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Introduction. When studying composite materials for construction purposes, it is needed to consider the mechanisms of formation of the structure and properties of modern concretes in the process of strength development. In studies of modern composite materials based on cement binder, there is no information about the development of structural defects and destruction of the material at the initial stages of strength development. This information can be obtained using X-ray computed tomography, a promising method of nondestructive testing of the state of the material. Therefore, the objective of this work was to study the formation and propagation of cracks in samples of fine-grained concrete with different fractional composition of sand due to natural processes of cement shrinkage, as well as the mechanics of destruction of samples of modified fine-grained concrete when applying a compressive load at the early stages of strength development. Materials and Methods. The study used fine-grained concrete mixtures of three compositions with different sand gradation. The tomography samples were made by placing fresh mixtures in polymer cylindrical containers. Tomography of the samples immediately after manufacture, as well as after 8 and 51 days, was performed in a YXLON Cheetah microfocus X-ray machine. The composition with two-fraction sand was modified by mechanical activation of the components, 20×20×20 mm cube samples were made. Further, compression tests were performed at the Instron installation after 3 and 7.5 hours, and then — tomography of the destroyed samples.Results. It was established that the destruction of contact zones depended on the ratio of the size of the fractions. In the presence of a bulk of coarse sand grains in concrete, the destruction of contact zones was more pronounced and had a main mode. When using fine or polyfraction sand, contact zones were destroyed locally and had a visually smaller area. The images of the destroyed modified sample, tested 3 hours after manufacturing, showed clear cracks and indents on the edges, which indicated the elastic-plastic nature of the destruction. In 7.5 hours, the edges of the sample upon destruction were covered with a network of small cracks; inside the sample there were also numerous cracks and microcracks, which indicated brittle fracture. Based on the obtained images of the deformed structure of modified concrete, the mechanism of transition from elastic-plastic destruction of the material to brittle one was clearly visible.Discussion and Conclusion. The studied dependences of the influence of the size of fine aggregate on the mechanisms of formation and propagation of structural defects contribute to the theory of the processes of destruction of fine-grained concretes. The results obtained prove the prospects of using X-ray computed tomography as a method of nondestructive testing of the internal structure of fine-grained concrete, including at the early stages of strength development.

Alkali‐activated cement (AAC) is an alternative binder with a promising potential to replace ordinary Portland cement (OPC) and mitigate its environmental issues. The use of fiber‐reinforced polymer (FRP) reinforcements with AAC concrete enables the development of corrosion‐resistant, environmentally friendly reinforced concrete structures. Bond behavior is critical in reinforced concrete structures and must be thoroughly studied for such new alternative materials. This study employs pullout tests to investigate the bond behavior between FRP bars and fine‐grained AAC concrete. Three fine‐grained AAC concretes with low to high strength, glass and carbon FRP bars and wrapped, milled, and smooth surface treatments were examined. The effect of bar casting position was also investigated. The compressive strength showed a significant influence on the bond strength. An average bond strength of approximately 18 MPa was observed for both glass and carbon FRP bars when used with 65 MPa concrete. Both the glass and carbon FRP bars with wrapping showed a lower bond strength than their milled FRP bars counterparts. The carbon bars without surface preparation (smooth bars) resulted in a much lower bond strength, around 4 MPa. In terms of casting positions, the bars cast in the middle section of the concrete block showed a higher bond strength than those at the bottom and top. [ABSTRACT FROM AUTHOR]

Introduction. Requirements, applied to materials used for 3D construction printing, can be met by adding modifiers at the stage of designing fine-grained concrete compositions. Natural zeolites can be considered highly effective finely dispersed additives capable of adjusting properties of concrete mixtures, that are particularly relevant for remote development areas having deposits of this mineral raw material.Materials and methods. Research works and experimental data are reviewed to analyze the use of zeolites as a mineral additive in construction. Zeolites can partially, to different extents, replace cement in concrete to optimally meet the requirements of construction 3D printing. The setting time, the density and the dynamic shear stress of various compositions of concrete mixtures, as well as the density, ultimate compressive and bending strength values were determined for mature concrete. Results of strength and calorimetric testing were used to evaluate the activity of zeolite.Results. The best performance was demonstrated by the specimens with 5–15 % zeolite content by the cement weight. The information is provided about the time it takes for the density of concrete mixture to become critical to ensure its suitability for a 3D printer, which is reduced by 60 to 80 minutes for compositions with the 5–15 % zeolite content compared to the controlling composition, provided that and the mixture suitability period can be optimized by choosing the concentration of zeolite. The initial density of the concrete mixture, evaluated using the Vicat cone penetrometer, did not demonstrate any great changes when the share of zeolite was increased.Conclusions. Zeolite improves mechanical characteristics of fine-grained concrete and adjusts the setting time depending on the concentration of the mineral filler. Hence, zeolite can be considered as an effective component of concretes suitable for additive technologies.

