The results of studies of the influence of various hardening conditions, incl. simulating conditions in the center of massive monolithic structures, for compressive and tensile strength, initial modulus of elasticity, hardening kinetics of self-compacting concrete of classes from B25 to B55. A classification of concretes according to the hardening kinetics is proposed, using as a criterion the time to reach a certain level from the design compressive strength under normal hardening conditions. The invariance of the dependence of the initial modulus of elasticity of the investigated concretes on the compressive strength on the age and type of concrete is established. A certain difference between the dependence "initial modulus of elasticity - ultimate compressive strength" and a lag in the rate of increase in tensile strength from the rate of increase in compressive strength of the studied concretes in the early period of hardening relative to traditional concretes of vibration compaction were revealed. It is shown that in the early period of hardening the studied concretes are characterized by a higher level of the conditional crack resistance criterion E0/Rt, which indicates to a certain extent a higher fragility of the studied concretes in this period.

Keywords: self-compacting concrete, compressive and tensile strength, hardening kinetics, modulus of elasticity, crack resistance criterion

The influence of redispersible polymer powders on the change in the properties of building mortars after holding at high temperature in accordance with GOST R 56387-2018 has been studied. An increase in the compressive strength by 10 – 15% was revealed at a dosage of RPP 2 – 3%. It was found that the introduction of an air-entrapping admixture into the composition has a negligible, up to 5%, negative effect on increasing the compressive strength. The increase in the flexural strength after holding at high temperature was 1.5 – 1.8 times. The decrease in the E-modulus practically does not depend on the dosage of RPP. After holding at high temperature, the E-modulus was 0.8 – 0.84 relative to normal hardening conditions. A decrease in the adhesion strength to the concrete base to values of 0.39 – 0.71 relative to normal hardening conditions was revealed. It is noted that the decrease in adhesion strength significantly depends on prescription factors, with the exception of formulations containing an air-entrapping additive, in which there is practically no decrease in adhesion strength after holding at high temperature.

Keywords: redispersible polymer powders, adhesion strength, E-modulus, compressive and flexural strength, high temperature

Numerous factors that determine the quality of monolithic reinforced concrete massive foundations predetermine the relevance of a thorough study of a set of issues related to the intensity of concreting, the technical capabilities of the contractor, temperature and humidity conditions, the characteristics of the technological properties of the concrete mixture and the kinetics of concrete hardening. To ensure the solidity of the structure, the expediency of determining the thickness of the layer to be laid is justified not only by the length of the working part of the vibrator, as prescribed by the standards, but also by the indicators of the intensity of concreting, determined by the design parameters, temperature and humidity conditions and the capabilities of the worker. The influence of the temperature and humidity conditions of the environment and the recipe features of the concrete mixture on the rational time of overlapping layers is shown. The results of modeling the level of tensile stresses on such recipe-technological factors as the class and kinetics of concrete hardening, temperature conditions and conditions of heat exchange with the environment are given on the example of a temperature-shrinkage block 20x20x2 m. the values ​​of its parameters. An equation is given for relative moisture loss over the thickness of the structure in dry hot weather when self-compacting concrete mixtures are used.

Keywords: intensity of concreting, quality of massive monolithic foundations, overlapping time of layers, moisture loss, level of tensile stresses

The relevance of modeling the temperature regime and the stress-strain state in the early period of the construction of massive monolithic reinforced concrete structures is shown. Some data are given on the temperature and time parameters of the formation of temperature fields in structures with a surface modulus from less than 1.1 to 2.4 from concrete classes from B25 to B70, both fast and slowly hardening. Based on the results of processing numerous data, the quantitative values ​​of the parameters of the heat release kinetics for the proposed dependence are substantiated. A simplified method for calculating thermal stresses is proposed, based on the proposed and substantiated dependences of concrete properties on its degree of maturity, which are confirmed by numerous experimental data, incl. obtained by other researchers. The calculated values ​​of stresses obtained during the construction of a temperature-shrinkage block 20x20x2 m from concrete of fast and slow hardening classes B25 and B45 were compared with some experimental results and modeling data. The conclusion is made about the inexpediency of using concrete of class B45 due to the high risk of cracking in the period of 1.5 - 3 days. When using concrete of class B25, preference should be given to fast-hardening.

