Small incinerators of dangerous wastes, including those from hospitals, are a source of emissions of highly variable compositions and concentrations. Mercury is a very dangerous pollutant causing neurotoxicity in human organism. The effect of HCl concentration on adsorption of mercury on activated carbon-based sorbent was studied for the incineration of hospital waste in a 250 kg/h capacity unit. The maximum concentration of adsorbed mercury on activated carbon was determined as a function of concentration of Hg and HCl in combustion products. Based on the expected chemical reactions and the thermodynamics, the adsorption of mercury from flue gases in oxidising atmosphere has been explained. The activated carbon-based sorbent was also capable of adsorbing acid gases like HCl. The efficiency of removal of mercury from combustion products increased up to 85-87% with the concentration of HCl in flue gases. The addition of calcium hydroxide increased the amount of mercury trapped on the sorbent only by about 10%. These tests proved that an entrained flow adsorber is a suitable unit for the removal of mercury from combustion products. The consumption of activated carbon for the mercury removal was from 0.1 to 0.15 mg/Nm3 of flue gas. The advantage of an entrained flow adsorber lies in its easy continuous operation. Therefore, it is a suitable unit for small and medium size incinerators of municipal and hospital waste.

Mercury concentrations in ash taken from electrostatic precipitations (ESPs) installed in bituminous coal and lignite power plants have been analyzed by the X-ray fluorescence (XRF) method and leaching test supported by detailed coal and ash compositions' analyses, surface scans and particles size distribution studies. The results showed that mercury was present on the surface of ash particles. Its concentration decreased upon increasing size of ash particles. Leaching tests showed that only small part of mercury was removed with water which suggests the fact that it occurred mostly in the form of insoluble compounds such as Hg, HgO, HgS and Hg2Cl2. There existed ash particles of the diameters from 50 to 60 gm, characterizing by the maximum capability to adsorb mercury and its compounds. The authors suggest that metals like copper and lead formed ash active centers that were preferably occupied by molecules containing atoms of mercury. It was highly possible to expect that content of these elements in ash would have a significant effect on sorption of mercury from combustion gases.

Structural concrete of the building of Vítkovice railway station from 1970s is characterized via mechanical fracture parameters. In this paper, six core-drilled cylindrical specimens with diameter of 75mm were provided by a Chevron type notch of depth of 12 mm. Three-point bending fracture tests were conducted on these specimens supported as beams with the span of 170mm. Load vs. deflection and load vs. crack mouth opening displacement diagrams were recorded from which modulus of elasticity (E), fracture toughness (KIc) and fracture energy (GF) were determined using linear elastic fracture mechanics approach and work-of-fracture method. Mean values of these parameters (with their coefficient of variability) were obtained as follows: E = 39.4GPa (29.1%), KIc = 0.90 MPa·m1/2 (18.5%), GF = 174.0J/m2 (26.0%). These parameters can serve as first guess or indicative values for structural assessment and life-time predictions.

In this paper, the attention is paid to investigation of the importance of the Interfacial Transition Zone (ITZ) in concrete for the global fracture behaviour. A simplified cracked geometry (consisting matrix, ITZ and aggregate) is modelled by means of the finite element method with a crack terminating at the matrix-ITZ interface. Numerical studies assuming two various ITZ thicknesses and several various ITZ elastic moduli are performed. Based on the values of the opening stress ahead of the crack tip (its average value and stress range) a few conclusions are discussed. The pilot analyses dealing with the effect of ITZ on the stress distribution should contribute to better description of toughening mechanisms in silicate-based composites.

This paper discusses numerical models of the wedge-splitting test on cylindrical specimens made of quasi-brittle material. These models are used for predictions of crack/damage propagation and are crucial for the further evaluation of fracture parameters. Validation of these models is made based on experimentally obtained data. To assemble the numerical models, ATENA FEM software was used. In this software, damaging of the structure/specimen caused by cracks, their initiation and progressive propagation throughout the loading process, can be modelled. In this case, the material model for concrete based on cohesive crack approach was used. The analysis is focused on comparison of the 2D and 3D numerical models created with real material properties obtained from experimental data.