Introduction. To restore and develop the Solovetsky Islands road network concrete slabs could be used. Currently, compositions with additives of industrial waste, including micro- and nanoscale, are being actively introduced to improve the performance characteristics of the road slabs. For the Arkhangelsk region, saponite-containing waste from the mining industry could be used as a material. At the same time, one of the ways to accelerate the process of gaining strength of composites is steaming. However, studies related to the effect of temperature and humidity treatment on the hardening process of a composite with a highly dispersed saponite-containing material (SCM) have not been previously studied. It is known that the process of moisture sorption with a finely dispersed additive allows controlling structure formation during concrete hardening, improving its operational characteristics. However, under conditions of high humidity, the quantitative content of tempering water, calculated based on the value of water absorption of the SСM, can significantly change and affect the process of gaining the strength of the composite. Therefore, the purpose of this research is to study the effect of temperature and humidity treatment on the strength gain of fine-grained concrete with the addition of SCM. It has been established that the accelerated method of concrete hardening has only a positive effect on the formation of a strong and dense structure. However, in the case of using highly dispersed saponite-containing material as an additive, the opposite effect (decrease in strength) associated with sorption properties and features of its structure can be observed. Methods and Materials. The SCM recovered from the recycled water was dried to constant weight and dispersed on a planetary ball mill. Particle size was determined by dynamic and electrophoretic light scattering, and specific surface area was determined by nitrogen sorption (BET theory). The strength of samples of fine-grained concrete of the control and experimental (with the addition of SCM) compositions was gained in two ways: under normal conditions and by an accelerated method using steaming. Strength tests of cube samples with dimensions of 70×70×70 mm were carried out on an automatic test press according to GOST 10180. The microstructure of the samples was examined by scanning electron microscopy. Results and Discussion. The separated, dried and ground saponite-containing material (powder) had an average particle size of 445±40 nm and a specific surface area of 50 670±10 m2/kg. In continuation of the studies, control (FGC) and test samples of fine-grained concrete (FGCscm) were made. The amount of a highly dispersed additive was introduced into the concrete mixture based on previously obtained results of kinetic studies of the water absorption process of saponite-containing material. The determination of the strength characteristics of 1-FGC and 1-FGCscm, hardening under normal conditions, was carried out on day 28. Samples of 2-FGC and 2-FGCscm, a day after sealing with water, were placed in a steaming chamber. After the expiration of the holding time, they were gradually cooled and their strength characteristics were determined. It has been established that the accelerated method of gaining strength of finegrained concrete, by steaming, has a positive effect only on control samples. For a composite with the addition of SCM, temperature and humidity treatment has the opposite effect. Thus, the dynamics of strength gain of 1-FGC and 1-FGCscm are of the same type. At the initial moment of time, an active site of strength gain is observed, but on day 7, the compressive strength of the samples of the experimental composition is 40% higher than that of the control one. At the accelerated curing method in the first two hours (isothermal heating stage), an active strength increase was observed in 2-FGC and 2-FGCscm. The subsequent curing of the control specimens has a linear dependence with a gradual increase in strength to the design strength in six hours. For 2-FGCscm, after two hours of steaming, the strength sharply begins to decrease, and after six hours – visible destruction of the concrete structure occurs. Consequently, prolonged temperature and humidity exposure of concrete with highly dispersed SSM admixture leads to a decrease in the strength characteristics of the specimens. Most likely, it is connected with moisture oversaturation of the composite structure. Therefore, in continuation of the research, electronic photographs were taken of the microstructure of concrete after three hours of holding in the steaming chamber. Thus, the microstructure of 2-FGCscm is mainly represented by spongy particles, and the number of formed needle-like (tobermorite crystals) – decreased significantly, compared to the control. A significant formation of voids in the experimental sample is also observed, which can be attributed to defects in the structure of the obtained composite. In addition, saponite has a three-layer crystalline structure, and the distance between its packets can easily change, since only Van-der-Waals forces condition the connection between them. This can lead to significant swelling of SСM under the action of moisture and create excessive pressure in the microstructure of hardening fine-grained concrete, causing the destruction of the internal structure with a subsequent decrease in strength characteristics. Therefore, in continuation of the research, the following experiment was performed: a suspension of saponite-containing material was placed in a glass-measuring cylinder and water was added to it. The swelling data obtained showed that all samples of highly dispersed SСM actively absorbed water. Thus, after 30 minutes the material increased in volume by 3.6 times, and after three hours – by 5.5 times, compared to the original. Conclusion. An important practical conclusion of the presented experimental results is the following provision: in the case of using a highly dispersed addition of saponitecontaining material to obtain fine-grained concrete, it is impossible to change the curing conditions of the samples from normal to accelerated method by steaming them. In the latter case, after exposure of experimental samples to temperature and humidity for two to three hours there is a loss of their strength properties. This fact is associated with the processes of saturation of saponite particles with water and their subsequent significant swelling.

