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Use of acoustic methods for assessing the influence of the composition of cement composites on the course of degradation caused by high temperature
Dvořák, Richard ; Medveď,, Igor (referee) ; Bílek,, Vlastimil (referee) ; Bodnárová, Lenka (referee) ; Chobola, Zdeněk (advisor)
The aim of this work is to optimize the acoustic nondestructive method (NDT) Impact-Echo (IE) as a complementary diagnostic tool for the assessment of concrete structures exposed to high temperatures. Commonly used diagnostic procedures for concrete structures affected by fire are most often limited to in-situ core drilling and subsequent testing of the drilled holes by destructive testing. The Impact-Echo method allows non-destructive testing of variously degraded test specimens and concrete structures, but its use for the assessment of temperature-degraded cementitious composites is hardly ever used in practice. Many studies suggest that Impact-Echo is one of the most suitable non-destructive methods for assessing the rate of degradation by elevated temperature. The dissertation work consists of three main stages, where the first stage is devoted to the production of test specimens of different mixtures and their subsequent degradation by elevated temperature and testing by the IE method and by reference destructive and non-destructive methods. The second stage consists of manufacturing a reinforced concrete element with built-in thermocouples and its subsequent inhomogeneous degradation by a local source of elevated temperature. The controlled degraded element thus prepared contains all grades of thermally degraded concrete tested in the first stage. Subsequent testing consists of verification of standard procedures in the field of acoustic NDT with simultaneous use of experimental modal analysis. In the third stage, the objective is to interpret the measured data from both previous parts, to verify whether within the closed element temperature-degraded areas can be distinguished from one another, or the degree of thermal degradation of concrete. Based on these results, the optimal testing procedure was determined and the basic assumptions for the creation of degrees of damage of the concrete by increased temperature were determined.
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Rehydration of alkali-activated slag after high temperature loading
Fialová, Barbora ; Bayer, Patrik (referee) ; Rovnaník, Pavel (advisor)
Ground granulated blast furnace slag is a by-product of the steel industry and is often used in combination with ordinary Portland cement as a binder in concrete. When concrete is exposed to high temperatures, physical and chemical transformations lead to significant loss of mechanical properties. This study aims to investigate the effect of high temperatures and rehydration on the mechanical properties, microstructure and phase composition of alkali activated slag. The results of the research could make an important contribution to decisions made concerning the reconstruction of structures affected by fire. In suitable cases it would be possible to regenerate parts of a structure instead of totally rebuilding it.
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Fatigue-creep interaction in Ni superalloys and TiAl alloys
Šmíd, Miroslav ; Věchet, Stanislav (referee) ; Kunz, Ludvík (referee) ; Obrtlík, Karel (advisor)
The present doctoral thesis is focused on the effect of dwells on the low cycle fatigue behaviour of advanced high temperature materials. 10 minutes strain holds are introduced into the cyclic straining of cast Ni-based superalloy IN792-5A and cast intermetallic alloy TiAl-7Nb which were chosen as experimental material. Fatigue experiments were conducted in strain control mode with constant total strain amplitude and strain rate. IN792-5A was subjected to continuous cyclic loading test as well as fatigue tests either with tensile peak strain holds or compressive peak strain holds at temperature 800 °C. TiAl-7Nb was experimentally examined by continuous cyclic loading tests and also by fatigue test with tensile peak strain hold times at temperature 750 °C. Cyclic hardening/softening curves, cyclic stress-strain curves, Manson-Coffin curves and Basquin curves were obtained. Stress relaxation data were measured during hold times. Mean stress evolution was observed and documented. Microstructure of both alloys was observed in as-received state and also after cyclic loading by means of SEM and TEM. Surface relief investigation revealed cyclic plastic strain localisation as well as fatigue crack initiation sites. Fatigue crack propagation was described using fracture surface and longitudinal gauge section observations. Dislocation structures developed in the course of fatigue tests were studied and documented.
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RESEARCH AND DEVELOPMENT COMPOSITE MATERIAL WITH A HIGHER RESISTANCE TO HIGH TEMPERATURES
Válek, Jaroslav ; Durica,, Tibor (referee) ; Kolář,, Karel (referee) ; Sitek,, Libor (referee) ; Bodnárová, Lenka (advisor)
Concrete has many advantageous properties as regards resistance to fire. It is non-flammable and it has a low thermal conductivity. However, concrete structures, which are not designed for resistance against fire, show significant damage after heating. In particular, the explosive flaking with the consequence of weakening the reinforced concrete cross-section and exposing the steel reinforcement to the temperatures higher than critical temperature of reinforcement. There are only a few possible measures of preventing or mitigating the effects temperature load used. Ways of protection can be divided into two systems: active and passive. Active systems are designed to ensure the greatest possible reduction of temperatures the concrete is exposed to. Passive systems directly resist to high temperatures and fire. Design of composition of concrete with the aim of higher resistance to exposition to high temperatures belongs among the passive systems. A part of the work focuses on summary searches of the problems of concrete and reinforced concrete structures exposed to high temperatures and fire. The goal of the work is defining requirements for cement matrix based composite material and its design ensuring the highest possible resistance to high temperatures or direct fire.