Keywords: massive monolithic structures, thermal stresses, cracking, degree of concrete maturity, kinetics

As a result of processing the experimental data of the authors and other researchers, a relationship was obtained between the adhesion strength of the Acc with a concrete base and the axial tensile strength Rt of solutions without RPP Acc = 0.23ˑRt at R2 = 0.996. A tendency to increase the adhesion strength to 30% with an increase in the dosage of RPP to 2% and up to 130% with the dosage of RPP 3%, while a significant effect on the adhesion strength of the type of cement and RPP has been established. The data on the role of the age of the concrete base and its treatment before applying a repair (restoring) mortar or concrete mixture on the adhesion strength are given. The dependence of the elastic modulus of mortar or fine-grained concrete on the axial tensile strength, invariant to prescription factors, is obtained. The discrepancy of some standards in terms of the requirements for the base during tests for adhesion strength was noted.

Keywords: dry building mixes, adhesion strength, modulus of elasticity, redispersible polymer powders, repair mixes

The expediency of using modeling using the finite element method to study the influence of certain prescription-technological factors on the resulting temperature fields and temperature stresses during the construction of massive foundation slabs is substantiated. A simplified method for determining thermal stresses based on the reduction of a three-dimensional problem to a one-dimensional one based on the hypothesis of flat sections is considered. The dependence is proposed and the quantitative values ​​of the parameters for calculating the kinetics of heat release of concrete in the temperature-shrinkage block are substantiated. As a result of the implementation of a numerical experiment on the influence of the duration of breaks between overlapping layers, the temperature of the environment and the concrete mixture, the class and kinetics of concrete hardening, and heat transfer parameters, the dependences of the level of tensile stresses on these factors over time were obtained. It is shown that when developing technological regulations for concreting, the determination of technological parameters (the intensity of laying the mixture, the thermal resistance of the formwork, the arrangement of working joints, etc.) is impossible without taking into account the kinetics of concrete hardening, determined by the prescription features of concrete mixtures.

Keywords: massive monolithic structures, temperature fields and stresses, prescription-technological factors, heat release of concrete, stress-strain state

A method of selecting admixtures for the production of dry building mixes is proposed based on the results of an expert assessment of the complex indicator of the workability of application (connectivity, continuity when pulling with a toothed trowel, elasticity, quality of rollers) on a 100-point scale and the strength of adhesion to the concrete base, followed by the calculation of the specific cost of admixtures in the mixture per ton of products, respectively, by one point of workability of application and 1 MPa of adhesion strength, developed as a result of studies of adhesive mixtures made using 4 different cements, 5 water-retaining additives and 12 redispersible polymer powders. The possibility of obtaining adhesive compositions of adhesion class C 2 using redispersible polymer powders of domestic production is shown.

Keywords: dry building mix, adhesion strength to the base, unit cost, processability of application

When accepting finished reinforced concrete structures, incl. monolithic, they are subject to requirements for strength, stiffness, crack resistance and durability. The quality of a monolithic reinforced concrete structure depends on the quality of work, the quality of materials, the quality of design solutions and the quality of regulatory documentation. According to SP 70.13330.2012, clause 5.18.1, "when accepting finished concrete and reinforced concrete structures ... you should check ... the quality of concrete for strength, and, if necessary, for frost resistance ...". Particular attention is required for massive monolithic reinforced concrete structures, during the construction of which, due to temperature-shrinkage deformations, it is possible to form an own stress field that exceeds its strength indicators at the stage of formation of the concrete structure, which may result in early cracking with subsequent development of cracks, which will not only negatively affect operational properties of the structure, but in principle can raise the question of the impossibility of its operation. The quality of concrete of a monolithic reinforced concrete structure is determined by both recipe and technological factors, the assessment of the degree of influence of which is an urgent task.