The object of this study is fine-grained concrete from the waste of mining and processing plants. These wastes are fine-grained dusty sands of polymineral composition. Their use as an aggregate for concrete is restrained by high water consumption, which does not make it possible to obtain sufficient strength for concrete. Therefore, the problem to be solved was the substantiation of the composition of fine-grained concrete from these wastes with mineral and chemical additives, which would ensure obtaining physical-mechanical and hydrophysical properties reasonable for the production of construction articles. The composition of fine-grained concrete from the waste at the Poltava Mining and Processing Plant and slag Portland cement in a ratio of 3:1 and additives of micro silica (15 % of the cement mass) and polycarboxylate superplasticizer (2 % of the cement mass) with W/C=0.5 was obtained. The compressive strength of this concrete reaches 40 MPa, which exceeds the strength of fine-grained concrete of a similar composition on natural fine-grained sand by 3 times. This is due to a greater degree of cement hydration, a greater number of formed hydration products, the presence of silica and quartz particles, a more uniform alternation of gel, silica particles, and crystal hydrates in the structure of cement stone. As a result, the structure contains a larger number of electro heterogeneous contacts between particles and grains that have negative (quartz, calcium hydro silicates) and positive (crystal hydrates of portlandite, hydro aluminates and calcium hydrosulfoaluminate) surface charges. This is what determines the achieved strength and water resistance of fine-grained concrete. The resulting composition of fine-grained concrete is recommended for the production of construction articles [ABSTRACT FROM AUTHOR]

The article considers the specific features of the concreting process using shrinkage-compensating cast-in-place concrete mixtures when replacing hydraulic power equipment at the Saratov Hydroelectric Power Station in the context of changing the raw material base and the working conditions. [ABSTRACT FROM AUTHOR]

Compositions of fine-grained concrete mixtures that provide the minimum required strength values in 1 day (7.5 MPa) have been developed. A comparison was made of the test results of the properties of samples printed on a 3D printer with samples made according to the same recipes on a vibrating platform. A laboratory printer was designed and constructed to study the properties of extruded mixtures. The method was also proposed for measuring concrete mixes' structural strength. Analysis of experimental data allowed the establishment of the features of the influence of the mineral additives and slag–alkaline binders for a comparison of basic physical and mechanical properties of concretes for 3D printing. It has been experimentally shown that possible undercompaction of the fine-grained mixtures formed on a 3D printer and decrease of properties are compensated by the introduction of hardening activator and superplasticizer additives. The novelty of this work lies in determining the comparative effect of various products of technogenic origin on the properties of mixtures for 3D printing. [ABSTRACT FROM AUTHOR]

The article focuses on investigating the influence of various fillers on the physicomechanical characteristics of fine-grained concrete modified with a mixed binder. River sand and lithoid pumice were chosen as fillers for the study. Compositions of aerated concrete with different fillers based on mixed or modified binders were developed, and their properties were experimentally studied. Superplasticizers, micro silica, and lime were used to modify the binder, which was introduced in varying quantities. The results obtained due to the type of fillers used are as follows: the compressive strength of the mortar made using of river sand, compared to the sample with the reference sample, increased by 27.5% at the age of 28 days, and by 12.1% in the case of lithoid pumice. The effect of lime definitely leads to an increase in the density of cement stone with heavy aggregate, and with light aggregate, the density decreases, and as a result, no significant increase in strength is observed.

Insufficient determination of the effect of surface-active substances introduced in extremely low concentrations on the formation of the stability of the properties of fine-grained concrete based on composite Portland cements determined the feasibility of conducting research in this area. The studies investigated the influence of the modification of composite cement with water activated by the use of the hydrophilic hydration mechanism. It has been proven that these factors include the type and amount of applied water nano modifiers. The analysis of the study results confirmed that the introduction of water activated by the mechanism of hydrophilic hydration into concrete in extremely small doses significantly increases the rate of formation of concrete strength and reduces the coefficient of its variation. Owing to this, the strength of the resulting modified fine-grained concrete based on composite cement at the age of 3 days exceeds the strength of the similar concrete without additives by 300 %, and at the age of 27 days – by 25 %. At the same time, the coefficient of variation of strength has a minimum value at the optimal amount of nano additive – water modifier. This allows us to assert the effectiveness of the revealed mechanism of modification of composite cement. Thus, there are reasons to assert the possibility of targeted regulation of the processes of formation of a strong homogeneous structure of fine-grained concrete based on composite cement by using water activated by the mechanism of hydrophilic hydration. [ABSTRACT FROM AUTHOR]

The goal of this work is to investigate the properties of finegrained modified concrete using various aggregates and blended binders. Considering that there is a tendency to reduce the use of Portland cement worldwide, the goal was to develop fine-grained concrete compositions with low clinker binders (mixed binders), which would preserve and even improve the physico-mechanical properties of fine-grained concrete. River and lithoid pumice sands were investigated and used as fillers. Superplasticizer, microsilica, and quicklime were used in different quantities to modify fine-grained concrete. The research results show that, depending on the type of fillers used, the compressive strength of the river sand-based mortar increased by 15.33 % after 28 days, and by 6.34 % with lithoid pumice, compared to the reference sample. The use of lime resulted in an increase in the density of cement stone for heavy aggregates but a decrease in density for light aggregates, with no significant increase in strength observed. [ABSTRACT FROM AUTHOR]

