Spelling suggestions: "subject:"young concrete"" "subject:"joung concrete""
1 |
Models for analysis of young cast and sprayed concrete subjected to impact-type loadsAhmed, Lamis January 2015 (has links)
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. / <p>QC 20150529</p>
|
2 |
Influência dos ciclos de execução nas deformações das estruturas de concreto armado de edifícios de andares múltiplos / Construction cycles influence on the deformation of reinforced concrete structures of multistory buildingsSalvador, Paulo Fernando January 2007 (has links)
Este trabalho discute as conseqüências nas deformações de estruturas de concreto armado de edifícios de andares múltiplos, moldadas através de ciclos de execução rápidos. Em recente revisão, as normas da ABNT sugerem uma maior preocupação com as solicitações aplicadas nas estruturas, quando o concreto apresenta idade inferior a vinte e oito dias. A literatura pesquisada também indica a ocorrência de manifestações patológicas decorrentes de deformações excessivas nas estruturas devido às ações de construção, que inclusive podem superar o que é previsto em projeto. Neste trabalho é mostrado um estudo experimental de caráter exploratório, buscando analisar a influência do carregamento precoce, nas deformações de vigas de concreto armado. Foram propostos dois tipos de pré-carregamento, um de curta duração, onde são aplicados três níveis de carregamento aos três dias de idade, e outro de longa duração, onde são aplicados carregamentos em idades de acordo com as ações construtivas decorrentes de dois ciclos de execução rápidos.Foram utilizados concretos produzidos com dois tipos de cimento, um de cura rápida (CP V-ARI) e outro de cura lenta (CP IV-32RS). Os resultados apontam para a confirmação do que preconizam a literatura e as normas nacionais e internacionais, se consideradas as deformações resultantes em valores absolutos. No entanto, o pré-carregamento parece afetar favoravelmente as taxas de deformação por fluência por um determinado período, podendo inclusive promover um ganho de rigidez instantânea em níveis de carregamento próximos aos de projeto. / This paper discuss the consequences on deformations of reinforced concrete structures of multistory buildings, cast during cycles of quick construction. ABNT codes, in recent reviews, suggest more attention with load application on structures when concrete presents a lower age than twenty-eight days. The literature also indicates the incidence of pathological manifestations resulting from excessive deformation on structures due to the actions during construction that could even overcome those predicted on designs. It is shown in this paper an experimental study of exploratory character, trying to analyze the influence of early loading on the deformability of reinforced concrete beams. Two types of preloading were considered: an instantaneous one, where three levels of loading for three days of age each were applied, and a continuous one, where loadings where on ages applied in accordance with construction steps due to two cycles of quick construction. Concretes produced with two types of cement were used: one of fast curing (CP V-ARI) and another of slow curing (CP IV- 32RS). The results confirm what the literature and national and international codes specify, when deflections in absolute values are considered. Nevertheless, preloading seems to positively affect creep for a determined period, even promoting a gain in instantaneous stiffness on levels of loading near those of design.