Keywords: massive monolithic reinforced concrete structures, crack resistance, durability, frost resistance, temperature-shrinkage deformation

Ensuring the regulation of the temperature regime of concrete curing to prevent cracking due to temperature gradients and shrinkage deformations is one of the key points in the construction of massive monolithic reinforced concrete structures. A combination of prescription and technological ones are considered as the main methods for regulating the temperature regime of holding during concreting of massive monolithic structures. The analysis of temperature fields and an assessment of possible cracking of concrete during concreting of grillages of bridge supports, holding of concrete during the erection of structures, in which it was carried out by different methods. It was proposed to clearly stipulate in the normative documents the cases of mandatory tests for concrete frost resistance based on samples taken from the structures. When developing design documentation for standardizing concrete quality indicators, it was proposed that the values of the class of concrete in terms of compressive strength should first of all be assigned according to the condition of ensuring the durability of the structure, depending on the class of the operating environment.

Keywords: massive monolithic reinforced concrete structures, temperature conditions of concrete curing, cracking, frost resistance, compressive strength class of concrete

It is shown that the given ratio of the modulus of deformation and the initial modulus of elasticity of concrete depends not only on the type of the function ε = f(𝜎), and from the initial modulus of elasticity of the concrete, i.e. concrete with different values of initial modulus at equal strength, for example, with the introduction of the composition of concrete organo-mineral modifiers, the dependence of the ratio E𝜎/E0 from the loading level will vary. To describe the "stress-strain" or "modulus of deformation-level of loading" for concretes with organo-mineral modifiers, it is advisable to use The sargin diagram, taking into account the influence of modifiers on the main deformation constants: the initial modulus of elasticity, the coefficient of elasticity, the relative deformation corresponding to the short-term strength limit. The nature of deformation of concretes with organo-mineral modifiers at a loading level above 0.9 may differ from the known patterns for traditional concretes, in connection with which this issue requires special study.

Keywords: modulus of deformation, stress-strain diagram, modified concretes, loading level, modulus of elasticity

Durability of concrete, including fine-grained, with alternating cyclic temperature effect of the external environment is normalized according to GOST 26633 mark on frost resistance, and frost resistance criterion for all concrete in accordance with GOST 10060-2012 is the ratio of ultimate strength on compression of the main and control samples not less than 0.9 with a coefficient of variation of strength in the series, while for concrete road and airfield pavements is another additional criterion is the loss of mass. In the previous edition of GOST 10060, the ratio of the compressive strength of the main and control samples not less than 0.95 was adopted as the criterion of frost resistance for all concretes. GOST 31357 normalizes the indicator "frost resistance of the contact zone" for a number of mortars or fine-grained concretes obtained from dry building mixtures. This indicator characterizes the ability of the solution or fine-grained concrete to maintain the strength of adhesion to the concrete base after a certain number of freeze-thaw cycles. For the brand for frost resistance of the contact zone from Fkz25 to Fkz100, the number of cycles is taken, after which the decrease in the adhesion strength to the base does not exceed 20% according to GOST 31356, and in this case the coefficient of variation of the clutch values measured in the series is not taken into account. Due to the limited information on the change in the in-series coefficient of variation of strength after cyclic freezing-thawing, as well as the ratio of frost resistance by the criterion of compressive strength and adhesion strength to the base, the purpose of this study was to identify patterns of change in the in-series coefficient of variation of strength and adhesion strength to the base of fine-grained concretes obtained from dry building mixtures based on various Portland cements with the content of various redispersible polymer powders from 0 to 3%. The studies were carried out on the basis of 75 freeze-thaw cycles. This paper presents the results on the change in the coefficient of variation of strength after cyclic freezing-thawing and the ratio of frost resistance coefficients

Keywords: frost resistance of concrete, coefficient of variation of strength, coefficient of frost resistance, fine-grained concrete, dry mixes