The development of drilling waste recycling technologies is a crucial task, primarily due to their negative impact on the environment, the increasing need for state control over compliance with environmental legislation by oil production companies, and the absence of universal technological solutions for their recycling and neutralization. This article provides a brief overview of various methods to recycle drilling waste for the production of different materials. Using drilling waste located in the Tyumen region as an example, the study demonstrates the potential of incorporating them as additives in fine-grained concrete. [ABSTRACT FROM AUTHOR]

Multi‐axial performance of high strength fine‐grained concrete for carbon reinforced concrete applications High‐strength fine‐grained concretes have been developed for carbon concrete structures whose uniaxial behaviour has been extensively investigated. In contrast, little is known about the material behaviour under multi‐axial loading. In this paper the biaxial capacity of one of these fine‐grained concretes is investigated under different loading conditions and compared with the behaviour of other biaxially loaded high‐strength concretes. The influence of specimen geometry is also discussed, with tests carried out on both cubic and disc‐shaped concrete specimens. Finally, the influence of the production process, in particular the laminating process typical of carbon concrete construction, is examined in more detail. These investigations serve as a basis for future studies on the biaxial compressive strength of carbon concrete. [ABSTRACT FROM AUTHOR]

Fine-grained high-strength concrete has already been tested extensively regarding its uniaxial strength. However, there is a lack of research on the multiaxial performance. In this contribution, some biaxial tests are investigated in order to compare the multiaxial load-bearing behaviour of fine-grained concretes with that of high-strength concretes with normal aggregate from the literature. The comparison pertains to the general biaxial load-bearing behaviour of concrete, the applicability of already existing fracture criteria and the extrapolation for the numerical investigation. This provides an insight into the applicability of existing data for the material characterisation of this fine-grained concrete and, in particular, to compensate for the lack of investigations on fine-grained concretes in general. It is shown, that the calibration of material models for fine-grained concretes based on literature results or normal-grained concrete with similar strength capacity is possible, as long as the uniaxial strength values and the modulus of elasticity are known. For the numerical simulation, a Microplane Drucker–Prager cap plasticity model is introduced and fitted in the first step to the biaxial compression tests. The model parameters are set into relation with the macroscopic quantities, gained from the observable behaviour of the concrete under uniaxial and biaxial compressive loading. It is shown that the model is able to capture the yielding and hardening effects of fine-grained high-strength concrete in different directions. [ABSTRACT FROM AUTHOR]

The object of research is fine-grained concrete on slag Portland cement. Owing to the significant content of metallurgical industry waste in its composition, slag Portland cement belongs to ecologically acceptable products. However, the insufficient rate of formation of its structure and, as a result, the main quality indicator, which is compressive strength, limits the field of use of slag Portland cement. Therefore, in the studies, the goal was to increase the speed of formation of the strength of fine-grained concrete made on slag Portland cement. The studies of the influence of modification of slag Portland cement with water, activated by the use of the hydrophobic hydration mechanism, established the factors affecting the speed of formation and the value of compressive strength of fine-grained concrete made on slag Portland cement. It has been proven that these factors include the type and amount of applied water nanomodifiers, as well as the type and amount of fine concrete aggregate. The analysis of the study results confirmed that the introduction of water activated by the mechanism of hydrophobic hydration into concrete in ultra-small doses significantly increases the rate of formation of concrete strength. Given this, the strength of the resulting modified fine-grained concrete based on slag Portland cement at the age of 2 days exceeds the strength of the similar concrete without additives by 60 %, and at the age of 210 days – by 25 %. This allows us to assert the effectiveness of the identified mechanism of modification of slag Portland cement. Thus, there are reasons to assert the possibility of targeted regulation of the processes of forming a strong structure of fine-grained concrete based on slag Portland cement by using water activated by the mechanism of hydrophobic hydration. [ABSTRACT FROM AUTHOR]

Uniaxial tensile behaviors of basalt fiber and basalt textile reinforced fine-grained concrete composites under the same fiber volume content with different curing cycles (7 days, 14 days, 21 days and 28 days) were investigated using acoustic emission technology (AE), specifically, including three types of basalt fibers (0.34%, 0.68% and 1.02%) and basalt textiles (1, 2 and 3). The results indicated that the tensile strength of specimens was positively correlated with the curing cycles. As the volume content of basalt fiber increased, the reinforcing effect on fine-grained concrete was improved. The Aveton-Cooper-Kelly (ACK) model was the starting point for analyzing the reinforcement mechanism of basalt textiles. Compared with basalt fibers, basalt textiles could significantly improve the cracking load, ultimate load, cracking displacement and ultimate displacement of fine-grained concrete. The specimens had obvious characteristics of strain hardening during the entire fracture process. The AE signals had a high correlation with the the load-displacement curves during entire experimental process. AE signals were classified to illustrate different fracture mechanisms: textiles slipping, textiles pulling out, matrix-textiles debonding and textiles breaking. AE technology was comprehensively used for the first time to compare and describe the uniaxial tensile properties of basalt fibers and basalt textiles reinforced fine-grained concrete composites. [ABSTRACT FROM AUTHOR]

