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The paper deals with selected alkali-activated aluminosilicate (AAAS) composites based on ceramic precursors in terms of characterization by mechanical fracture parameters. Two composites made of brick dust as a precursor and an alkaline activator with a silicate modulus Ms = 1.0 were investigated. The composites differed in the fineness of grinding of the precursor – in the first set it was 0 to 1 mm, in the second set 0 to 0.3 mm. The filler was crushed brick. The test specimens had nominal dimensions of 40 × 40 × 160 mm and were provided with notches in the middle of the span up to 1/3 of the depth of the specimens after 28 days of hardening. Five to six specimens from each composite set were tested. The specimens were subjected to three-point bending tests, in which force vs. displacement (deflection in the middle of the span) diagrams (F–d diagrams) and force vs. crack mouth opening displacement (F–CMOD) diagrams were recorded. After correction of these diagrams, the values of static modulus of elasticity, effective fracture toughness, effective toughness and specific fracture energy were determined using the Effective Crack Model and the Work-of-Fracture method. After the fracture experiments, the values of informative compressive strength were determined on one of the fractured parts. At the same time, the values of static modulus of elasticity, tensile strength and specific fracture energy were identified using artificial neural networks and F–d diagrams measured and simulated in the ATENA FEM software. All evaluations included the determination of basic statistics of parameters.

The paper deals with selected alkali-activated aluminosilicate (AAAS) composites based on ceramic precursors in terms of their characterization by mechanical fracture parameters. Composites made of brick dust as a precursor and an alkaline activator with a silicate modulus of Ms = 0.8, 1.0, 1.2, 1.4 and 1.6 were investigated. The filler employed with one set of composites was quartz sand, while for the other set it was crushed brick. The test specimens had nominal dimensions of 40 × 40 × 160 mm and were provided with notches at midspan of up to 1/3 of the height of the specimens after 28 days. 6 samples from each composite were tested. The specimens were subjected to three-point bending tests in which force vs. displacement (deflection at midspan) diagrams (F–d diagrams) and force vs. crack mouth opening (F–CMOD) diagrams were recorded. After the correction of these diagrams, static modulus of elasticity, effective fracture toughness, effective toughness and specific fracture energy values were determined using the Effective Crack Model and the Work-of-Fracture method. After the fracture experiments, informative compressive strength values were determined from one of the parts. All of the evaluations included the determination of arithmetic means and standard deviations. The silicate modulus values and type of filler of the AAAS composites significantly influenced their mechanical fracture parameters.

Mechanical fracture parameters were obtained for special fine-grained cement-based composites from three-point bending tests. A total of four sets of composite specimens were tested. All of the sets of composites were based on a general recipe and differ in the amount of high-strength aggregate and/or dispersed steel fibres present. Standardized prism specimens with a nominal size of 40 × 40 × 160 mm were used for the fracture tests after 131 days of curing. An initial notch was cut in the centre of the prisms with a depth approximately equal to one third of the specimen’s height. Three specimens from each set of composites were tested in the three-point bending fracture test configuration. Load versus midspan deflection diagrams were recorded. Experimentally obtained load vs displacement diagrams were corrected and analysed using the Effective Crack Model, the Work-of-Fracture method and an independent identification technique using numerical modelling. The most important mechanical fracture parameters, such as static modulus of elasticity, effective fracture toughness, specific fracture energy and effective tensile strength, were determined. Compressive and splitting tensile strength values were obtained from the halves of the specimens left over after the bending tests.

This paper concerns the results of research into the influence of the composition of rock inclusions on the fracture response of cement-based composite specimens. Specially designed specimens of the nominal dimensions 40 × 40 × 160 mm with inclusions in the shape of prisms with nominal dimensions of 8 × 8 × 40 mm were provided with an initial central edge notch with a depth of 12 mm. These specimens, which were made of fine-grained cement-based composite with different types of rock inclusion – amphibolite, basalt, granite, and marble – were tested in the three-point bending configuration. Fracture surfaces were examined via scanning electron microscopy and local response in the vicinity of rock inclusions was characterized via the nanoindentation technique. The aim of this paper is to analyse the influence of the chemical/petrographic composition of rock inclusions on the effective mechanical fracture parameters of cement-based composites, as well as on the microstructural mechanical parameters of the interfacial transition zone. The results of this research indicate the significant dependence of the effective fracture parameters on the petrographic and related chemical composition of the rock inclusions.

