Ondřej Holčapek

Fracture characteristics of refractory composites containing metakaolin and ceramic fibers

The aim of present article is to describe influence of composition of refractory composites on its response to gradual thermal loading. Attention was focused on the impact of ceramic fibers and application of metakaolin as an aluminous cement supplementary material. Studied aluminate binder system in combination with natural basalt fine aggregates ensures sufficient resistance to high-temperature exposure. Influence of composition changes was evaluated by the results of physical and mechanical testing—compressive and flexural strength, bulk density, and fracture energy were determined on the different levels of temperature loading. Application of ceramic fibers brought expected linear increase of ductility in studied composites. Metakaolin replacement showed the optimal dose to be just about 20% of aluminous cement weight.

Basic and Hygric Properties of Concrete Containing Fine Ceramic Powder

The motivation for utilization of active admixtures in concrete lies primarily in a positive effect on properties of hardened concrete with regard to its durability. Fine parts complement the grain size distribution curve, but also due to subsequent hydration arise phases with better resistance to aggressive substances from surrounding environment. Process of pozzolanic reaction is also associated with a reduction in open capillary porosity, causing a gradual reduction of the permeability of concrete. The paper presents an experimental program focused on the monitoring of evolution of basic and hygric properties of concrete with fine ceramic powder addition.

Time Progress of Compressive Strength of High Performance Concrete

Development in the field of concrete engineering is increasingly focused on the practical application of high performance concrete (HPC) or ultra-high performance concrete (UHPC) in construction practise. Newly developed kings of concrete are newly using in transport and building structures. The process of hydration of hydraulic binders based on Portland cement doesn ́t stop after 28 days, when the test of compressive strength take place, but it ́s a long time process that takes for many months. For design we use the values of strength of 28 days. This paper explorers how does the long-term development of compressive strength of HPC runs. The composition of HPC is significantly different from the common concrete lower strength classes. The question of the influence of additives, filler on microsilica based, silica flour to the time development of compressive strength is being explored in this paper. There is also recorded the influence of curing condition of the test specimens to the compressive strength. The age of testing samples starts at a very early ages 1, 3, 7, 21, 28, 45, 90 and 180 days. The strength in uniaxial compression was measured on cubes with dimension 100 mm.

ANALYSIS OF MECHANICAL PROPERTIES OF HYDROTHERMALLY CURED HIGH STRENGTH CEMENT MATRIX FOR TEXTILE REINFORCED CONCRETE

The main objective of this article is to describe the influence of hydrothermal curing conditions in an autoclave device (different pressure and temperature), which took place at various ages of a fresh mixture (cement matrix – CM, and fibre-reinforced cement matrix – FRCM), on textile reinforced concrete production. The positive influence of autoclaving has been evaluated through the results of physical and mechanical testing – compressive strength, flexural strength, bulk density and dynamic modulus of elasticity, which have been measured on specimens with the following dimensions: 40×40×160mm 3 . In addition, it has been found that increasing the pressure and temperature resulted in higher values of measured characteristics. The results indicate that the most suitable surrounding conditions are 0.6MPa, and 165 °C at the age of 21 hours; the final compressive strength of cement matrix is 134.3MPa and its flexural strength is 25.9MPa (standard cured samples achieve 114.6MPa and 15.7MPa). Hydrothermal curing is even more effective for cement matrix reinforced by steel fibres (for example, the compressive strength can reach 177.5MPa, while laboratory-cured samples achieve a compressive strength of 108.5MPa).

Frost Resistance of Concrete Screed with the Fly Ash Addition

Using of fly ash in concrete screed is becoming a common practice. This situation entails both environmentally and economically positive effect. The problem may occur with the durability and life of such materials. Therefore, it is important to correctly grasp these materials and final properties sufficiently verify. One of these properties is frost resistance. This paper presents the results of measurements on the frost resistance of concrete screed with the substitution fly ash 0-50% by weight of clinker. Specimens were subjected to destructive and non-destructive test of frost resistance after 28 days of ripening and the results were evaluated.

