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Ingår i: Tunnelling and Underground Space Technology. - : Elsevier BV. - 0886-7798 .- 1878-4364. ; 42, s. 105-111
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Tidskriftsartikel (refereegranskat)abstract
- The effect on shotcrete from blasting operations during tunnelling is studied, with focus on young and hardening shotcrete. A finite element model specially adapted for analysis of the shotcrete behaviour is tested, it is able to describe stress wave propagation in two dimensions which is important for cases where shear stresses are dominant. The modelling results are compared with in situ measurements and observations, from construction blasting during tunnelling through hard rock. The comparison shows that the model gives realistic results and can be used to investigate the vulnerability of shotcrete, aiming at compiling recommendations and guidelines for practical use. The given recommendations emphasize that blasting should be avoided during the first 12 h after shotcreting and that distance and shotcrete thickness are important factors for how much additional time of waiting is possibly needed.
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| 26. |
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| 27. |
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| 28. |
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| 29. |
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| 30. |
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| 31. |
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| 32. |
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| 33. |
- Ansell, Anders, et al.
(författare)
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Betongkonstruktioner
- 2010
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Rapport (övrigt vetenskapligt/konstnärligt)
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| 34. |
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| 35. |
- Ansell, Anders, et al.
(författare)
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Concrete Structures
- 2012
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Rapport (övrigt vetenskapligt/konstnärligt)
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| 36. |
- 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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| 37. |
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| 38. |
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| 41. |
- Ansell, Anders, et al.
(författare)
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Durability of shotcrete tunnel linings due to steel fibre corrosion in cracks
- 2020
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Konferensbidrag (refereegranskat)abstract
- Steel fibre reinforcement in homogenous, intact, wet-mix shotcrete show an excellent durability against corrosion. The alkaline concrete environment gives good protection but the relatively thin shotcrete layers may crack due to shrinkage and other deformations. Durability requirements today often demands service-life of more than 100 years, which is not realistic with a maintained load-bearing capacity. Special measures must therefore be taken in the design of shotcrete rock support, such as addition of extra amount of fibres if the shotcrete is cracked or increased structural thickness, which is here demonstrated with examples. Due to the complex situation with requirements on service-life, climate impact and cost-efficiency, the design of future shotcrete supports will be optimized based on life cycle and life cycle cost analyses.
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| 42. |
- Ansell, Anders
(författare)
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Dynamic finite element analysis of young shotcrete in rock tunnels
- 2007
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Ingår i: ACI Structural Journal. - 0889-3241 .- 1944-7361. ; 104:1, s. 84-92
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Tidskriftsartikel (refereegranskat)abstract
- The problem with shotcrete on rock subjected to vibrations has previously been studied in place and through numerical modeling. Within the present project, an elastic finite element model is presented that consists of beam elements used to model the flexural stiffness and mass of the shotcrete lining and a section of rock. Spring elements are added for elastic coupling between shotcrete and rock. The loads on the model are acceleration-time series. Scaling laws for the magnitude of vibration velocities in rock as a function of distance and amount of explosives are used to study the damage to shotcrete at various. distances from the source of an explosion. Age-dependent shotcrete material properties are varied to investigate the vulnerability of young shotcrete exposed to explosion-induced vibrations. Recommended minimum shotcrete ages, based on the numerical results, are given for practical use.
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| 43. |
- Ansell, Anders
(författare)
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Dynamic testing of steel for a new type of energy absorbing rock bolt
- 2006
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Ingår i: Journal of constructional steel research. - : Elsevier BV. - 0143-974X .- 1873-5983. ; 62:5, s. 501-512
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Tidskriftsartikel (refereegranskat)abstract
- Rock bolts of steel are the most frequently used components in systems for rock reinforcement. Special types are used as energy absorbing elements in systems exposed to dynamic loads and these must yield plastically under high loading velocities. A new type of energy absorbing rock bolt of soft steel has been suggested and tested. To investigate the strain rate effects on the yield stress and ultimate strength of the steel were bars dynamically loaded in a high speed testing machine. The observed tensile forces can be divided into one part from impact and another from quasi-static straining. The tests showed that there was a considerable strain rate effect on the yield stress of the steel. It is recommended that higher dynamic yield stresses are utilized in the dynamic design which has to incorporate the risk of high peak forces from impact loading.
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| 44. |
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| 45. |
- Ansell, Anders, et al.
(författare)
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Dynamically loaded young shotcrete linings
- 2001
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Ingår i: Shotcrete: Engineering developments. - Lisse : Swets & Zeitlinger. - 90 5809 176 7 ; , s. 33-40
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Konferensbidrag (refereegranskat)
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