Four sets of alkali-activated aluminosilicate (AAAS) composites based on ceramic precursors were studied in terms of their characterization by mechanical fracture parameters as a basis for considerations of durability. AAAS composites made of brick powder as a precursor and alkaline activator with various silicate moduli (Ms = 0.8, 1.0, 1.2, 1.4, and 1.6) were investigated. The sets of AAAS composites differed in terms of the used filler: quartz sand or brick rubble. Two different precursor particle size ranges of 0–1 mm and 0–0.3 mm were used for both types of filler. The test specimens had nominal dimensions of 40 × 40 × 160 mm and were provided with a notch at midspan after 28 days of hardening. The notches were cut up to 1/3 of the height of the specimens. The specimens were subjected to three-point bending fracture tests during which force vs. deflection (F–d) and force vs. crack mouth opening displacement (F–CMOD) diagrams were recorded. Tensile strength ft,ID and specific fracture energy GF,ID values were identified using the inverse method based on a neural network ensemble. The obtained F–CMOD diagrams were subsequently evaluated using the double-K fracture model supported by the ft,ID and GF,ID values. The double-K model allows the quantification of two different levels of crack propagation: initiation, which corresponds to the beginning of stable crack growth, and unstable crack propagation.

This paper deals with the pilot characterization of a special alkali-activated aluminosilicate composite composed of waste brick powder, brick rubble and a solution of potassium water glass. Fracture tests were conducted on the specimens via three-point bending and fracture parameters were evaluated. Selected specimen was investigated using micro-tomography to supplement the results with visual information about the inner structure of this newly designed material before and after the mechanical loading. Tomographic measurements and image processing were conducted for a qualitative and quantitative assessment of changes in the internal structure with an emphasis on the calculation of porosimetric parameters and visualization of the fracture surface. Fractal dimension of fracture surface was estimated.

This paper concerns research into the importance of the interfacial transition zone around inclusions of selected materials in fine-grained cement-based composite. Tests were performed on eight sets of prismatic test specimens. The sets differed in the inclusion materials used, which were placed at midspan above the initial central edge notch. The first was a reference set without any inclusion, the second contained a steel inclusion, four more contained different types of rock inclusion, the seventh contained an inclusion of extruded polystyrene, and the last contained a space of the same dimensions as that occupied by the inclusions in sets 2 to 7. The test specimens were subjected to three-point bending fracture tests at the age of (usually) 28 days. The fracture response was analysed by means of fracture mechanics theory, and apparent mechanical fracture parameters (modulus of elasticity, fracture toughness and fracture energy) were evaluated. The conclusion shows that a possible relationship exists between the differences in the mechanical fracture parameters of specimens with/without an inclusion and the existence of the interfacial transition zone.

This paper presents the results of fracture tests conducted on nine concrete specimens from drill-cores taken from three selected objects located at the former Transgas Gas Control Center in Prague. The specimens were provided with a chevron notch before being tested in three-point bending. The evaluation included arithmetic mean values and basic statistics for the static modulus of elasticity, fracture toughness and specific fracture energy of the assessed concrete specimens.

The paper is focused on the advanced evaluation of fracture tests of selected rock specimens. Assessment of fracture response of chevron notch cylindrical specimens of Silesian granite subjected to three-point bending test was performed. The effective crack model is used and academic software chevroncylinder (author Petr Frantík) based on the finite element method is applied for the analyses. Based on the described procedures, estimation of maximum values of the effective fracture toughness are established and discussed. discussed. Článek se zabývá pokročilým vyhodnocením lomových testů vybraných horninových vzorků. Je provedeno hodnocení lomové odezvy válcových vzorků s šípovým vrubem ze Slezské žuly vystavených tříbodovému ohybu. Je použit model efektivní trhliny spolu s akademickým programem chevroncylinder (autor Petr Frantík) založeným na metodě konečných prvků. Na základě uvedených postupů je odhadnuta a diskutována maximální hodnota efektivní lomové houževnatosti.

Mechanical fracture properties of interfacial transition zone (ITZ) of fine-grained composite based on cement matrix with different types – basalt, granite, marble and amphibolite – of rock inclusion were studied. Specimens with the initial stress concentrator were tested in standard three-point bending configuration. Fracture surfaces were examined with light and electron microscopes. Local ITZ response was characterized by nanoindentation in the vicinity of rock inclusions. Local elasticity, hardness and viscous properties were assessed. It has been shown that the ITZ is mechanically weaker compared to the bulk matrix in the region of ca. 0–20 μm from the inclusion for all specimen’s types. It exhibits gradual increase of elastic modulus and hardness, which can be approximately expressed by a power law. On the other hand, the creep in ITZ was found to be higher compared to the bulk matrix. The results of nanoindentation measurements are in a good agreement with overall mechanical properties, fracture response and microstructure measurements done by scanning electron microscopy.