|
3 |
Influência dos ciclos de execução nas deformações das estruturas de concreto armado de edifícios de andares múltiplos / Construction cycles influence on the deformation of reinforced concrete structures of multistory buildingsSalvador, Paulo Fernando January 2007 (has links)
Este trabalho discute as conseqüências nas deformações de estruturas de concreto armado de edifícios de andares múltiplos, moldadas através de ciclos de execução rápidos. Em recente revisão, as normas da ABNT sugerem uma maior preocupação com as solicitações aplicadas nas estruturas, quando o concreto apresenta idade inferior a vinte e oito dias. A literatura pesquisada também indica a ocorrência de manifestações patológicas decorrentes de deformações excessivas nas estruturas devido às ações de construção, que inclusive podem superar o que é previsto em projeto. Neste trabalho é mostrado um estudo experimental de caráter exploratório, buscando analisar a influência do carregamento precoce, nas deformações de vigas de concreto armado. Foram propostos dois tipos de pré-carregamento, um de curta duração, onde são aplicados três níveis de carregamento aos três dias de idade, e outro de longa duração, onde são aplicados carregamentos em idades de acordo com as ações construtivas decorrentes de dois ciclos de execução rápidos.Foram utilizados concretos produzidos com dois tipos de cimento, um de cura rápida (CP V-ARI) e outro de cura lenta (CP IV-32RS). Os resultados apontam para a confirmação do que preconizam a literatura e as normas nacionais e internacionais, se consideradas as deformações resultantes em valores absolutos. No entanto, o pré-carregamento parece afetar favoravelmente as taxas de deformação por fluência por um determinado período, podendo inclusive promover um ganho de rigidez instantânea em níveis de carregamento próximos aos de projeto. / This paper discuss the consequences on deformations of reinforced concrete structures of multistory buildings, cast during cycles of quick construction. ABNT codes, in recent reviews, suggest more attention with load application on structures when concrete presents a lower age than twenty-eight days. The literature also indicates the incidence of pathological manifestations resulting from excessive deformation on structures due to the actions during construction that could even overcome those predicted on designs. It is shown in this paper an experimental study of exploratory character, trying to analyze the influence of early loading on the deformability of reinforced concrete beams. Two types of preloading were considered: an instantaneous one, where three levels of loading for three days of age each were applied, and a continuous one, where loadings where on ages applied in accordance with construction steps due to two cycles of quick construction. Concretes produced with two types of cement were used: one of fast curing (CP V-ARI) and another of slow curing (CP IV- 32RS). The results confirm what the literature and national and international codes specify, when deflections in absolute values are considered. Nevertheless, preloading seems to positively affect creep for a determined period, even promoting a gain in instantaneous stiffness on levels of loading near those of design.
|
4 |
Influência dos ciclos de execução nas deformações das estruturas de concreto armado de edifícios de andares múltiplos / Construction cycles influence on the deformation of reinforced concrete structures of multistory buildingsSalvador, Paulo Fernando January 2007 (has links)
Este trabalho discute as conseqüências nas deformações de estruturas de concreto armado de edifícios de andares múltiplos, moldadas através de ciclos de execução rápidos. Em recente revisão, as normas da ABNT sugerem uma maior preocupação com as solicitações aplicadas nas estruturas, quando o concreto apresenta idade inferior a vinte e oito dias. A literatura pesquisada também indica a ocorrência de manifestações patológicas decorrentes de deformações excessivas nas estruturas devido às ações de construção, que inclusive podem superar o que é previsto em projeto. Neste trabalho é mostrado um estudo experimental de caráter exploratório, buscando analisar a influência do carregamento precoce, nas deformações de vigas de concreto armado. Foram propostos dois tipos de pré-carregamento, um de curta duração, onde são aplicados três níveis de carregamento aos três dias de idade, e outro de longa duração, onde são aplicados carregamentos em idades de acordo com as ações construtivas decorrentes de dois ciclos de execução rápidos.Foram utilizados concretos produzidos com dois tipos de cimento, um de cura rápida (CP V-ARI) e outro de cura lenta (CP IV-32RS). Os resultados apontam para a confirmação do que preconizam a literatura e as normas nacionais e internacionais, se consideradas as deformações resultantes em valores absolutos. No entanto, o pré-carregamento parece afetar favoravelmente as taxas de deformação por fluência por um determinado período, podendo inclusive promover um ganho de rigidez instantânea em níveis de carregamento próximos aos de projeto. / This paper discuss the consequences on deformations of reinforced concrete structures of multistory buildings, cast during cycles of quick construction. ABNT codes, in recent reviews, suggest more attention with load application on structures when concrete presents a lower age than twenty-eight days. The literature also indicates the incidence of pathological manifestations resulting from excessive deformation on structures due to the actions during construction that could even overcome those predicted on designs. It is shown in this paper an experimental study of exploratory character, trying to analyze the influence of early loading on the deformability of reinforced concrete beams. Two types of preloading were considered: an instantaneous one, where three levels of loading for three days of age each were applied, and a continuous one, where loadings where on ages applied in accordance with construction steps due to two cycles of quick construction. Concretes produced with two types of cement were used: one of fast curing (CP V-ARI) and another of slow curing (CP IV- 32RS). The results confirm what the literature and national and international codes specify, when deflections in absolute values are considered. Nevertheless, preloading seems to positively affect creep for a determined period, even promoting a gain in instantaneous stiffness on levels of loading near those of design.