A study on the accuracy of an approximation of the stress field in a cracked body is presented. Crack-tip stress tensor is expressed using the linear elastic fracture mechanics (LEFM) theory in this work, more precisely via its multi-parameter formulation, i.e. by Williams power series (WPS). Determination of coefficients of terms of this series is performed using a least squares-based regression technique known as over-deterministic method (ODM) for which results from finite element (FE) method computation are usually taken as inputs. Main attention is paid to a detailed analysis of a suitable selection of FE nodes whose results serve as the inputs to the employed method. Two different ways of FE nodal selection are compared – nodes selected from the crack tip vicinity lying at a ring of a certain radius versus nodes selected more or less uniformly from a specified part of the test specimen body. Comparison of these approaches is made with the help of procedures developed by the authors which enable both the determination of the coefficients of terms of the analytical WPS approximation of the stress field based on the FE results and the backward reconstruction of the field (again using WPS) from those determined terms’ coefficients/functions. The wedge-splitting test (WST) specimen with a crack is taken as example for the study.

Quasi-brittle type of fracture behaviour is viewed from a unique perspective in the paper. This view explains and extends the classification of fracture of materials, as it is commonly understood, and might provide a way to a more general understanding of this phenomenon. The proposed approach separates the ‘effective’ (brittle) fracture phenomenon from the evolution of the nonlinear zone at the crack tip. This separation is performed on the energetic level, the evaluated work of fracture, as well as on the level of geometrical description of the two separated entities which are the products of the fracture phenomenon: The crack surface and the nonlinear zone around the crack front. The approach is introduced here mainly from the philosophical and theoretical standpoint, its verification and validation is included only partially.

Fracture behaviour (crack deflection) of a crack in a silicate-based composite is investigated. The three-point bending test is simulated numerically by means of the finite element method and the influence of existence of a stiff aggregate ahead of the crack tip is studied and discussed. The maximum tangential stress criterion is applied in order to calculate the initial crack propagation angle. Various geometrical configurations are investigated (the aggregate eccentricity and depth are varied) and the generalized MTS criterion based on the Williams expansion is tested considering various numbers of initial terms of the series. Unfortunately, the results show that using the generalized MTS criterion is strongly limited by the elastic mismatch caused by the presence of the stiff aggregate near the crack tip. (C) 2017 Published by Elsevier Ltd.

An analysis focused on capturing the phenomenon of quasibrittle fracture is presented. Selected parameters relevant for quasi-brittle fracture are evaluated and an assessment of their dependence on the size and shape of the test specimen is studied. Determination of these fracture characteristics is based on the records of the fracture tests on notched specimens, particularly from recorded loading diagrams. A method of separation of the energy amounts released for the propagation of the (effective) crack and that dissipated within the volume of a large nonlinear zone at the crack tip – the fracture process zone – is introduced and tested on selected data from experimental campaigns published in the literature. The work is accompanied with own conducted numerical simulations using commercial finite element code with implemented cohesive crack model. Results from three-point bending tests on specimens of different sizes and relative notch lengths are taken into account in this study. The proposed model has only two parameters whose values are constant for all specimen sizes and notch lengths.

A numerical study of the structural response of the prestressed monoblock railway sleepers used on Czech railway tracks is presented. The build quality of prestressed railway sleepers is assessed using three-point bending tests as stipulated by European Standards. Two sleeper cross-sections are evaluated with respect to static loading. The tests are modelled herein using ATENA nonlinear finite element method (FEM) software. This programme enables the realistic estimation of failure load, pre/postpeak behaviour and crack pattern using non-linear fracture mechanics methods as well as damage and plasticity models. The main purpose of this study is to quantify the influence of prestressing reinforcement position on sleeper response during evaluative tests. A comparison of the simulation results with ex-perimental data is also performed.