Extruded polystyrene (XPS) is a material with applications in the building industry, where it is typically used as thermal insulation. Fracture experiments in the three-point bending configuration were conducted on XPS beam specimens with an initial stress concentrator made before testing. The nominal dimensions of the beams were 40 × 40 × 160 mm. The depth of the initial edge notch on the bottom side of the specimen was approximately 1/3 of specimen height. The span length was 120 mm. Load vs. displacement diagrams were recorded during fracture tests, and subsequently the modulus of elasticity (E), effective fracture toughness (KIce) and specific fracture energy (GF) of the XPS were determined. The mean values obtained for the mechanical fracture parameters and coefficients of variation (number of specimens) were the following: E = 10.7 MPa, 9.2 % (7); KIce = 0.0547 MPa∙m1/2, 16.7 % (3); GF = 183.2 J∙m–2, 34.3 % (3).

The structural concrete can be also characterized via parameters obtained by evaluation of fracture tests. Therefore, this paper presents software CheF developed in Java programming language, designed for evaluation of three-point bending fracture tests of core based concrete specimens with chevron type notch. The records of fracture tests in form load vs. displacement and load vs. crack mouth opening displacement diagrams are analysed by software CheF to obtain values of selected mechanical fracture parameters: modulus of elasticity E, fracture toughness KIc, and fracture energy GF, determined based on the linear elastic fracture mechanics approach and work-of-fracture method.

The aim of this paper is to describe the procedure of determining the mechanical fracture parameters of selected concrete specimens taken from panels after the fire experiments. The records (in form load vs displacement diagrams) of three-point bending fracture tests of these specimens with initial stress concentrators was first advanced corrected and subsequently evaluated using the Effective Crack Model and the work-of-fracture method. The increasing temperatures during the fire experiments ranging between 550 to 1000 °C led to a decrease of modulus of elasticity and fracture toughness values and to the increase of fracture energy value. The 2D laser profile scanner was used to estimate the degree of complexity of fracture surfaces; its statistical dependence on the mechanical fracture parameters proved to be moderate – the absolute value of the correlation coefficient was about 0.5°[–].

In this paper, the attention is paid to investigation of the importance of the interfacial transition zone (ITZ) in selected fine-grained cement-based composites for the global fracture behaviour. This is a region of cement paste around the aggregate particles which specific features could have significant impact on the final behaviour of cement composites with a crack tip nearby this interface under applied tension. The aim of this work is to show the basic interface microstructure by scanning electron microscopy (SEM) done by MIRA3 TESCAN and to analyse the behaviour of such composite by numerical modelling. Numerical studies assume two different ITZ thicknesses taken from SEM analysis. A simplified cracked geometry (consisting of three phases – matrix, ITZ, and aggregate) is modelled by means of the finite element method with a crack terminating at the matrix–ITZ interface. ITZ’s modulus of elasticity is taken from generalized self-consistent scheme. A few conclusions are discussed based on comparison of the average values of the opening stress ahead of the crack tip with their critical values. The analyses dealing with the effect of ITZ’s properties on the stress distribution should contribute to better description of toughening mechanisms in silicate-based composites.

In this paper, authors focus attention on mechanical fracture parameters obtained from records of three-point bending fracture tests on concrete specimens with initial notch. Total eight sets of specimens were tested. Three specimens at the age of 28 days were tested in each set. Concrete was different in dosage of Portland cement CEM I 42.5 R and amount of used superplasticizer. The Effective Crack Model (ECM) was used to evaluate the load vs deflection diagrams to obtain modulus of elasticity and effective fracture toughness; specific fracture energy was determined using work-of-fracture method. Modulus of elasticity, tensile strength and fracture energy were also subject of identification via inverse analysis based on artificial neural network, which aim is to transfer the input data obtained from the fracture test to the desired material parameters.

In this paper, authors concentrate attention on crack initiation and acoustic emission (AE) parameters obtained from records of three-point bending fracture tests on eight sets of concrete specimens with initial stress concentrator at the age of 28 days. Resistance to stable and unstable crack propagation was quantified via evaluation of load vs crack mouth opening displacement diagrams using Double-K fracture model. The AE technique was used to monitor damage process taking place during testing in specimens.