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Fracture toughness testing at high temperature range using miniaturized CT specimens
Lokvenc, Martin ; Chlup, Zdeněk (referee) ; Stratil, Luděk (advisor)
This thesis deals with a high temperature testing of fracture toughness and studies the size effect on measured values using miniature size CT specimen. Two types of specimen geometry were manufactured from P91 steel, the standard size and the quarter size specimen. J-R curves were obtained in the temperature range from 23°C to 600°C. No specimen size effect was observed at room temperature tests. The realized experiments together with fractography analysis demonstrated the drop of toughness at 400°C caused by the effect of dynamic strain aging.
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Study of thermal insulation properties of cement concrete exposed to high temperature
Nováková, Iveta ; Sitek,, Libor (referee) ; Bodnárová, Lenka (advisor)
Master`s thesis is divided in to two parts, practical and theoretical. In theoretical part are listed basic information’s about light weight concrete, special emphasis are given to characteristic and practical application of compact light weight concrete with Liapor aggregates. In this study is described influence of high temperature on concrete structure and chemical, mechanical and physical changes, which take place during exposal to high temperatures. Further is evaluated surface permeability of concrete and addition of polypropylene fibres to concretes resistive to high temperatures. The practical part deals with design, production and testing of cement based concrete with use of different aggregates (light weight aggregates Liapor, basalt). The properties and use for applications in high temperatures is also mentioned. The influence of high temperature on strength, absorption, thermal conductivity, changes of surface permeability and degradation of testing specimens due to heat loads according to normative heat curve (ISO 834). For better transparency are experimental tests divided in to five phases and the measured values are evaluated on the end of each phase. In conclusion are resumed all knowledge’s obtained by testing and evaluated the most suitable formulation. The approach for further research is also mentioned.
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Properties of alkai-activated aluminosilicates subjected to high temperatures
Halasová, Kristýna ; Bayer, Patrik (referee) ; Rovnaník, Pavel (advisor)
Currently the concept of geopolymer or alkali activated aluminosilicate is very mentioned topic. Many studies concerning utilization of these materials have already been made, but if we want to use these materials in building industry in the future, further research will be needed. This thesis deals with the influence of high temperatures on alkali-activated slag, fly ash and metakaolin-based composites. The work compares mechanical and technological properties, microstructural changes and effect off iller on the geopolymer composites heated at 200–1200 °C. In conclusion, the individual properties are compared and application possibilities of these materials are evaluated.
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Properties of cement composites with fiber reinforcement
Jankech, Filip ; Sitek,, Libor (referee) ; Bodnárová, Lenka (advisor)
This thesis summarizes the knowledge of the possibility of using organic fibres as dispersed reinforcement into cement composites. It is concretely a summary of knowledge about the types and properties of synthetic and cellulosic fibres, their influence on the properties of fresh and hardened cement composite and lastly a description of the fire resistance of cement composites and the effect of organic fibres on the action of those composites at high temperatures and in the presence of fire. The experimental part is supposed to verify the effect of the cellulose fibres on increasing the durability of the cement composite at high temperatures.
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Fatigue properties of nickel-base superalloy inconel 792-5A at 800°C
Šmíd, Miroslav ; Obrtlík, Karel ; Petrenec, Martin ; Polák, Jaroslav ; Hrbáček, K.
Smooth specimens were cyclically strained under strain control with constant strain amplitude and constant strain rate. Low cycle fatigue tests were conducted in servo-hydraulic pulsator MTS equipped with a three zone resistance furnace at temperature 800°C in air. Fracture surface was studied in SEM after fatigue test termination. Selected specimens were used to prepare foils for the transmission electron microscope (TEM) observation of microstructure and dislocation arrangement. They were used to obtain cyclic hardening/softening curves, cyclic stress-strain curve and fatigue life curves in the representation of stress amplitude, total strain amplitude and plastic strain amplitude versus number of cycles to fracture. Experimental points can be approximated with the Manson-Coffin law and the Basquin law. Fracture surface examinations revealed fatigue crack initiation sites.
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