Radoslav Sovják

TRIAXIAL COMPRESSIVE STRENGTH OF ULTRA HIGH PERFORMANCE CONCRETE

The aim of this work is to describe the strength of Ultra High Performance Concrete (UHPC) under triaxial compression. The main goal is to find a trend in the triaxial compressive strength development under various values of confinement pressure. The importance of triaxial tests lies in the spatial loading of the sample, which simulates the real loading of the material in the structure better than conventional uniaxial strength tests. In addition, the authors describe a formulation process for UHPC that has been developed without using heat treatment, pressure or a special mixer. Only ordinary materials available commercially in the Czech Republic were utilized throughout the material design process.

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.

AN EXPERIMENTAL INVESTIGATION INTO MOISTURE-INDUCED EXPANSION OF PLASTERS

This paper presents an experimental study on moisture-induced expansion of selected plasters. Contactless measurement is introduced and a coefficient of moisture expansion for different building plasters is established. It is found that stresses which might develop in building materials due to moisture variations are equal to or higher than stresses which might be caused by temperature variations.

EFFECTIVE FRACTURE ENERGY OF ULTRA-HIGH-PERFORMANCE FIBRE-REINFORCED CONCRETE UNDER INCREASED STRAIN RATES

The main objective of this paper is to contribute to the development of ultra-high performance fibre reinforced concrete (UHPFRC) with respect to its effective fracture energy. Effective fracture energy was investigated in this paper considering different fibre volume fractions and different strain rates. It was concluded that the effective fracture energy is dependent on the strain rate. In addition, it was found that higher fibre volume fractions tend to decrease the sensitivity of the UHPFRC to increased strain rates.

EFFECT OF FIBRE ASPECT RATIO AND FIBRE VOLUME FRACTION ON THE EFFECTIVE FRACTURE ENERGY OF ULTRA-HIGH-PERFORMANCE FIBRE-REINFORCED CONCRETE

This paper investigates the effective fracture energy of UHPFRC with various fibre volume fractions and various fibre aspect ratios. We have concluded that the effective fracture energy is dependent on both the fibre volume fraction and the fibre aspect ratio. In addition, we have found that both dependencies follow a linear trend.

Experimental Study of Primary Lining Tunnel Concrete after Thirty Years of Operation

This article shows the acquisition and testing of concrete from tunnel primary lining. The main objective of this article was to describe the mechanical properties of primary lining tunnel concrete at higher age. The primary lining was made from dry mixture sprayed concrete. The primary lining concrete was in contact with the surrounding rock continuously. The surrounding rock could be dry or with ground water. Some surrounding rocks could produce a highly aggressive environment for concrete. The question was, whether the sprayed concrete is still in service after 30 years in this environment.

Residual velocity of the non-deformable projectile after perforating the ultra-high performance fibre reinforced concrete

This contribution is focused on the optimal fibre content in the ultra-high performance fibre reinforced concrete (UHPFRC) mixture with respect to the residual velocity of the non-deformable projectile after perforating the UHPFRC slabs. Impact velocity of the non-deformable projectile was in the range of 700 m/s. The UHPFRC used in this study exceeded 150 MPa in uniaxial compression and tensile strength was around 10 MPa. In total 24 UHPFRC slabs with different fibre content were tested for impact loading. In addition, 8 slabs were tested for comparison including high strength concrete (HSC) and conventional fibre reinforced concrete (FRC). It was verified experimentally that UHPFRC had an excellent impact resistance compared to conventional materials such as FRC or HSC. Further it was found that optimal fibre content in UHPFRC for impact resistant structures is 2% by volume. Usage of less than 2% of fibre concrete by volume led to higher residual velocity of the projectile after perforating the slab and also to higher debris fragment mass.

Design of a novel horizontal impact machine for testing of concrete specimens

A novel design approach of an impact testing apparatus for concrete and highperformance cementitious composite specimens is described in this paper. To date, various approaches have been adopted when measuring the behaviour of concrete under impact loading. Most of the research teams used machines based on a guided drop-weight falling down freely on a concrete specimen. This paper describes the development process of a novel machine that is based on the pendulum principle. This experimental setup allows a horizontal placement of a specimen which has several advantages, such as easy elimination of a so called double hit, easy access to the sample and free space for the sensor mounting. The machine in the current setup is used for testing beam specimens, but, due to its modular concept it is also possible to rearrange the setup to test slabs utilizing a proprietary load transformer. The preliminary experimental results are shown in this paper as well as a description of the data acquisition system and adopted method of data filtration.

Experimental Tests of the UHPC in Triaxial Compression

An experimental study of the ultra high performance concrete (UHPC) subjected to triaxial compression is presented in this paper. The examined ultra high performance concrete was developed at the Faculty of Civil Engineering at CTU in Prague from the components locally available in the Czech Republic without using any special mixing technique or curing procedure. The uniaxial compressive strength was determined to be 123 MPa and 149 MPa for cylinders and cubes, respectively. Behaviour of the UHPC in triaxial compression was investigated in two different ways. In the first way cylinders were tested in triaxial chamber. The cylinders were 200 mm high and 100 mm in diameter. The cylinders were tested in triaxial chamber under lateral stresses equaled to 10, 20 and 30 MPa. The second way of testing UHPC in the triaxial compression was a triaxial loading machine where cubes were tested. The size of the cubes was 100 mm. The cubes were tested under the lateral stresses equaled to 15, 30, 60 and 90 MPa. Two equations for both cylinders and cubes showing the strength development were provided using power law regressions. The difference between triaxial strength development of cubes and cylinders was slight. The results obtained in this study were in good agreement with the results gained from the literature. It was experimentally verified that the strength development of the UHPC under different levels of the confinement pressure is not linear and tends to follow the power law function. In addition it was stated that the permissible stress surface of the UHPC falls below the permissible stress surface of the normal strength concretes, especially within the higher confinement pressures.

Properties of innovative renders on a lime basis for the renovation of historical buildings

Mechanical and hygric properties, namely compressive strength, bending strength, water sorptivity, moisture diffusivity and the water vapor diffusion coefficient, of two innovative lime plasters with pozzolanic admixtures (metakaolin, metashale) and two commercial renovation plasters are studied in this paper. Reference measurements with common lime plaster are done as well. Experimental results show that the analyzed lime-pozzolana plasters are suitable for an application in reconstruction of historical buildings. Their mechanical properties are superior to lime plaster and better or similar to commercial renovation plasters. Their hygric properties are similar to lime plaster or slightly better and comparable with commercial plasters.