Alkali-activated slag is a building material considered as alternative to ordinary Portland cement based materials. Addition of graphite powder increases its electric conductivity, hence, introducing new functionality to building materials such as self-sensing and self-heating properties. In this study, the effect of graphite filler on the mechanical fracture properties of alkali-activated slag composite is investigated. Graphite powder was added in the amount of 5, 10 and 15% with respect to the slag mass. Modulus of elasticity, fracture toughness and fracture energy were determined using standard three-point bending test on prismatic specimens with central edge notch. The course of fracture tests was also monitored by acoustic emission (AE) method. Compressive strength was determined on the fragments remaining after the fracture tests. Results showed that addition of graphite caused a decrease in compressive strength, fracture toughness and fracture energy but modulus of elasticity increased. Addition of graphite also caused a decrease in AE events and increase in amplitude of signals. This implies that mortars with graphite filler is more brittle than the reference mortar, and lower number of much larger cracks is formed during the fracture test.

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.

Knowledge of the mechanical and primarily fracture parameters of composites with a&nbsp, brittle matrix is essential for the quantification of their resistance to crack initiation and growth, and also for the specification of material model parameters employed for the simulation of the quasi-brittle behavior of structures made from this type of composite. Therefore, the main target of this paper is to quantify the mechanical fracture parameters of alkali-activated slag composites with steel microfibers and the contribution of the matrix to their fracture response. The first alkali-activated slag composite was a reference version without fibers, the others incorporated steel microfibers amounting to 5, 10, 15 and 20% by weight of the slag. Prism specimens with an&nbsp, initial central edge notch were used to perform the three-point bending fracture tests. Load vs. displacement (deflection at midspan) and load vs. crack mouth opening displacement diagrams were recorded during the fracture tests. The obtained diagrams were employed as inputs for parameter identification, the aim of which was to transfer the fracture test response data to the desired material parameters. Values were also determined for fracture parameters using the effective crack model, work-of-fracture method and double-K fracture model. All investigated mechanical fracture parameters were improved by the addition of steel microfibers to the alkali-activated matrix. Based on the obtained results, the addition of 10 to 15% of microfibers by weight is optimal from the point of view of the enhancement of the fracture parameters of alkali-activated slag composite.

Fine-grained cement-based composites used in civil engineering and construction industry are usually made of cement-based matrix and natural aggregate (such as sand, gravel, crushed stone, etc.). Red ceramic waste aggregate is considered as a perspective replacement of a part of natural aggregate in modern environmentally oriented building materials. Fine-grained cement composite with natural aggregate partially replaced by ceramic waste aggregate usually show different mechanical fracture characteristics from ordinary fine-grained concrete. The specimens were tested at six different ages. This was the reason for conducting the research programme. Altogether, 6 fine-grained cement mixtures with various proportions of natural and red ceramic waste aggregate were prepared. The aim of this paper is to present and compare mechanical fracture properties obtained from static and fatigue tests. Bulk density, flexural and compressive cube strength, fracture toughness and fatigue properties (S−N − Wöhler curve) were of special interest. All of these tests are important for a practical application of concrete with ceramic aggregate for structures. All the results were statistically analysed and they showed that the fatigue and mechanical fracture properties were improved or at least kept up with the increasing levels of red ceramic waste aggregate. Environmental impact of application in construction industry of composites in question is discussed.

This paper deals with the results obtained from the employment of a selected fracture model to evaluate wedge splitting fracture tests carried out on hemp fibre concrete specimens. The research work was focused mainly on the effect of the dosage and length of hemp fibres on the initiation part of crack propagation in concrete specimens, and on critical crack opening displacement. Concrete mixtures with different volumetric dosages (0.5, 1.0 and 2.0 %) and fibre lengths (10, 20 and 40 mm) were prepared, and six identical specimens were cast from each mixture. Specimens were also cast from a reference mixture, which was without fibres. The specimens were provided with an initial notch and tested using the wedge splitting test method. Load versus crack mouth opening displacement diagrams were recorded during testing and (after data filtering and appropriate modifications) subsequently evaluated using the Double-K fracture model. This model allows the evaluation of two material parameters – the initiation fracture toughness, which defines the onset of stable crack propagation, and the unstable fracture toughness, which defines the onset of unstable cracking or failure. Finally, the critical crack opening displacement was determined with the assumption of the bilinear function of softening in tension.

