Zdeněk Bittnar

Material and structural characterization of alkali activated low-calcium brown coal fly ash

The waste low-calcium Czech brown coal fly ash represents a considerable environmental burden due to the quantities produced and the potentially high content of leachable heavy metals. The heterogeneous microstucture of the geopolymer M(n) [-(Si-O)(z)-Al-O](n).wH(2)O, that forms during the alkaline activation, was examined by means of microcalorimetry, XRD, TGA, DSC, MIP, FTIR, NMR MAS ((29)Si, (27)Al, (23)Na), ESEM, EDS, and EBSD. The leaching of heavy metals and the evolution of compressive strength were also monitored. The analysis of raw fly ash identified a number of different morphologies, unequal distribution of elements, Fe-rich rim, high internal porosity, and minor crystalline phases of mullite and quartz. Microcalorimetry revealed exothermic reactions with dependence on the activator alkalinity. The activation energy of the geopolymerization process was determined as 86.2kJ/mol. The X-ray diffraction analysis revealed no additional crystalline phases associated with geopolymer formation. Over several weeks, the (29)Si NMR spectrum testified a high degree of polymerization and Al penetration into the SiO(4) tetrahedra. The (23)Na NMR MAS spectrum hypothesized that sodium is bound in the form of Na(H(2)O)(n) rather than Na(+), thus causing efflorescence in a moisture-gradient environment. As and Cr(6+) are weakly bonded in the geopolymer matrix, while excellent immobilization of Zn(2+), Cu(2+), Cd(2+), and Cr(3+) are reported.

Effect of transversal reinforcement in normal and high strength concrete columns

This paper deals with an experimental investigation and numerical simulation of reinforced concrete columns. Behavior of normal and high strength columns is studied with special attention paid to the confinement effects of transversal reinforcement in columns with square cross section. Character of a failure, strengths, ductility and post-peak behavior of columns are observed in experiments and also in numerical solution. Three-dimensional computational model based on the microplane model for concrete was constructed and compared with experimental data. Results of numerical model showed good agreement in many aspects and proved capabilities of the used material model.RésuméCet article traite de la recherche expérimentale d’échantillons de colonnes en béton armé et de leur simulation numérique. Le comportement des colonnes à résistance normale et à haute résistance est étudié avec une attention particulière pour ce qui est des effets du confinement dû aux armatures transversales à section carrée. Le caractère de la rupture, les résistances, la ductilité et le comportement après-pic des colonnes sont observés de façon expérimentale et numérique. Le modèle mathématique tridimensionnel qui prend ses racines dans le modèle microplan pour le béton a été établi et comparé aux données expérimentales. Les résultats du modèle numérique sont en bon accord avec les expériences et témoignent de la grande capacité du modèle utilisé.

Multiscale simulations of concrete mechanical tests

In civil engineering, computational modeling is widely used in the design process at the structural level. In contrast to that, an automated support for the selection or design of construction materials is currently not available. Specification of material properties and model parameters has a strong influence on the results. Therefore, an uncoupled two-step approach is employed to provide relatively quick and reliable simulations of concrete (mortar) tests. First, the Mori-Tanaka method is utilized to include the majority of small aggregates and air voids. The strain incremental form of MT approach serves for the prediction of material properties subsequently used in the finite element simulations of mechanical tests.

Parallel explicit finite element dynamics with nonlocal constitutive models

Abstract The present paper deals with the parallelization of an explicit time stepping algorithm in a general finite element environment. Particular attention has been paid to nonlocal constitutive models. A central difference method has been used to discretize the governing equations in time. Modifications of both node-cut and element-cut strategies have been developed to provide an efficient support for nonlocal constitutive models. Efficiency of the proposed approach is demonstrated on different hardware platforms.

Using the recursive subdivision and the advancing front technique for the discretization of multi-phase microstructures

The present paper deals with the discretization of microstructure initially represented by a digital image obtained from computer tomography or any other similar scanning device. The boundary voxels in the digital representation are identified and then replaced by a boundary triangulation of the similar resolution as the original digital image. This triangulation is then subjected to recursive subdivision to recover a smooth surface of the microstructure which is then retriangulated according to a user specified resolution. Finally, the interior of the microstructure is discretized. The performance of the proposed approach is shown on an example.

Nanoindentation of Cement Pastes and Its Numerical Modeling

This paper focuses on experimental investigations and numerical modeling of micromechanical behavior of cement paste. Cement paste is taken as a fundamental representative of building materials with heterogeneous microstructure. The main studies are devotes to experimental nanoindention and its implications to evaluation of material properties. The paper concerns the appropriateness of conventional methods used for evaluation of micromechanical properties and investigates possibilities of the use of enhanced methods for better description of the nanoindentation process. Limitation of traditional elastic solution is shown on the unique experimental program. Better descriptions of indentation based on analytical viscoelastic solution and finite element model with general visco-elasto-plastic constitutive relation are proposed. These models are used for simulation of indentation and for estimation of material parameters at micrometer scale.

The Behavior of Fly Ash in Concrete Mixture

It is desirable to use secondary raw materials with regard to the sustainable development. One such suitable material is fly ash. Still enough unused possibility of using fly ash is the use in cement and concrete. This use brings a positive ecological and economic effect. However, it is important to devote to the behavior of fly ash in concrete mixture at the macro and micro level. This paper deals with selected properties as a positive influence on the development of hydration heat, the increase in compressive strength or problems with water absorption fly ash.