Using thermogravimetric analysis methods, it has been established that the use of nano- and complex modifiers of concrete mixtures leads to an acceleration of hydration processes in the initial period of material hardening, which correlates with the data on the dynamics of specimen hardening and the results of structural studies. Optimal formulations of modified fine-grained concretes made from local mineral raw materials of Yakutia have been developed, recommended for the production of concrete products and structures operating in extreme climatic conditions. [ABSTRACT FROM AUTHOR]

The construction of energy-efficient buildings is very important as it can lower the energy consumed for construction and reduce the energy required for the heating and cooling effects of buildings and reduce the global warming caused by carbon emissions in buildings. It is therefore necessary to check the thermal characteristics of any construction material. Chopped Basalt Fiber (BF) is dispersed to the fine-grained concrete matrix and after determining the optimum percentage of fiber content cement is partially replaced in the matrix with ground granulated blast furnace slag (GGBS) by different percentages. GGBS helps in providing improved structural performance and also reduces the waste material generated by the steel industry. In addition, thermogravimetric analysis (TGA) is done to ascertain the thermal stability of the material when subjected to higher temperatures. The thermal conductivity of fine-grained concrete is determined to check the conductance of heat through the sample for thermal evaluation of the structure. Furthermore, the specific heat capacity, thermal resistance and thermal diffusivity of the material are also studied. The fine-grained concrete with basalt fiber and GGBS shows better thermal properties than controlled fine-grained concrete. [ABSTRACT FROM AUTHOR]

Compositions of fine-grained concrete mixtures that provide the minimum required strength values in 1 day (7.5 MPa) have been developed. A comparison was made of the test results of the properties of samples printed on a 3D printer with samples made according to the same recipes on a vibrating platform. A laboratory printer was designed and constructed to study the properties of extruded mixtures. The method was also proposed for measuring concrete mixes’ structural strength. Analysis of experimental data allowed the establishment of the features of the influence of the mineral additives and slag–alkaline binders for a comparison of basic physical and mechanical properties of concretes for 3D printing. It has been experimentally shown that possible undercompaction of the fine-grained mixtures formed on a 3D printer and decrease of properties are compensated by the introduction of hardening activator and superplasticizer additives. The novelty of this work lies in determining the comparative effect of various products of technogenic origin on the properties of mixtures for 3D printing.

The effects of fiber structure and curing period on the flexural properties of basalt fibers reinforced fine-grained concrete were investigated in this paper. The damage and fracture processes of composites were monitored using acoustic emission technology. The flexural behavior of composites in terms of load-displacement curves, ultimate flexural load, and toughness was studied. The damage and fracture patterns of composites were described using acoustic emission parameters such as ringing count, amplitude, and accumulative energy. In the middle and late periods, basalt textiles outperformed basalt fibers in terms of improving the flexural load of fine-grained concrete. The flexural load improved at first and then decreased as the amount of basalt fibers increased, while the flexural load continued to improve as the amount of basalt textiles increased. Compared with basalt fibers reinforced fine-grained concrete, the flexural load of basalt textiles reinforced fine-grained concrete with 28 days curing period increased by 13.40%, −2.13%, and 75.96%, respectively. In terms of acoustic emission, the basalt textiles reinforced fine-grained concrete released more energy, the intensity and frequency of the acoustic emission signal were higher, and the cumulative ringing count was also greater. The basalt textiles reinforced fine-grained concrete exhibited greater flexural deflection and ductility.

Fine-grained high-strength concrete has already been tested extensively regarding its uniaxial strength. However, there is a lack of research on the multiaxial performance. In this contribution, some biaxial tests are investigated in order to compare the multiaxial load-bearing behaviour of fine-grained concretes with that of high-strength concretes with normal aggregate from the literature. The comparison pertains to the general biaxial load-bearing behaviour of concrete, the applicability of already existing fracture criteria and the extrapolation for the numerical investigation. This provides an insight into the applicability of existing data for the material characterisation of this fine-grained concrete and, in particular, to compensate for the lack of investigations on fine-grained concretes in general. It is shown, that the calibration of material models for fine-grained concretes based on literature results or normal-grained concrete with similar strength capacity is possible, as long as the uniaxial strength values and the modulus of elasticity are known. For the numerical simulation, a Microplane Drucker–Prager cap plasticity model is introduced and fitted in the first step to the biaxial compression tests. The model parameters are set into relation with the macroscopic quantities, gained from the observable behaviour of the concrete under uniaxial and biaxial compressive loading. It is shown that the model is able to capture the yielding and hardening effects of fine-grained high-strength concrete in different directions.

