Petr Daněk

During the service life of a structure, a number of phenomena occur that can affect the ability of the concrete structure to continue to perform its function. In the case of concrete load-bearing elements, fire and its effects represent significant influencing factor. Exposure to fire can, among other things, cause a great reduction of material properties of both concrete and reinforcement itself, but also affect their interaction. After a fire incident, it is necessary to assess the residual load capacity of the reinforced concrete elements. As part of the diagnostic work, material samples are usually taken from the structure after a fire. The purpose of the present paper was to investigate whether the experimentally obtained material characteristics can be used to verify the basic behaviour of concrete structures after fire. The structural material properties were used as input values for simplified numerical models of the structures. ATENA Cervenka Consulting software was chosen to construct a numerical model simulating the behaviour of the structural elements after exposure to fire loads. Finally, the effect of high temperatures on the residual load capacity of the beams determined by the numerical approach was compared with the experimental values of the load capacity of the beams after fire. The behaviour of concrete beams of dimensions 100 × 150 × 1800 mm reinforced with composite reinforcement was investigated. The specimens exposed to temperatures of 1000°C were subjected to mechanical bending tests after cooling to determine their residual load capacity.

This paper describes the development of a sandwich composite beam based on FRP materials which is a suitable alternative to reinforced concrete beams in specific applications. These FRP materials show higher durability and stability compared to reinforced concrete in aggressive environments (e.g. wastewater treatment plants). Compared to pultruded FRP beams, the developed solution better utilizes the properties of the sub-components. The results of this research suggest that the optimum use of composite materials is when the upper and lower flanges of the beam consist of pultruded composite profiles in the TT cross-section and the standing composite grating; this creates a relatively stiff beam with a high load-bearing capacity and resistance to aggressive environments at a very low self-weight. The final properties of this beam can be adjusted thanks to the variability of the dimensions of the web as well as the variability of a suitable laminate surface treatment. This in the final combination creates a sandwich composite structure. The behaviour of the composite beam is then confirmed in this paper using a four-point bending test. Different configurations of the beam design allowed us to determine the influence of the laminate surface layer (verification of the sandwich functionality), but also the influence of the beam connection at the standing point on the resulting behaviour. The results of the experiments demonstrated the optimal physical and mechanical parameters of the sandwich composite beam structure and gave us insights for further use of this type of structure.

Advanced experimental evaluation of the mechanical fracture properties of quasi‐brittle materials is an important part of the design, assessment, and computational modeling of structures made of these materials. The aim is to determine parameters independent of the test configuration or the size of the test specimens. This paper presents the results of an extensive experimental program which is part of the development of a comprehensive multilevel approach for experimental–computational determination of the mechanical fracture parameters of concrete. Three‐point bending and wedge‐splitting tests were performed simultaneously using three different geometrically similar specimen sizes and two well‐separated depths of initial notches. Before the start of the main testing program, the pilot tests that were performed were aimed at the design of the most appropriate concrete mixture. Special attention was paid to the selection of a maximum aggregate grain size. In order to study the process of concrete aging and the development of its mechanical properties, the accompanying destructive and non‐destructive tests were carried out at different ages of hardening. Fracture parameters were evaluated using the effective crack‐length formulation and work‐of‐fracture method. Finally, the size‐independent specific fracture energy was back‐calculated from the measured specific fracture energy.

The paper describes an experiment focusing on the way the material system influences the bond strength of large-format tiles installed on concrete substrate during mechanical loading under conditions that correspond to real-life application. This involves a controllable mechanical load applied over an area of a test model while observing its condition using non-destructive methods (ultrasonic pulse velocity test, acoustic emission method, strain measurement, and acoustic tracing). The model consisted of a concrete slab onto which were mounted four different systems with large-format tiles with the dimensions of 3 m × 1 m. The combinations differed in the thickness of the tile, the adhesive, and whether or not a fabric membrane was included in the adhesive bed. The experiment showed that the loading caused no damage to the ceramic tile. All the detected failures took place in the adhesive layer or in the concrete slab.

This paper focuses on the experimental determination of the shrinkage process in Self-Compacting High-Performance Concrete (SCC HPC) exposed to dry air and autogenous conditions. Special molds with dimensions of 100 mm × 60 mm × 1000 mm and 50 mm × 50 mm × 300 mm equipped with one movable head are used for the measurement. The main aim of this study is to compare the shrinkage curves of SCC HPC, which were obtained by using different measurement devices and for specimens of different sizes. In addition, two different times t₀ are considered for the data evaluation to investigate the influence of this factor on the absolute value of shrinkage. In the first case, t₀ is the time of the start of measurement, in the second case, t₀ is the setting time. The early-shrinkage (48 h) is continuously measured using inductive sensors leant against the movable head and with strain gauges embedded inside the test specimen. To monitor the long term shrinkage, the specimens are equipped with special markers, embedded into the specimens’ upper surface or ends. These markers serve as measurement bases for the measurement using mechanical strain gauges. The test specimens are demolded after 48 h and the long term shrinkage is monitored using the embedded strain gauges (inside the specimens) and mechanical strain gauges that are placed, in regular intervals, onto the markers embedded into the specimens’ surface or ends. The results show that both types of measurement equipment give a similar result in the case of early age measurement, especially for the specimens cured under autogenous conditions. However, the early age and especially long term measurement are influenced by the position of the measurement sensors, particularly in the case of specimens cured under dry air conditions. It was proven that the time t₀ have a fundamental influence on the final values of the shrinkage of investigated SCC HPC and have a significant impact on the conclusions on the size effect.

The paper deals with the experimental study of the behaviour of large concrete beams reinforced with glass fibre reinforced polymer (GFRP) rebars exposed to high temperatures equivalent to fire load. The four-point bending test was carried out on the beams after cooling. This study provided values for the load bearing capacity of the beams.

The paper deals with results of the experiment when ordinary and lightweight concrete was subjected to cyclic loading. Each test specimen was loaded with the force equivalent to one third of the expected compressive strength value similarly to the static modulus of elasticity test with the difference that the total number of loading cycles was nearly 4500. Dynamic modulus of elasticity and compressive strength was measured before and after cyclic loading. The result of this experiment is statistical analysis of the test results and assessment of the influence of multiple cyclic loading.

The paper discusses the results of an experiment that focused on measuring Young's modulus in compression using several different specimens. They were made from lightweight concrete with porous aggregate and differed in shape (cylinder × prism), type (casting × core drilling), and size. Each type category counted a minimum of 6 specimens. The determination of Young's modulus was supplemented by a measurement of the dynamic modulus of elasticity determined by the ultrasonic pulse velocity test. The experiment was concluded by a statistical analysis of the measured values, which focused on the influence of each specimen variety on the value of the elastic modulus.