This paper presents selected results from a study concerning mechanical fracture parameters related to the formation of the solid structure of cement mortars. The tested mixtures were composed of an invariable quantity of cement (type CEM I 42.5 R) and an increasing amount of amorphous SiO2 (marked with a P), or a variable quantity of cement replaced by varied quantities of amorphous SiO2 (marked with an N). It was decided that the consistencies of the fresh mixtures should be identical: to ensure this, the water-cement ratio was increased as necessary. Quantification of the mechanical fracture parameters of the studied composites (fracture toughness, fracture energy, compressive strength, modulus of elasticity) was performed via the evaluation of fracture tests performed on beam specimens with an initial stress concentrator loaded in three-point bending. Subsequently, the results of the experiments in the form of load versus displacement diagrams were processed using the effective crack fracture model and the work of fracture method.

From the volumetric point of view, aggregate is the most important ingredient in any kind of concrete. It is impossible to use raw soil instead of aggregate to produce concrete. There are numerous reasons for not using soil for concrete production on Earth, and we should not use lunar soil for concrete production on the Moon for the same reasons. Nevertheless, almost all developed lunar concrete-like composites, such as sulphur or polymeric concretes, are based on raw lunar soil. In the research programme, cement composite based on lunar aggregate simulant was tested. The mechanical fracture properties of the composite were the key point of interest. It was proven that the tested lunar concrete is characterized by stable and uniform properties. The obtained results were compared with the properties of other ordinary cement composites.

The paper is focused on the mechanical fracture properties of hardened cement mortars in which Portland cement is partially replaced by reactive admixtures. These properties are evaluated in relation to the mortars’ microstructure through the size and content of pores. The cement in the mortars was partially replaced by a diatomite that contains amorphous SiO2. The three-point bending fracture tests were performed on beams with a central edge notch. The investigated mechanical fracture properties of the aforementioned cement-based composites included flexural strength, compressive strength, elasticity modulus, effective fracture toughness and specific fracture energy.

The contribution is devoted to investigation of the fracture process zone (FPZ) in a fine-grained cement-based composite made from hydrated Portland cement. Particularly, experimental investigations and description of the stable crack propagation using fracture mechanics model are conducted. Three-point bending tests on small composite beams with a central edge notch were performed. The damage due to fracture was monitored by means of nanoindentation performed around the macroscopically observable crack. Acoustic emission events were recorded during the three-point bending test and correlated with load–displacement data. The beneficial effect on the fracture resistance of fine-grained mortar specimens compared to plain cement pastes was quantified.

This paper examines the analysis of acoustic emission signals captured during three-point bending fracture tests of concrete specimens with similar water-cement ratio, but one set of specimens was prepared from aerated concrete. The acoustic emission method is an experimental tool well suited for monitoring fracture processes in material. The typical parameters of acoustic emission signals were identified from the acoustic emission records for two different concrete mixtures to further describe the under-the-stress behaviour and failure development. An understanding of microstructure–performance relationships is the key to true understanding of material behaviour. The crack growth was continuously monitored using four acoustic emission sensors mounted on the specimen. The acoustic emission results are accompanied by selected fracture mechanical parameters determined via evaluation of load versus displacement diagrams recorded during three-point bending fracture tests.

The use of stochastic nonlinear computational mechanics in real-world applications faces a fundamental obstacle – the lack of detailed information about the stochastic properties of material parameters involved in the problem. The current paper describes the results of an extensive experimental program which focused on determining fracture-mechanical parameters and their stochastic models of concrete C25/30. The testing program consisted of compression tests on cubic specimens, three-point bending tests on beams with notch and wedge-splitting tests on cubic specimens with notch. In the case of the three-point bending tests, along with the standard evaluation of fracture-mechanical properties according to code specifications material parameter identification, artificial neural network based inverse analyses were carried out. In order to quantify the influence of the consistency of freshly mixed concrete on its fracture-mechanical properties, two concrete mixtures of the same strength class were tested: (i) mixture with a slump value of 45 mm and (ii) mixture with a slump value of 70 mm. In addition, the time development of fracture-mechanical parameters and their variability was studied. All results obtained from individual tests are presented, compared and discussed here. Stochastic models of selected parameters of the analyzed concrete for stochastic nonlinear FE-model analysis will be recommended.