The paper is devoted to developing high-performance nanostructured concrete for 3D printing and study-ing its strength and operational characteristics. The research addresses issues related to the use of modern concrete chemistry, such as plasticisers and concrete hardening accelerators, and nanomodifica-tion by two types of nanocarbon: nanotores (Astralene) and sulphur graphene (Ugleron). The designed lightweight concrete for 3D printing has a compressive strength of about 70 MPa and 9 MPa after extru-sion at a density of about 1.55–1.6 kg/m³. Pozzolan additives from industrial waste in concrete are used, such as silica fume and oil shale ash. Microspheres, which are also industrial wastes, are used as light-weight aggregate and thixotropy regulators. As a result, lightweight, high-performance concrete for 3D printing was developed, which allows the disposal of industrial waste inside it.

This paper continues the authors' previous research on developing high-performance concretes based on low-strength aggregates. Unlike the previous work, where a low-strength crushed stone was used, the current research investigates a low-strength fine aggregate-based concrete. With modern construction chemistry in combination with pozzolan and high dispersed ground additives and nanomodification, it is possible to obtain high strength and operational characteristics concretes. The work carried out studies of three cement manufacturers for suitability in high-strength hydrotechnical concrete and phase analysis of the aggregates used in this study. Strength (7, 28, 180 days), density, waterproofness, freeze-thaw resistance of fine-grained concrete based on low-strength and high-strength aggregates were compared in the research. The final result is a concrete recipe with freeze-thaw resistance class F400, Waterproofing class W20 and compressive strength of 60.5 MPa at the age of 28 days (71.9 MPa at 180 days).

The technical feasibility of the recycling of specific polymeric composite materials was evaluated. Two types of carbon composites, both with phenol-formaldehyde resin but with different reinforcement, were studied. It was discovered that the solvolysis with the oxidizing agents used in an acidic environment allowed for the achievement of a high-efficiency fiber extraction. The extracted secondary carbon fibers had a high degree of purity (95–99.5% of resin was removed). Fiber thickness slightly decreased during the process (on average, by 20%). The use of chopped secondary fibers (3–9 mm fiber length) for concrete reinforcement produced a positive effect. Hence, the compressive and bending strength of the concrete blocks were accordingly 1.5% and 16% higher in comparison with the control sample. The use of secondary carbon fabric for the production of composite materials a good result: the effective tensile strength of CFRP samples reinforced with recovered fabric is only lower by 4.5% in comparison with virgin fabric. [ABSTRACT FROM AUTHOR]

The problem of increasing the service life of buildings and structures for agricultural purposes operated in aggressive environments is relevant. The aim and scientific novelty of the work were to determine the relationship between the structure and properties of geopolymer concretes in aggressive environments. The properties of various concrete compositions under the influence of a solution of lactic, acetic, and oxalic acids were studied. With an exposure time of 90 days in an aggressive environment, samples of concrete based on a geopolymer binder had up to 6% less loss of strength and up to 10% less weight loss than concrete based on a cement binder. The effectiveness of the developed composition and technological solutions was confirmed, and it was quantitatively expressed in increased compressive strength and tensile strength in bending by 81.0% and 73.5%, respectively. It has been established that raising the heat treatment temperature to 80 °C leads to increased compressive strength for all compositions of geopolymer binders. The most favorable heat treatment conditions are created at 80 °C. The relations of the strength characteristics of geopolymer binders are revealed, which allow a detailed quantitative and qualitative assessment of the influence of the studied factors on the change in the system "composition—hardening conditions—properties" and can be used in the development of production compositions of binders and composites based on them, as well as their regulation—physical, mechanical, and operational characteristics. [ABSTRACT FROM AUTHOR]

The paper considers the dependences of the properties of self-cleaning fine-grained concrete on the composition of the concrete mixture: the ratio of sand to white Portland cement, the content of plasticizer, the content of photocatalytic composite material. The photocatalytic composite material (PCM) is a diatomite-based anatase-silica material obtained by sol-gel deposition of nanoscale TiO2 of anatase modification on a silica carrier. The investigated properties of fine-grained concrete were: average density, compressive strength and self-cleaning ability, which was determined by evaluating the photocatalytic decomposition of an organic dye – Rhodamine B. It was shown that an increase in the content of PCM to increase the ability of the coating to self-cleaning leads to a loosening of the structure of fine-grained concrete, but this can be successfully offset by an increase in the content of plasticizer, which additionally improves the distribution of the photocatalytic agent in the cement-sand matrix. An increase in the proportion of sand and the content of plasticizer also contribute to an increase in the ability to self-cleaning. The pozzolan activity of the PCM provides an increase in the strength of concrete. The results of comparing the self-cleaning ability of fine-grained concrete of various compositions indicate a high photocatalytic activity of samples with PCM, which allows its application for creating self-cleaning coatings.