Fracture properties of quasi-brittle cementitious composites are typically determined from the load–displacement response recorded during a fracture test by using the work-of-fracture method or possibly other relevant fracture models. Our contribution is focused on a set of experimental tests which are used to study the fracture behaviour on notched dog-bone-shaped specimens made of cementitious materials. These specimens are subjected to modified compact tension (ModCT) test under a specific range of eccentricity of the tensile load. This type of test generates a stress state in the specimen ligament which combines a direct tension with a defined level of bending due to eccentricity of the tensile load. Several values of relative notch length are also considered. While the crack propagates, a variety of stress states, resulting in variations in the crack-tip stress and deformation constraint, appears in the ligament zone because of the changes in the eccentricity of the applied load, which influences the fracture behaviour of the investigated specimens. The K-calibration, T-stress, CMOD and COD curves for ModCT specimens are introduced and variations of these curves with varying load eccentricity are discussed.

Cement-based composites are traditionally used building materials. Concrete is the basic representative of this type of materials which exhibit the so called quasi-brittle response. Quantification of mechanical fracture parameters is performed using fracture tests on specimens with a stress concentrator. Load versus crack mouth opening displacement (P–CMOD) diagrams are recorded during these tests. In order to correctly evaluate these diagrams, an advanced own developed software tool was used for the data filtering and appropriate modifications. In this paper, the programmed Java utility is generally introduced and its utilization demonstrated on the set of recorded P–CMOD diagrams, which are further evaluated using Double-K fracture model.

Records of fracture tests on steel fibre reinforced concrete notched specimens in a three-point bending configuration are evaluated in detail and selected results are discussed in the paper. The values of fracture parameters are determined using work of fracture method and double-K fracture model. Primarily, the role of plain concrete as a matrix in steel fibre reinforced concrete specimens is studied with regard to the recorded fracture response.

The paper presents data obtained in own experimental campaign focused on the investigation of the effect of fracture propagation on the change in values of electrical resistivity and ultrasound pulse passing time in notched beam specimens. Test geometry of three-point bending was utilized to propagate the crack through the specimen ligament in several loading−unloading cycles, between which the electrical resistivity and ultrasonic measurements over the fractured region in the centre of the beam were performed. Equivalent elastic crack length was estimated at those loading stages. Relationships among the changes in the investigated, non-destructively gained parameters, the effective crack length and the distance from crack tip are discussed with respect to two variants of treatment of the specimens’ surface during testing: the pre-dried and the wet one.

The paper presents a fracture-mechanics study conducted on a novel test configuration based on a combination of the wedge splitting and the bending test geometry.Four variants of this configuration differing in parameters of the specimen shape and boundary conditions were considered in the study.These selected variants exhibit significantly different stress state conditions at the crack tip, or, more generally, in the whole specimen ligament.The variation of the crack-tip stress constraint is of a crucial importance for the study because the expected differences in the fracture process zone extent and the amount of energy dissipation related to this zone is of particular interest here.An extensive experimental campaign was conducted on concrete specimens made of the same mixture.Processing and evaluation of the test data using the standard work of fracture technique was accompanied with numerical simulations by means of finite elements with implemented cohesive crack model to correlate the amount of energy dissipation with the simulated damaged zone extent.Variations of the fracture energy and also the stress/inelastic strain distribution with the change of the crack-tip constraint parameter and the notch length are observed. INTRODUCTION, MOTIVATIONA novel method for evaluation of fracturemechanical parameters of quasi-brittle cementitious composites was introduced recently [1] by the Brno University of Technology (BUT) partner of the joint research presented in this paper.The method should eliminate drawbacks of most of the established models for determination of those characteristics, which is the dependence of values the fracture properties determined from fracture tests on laboratory specimens on the specimen size, geometry and its boundaries.Capturing these effects shall be achieved via a detailed incorporation of the fracture process zone (FPZ) existence into the evaluation

