Petr Konvalinka

Development of Ultra High Performance Fiber Reinforced Concrete mixture

Formulation process of Ultra High Performance Fiber Reinforced Concrete (UHPFRC) is described in this paper. Materials locally available in the European Union were used throughout the optimization process. The mixture was also developed without any special curing, such as elevated temperature, pressure or vapor. The optimization process consisted of two steps. In the first step a cementitious matrix was optimized with respect to its maximal compressive strength, flexural strength and workability. The key element in the optimization process was to achieve maximal particles packing density, to choose efficient enough high-range water reducer (HRWR) and to decrease water binder ratio as much as possible. In the second step of the optimization process short, high tensile strength steel fibers were added into the matrix that showed highest workability and strength. The resulting compressive strength of UHPFRC mixtures exceeded 150 MPa after 28 days, average secant modulus of elasticity was in the range of 55 GPa and direct tensile strength in range of 10 MPa. During the optimization process mixtures with 1, 2 and 3% of fibers by volume were tested. It was found that with respect to acceptable workability and superior mechanical performance the optimal fiber content is between 2 and 3% by volume.

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.

Multi-Functional High-Performance Cement Based Composite

The paper introduces development of new type of high-performance Portland cement based composite applicable for number of practical utilization. The fundaments of performed research was to design mixture with controlled process of hydration, easy production, suitable time of setting, good workability and rapid evolution of mechanical properties as well as satisfactory long-term stability of hardened composite. Selected mixture were evaluated by means of mechanical properties and volume changes determination.

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).

Numerical analysis of projectile impact on cementitious composite

This paper describes the numerical modeling of projectile impact on cement based composite slabs. In the simulated experiment, local damage is inflicted by impact of defined projectiles on specimens made from normal strength concrete (NSC), ultra-high performance concrete (UHPC) and ultra-high performance fiber-reinforced concrete (UHPFRC). Deformable ogive-nose projectiles with diameter of 7.92 mm and mass of 8.0 g with impact velocity about 700m/s were used in the modeled experiment, hitting center of the specimens. Data from the measured and visual evaluation of specimen damage were used for comparison of specimen projectile impact resistance in relation to the used material and compared to the numerical prediction.

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.

Behaviour of Different Types of Concrete under Impact and Quasi-Static Loading

This paper describes mixture formulation of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) with 2% of fibres by volume and its response to quasi-static and dynamic impact loading. The UHPFRC mixture was prepared using locally available constituents and no special curing or mixing methods were used for its production. In addition, the mechanical parameters of three other types of concrete, i.e. normal strength concrete (NSC), fibre reinforced concrete (FRC) and high performance concrete (HPC) is compared. The main properties assessed throughout the experimental work are compressive, flexural and direct tensile strength as well as response of tested concretes to impact flexural loading. The impact loading is produced by a vertically falling weight of 24 kg from the height of 1 m on concrete prisms. The strain rate increase corresponds to low-velocity impacts such as vehicle crash or falling rocks. Compressive strength of UHPFRC exceeded 130 MPa and its direct tensile strength was 10.3 MPa. This type of concrete also exhibited strain hardening both in flexure under quasi-static conditions and during impact. Based on the comparison of impact reactions, it was concluded that the resistance of UHPFRC to impact loading is superior compared to the referent types of concretes (NSC, FRC, HPC).

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.

Long-term behaviour of concrete structures reinforced with pre-stressed GFRP tendons

Nowadays, composite materials are used more often in every part of industry, including civil engineering. Using these composite materials in civil engineering is innovational and there are many unanswered questions about these materials and the relaxation of the glass fibre reinforced polymers (GFRP) tendon in pre-stressed concrete is one of them. Knowing the long-term behaviour of the pre-stressed GFRP tendons is very important for the right design. Underestimating the long-term changes in the GFRP tendons can lead to serious problems or collapse of a structure. This paper shows two long-term experiments. One of them is the relaxation of pre-stressed GFRP tendons and the second one is creep of a concrete slab reinforced with pre-stressed GFRP tendons. The first experiment shows that relaxation of pre-stressed GFRP tendons is very high. A GFRP tendon was pre-stressed up to 37% (237,9 MPa) of its tensile strength (654,0 MPa). The decrease of tensile stress when the experiment was closed (after 132 days) was about 10,5%. Based on the experimental data, the numerical viscoelastic model consisting of Kelvin links was developed. The modulus of elasticity of the fibres and matrix was determined with the nanoindentation method. Others parameters were fitted from the experimental data. The chosen numerical model corresponds very well with the experimental data, but for the best outcome a longer experiment should be carried out. The numerical model and fitting of parameters were made in MATLAB 2007a software. The creep of the slab shows the long-term behaviour of a structure reinforced with GFRP tendons. The creep test was ended after one year. A concrete slab pre-stressed with GFRP tendons was subjected to a four point loading test with a constant load. During the year deflections and strain were recorded and hence the creep curve is plotted.