|
5 |
Temperatursprickor i ung betong : Utvärdering av projektering och utförande av åtgärder, samt utveckling av dimensioneringsmetod för kylsystem / Thermal cracks in young concreteTilfors, Sara, Nezhad Arya, Nessa Yosef January 2011 (has links)
The report contains three parts. The first part is a summary and overall presentation of thermal cracks and preventing measures against such cracks. This is a theoretical part where the interested designer can get acquainted with various alternatives, be guided to more detailed literature, and receive some practical information regarding the design of the respective measures. The second part deals with calculation of the risk for thermal cracks and design of water cooling as a limiting measure. Two complementary methods are reviewed for efficient calculation: • the handbook method CraX1 (by Lulea University of Technology) is used for quick verification • finite differential analysis in the calculation tool ConTeSt Pro for more precise projecting Here is also presented a development of existing standards through design of the cooling system by means of the Bernoulli equation. Comparison is made with the approximation with Manning’s formula, which occurs in the field today. The calculation part is concluded with a calculation example. In addition to prescribing good project planning, the example also highlights some of the calculation programs’ possibilities and limitations. The third part of the report evaluates the current calculation of the risk for thermal cracking in young concrete, as well as the design and implementation of thereto related measures. This is done through studies of reference objects and interviews with all parts involved, i.e. clients, consultants, contractors and suppliers. The report aims to a compilation and transfer of skill and experience, so that potential for improvement in the prevention of thermal cracking can be identified, and the design can be improved with regard to the conditions of the construction phase. / Rapporten innehaller tre delar. Den forsta delen utgor en sammanstallning och oversiktlig presentation av temperatursprickor samt forebyggande atgarder mot sadana sprickor. Det ar en teoretisk del dar den intresserade projektoren kan satta sig in i olika alternativ, vagledas till utforligare litteratur, samt fa en del praktisk information infor projekteringen av respektive atgard. Den andra delen avhandlar berakning av risk for temperatursprickor samt projektering av vattenkylning som sprickbegransande atgard. Tva kompletterande metoder gas igenom for effektiv berakning: • handboksmetoden CraX1 (fran Lulea tekniska universitet) for snabbare kontroller • finita differensanalyser i berakningsprogrammet ConTeSt Pro for noggrannare projektering Har presenteras aven en utveckling av nuvarande standarder via dimensionering av kylsystem for vatskekylning med hjalp av Bernoullis ekvation. Jamforelse gors med approximationen med Mannings formel, vilken i dagslaget forekommer i branschen. Berakningsdelen avslutas med ett berakningsexempel. Forutom att foreskriva god projektering, belyser exemplet dessutom nagra av berakningsprogrammens mojligheter och begransningar. Rapportens tredje del utvarderar den nuvarande berakningen av risk for temperatursprickor i ung betong, samt projekteringen och genomforandet av tillhorande atgarder. Detta gors via studier av referensobjekt och intervjuer med samtliga involverade aktorer, d.v.s. bestallare, konstruktorer, entreprenorer och leverantorer. Rapporten syftar till en sammanstallning och aterforing av kompetens och erfarenhet, sa att forbattringspotential i forebyggandet av temperatursprickor kan identifieras, och projekteringen kan forbattras med hansyn till produktionens forutsattningar.
|
6 |
Thermal Crack Risk Estimation and Material Properties of Young ConcreteHösthagen, Anders January 2017 (has links)
This thesis presents how to establish a theoretical model to predict risk of thermal cracking in young concrete when cast on ground or an arbitrary construction. The crack risk in young concrete is determined in two steps: 1) calculation of temperature distribution within newly cast concrete and adjacent structure; 2) calculation of stresses caused by thermal and moisture (due to self-desiccation, if drying shrinkage not included) changes in the analyzed structure. If the stress reaches the tensile strength of the young concrete, one or several cracks will occur. The main focus of this work is how to establish a theoretical model denoted Equivalent Restraint Method model, ERM, and the correlation between ERM models and empirical experiences. A key factor in these kind of calculations is how to model the restraint from any adjacent construction part or adjoining restraining block of any type. The building of a road tunnel and a railway tunnel has been studied to collect temperature measurements and crack patterns from the first object, and temperature and thermal dilation measurements from the second object, respectively. These measurements and observed cracks were compared to the theoretical calculations to determine the level of agreement between empirical and theoretical results. Furthermore, this work describes how to obtain a set of fully tested material parameters at CompLAB (test laboratory at Luleå University of Technology, LTU) suitable to be incorporated into the calculation software used. It is of great importance that the obtained material parameters describe the thermal and mechanical properties of the young concrete accurately, in order to perform reliable crack risk calculations. Therefore, analysis was performed that show how a variation in the evaluated laboratory tests will affect the obtained parameters and what effects it has on calculated thermal stresses.