At combustion of hospital waste numerous harmful pollutants were formed inclusive persistent PCDD/F (polychlorinated dibenzodioxines and dibenzofuranes) compounds.To eliminate these substances an existing incinerator consisting of the furnace heat exchanger with flue gas cleaning was used.This incinerator had a capacity of 250 kg of waste per hour.Elimination od acid gases was executed by dosing of solid sorbents (calcium hydrate and bicarbonate) at two points of the waste gases duct.Local temperatures of waste gases at these two points were different.Sorbents were dosed at different distances from the textile filter by which ash and sorbent were separated from the stream of waste gases.By this arrangement it was possible to alter the residence time of sorbents in waste gases.The effect of addition of bicarbonate on dechlorination and desulphurisation was also studied.The executed experiments indicated that at dosing of calcium hydroxide into waste gases at higher temperatures sulphur oxides reacted preferably to hydrogen chloride.The strong effect of HCl in waste gases on formation of PCDD/F was proved.For sufficient elimination of persistent compounds from waste gases it was necessary to add beside calcium hydrate also active carbon.Two types of sorbents based on carbon were tested, namely Norit and Chezacarb B. Both were macroporous.Chezacarb B was carbon black from the process of hydrogen production by partial oxidation.Efficiency of both sorbents for removal of PCDD/F was similar.

The accuracy of the multi-parameter approximation of the stress fields in a cracked body is studied in the paper. This approximation, which uses the Williams power series expansion (WE), is intended to be used to estimate the extent of the nonlinear zone, which plays a significant role within tensile failure and the fatigue assessment of non-brittle materials. The characteristics of this zone could be potentially incorporated into methods of determining the true values of fracture parameters and the fatigue behaviour descriptors of materials exhibiting nonlinear failure. Considering the fact that in the case of elastic–plastic and especially quasi-brittle materials the size of this zone is substantial in comparison to specimen dimensions, it is necessary to consider a large region around the crack tip for this task. An automatic routine implemented as a Java application was created to determine the values of coefficients of the higher order terms of the WE that describe crack-tip fields. These values are calculated using the Over-Deterministic Method (ODM), which is applied to the results of the finite element (FE) analysis of an arbitrary mode I test geometry. Furthermore, another Java application developed by the authors provides an analytical reconstruction of the crack-tip stress field by means of the truncated WE, and enables detailed analysis of the crack-tip stress field approximation. The developed procedures simplify the analysis of the description of mechanical fields at a greater distance from the crack tip considerably. The presented research is focused on the optimisation of the selection of FE nodal results entering the ODM procedure used to determine the values of coefficients of the higher order terms of the WE. The aim is to improve the accuracy of the approximation.

The paper presents a study on accuracy of the multi-parameter approximation of the stress field in a cracked body using Williams power series calibrated according to results of FEM computation. Main attention is paid to a detailed analysis of suitable selection of FE nodes, whose results serve as inputs to the over-deterministic method (ODM) employed for determination of the coefficients of terms of the Williams expansion describing the crack-tip field. Two different ways of FE nodal selection are compared – nodes selected from the crack tip vicinity versus nodes selected from a specific part of the test specimen body in a specific way. Comparison is made with the usage of the authors’ own developed procedures enabling both the determination of the coefficients of the analytical approximation of stress field based on the FE results and the backward reconstruction of the field from those determined terms’ coefficients/functions. The wedge-splitting test (WST) specimen with a crack is taken as example for the study.

Measurement and visualization of the fracture process zone (FPZ) in the silicate composites is essential for understanding how the crack is formed and how it is propagating through the material. Although several time-lapse radiography and tomography experiments during the three-point bending of silicate composites with conventional imaging techniques have been already realized, the crack initiation and propagation is so fast process, that it is very difficult to capture sufficient spatial information in the exact moment when this process is occurring. Nowadays progress in X-ray imaging instrumentation, especially single photon counting CdTe detectors with a fast parallel readout, allows speeding up the acquisition of the tomographic data while keeping the intrinsic resolution of tens of micrometers and high efficiency even for photons with higher energy. Tomography of relatively fast processes is becoming realistic even in laboratory conditions using commercially available microfocus X-ray tubes. As an example of such a fast tomography, results of "on-the-fly" series of tomographic experiments performed on the specimen subjected to continual three-point bending test is presented.

The paper presents an analysis aiming at capturing the phenomenon of quasi-brittle fracture based on the record of the fracture tests on notched specimens. A method of separation of the energy amounts released for the crack advance and that dissipated within the volume of the sizeable nonlinear zone at the crack tip – the fracture process zone– is introduced. The approach is tested on selected data of published experimental campaigns accompanied with own conducted numerical simulations.