In the present paper, large amounts of bottom ash (BA) and fly ash (FA) in a Vung Ang thermal power plant in Vietnam were used to substitute crushed sand (CS) to produce fine-grained concrete. The FA content was fixed at 20 %, the BA content increased from 20 % to 50 % corresponding to the CS content decreased from 60 % to 30 %. Four mixtures of fine-grained concrete were prepared to produce concrete. It was found that the compressive and splitting tensile strengths decreased when the amount of FA and BA increased from 40 % to 70 %. The compressive and the splitting tensile strengths were comparable to those in conventional concrete containing bottom ash. The larger content of bottom ash caused higher water absorption and resulted in lower chloride resistance, which is because of the porous structure of BA. Based on the results of chloride resistance, this fine-grained concrete is classified as moderately permeable concrete. The results of this study indicated that crushed sand (fine aggregate) of fine-grained concrete can be replaced by fly ash incorporated with bottom ash up to 60 % and this fine-grained concrete can be applied for construction works as conventional concrete.

Objectives. The aim of this study was to identify variation patterns of the in-series variation coefficient of adhesive strength to base and the dependency of the frost resistance coefficient on the criterion of adhesion on prescription factors of fine-grained concrete obtained from dry building mixtures based on various Portland cements containing various low-modulus inclusions and redispersible polymer powders from 0 to 3%. According to GOST 31357, the main quality indicators of the hardened mixture or fine-grained concrete, in addition to frost resistance, include the indicator of “frost resistance of the contact zone”, which characterises the ability of the hardened concrete to maintain adhesion to the base during normal separation following a certain number of freeze-thaw cycles.Method. The tests of the main samples were carried out after 75 freeze-thaw cycles in accordance with GOST 31356. The marks of the frost resistance of the contact zone are set from FKZ25 to FKZ100. The number of freeze-thaw cycles is taken as a mark, after which the adhesion strength to the base, determined according to GOST 31356, is at least 80% of the strength of the control samples. The arithmetical average is taken at between at least 5 values, while the variation coefficient measured across a series of adhesion strength values is not considered.Results. The average values of the variation coefficient of adhesion strength were: following 75 cycles of freezing-thawing – 0.224 (in a range from 0.058 to 0.616); following 25 cycles of freezing-thawing – an average of 0.129 (in a range from 0.016 to 0.352). The variation coefficient of adhesion strength can exceed the variation coefficient of compression strength by approximately 2.5 to 3 times. When introducing the redispersible powders into the structure of fine-grained concrete, there is no unambiguous pattern of change in the values of the variation coefficient of adhesion to base and it is possible to increase or decrease this value, while the values of the variation coefficient of adhesion can vary by an order of magnitude.Conclusion. For all concretes, there is a tendency of increase of the coefficient of frost resistance with a decrease in the adhesion variation coefficient both after hardening under normal conditions as well as during cyclic freezing and thawing. When conditions VF/V28A

Measuring and calculating methods of critical stress intensity factors (SIFs) have become hot topics which attracted large attention recently. In this work, anti-symmetrical four-point bending tests of cracked fine-grained concrete specimens were conducted experimentally and numerically by using a computer-controlled universal testing machine and ABAQUS code. A comparative study of the calculation method of pure mode II stress intensity factor of a fine-grained concrete was performed by utilizing the conventional finite element method (FEM) in two and three dimensions as well as the extended finite element method (XFEM) in three dimensions. The results show that in three-dimensional models, the crack mode is closest to the pure mode II at the center of specimen thickness. Pure mode II stress intensity factors obtained by SEAM2D and XFEM3D are 1.013 and 1.0617 times that by SEAM3D, respectively. Pure mode II stress intensity factors of the fine-grained concrete obtained by the conventional FEM are more stable than that by XFEM. The number of mesh circles has slight influence on the calculation results of pure mode II stress intensity factor. [ABSTRACT FROM AUTHOR]

Introduction. Diffusion of aggressive components of the external environment deeper into the concrete intensively flows through the contact surfaces and between the filler and the cement matrix. Therefore, it is better to apply active fillers that interact with the cement matrix on certain mechanisms for reducing the conductivity of the contact surfaces, which leads to the products durability.Materials and methods. Methods of mechanical testing, x-ray phase analysis, scanning electron microscopy were used for comparative studies of concrete corrosion resistance with the active filler (urtite) and with the inactive one (quartz sand).Results. This research indicates that the urtite filler significantly increases the strength and corrosion resistance of concrete in comparison with the traditional filler with quartz sand. The investigation of the contact zone of cement stone-aggregate with the usage of REM demonstrates that there are no corrosion products in the contact zone of urtite with the cement matrix, while the contact zone of quartz sand and its surface acquires corrosion products represented mainly by gypsum.Discussion and conclusion. The increased corrosion resistance of the fine aggregate concrete through the application of active filler on the basis of urtite provides lower conductivity of the contact surfaces between cement matrix and filler due to the chemical affinity of the rock-forming minerals of nepheline to calcium hydroxide. The additional factor that enhances the self-inhibition of acid corrosion is the formation of the gel-like layer of silica on the surface of nepheline, which inhibits the advancement of hydroxide ions due to the flow of electro-surface processes. Thus, the nepilnameciais fillers are appropriate to apply in concrete, which is used in the conditions of the chemical aggression of high intensity.