|
7 |
Basic Creep of Young Concrete - Sensitivity in the Evaluation MethodEkmat, Benar, Hermes, Natalea January 2019 (has links)
Creep is defined as deformation that takes place under constant load after an initial elastic response. This thesis focuses on a material property problem area that concerns stress analysis. Focus is on stress development considering creep deformations occurring when a concrete structure is under load, i.e. stress analysis with viscoelastic properties of the material.From laboratory tests, both elastic modulus and deformations over time are estimated in an evaluation process. Usually, deformations of moist sealed samples are denoted basic creep. At Luleå Technical University creep measurements are evaluated according to the theory and methodology in Larson and Jonasson (2003a, 2003b). The model is denoted Linear Logarithmic Model, used for moist sealed concrete samples. This thesis involves an investigation of the evaluation procedure for basic creep performed in Thysell laboratory at LTU, to examine how sensitive the evaluation process is for the outcome from stress calculations. The calculations are performed in the Finite Element Method software ConTeSt Pro.The aim of the thesis is to analyze the sensitivity of evaluation of basic creep and of the Linear Logarithmic Model (LLM) by making changes in the evaluation process to see how different parameters sets effect calculated stresses/strains during through crack analysis. The changes are solely done in the relaxation spectra.The purpose is also to analyze how sensitive the changes made in the evaluation process are when typical cases are studied. The typical cases are defined with a structure of a newly cast wall on a mature slab, where various thickness of the wall during different temperature conditions are analyzed. The temperature conditions are named standard, winter and summer. With this, concrete is tested and evaluated to yield two material parameter sets useful for temperature - and stress calculations for young concrete.The material parameter sets were analyzed and their creep values were converted into relaxation values, i.e. relaxation spectra, according to Maxwell-chain formulation for LLM. ConTeSt calculations generate temperature development for the walls and slabs. Colour maps and values of the strain ratio for each studied case are also obtained.It can be established that the evaluation process of basic creep is sensitive. A conclusion to be drawn is that small changes in the relaxation spectra, gives changes in the results of stress calculations for the typical cases. As soon as we deviate from the temperature development for the test performed in the laboratory, either by changing the thickness of the wall or by testing different temperature conditions we get a different temperature development than the tested one. With the deviation in the calculated temperature development compared to the measured one, a difference in the calculated strain ratios for the two different material parameter sets created are found.The main discovery in this work is that when a geometry of the wall that is identical to the geometry of the concrete tested at the laboratory is analyzed, a small deviation in the calculations of strain is obtained. This is expected since the temperature development in the created wall will follow the temperature development of the tested concrete. When differing from this geometry and temperature case, differences in calculated strain ratios are observed. / Krypning är deformation som sker under en konstant belastning och i detta examensarbete är det fokus på deformationer av fuktförseglade betongprover. Detta examensarbete är inriktat på olika materialparametersuppsättningar som berör spänningsanalyser. Det är fokus på spänningsutveckling med avseende på krypdeformationer som uppstår när en betongstruktur är under belastning. Detta gällande spänningsanalyser med viskoelastiska egenskaper hos betongmaterialet. Från laboratorietester bestäms både elasticitetsmodulen och deformationer över tid i en utvärderingsprocess. På Luleå tekniska universitet, utvärderas krypningsmätningarna enligt teorin och metodiken som finns beskriven i Larson och Jonasson (2003a, 2003b). Modellen är benämnd Linjär Logaritmisk Modell (LLM), som används för fuktförseglade betongprover. Examensarbetet innehåller en undersökning av utvärderingsprocessen för krypning utförda i Thysell laboratoriet vid LTU. Detta för att undersöka hur känslig utvärderingsprocessen är för resultatet av spänningsberäkningar. Beräkningarna utförs i Finita Elementprogrammet