National Repository of Grey Literature 76 records found  1 - 10nextend  jump to record: Search took 0.00 seconds. 
Porušování jemnozrnného cementového kompozitu v blízkosti rozhraní plniva a matrice
Vyhlídal, Michal ; Kabele,, Petr (referee) ; Profant, Tomáš (referee) ; Keršner, Zbyněk (advisor)
The Interfacial Transition Zone (ITZ) between the aggregate grain/reinforcement bar and the matrix is considered to be the weakest element in cementitious composites and is, according to some authors, directly responsible for the nonlinear (more precisely, quasi-brittle) behavior of the composites. The aim of this work is verification of the generally accepted paradigm of the weakest element by means of fracture experiments and corresponding numerical simulations. In the experimental part, in addition to traditional fracture tests, modern methods of 3D scanning, scanning electron microscopy, chemical analysis using an electron microprobe and nanoindentation were used. In the numerical part, models based on generalized linear elastic fracture mechanics as well as modern models intended for the simulation of cement composites, namely crack band model with smeared cracks and the Microplane model, were used. Based on the results, the numerical part was complemented by the Cohesive Zone Material model simulating the behavior of the interface. All results are discussed and put into context with already published work. The main conclusion of the work is that the properties of the ITZ do not have such an influence on the behavior of cement composites as the adhesion between the inclusion and the ITZ, i.e. the matrix.
Stress state aspects of quasi-brittle fracture
Sobek, Jakub ; Němeček,, Jiří (referee) ; Keršner, Zbyněk (referee) ; Klusák,, Jan (referee) ; Veselý, Václav (advisor)
The presented dissertation thesis is focused, as the title suggests, on the analysis of stress state aspects of quasi-brittle fracture. That means the fracture of composite materials with cement matrix (such as concrete, mortar, plaster, etc.), ceramics and other composites. Used methods are based on the theory of multi-parameter linear elastic fracture mechanics, which highlights the importance of considering of several initial terms of Williams power series, approximating the stress and displacement fields in a cracked body, within conducted fracture analyses. Determination of values of coefficients of terms of this series, recalculated into the shape functions serving in most of the conducted stress state analyses, is performed via the so called over-deterministic method. Another tool for the problem solving is nonlinear fracture mechanics, represented primarily by the cohesive crack model, namely the crack band model implemented in the used ATENA software. For the backward reconstruction of stress field in the cracked bodies the application ReFraPro is used. The analytical part deals with various aspects of wedge-splitting test – from the boundary conditions, though various possibilities of nodal selection (required as input variables for the over-deterministic method) up to the advanced (automated) analysis of numerical model. Special chapter includes atypical test specimens designed for adjusting of various levels of constraint of stress and deformation at the propagating crack tip. The study of this geometry and also the subsequent detail analysis reveals important information for real experiments. Backward reconstruction of stress field presents analysis on suitable possibilities of nodal selections as inputs into the procedure of approximation of the crack tip fields and answers the question of the necessity of application of the multi-parameter linear elastic fracture mechanics for certain fracture analyses of specimens from quasi-brittle materials. The th
Investigation and Modelling of Concrete Carbonation
Rovnaníková, Markéta ; Keršner, Zbyněk (referee) ; Bajza, Adolf (referee) ; Havlica, Jaromír (advisor)
The corrosion of steel reinforcement is probably the most frequent type of degradation of reinforced concrete structures. Reinforcing bars are protected from corrosion by a thin iron oxide layer which is formed on their surface due to a high alkalinity of the surrounding concrete. Corrosion may start when this layer is destroyed (i) either by chloride ion penetration (when the concentration of dissolved chloride at the depth of the reinforcing steel exceeds a critical value) or (ii) by a reduction in the pH value of pore solution to values below 9.5. Such a reduction in alkalinity is the result of carbonation of the Ca(OH)2 in the concrete mass, i.e. of its reaction with the atmospheric CO2. The main aim of this work is an experimental investigation and modelling of carbonation process. The effect of relative humidity, curing period and replacement of Portland cement by supplementary cementing materials (SCM - fly ash, blast furnace slag and metakaolin) on carbonation process has been studied. Calcium hydroxide content in the presence of SCM and their pozzolanic activities have been also determined. Furthermore, lots of parametric studies and comparison of carbonation models have been provided. The software tools based on the introduced models and considering the input data to be statistically independent random variables have been described and used for other parametric studies.