Results of pilot experiments focused on monitoring of shrinkage, weight loss and mechanical fracture properties of two selected polymer-modified cement mortars (PCM) during their aging are presented in the paper. Comparison of the measurement results was carried out on the test specimens at the age of 3, 28, and 90 days. The results show that even the specimens made from PCMs, for which the manufacturer declares no shrinkage, shrank considerably. The results also show that the development of mechanical fracture parameters during ageing of these materials differs from generally expected trends and thus the monitoring of these parameters is recommended.

Fracture behaviour of a crack in a particle (silicate based) composite is studied. The crack propagation depends not only on mutual elastic mismatch of matrix and aggregate but also the influence of the interfacial transition zone (ITZ) between the matrix and the aggregate is discussed. Various combinations of materials and geometry of matrix, aggregate and ITZ can improve or degrade fracture properties of the composite. Extensive numerical simulations on a basic 3-point-bending cracked specimen via the finite element method are performed in order to analyze the stress field near the crack tip. Linear elastic fracture mechanics approach is utilized in order to assess the crack stability and summarize several conclusions. Studováno je lomové chování trhliny v částicovém (silikátovém) kompozitu. Šíření trhliny závisí nejen na vzájemných elastických vlastnostech matrice a agregátu ale současně je diskutován vliv mezifázové přechodové zóny mezi matricí a agregátem. Různé kombinace materiálů a geometrie matrice, agregátu a mezifáze mohou zlepšit nebo zhoršit lomové vlastnosti kompozitu. Pomocí metody konečných prvků jsou provedeny rozsáhlé numerické simulace na vzorku s trhlinou zatíženém tříbodovým ohybem a je zjišťováno pole napětí v blízkosti vrcholu trhliny. Pro hodnocení stability trhliny a shrnutí zjištěných závěrů je použit koncept lineárně elastické lomové mechaniky.

The experimental results for quasi-brittle materials such as concrete and fibre-reinforced concrete exhibit high variability due to the heterogeneity of their aggregates, additives and general composition. An accurate assessment of the fracture-mechanical parameters of such materials (e.g. compressive strength fc and specific fracture energy Gf) turns out to be much more difficult and problematic than for other engineering materials. The practical design of quasi-brittle material-based structures requires virtual statistical approaches, simulations and probabilistic assessment procedures in order to be able to characterize the variability of these materials. A key parameter of non-linear fracture mechanics modelling is the specific fracture energy Gf and its variability, which has been a research subject for numerous authors although we will mention only [1, 2] at this point. The aim of this contribution is the characterization of stochastic fracture-mechanical properties of four specific, frequently used classes of concrete on the basis of a comprehensive experimental testing programme.

Mechanical fracture parameters obtained from three-point bending tests on concrete specimens with a central edge notch are introduced in this paper. A total of four sets of specimens were tested. The concrete used in each set differed in cement dosage, which ranged from 250 to 455 kg per m3of fresh concrete. Three specimens in each set were tested at the age of 28 days. Increasing the dosage of cement influences the mechanical fracture properties of concretes in both positive and negative ways.

Knowledge regarding the values of fracture-mechanical parameters is critical for the virtual failure modeling of elements and structures made of concrete. A key parameter in nonlinear fracture mechanics modeling is the specific fracture energy of concrete, and its variability. Three-point bending tests on notched-beam specimens are fundamental experiments for the determination of fracture-mechanical parameters. In the present paper, two basic approaches are applied to determine fracture-mechanical parameter values from these tests: (i) the effective crack model/work-of-fracture method, and (ii) inverse analysis using artificial neural networks and stochastic simulations. First, the paper describes suitable methods for the determination of fracture-mechanical parameters. Second, the FraMePID-3PB software tool, which has been developed in order to automate the whole time consuming process of inverse analysis, is described. Finally, the verification of methodology and software is presented using two illustrative examples.

This paper presents a study on the influence of material parameters of concrete on the mechanical response of prestressed monoblock railway sleepers used on Czech railway tracks. The influence of selected mechanical/fracture parameters of concrete on the structural behaviour of the studied precast structural members is investigated by means of numerical modelling of standardized static loading tests, which are used as a tool for quality control during the manufacture of sleepers according to the European Standards for railway applications. The tests are numerically simulated using ATENA nonlinear finite element method software. The results of the simulations are compared with experimental data obtained via static loading tests performed on sleepers both before their use according to their designated purpose and after 17 years in operational service.

