Zdeněk Chobola

The study of the resistance of plain concrete to high temperatures is a current topic across the field of civil engineering diagnostics. It is a type of damage that affects all components in a complex way, and there are many ways to describe and diagnose this degradation process and the resulting condition of the concrete. With regard to resistance to high temperatures, phenomena such as explosive spalling or partial creep of the material may occur. The resulting condition of thermally degraded concrete can be assessed by a number of destructive and nondestructive methods based on either physical or chemical principles. The aim of this paper is to present a comparison of nondestructive testing of selected concrete mixtures and the subsequent classification of the condition after thermal degradation. In this sense, a classification model based on supervised machine learning principles is proposed, in which the thermal degradation of the selected test specimens are known classes. The whole test set was divided into five mixtures, each with seven temperature classes in 200 °C steps from 200 °C up to 1200 °C. The output of the paper is a comparison of the different settings of the classification model and validation algorithm in relation to the observed parameters and the resulting model accuracy. The classification is done by using parameters obtained by the acoustic NDT Impact-Echo method and image-processing tools.

The paper presents the results of experiments carried out on test samples made from reinforced concrete. Within this experiment, concrete beams with steel reinforcement in the middle were made. The concrete samples were tested by a non-destructive acoustic Impact-echo method before and after high temperature loading (after cooling to room temperature). We focused on the dominant frequencies shift in the frequency spectra obtained by this method. The aim was to assess the ability of aforementioned acoustic method to detect the thermal damage of steel reinforced concrete.

Nonlinear Ultrasonic Spectroscopy (NUS) methods investigates nonlinear phenomena that occur when the sound or ultrasound waves pass through the material. This paper focuses on the use of these methods for nondestructive testing of reinforced concrete damaged by high temperature. For this research, reinforced concrete beams with one steel rod were made and the NUS measuring equipment was assembled. An appropriate nonlinear parameter was selected to assess the damage. The nonlinear behavior of thermally damaged reinforced concrete was manifested by the deformation of ultrasound waves passing through the measured samples, resulting in nonlinear effects in the frequency spectra of the recorded signal.

The paper deals with the methodology, quantification and comparison of measurement deviations for two types of the ceramic shard. The ceramic material has better polarization properties than concrete material. For impedance measurement the ceramic shard is more appropriate. The IS method diagnostics based on processing of measured data which aims evaluating the absolute error and the relative error (deviation) of the loss factor values. Results show the spectrum parts which are useful in the lower frequency range only. When Agilent 33220A and Agilent 54645A are used to carry out the IS, the most important factor from the viewpoint of the tgδ(f) determination accuracy is the accuracy of the phase shift between the excitation signal and that at the measured system output. Other factors are the measurement accuracy of the electric capacity and resistance of the oscilloscope input channel.

This paper focuses on the use of nonlinear acoustic spectroscopy with one excitation signal for NDT of concrete parts exposed to high temperature. The rate of the temperature rise, the highest achieved temperature, time of exposure to elevated temperature and the cooling rate has an influence on the overall damage of concrete structures affected by fire. The test specimens described in this paper have been heated for various periods of time and at a different temperature. Nonlinear acoustic spectroscopy method was performed before and after heat load.

The paper presents the results of the experimental work which was focused on the use of the Impact-echo method for testing of concrete specimens damaged by high-temperature. The test specimens were heated to 600 and 800 °C for sixty, one hundred twenty and three hundred minutes. After heating, the samples were continuously cooled to room temperature and tested by Impact-echo method.

The use of different fiber types (polypropylene, steel, glass, natural fiber) to cement based materials for better performance is becoming more and more frequent. The addition of fibers increases the mecsshanical characteristics of these composites. This paper examines the cement based composites reinforced by steel fibers. We focused on the relationship between the amount of steel fibers (40, 80 and 120 kg on 1 m³ of concrete) and changes of parameters of acoustic waves which pass through the material. For non-destructive evaluation the two pulse-echo methods were used. First of these pulse-echo methods used as impactor the steel hammer and the second one used electronic ultrasound transmitter. The aim was to verify the ability of these methods to determine the amount of steel fibers in concrete.

When increasing the temperature of the concrete, there are various chemical processes that cause mechanical and chemical changes of material. These changes have a significant impact on the physical and mechanical properties of concrete structures (modulus of elasticity, toughness, surface hardness), which are essential to meet the basic building requirements, especially mechanical strength, stability and fire safety. The paper presents the results from non-destructive testing of concrete specimens degraded by high temperatures. The test specimens were heated to 600°C. This temperature was then maintained for one, two and five hours. After heating, the samples were slowly cooled down to room temperature and then tested by acoustic non-destructive methods (Impact-echo method and one of the nonlinear acoustic spectroscopy methods). The objective was to verify whether the parameters obtained by these methods are able to describe material degradation due to high temperature and residence time at this temperature.

The paper is focused on non-destructive measurement of high temperature degraded concrete test specimens of three mixtures different by the use of coarse aggregate. Testing is done by ultrasonicpulse velocitymethod and Impact-Echo method. Non-destructive results are compared with destructive tests. Ultrasonic pulse velocity, dominant resonance frequency and acoustic impedance are discussed and compared with changes in density, cubic compressive strength, and tensile strength of concrete. The paper suggests possible assessment of degraded concrete by the change in acoustic impedance dependent on residual tensile strength.

The paper is concerned with the technical aspects of the appraisal and retrofitting process of fire damaged reinforced concrete structures. The assessment of fire damaged structures is carried out along lines similar to those of the appraisal of existing structures. In practice, constructions are most often assessed by destructive tests in-situ and on core bore specimens. In addition to destructive tests, damaged structures are also assessed by non-destructive ones. The present paper shows the use of non-destructive methods of measurement using the acoustic-emission and impact-echo methods. Acoustic emission provides valuable data on the structural integrity of a material. This method has a significant potential to be used for in-situ monitoring and evaluation of the current state of structures. An impact-echo method is based on impact-generated stress waves that propagate through concrete and are reflected by internal flaws and external surfaces. Impact-echo can be used to determine the location and extent of flaws such as crack delamination, voids, honeycombing and deboning in plain, reinforced, and post-tensioned concrete structures. The paper presents a possible rehabilitation plan based on the potential results obtained by these non-destructive methods.