Physical and Mechanical Properties of Composites Made with Aluminous Cement and Basalt Fibers Developed for High Temperature Application

Present paper deals with the experimental study of the composition of refractory fiber-reinforced aluminous cement based composites and its response to gradual thermal loading. Basalt fibers were applied in doses of 0.25, 0.5, 1.0, 2.0, and 4.0% in volume. Simultaneously, binder system based on the aluminous cement was modified by fine ground ceramic powder originated from the accurate ceramic blocks production. Ceramic powder was dosed as partial replacement of used cement of 5, 10, 15, 20, and 25%. Influence of composition changes was evaluated by the results of physical and mechanical testing; compressive strength, flexural strength, bulk density, and fracture energy were determined on the different levels of temperature loading. Increased dose of basalt fibers allows reaching expected higher values of fracture energy, but with respect to results of compressive and flexural strength determination as an optimal rate of basalt fibers dose was considered 0.25% in volume. Fine ground ceramic powder application led to extensive increase of residual mechanical parameters just up to replacement of 10%. Higher replacement of aluminous cement reduced final values of bulk density but kept mechanical properties on the level of mixtures without aluminous cement replacement.

High Temperature Composite of Aluminous Cement with Addition of Metakaolin and Ground Bricks Dust

The following article deals with the study of mechanical properties of aluminous cement composites exposure to high temperatures. The newly designed mixtures that resist the action of high temperatures 1000 °C find their application in various fields of industrial production or in the form of fire wall for protection bearing structures. All the mechanical properties such as compressive strength and tensile strength in bending were measured on samples 160x40x40 mm. These samples were exposed to temperatures 600 °C and 1000 °C and one group of samples was reference and stayed in laboratory condition. Aluminous cement unlike the common Portland cement keeps sufficient strength even after high temperature exposure. For ensuring required ductility the basalt fibers were added to the mixture. In an effort to use of secondary raw materials as a replacement for cement as well as a suitable binder was used metakaolin and ground brick dust. Very convenient characteristics of these components are their latent hydraulic potential that makes interesting hydration products.

Resistance of Refractory Cement Composite to Cyclic Temperature Loading

The aim of this study was to describe mechanical properties decline and macroscopic changes after cyclic thermal load of refractory slabs. Investigated elements were made from refractory cement composite containing natural basalt aggregate, fine ceramic powder, aluminous cement with high volume of Al2O3, different dosage of basalt fibres, water and plasticizer. Slabs with dimension 300 x 200 x 38 mm were exposed to elevated temperature 600 °C for three hours (temperature gradient 10 °C/min) and cooled to laboratory condition. This loading cycle was repeated six times. Tensile characteristics were investigated by bending test with clear span of supports 200 mm. Maximum force and displacement increased with increasing amount of basalt fibres. Maximum flexural strength of slabs corresponded to material characteristics measured on specimens 40 x 40 x 160 mm. Slabs with 1% of basalt fibres achieved flexural strength 4.8 MPa (after six loading cycles). The highest weight decline took place after the first loading cycle. Successful design of original fibre-cement composite has been approved by cyclic loading of larger dimension specimens.

CYCLIC TEMPERATURE LOADING RESIDUAL FLEXURAL STRENGHT OF REFRACTORY SLABS

This paper describes the effect of cyclic elevated temperature loading on refractory slabs made from high performance, fibre reinforced cement composite. Slabs were produced from aluminous cement-based composites, reinforced by different dosages of basalt fibres. The composite investigated in this study had self-compacting characteristics. The slabs used were exposed to different thermal loading – 600 °C, 1000 °C, six times applied 600 °C and 1000 °C. Then, flexural strength was investigated in all groups of slabs, including group reference slabs with no thermal loading. The results show that the appropriate combination of aluminous cement, natural basalt aggregate, fine filler and basalt fibres in dosage 1.00% of volume is able to successfully resist to cyclic temperature loading. Tensile strength in bending of these slabs (after cyclic temperature loading at 600 °C) achieved 6.0 MPa. It was demonstrated that it is possible to use this composite for high extensive conditions in real industrial conditions.

Numerical Analysis of Effect of Cyclic Thermal Load on Concrete to Concrete Bonding

This paper presents a numerical simulation and preliminary experimental investigation of bonding of concrete structures using cement and polymer cement as adhesive materials. The numerical model of concrete plates bonded with the polymer cement is created and subjected to cyclic thermal loading. As a result, the critical stresses are obtained. Also all the necessary materials for carrying out high-quality, fast and inexpensive experimental investigation of the strength of cement and polymer cement bond are prepared. The experimental setup and the first results obtained are also described. The bonding quality of the polymer cement shows satisfactory results under this kind of loading, thus the use of the polymer cement for bonding of concrete structures of different properties is reasonable in the specific fields considered in this study.