Petr Máca

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.

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).

BOND STRESS-SLIP BEHAVIOUR OF CONCRETE AND STEEL UNDER HIGH-LOADING RATES

Understanding the bond behaviour between reinforcing steel and concrete under high-loading rates is becoming more and more important with increasing frequency of natural disasters, impact loadings and a threat of terrorism. This paper aims to obtain a better understanding of the material interactions between the steel rebar and the concrete in the bond zone under different loading rates. During the experimental program push-in tests were conducted under quasi-static and dynamic loading conditions. Both a servo-hydraulic machine as well as an instrumented drop tower were used during the investigation. Samples with short-bond zone in the middle of a cylindrical specimen were used and only a small reinforcement bar diameter (10 mm) was investigated. This approach was chosen to ensure constant bond stress distribution and that the failure occurs during the first pass of the stress wave through the bond zone. Throughout the experimental programme the loading rate was varied from 0.01 mm/s to 8.3 m/s. Bond stress–slip relationships in dependence on the bond stress rate are presented in this paper. The results indicate a bond stress dependence on the loading rate although the scattering of the results is quite high. The experimentally determined dynamic increase factor (DIF) for concrete-steel bond stress is around 1.5 which is a value comparable to other authors.

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.

Innovative Lime-Pozzolana Renders for Reconstruction of Historical Buildings

Bulk density, matrix density, open porosity, compressive strength, bending strength, water sorptivity, moisture diffusivity, water vapor diffusion coefficient, thermal conductivity, specific heat capacity and thermal diffusivity of two innovative renovation renders on limepozzolana basis are analyzed. The obtained results are compared with reference lime plaster and two commercial renovation renders, and conclusions on the applicability of the particular renders in practical reconstruction works are drawn.

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.

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.

Mechanical, hygric and thermal properties of innovative renovation renders

Basic physical properties, mechanical, hygric, and thermal properties of three lime plasters containing metakaolin as pozzolana admixture and provided with different amounts of hydrophobizing additive are investigated in the paper. As the designed plasters are intended for renovation of historical buildings, their properties are compared with two commercial renovation renders commonly used in the building practice. Experimental results show that using the hydrophobizing additive in the amount of 1% of the mass of binder is the most prospective solution. The porosity of this plaster meets the basic WTA requirement of 40%. Its mechanical properties are comparable with the commercial plasters. The liquid water transport properties are favourable, the water vapor transport is somewhat slower than optimal but still acceptable, the thermal properties are satisfactory. Taking into consideration that this level of properties is achieved without using any kind of cement which is in accordance with the present effort of conservators to use materials with traditional composition, one may expect that this plaster can find successful application in reconstruction of historical buildings in the future.