ConTeSt. Syftet med detta arbete är att analysera känsligheten i utvärderingen av krypning för fuktförseglade betongprover och för den Linjära Logaritmiska modellen genom att göra ändringar i utvärderingsprocessen för att se hur olika materialparametersuppsättningar påverkar beräknade spänningar under analys av genomgående sprickor. Ändringar görs endast i relaxationsspektra. Syftet är också att analysera hur känsliga förändringarna i utvärderingsprocessen är när olika typfall studeras. Typfallen definieras av ny gjuten vägg på en mogen betongplatta, där olika väggtjocklekar under olika temperaturförhållanden analyseras. Temperaturförhållandena benämns standard, vinter och sommar. Därvid testas och utvärderas betongen för att ge två materialparameteruppsättningar som är användbara för temperatur- och spänningsberäkningar för ung betong.Materialparameteruppsättningarna analyserades och deras krypvärden konverterades till relaxationsvärden, så kallade relaxations spektra, genom att använda Maxwell element för LLM. Från ConTeSt beräkningar erhålls värden för temperaturutveckling i vägg samt platta. Värmeutvecklingskarta tillsammans med värden på töjningskvoten för väggarna under varje studerat temperaturfall erhålls också från ConTeSt programmet. Det kan fastställas att den studerade utvärderingsprocessen för krypning är känslig. Små ändringar i relaxationsspektra ger variationer i resultatet av spänningsberäkningar för de olika typfallen. Så fort vi varierar den beräknade väggens temperaturutveckling från temperaturutvecklingen för testet som utförts i laboratoriet, antingen genom att ändra väggtjocklek eller genom att testa olika temperaturförhållanden, så erhålls en annorlunda temperaturutveckling än den som togs fram från laboratoriet. Med avvikelser i de beräknade temperaturutvecklingarna jämfört med den erhållna temperaturutvecklingen från den testade betongen i laboratoriet beräknas och analyseras skillnaden i töjningskvot. Den huvudsakliga upptäckten i detta arbete är att när den beräknade geometrin på väggen är identisk med den geometri som används för testriggen i laboratoriet, erhålls små variationer i de beräknade töjningskvoten. Detta är förväntat eftersom temperaturutvecklingen i den beräknade väggen är densamma som för betongen i testriggen i laboratoriet. När man avviker från den geometri eller de temperaturförhållandena som är identiska till dem i laboratoriet så erhålls större avvikelser i värden för den beräknade töjningskvoten.
|
8 |
Simulation of Hardening of the MahanaKhon Tower Mat FoundationKazi-tani, Zakaria January 2019 (has links)
Cement hydration is the result of a series of simultaneous chemical reactions occurring during the production of concrete. An excessive amount of heat is generated, which consequently may give rise to thermal stresses and cause early age cracks in concrete that may affect its structural integrity, and load bearing capacity. Incorporating fly ash into the concrete mixture has shown to be an efficient method to reduce the temperatures developed during early age hydration, especially for massive concrete structures. Fly ash does additionally affect the concrete's development of compressive strength, tensile strength and Young's modulus. The MahanaKhon tower's mat foundation is divided into 14 layers, with fly ash incorporated in the concrete mix. A finite element model was developed of the mat foundation with COMSOL Multiphysics to simulate the developed temperatures and thermal stresses during curing. The simulations were carried out as parametric studies with different strain reference temperatures. The simulated temperatures were compared with existing temperature measurements that were conducted in three different elevations in each concrete layer. The result of the temperature analyses showed that the measured temperatures were generally larger than the simulated ones, which may have been the result of the numerical model's heat conductivity and convective heat transfer coeffcient not reflecting the actual case. Furthermore, the numerical model did not take into account the effects of solar radiation, which would most likely have increased the temperature of the concrete. The maximum simulated temperatures were mostly found in the center level of the concrete, followed by the lower level, and the lowest at the top. It was also observed that the maximum temperatures in some of the mat foundation layers could exceed 70 °C, which is generally considered high since the risk of delayed ettringite formation may arise. The large temperature is partially a result of not using cooling