Numerical analysis of selected silicate based composites shrinkage
Drbušková, Magdaléna ; Vlk, Zbyněk (referee) ; Keršner, Zbyněk (advisor)
The thesis is divided into two main parts. In the first theoretical part is described the problems of shrinking including a comparison of Czech standard and Model Code 2010, Vol. 1. The second practical part of the master`s thesis is focused on the numerical analysis shrinkage primarily on the initial stage of this process. The experimentally obtained data are set approximations of the relative deformation using ShrCeC. Subsequently the numerical simulation of shrinkage of selected silicate specimens using a computer applications SpatiDist and FyDiK 2D. The real test specimens are modelled as two-component composite consisting of cement paste and aggregates. The result is a parametric study takes into account the influence of type and size of grain aggregate.
Ballistic-Proof Concretes
Koutný, Ondřej ; Bílek,, Vlastimil (referee) ; Keršner, Zbyněk (referee) ; Ptáček, Petr (advisor)
Doctoral thesis „Ballistic-proof concretes“ deals with description, design and development of material based on ultra-high performance fibre reinforced cementitious composite with increased ballistic resistance i.e. increased resistance against high-strain rate dynamic loading induced by interaction of high-velocity moving objects. High mechanical properties, essential for such a material, are reached especially by maximal reduction of water-to-binder coefficient using high-range water reducing agents, high-strength aggregates and dense structure by precise selection and dosage of raw materials in the recipe. The main goal is to prepare a methodology for design of such a materials, observation of material behaviour on ballistic loading and quantitative description of material response for protective structures design. Properties of designed materials within this thesis are comparing with properties of commercially available and commonly used cementitious composites in order to create a concept for material limits in the field of ballistic protection. This concept enables to estimate ballistic protection of present or newly-designed materials and structures.
Evaluation of fracture test of selected core-drilled cylindrical specimen
Halfar, Petr ; Sobek, Jakub (referee) ; Keršner, Zbyněk (advisor)
The diploma thesis deals with effective crack model for a cylinder specimen with an chevron-shaped notch, loaded by a three-point bending. The first part of the thesis is theoretical and the it describes the use of fracture mechanics for quasi-brittle materials – concrete and rocks. Furthermore there is described the linear elastic fracture mechanics and founding the geometry function for a different geometries of fracture tests. The second part describes the FEM program, which was used to calculate the fracture parameters, that were used to determine the length of the effective crack and how the program was tested before it was used. The last part describe calculations of fracture toughness by using the effective crack model on selected rock and concrete specimens.
Basic features of aggregate-matrix-interface fracture of selected building composites
Vyhlídal, Michal ; Seitl, Stanislav (referee) ; Keršner, Zbyněk (advisor)
The aim of this work is to analyze the behavior of cement composite, depending on the features of ITZ. The first part of the thesis is theoretical and deals with the origin and features of ITZ, defines the basic relationships and concepts of continuum mechanics and linear elastic fracture mechanics and presents crack tips posibilities in the three-phase composite, including their short characterization. The second part is practical and describes creation of numerical model in the ANSYS software, presentation of partial results and compare of them. The result of this work is a comparison of the mean values of stress and amplitude of stress depending on the features of ITZ.
Comparison of geometry of vaults of historical buildings with computational model
Dobrovolný, Tomáš ; Keršner, Zbyněk (referee) ; Frantík, Petr (advisor)
Thesis deals with the study of the shape of the actual vault and its modeling. Modeling is based on the computation of the pressure line using a chain model. It is investigated whether the builders could in the past derive the shape of the vault by means of a chain on which they suspended the weights. To compare the actual arch with the model, the entrance vault of St. Jacob's Church in Brno was selected.
Physical Discretization Modelling of Shrinkage
Bedáň, Jan ; Keršner, Zbyněk (referee) ; Frantík, Petr (advisor)
The aim of this thesis is design and programming of a numerical model of shrinkage of cement composites using physical discretization and solved by parallel processing. Introduction is about the shrinkage of cement composites and explanation of terms, which are included in this thesis. The main part deals with programming of the output model, the procedure of its creation and the results of simulations.
Modelling the response of building material specimens during fracture tests
Navrátil, Petr ; Veselý,, Václav (referee) ; Keršner, Zbyněk (advisor)
The thesis focuses on the computational simulation of wedge splitting test of a concrete specimen by using finite element method. Different levels of numeric model for different notch depth and for different position of support are solved. Depending on the depth of a notch and difference of configuration, the crack paths and responses to an exterior load on a crack mouth opening displacement are evaluated.

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