The partial replacement of cement in concrete by an alternative admixture is a current topic in materials engineering. In order to examine the effect of replacing 20 % of the weight of Portland cement in fine-grained concrete using selected admixtures on volume changes and mechanical fracture parameter values, a set of specimens was fabricated from these quasi-brittle materials. Granulated blast-furnace slag, high-temperature fly ash, metakaolin and specially selected combinations of them were used as admixtures. Three-point bending fracture tests were conducted on these specimens and load versus crack mouth opening displacement (PCMOD) diagrams were recorded during the testing. In this paper, the outputs of the double-K fracture model were used for the prediction of beginning of stable crack propagation in fine-grained cement-based composites with admixtures.

This paper introduces the results of fracture experiments on concrete specimens of different composition modified by zeolite (7.5 to 30 % by weight of cement dosage). It presents the determination and comparison of basic mechanical/fracture parameter values such as volume density, compressive strength, elasticity modulus, effective fracture toughness and fracture energy.

Alkali-activated slag is a material which has great potential for use in industrial applications. The main drawback of this material is the high level of autogenous and especially drying shrinkage that affects it, causing the deterioration of its mechanical properties. Mechanical and mechanical fracture parameter values show that an alkali-activated slag composite which is saturated with water is more brittle and exhibits lower strength in comparison with the same material after being surface-dried. These results are essential for the adjustment of testing conditions in order to obtain accurate and reproducible results.

This paper concerns an investigation into the effects of high temperatures acting on specimens made from specially designed fine-grained cement-based composite with spherical glass aggregate. The experimental programme was carried out on four sets of beam specimens with the dimensions 20 × 40 × 200 mm. The specimens were thermally loaded to pre-set temperatures of 100, 200 and 400 °C, after which these temperatures were maintained for 60 minutes (one of the sets contained three reference specimens which were placed in a laboratory at a temperature of 20 °C and were not exposed to heat). After the temperature loading had been performed, the specimens were left to cool down to ambient temperature. After that, fracture tests were used to determine the degree of damage caused to the specimens by temperature loading. Three-point bending tests were conducted on the previously loaded beam specimens, which were notched to provide them with an initial stress concentrator using a diamond blade saw before testing. The depth of the initial edge notch on the bottom side of the specimen was approximately 1/3 of the total specimen depth. The span length was 180 mm. Fracture response was evaluated via load versus deflection diagrams. The monitored parameters were modulus of elasticity, effective fracture toughness and specific fracture energy. Informative compressive strength values were also determined. Selected specimens were investigated using X-ray micro-tomography (micro-CT).

Conducted loading tests of plain concrete specimens are briefly introduced in the paper together with description of the performed numerical simulations of these tests in ATENA 3D software. The simulation results of concrete failure are analysed in detail and compared with the experimental results recording failure processes with the help of acoustic emission. The simulation results are in good agreement with the recorded data.

The fracture-mechanical parameter values of concrete, a quasi-brittle composite material, are determined via evaluation of records of experiments on specimens with stress concentrators. One of the fracture models applicable to concrete is the double-K model. This model combines the concept of cohesive forces acting on the effective crack increment with a criterion based on the stress intensity factor. The outputs of the model are critical crack tip opening displacement and fracture toughness values, including the initiation stress intensity factor value corresponding to the beginning of stable crack propagation. Outputs of three-point bending fracture tests of fibre reinforced concrete obtained using double-K fracture model are presented in this paper. The main aim is the determination and comparison of the above-mentioned parameter values of two types of the composite both without and with polypropylene fibres. Both types of tested composite had the same basic matrix consisting of cement, sand and water. In one case, gravel was used for normal weight concrete, in the other case lightweight aggregates were used for lightweight concrete. Both types of the testing matrix were designed with a similar value of compressive strength. Concretes were reinforced by spread polypropylene fibres of three lengths. There were made eight sets of testing concrete specimens: without fibres, and with fibres of 19 mm, 38 mm and 54 mm length. Dosage of fibres was 9 kg/m3 in all six cases.

Cement and alkali activated fly ash based concrete is analyzed.Fracture properties of various compositions are investigated both experimentally and numerically.Primary attention is accorded to the influence of the amount of fly ash replacing cement on fracture energy.The influence of short synthetic fibers on mechanical properties is also studied.Artificial neural network is introduced to provide parameters of the constitutive model. Several cement and alkali activated fly ash based concrete samples are examined in this paper with emphasis on their fracture properties. These are first obtained from an extensive experimental program. The measured loading curves are then compared with those derived numerically in the framework of an inverse approach. Here, the artificial neural network and the ATENA finite element code are combined to constitute the optimization driver that allows for a reliable determination of the modulus of elasticity, fracture energy, and tensile strength of individual concretes. A brief introduction to the numerical analysis of fiber reinforced specimens again in conjunction with inverse analysis is also provided.