methods, such as cooling pipes, but also due to the high initial and ambient temperatures. The result of the thermal stress analyses showed that no tensile stresses arose when the strain reference temperature, Tref, was specified to 30 °C, corresponding to the mean ambient temperature. This is due to the concrete temperature not falling below Tref, and the concrete will therefore be in expansion and only be subject to compressive stresses. Increasing Tref to 50 °C, which was considered a reasonable estimation, resulted in developed tensile stresses in all mat foundation layers, where the majority of the mat foundation layers showed a risk of superficial surface cracks. The maximum tensile stresses were found at the final time of the simulations, which was expected, since the temperatures were at their lowest as a result of removing the curing insulation. Finally, setting Tref to 70 °C, corresponding to the maximum temperature during hardening, increased the induced tensile stresses considerably, due to the large temperature gradient between Tref and the concrete temperature. The maximum stresses were, as expected, located at the top level and caused by internal restraint. The second largest tensile stresses were found in the center level, also subject to internal restraint. The lowest tensile stresses were located in the lower level, subject to external restraint. / Cementhydratation är resultatet av en serie kemiska reaktioner som sker under tillverkningen av betong. Stora mängder värme genereras, vilket följaktligen kan ge upphov till termiska spänningar och orsaka tidig sprickbildning som påverkar betongens hållfasthet, och bärförmåga. Inkludering av flygaska i betongblandningen har visat sig vara en effektiv metod avsedd att minska temperaturerna som utvecklas under hydratationen i ung betong, särskilt i massiva betongkonstruktioner. Flygaska påverkar också betongens utveckling av tryckhållfasthet, draghållfasthet och elasticitetsmodul. MahanaKhon towers bottenplatta är uppdelad i 14 lager, där flygaska inkluderades i bottenplattans betong. En finit elementmodell av bottenplattan skapades i COMSOL Multiphysics, där de utvecklade temperaturerna och termiska spänningarna i den unga betongen simulerades under bottenplattans härdningsfas. Simuleringarna genomfördes som parameterstudier med olika referenstemperaturer. De simulerade temperaturerna jämfördes vidare med befintliga temperaturmätningar som utfördes i tre olika elevationer i varje gjutetapp. Resultaten av temperaturerna visade att de uppmätta temperaturerna var generellt högre än de simulerade, vilket bland annat kan bero på att betongens värmeledningsförmåga, samt konvektiva värmeöverföringskoefficient inte återspeglade det aktuella fallet. Den numeriska modellen tog inte heller hänsyn till effekten av solinstrålning, som sannolikt skulle ökat betongens temperatur. De maximala temperaturerna hittades mestadels i betongens mittnivå, följt av den lägre nivån och slutligen lägsta nivåerna vid toppen. Det observerades även att de maximala temperaturerna i bottenplattan kunde överstiga 70 °C, vilket generellt anses vara högt då risken för fördröjd ettringitbildning kan uppstå. De höga temperaturerna beror delvis på avsaknad av kylmetoder, såsom kylrör, men även på den höga initialtemperaturen och omgivningstemperaturen. Resultaten av spänningsanalysen påvisade att inga dragspänningar uppstod när referenstemperaturen Tref denierades till 30 °C, som motsvarar den genomsnittliga omgivningstemperaturen. Detta förklaras av att betongen kommer att vara i expansion och följaktligen endast utsättas för tryckspänningar. Efter att Tref ökats till 50 °C, vilken ansågs vara en rimlig estimering i denna studie, uppstod dragspänningar i alla lager i bottenplattan, där vissa utsattes för risk för ytsprickor. De maximala dragspänningarna uppstod vid simuleringarnas slut, vilket var förväntat då temperaturerna var som lägst vid den tidpunkten till följd av att isoleringen avlägsnades. Slutligen höjdes Tref till 70 °C, vilket motsvarar den maximala temperaturen i bottenplattan under härdning. De inducerade dragspänningarna ökade avsevärt på grund av den stora temperaturgradienten mellan Tref och betongtemperaturen. Samtliga lager utsattes i detta fall för risk för genomgående sprickor. De maximala dragspänningarna påträffades på toppnivån och orsakades av inre tvång. De näst största dragspänningarna fanns i mitten av plattan och var också resultatet av inre tvång. De lägsta dragspänningarna påträffades vid plattans lägre nivå, som utsattes för yttre tvång.
|
Page generated in 0.0416 seconds