The expediency of replacing natural quartz Sands, whose reserves are gradually exhausted, by rock screenings as part of fine-grained concretes is shown. In particular, the characteristics of granite and quartzite crushing screenings in comparison with quartz river sand are presented, as well as the results of studying the strength and abrasion of the fine-grained concrete obtained. To exclude the effect of the grain composition on the properties of concrete, aggregate samples were pre-fractionated, and the conditions for producing concrete (cement consumption, water-cement ratio, concrete mix mobility) were the same. It was found that quartz sand having rounded grains with a relatively smooth surface promotes the formation of dense particle packing and increases the strength of fine-grained concrete by 9-14%, but at the same time reduces the abrasion of similar standard samples by 20-30% compared to screenings crushing of granite and quartzite. The maximum abrasion resistance showed concrete containing quartzite screening. Quartzite - metamorphic (metasomatic) dense rock crushing which produces angular particles with a rough surface. The absence of dense packaging reduces the compressive strength of concrete samples containing quartzite aggregate compared to quartz sand. At abrasion of similar samples in a thin layer the resistance to sliding of filler particles one relative to another increases. Therefore, when choosing crushed rock fines instead of natural quartz Sands, it is important to consider the purpose of the designed fine-grained concrete. [ABSTRACT FROM AUTHOR]

This article aims to study the effects of adding steel fibers and galvanized recycled spring on mechanical properties and crack development in the fine-grained concrete exposed to the acidic environment containing magnesium sulfate. To this end, specimens containing 0.3% and 0.6% of steel fibers and springs, respectively, by concrete volume, were built in normal temperature using 10 cm × 20 cm standard steel formworks so as to conduct the compressive and tensile strength tests. All specimens were cured in 28 days exposed to the environment containing 0%, 5%, and 10% of magnesium sulfate. Based on the results, addition of steel fibers and recycled spring improves the compressive and tensile strength by 50% and 60%, respectively. Moreover, the specimens containing recycled spring better withstood against the acidic environments in comparison with the specimens including steel fibers. In general, it was found that due to the negligible difference between the strength of the specimens, the application of metal-recycled spring in the fine-grained concrete is technically and economically justifiable. [ABSTRACT FROM AUTHOR]

The problem of increasing the service life of buildings and structures for agricultural purposes operated in aggressive environments is relevant. The aim and scientific novelty of the work were to determine the relationship between the structure and properties of geopolymer concretes in aggressive environments. The properties of various concrete compositions under the influence of a solution of lactic, acetic, and oxalic acids were studied. With an exposure time of 90 days in an aggressive environment, samples of concrete based on a geopolymer binder had up to 6% less loss of strength and up to 10% less weight loss than concrete based on a cement binder. The effectiveness of the developed composition and technological solutions was confirmed, and it was quantitatively expressed in increased compressive strength and tensile strength in bending by 81.0% and 73.5%, respectively. It has been established that raising the heat treatment temperature to 80 °C leads to increased compressive strength for all compositions of geopolymer binders. The most favorable heat treatment conditions are created at 80 °C. The relations of the strength characteristics of geopolymer binders are revealed, which allow a detailed quantitative and qualitative assessment of the influence of the studied factors on the change in the system “composition—hardening conditions—properties” and can be used in the development of production compositions of binders and composites based on them, as well as their regulation—physical, mechanical, and operational characteristics.

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.

The paper deals with perspectives of modification of cement composites at different scale levels (nano-, micro-, macro-). Main types of micro- and nanomodifiers used in modern concrete technology are presented. Advantages of fullerene particles applied in nanomodification of cement concretes have been shown. Use of complex modifiers based on dispersed fibers, mineral additives and nanoparticles is proposed. These are the basic components of the fiber fine-grained concretes: cement of class CEM I 42,5R produced by JSC «Mordovcement», river sand of Novostepanovskogo quarry (Smolny settlement, Ichalkovsky district, Republic of Mordovia), densified condensed microsilica (DCM-85) produced by JSC «Kuznetskie Ferrosplavy» (Novokuznetsk), highly active metakaolin white produced by LLC «D-Meta» (Dneprodzerzhinsk), waterproofing additive in concrete mix «Penetron Admix» produced by LLC «Waterproofing materials plant «Penetron» (Ekaterinburg), polycarboxylate superplasticizer Melflux 1641 F (Construction Polymers BASF, Germany). Dispersed reinforcement of concretes was provided by injection of the fibers of three types: polypropylene multifilament fiber with cutting length of 12 mm, polyacrylonitrile synthetic fiber FibARM Fiber WВ with cutting length of 12 mm and basalt microfiber «Astroflex-MBM» modified by astralene with length about 100÷500 microns. Analysis of results of the study focused on saturated D-optimal plan was carried out by polynomial models «mixture I, mixture II, technology – properties» that considers the impact of six variable factors. Optimum fields of variation of fine-grained modified fiber concrete components have been identified by the method of experimental-statistical modeling. Polygons of distribution levels of factors of modified cement fiber concretes are constructed, that allowed tracing changes in fields of tensile in compressive strength and tensile strength in bending at age of 28 days depending on target characteristics and variable parameters.