An extensive set of parameters of several lime–metakaolin plasters, including basic material characteristics, mechanical and fracture-mechanical properties, durability characteristics, hydric parameters and thermal properties, is presented. In a comparison with the reference lime plaster without any additives, the experimental results show three- to five-fold increase in strength, about 25% decrease of water vapor diffusion coefficient, 20% decrease of water absorption coefficient and a very high increase of freeze/thaw resistance. Using the lime of very high purity in lime–metakaolin plasters (98.0% of CaO + MgO) is found preferable to lower-purity limes; the main differences are observed in the freeze/thaw resistance, mechanical and fracture-mechanical parameters. A comparison with two commercial renovation plasters shows that the only notable advantage of the best commercial plaster studied over the best lime–metakaolin plaster is in its lower moisture diffusivity but it can be matched relatively easily by using hydrophobization admixtures. Therefore, it can be concluded that the lime–metakaolin plasters can find a wider application range in the renovation of historical buildings in the future and may at least partially replace the current renovation plasters as a cheaper and equally effective solution.

The paper is focused on the utilization of electromagnetic emission (EME) and acoustic emission (AE) phenomena within the non-destructive assessment of damage and fracture in cementitious composites. In the first part of the paper, the EME and AE responses from tests performed in two testing configurations differing considerably in stress distribution are investigated. Typical EME and AE response waveforms appear not to differ from each other for either of the loading configu-rations. However, a significant difference is observed in the frequency distribution of the time lag of the acoustic signal behind the electromagnetic one, which results from the different distribution of the fracture events over the specimen’s volume. In the second part of the paper, the failure process within the specimens in the used testing configurations is assessed from the fracture-mechanical point of view. A numerical 3D analysis of the stress distribution and crack propagation within the specimens was performed by using ATENA non-linear FEM code based on a fracture- and damage-mechanics, and plasticity approach. The predictions of the numerical model are in good agreement with the results obtained by AE/EME experimental measurements. The comparison of the AE/EME records from the tests with the results of performed numerical simulation can help in the understanding, interpretation and exploitation of AE/EME in research into damage to and failure of non-electric quasi-brittle silicate-based building materials.

For application in civil engineering structures and also for strengthening existing structures cement-based composites are one of the most frequently used materials. Although many studies have been conducted on the mechanical properties of concrete from static tests, very little research has been reported on concrete fatigue properties. The purpose of this paper is to investigate the static and fatigue behavior of new cement-based composites, developed by ZPSV, a.s. a company producing bridge/train/building structures, are presented. To this end specimens were prepared and tested under static (compressive strength, bending strength, volume density and freezing and thawing resistance index) and cyclic loading (fatigue parameters — Wohler/ S − N curve). The experimentally obtained results (both static and fatigue) of cement-based composites are compared and the suitability of the composites in terms of their application for thin building elements is discussed.

Publisher Summary Concrete is considered as a two-phase material and the problem of cracking of coarse aggregate in cement paste is analyzed in this chapter. The boundary between the fractured matrix and the aggregate particle to be fractured is considered as an interface between two different materials and characterized by the corresponding elastic mismatch of both materials. A fracture model that deals with the influence of aggregate particle dimension and elastic mismatch between matrix and aggregate is suggested and discussed in the chapter. The crack is postulated to grow and to fracture the particle if the stress averaged over the particle exceeds its critical value. The procedure has been applied to concrete, where the matrix consists of cement paste and the particles correspond to fine and coarse aggregates. Calculations are performed by a combination of analytical and numerical approaches. A parametric study, showing the influence of particles dimension and elastic mismatch between the paste and aggregate is also discussed.

The paper deals with the effect of structural geometry on the description of fracture behaviour of quasi-brittle materials by equivalent elastic crack approach. Particularly it is focused on the geometry effect on an R- curve shape. The effect is investigated in the paper using characteristics of stress constraint at the crack tip, for which the theory of two-parameter fracture mechanics is employed. The paper is conceived as a study of the dependence of an R -curve on stress constraint described here by biaxiality factor B. The methods by Bažant a Kazemi and Ouyang, Mobasher a Shah for determination of size independent R- curve is taken into account in the study. For both methods, R -surfaces are constructed and determination of "true" R -curve appropriate to structural size, geometry and material from the R -surface is explained. Finally, the structural analysis from determined R-curves is performed.