Low-modulus inclusions in the form of entrained air or ash microspheres in the composition of fine-grained concrete practically do not affect the ratio of the ultimate strength in bending and compression during hardening of concrete under normal conditions. After 75 freezing-thawing cycles, the ratio of the flexural and compressive strengths of fine-grained concrete with ash microspheres did not change, and in concretes with involved air, the influence of factors such as the type and dosage of redispersible powder and the type of cement increased sharply. Low-modulus inclusions have practically no effect on the ratio between the initial elastic modulus and the compressive strength of fine-grained concrete during hardening under normal conditions. After 75 cycles of freezing and thawing in concrete with entrained air, the influence of such factors as the type and dosage of redispersible powder and the type of cement on the ratio of the initial elastic modulus to compressive strength increases sharply. The cyclic freezing-thawing does not affect on the ratio of module and strength in concretes with an ash microsphere. Regardless of the presence of low-modulus inclusions, there is no clear relationship between adhesion to a concrete base and flexural strength of fine-grained concrete. In concretes with entrained air, the increase in adhesion to the concrete base after 75 freezing-thawing cycles, depending on the type of cement and dosage of redispersible powder, was up to 82%, and for compositions with an ash microsphere, up to 62%. Regardless of the presence of low-modulus inclusions, after 75 freeze-thaw cycles, adhesion to the base corresponds to class C-1. With an increase in the dosage of redispersible powder from 0 to 3% in compositions with low-modulus inclusions, an increase in adhesion to the base was noted up to 62%.

Keywords: dry mortar mixes, redispersible polymer powders, low modulus inclusions, adhesion to the base, elastic modulus, tensile strength, freezing and thawing cycles

The introduction of RPP to 3% by weight of the dry concrete mixture is accompanied by a decrease in the compressive strength of fine-grained concrete aged from 28 days to 37%, and after 75 cycles of freezing and thawing - up to 46%. The decrease in tensile strength in bending at the age of 28 days was 25%, after 75 cycles - up to 23%. The dependence of the initial modulus of elasticity of concrete on the compressive strength remains virtually unchanged after 75 cycles of freezing and thawing. The dependence of the adhesion of MZB to the concrete base with increasing dosage of RPP is ambiguous. The maximum increase in adhesion to the concrete base after 28 days of curing in NU and after 75 cycles of freezing and thawing was 26%. An increase in RPP dosage to 3% leads to a decrease in the initial elastic modulus of MZB to 26% after 28 days of curing in NU and to 32% after 75 cycles of freezing and thawing.

Keywords: dry building mixes, redispersible polymer powders, adhesion to the base, elastic modulus, tensile strength, freezing and thawing cycles

An assessment was made of the possibility of obtaining high-strength concrete based on certain materials of SRV. The analysis of the effect on the ultimate strength of concrete in compression for the duration of hardening, the magnitude of the water-cement ratio, and the properties of materials was performed. A technique for selecting efficient materials for the production of high-strength concrete based on an estimate of the specific consumption of cement is given. It is shown that on the basis of materials available in the northern regions of NRW, high-strength concrete with a specific cement consumption of about 6 (kg / m3) / MPa can be obtained. It was concluded that, in the production of high-strength concrete, it is advisable to increase the adhesion of cement stone to coarse aggregate, for example, by introducing active microsilica in the form of rice husk ash into the concrete mix. It has been suggested that it is necessary to study the effect of superplasticizing additives used in SRV on the kinetics of hardening of cements in order to determine the group of additives that have a minimal retarding effect on the growth of concrete strength in the early period

Keywords: specific consumption of cement, high-strength concrete, tensile strength, cement, coarse aggregate, fine aggregate, adhesion of cement stone with aggregate, hardening kinetics

The experience of applying centrifugation as a molding and compacting method has opened new prospects for the production of highly efficient reinforced concrete products. The experience of production of centrifuged power line supports at one of the enterprises of the Socialist Republic of Vietnam is analyzed. The studies aimed at studying the change in the composition of the mixture during centrifugation and the kinetics of the strength of centrifuged concrete were carried out. The dependence of the ultimate strength of centrifuged concrete on compression on the value of B / W after HMT (heat and moisture treatment) and at the age of 3-7 days was studied. Conclusions are made on the correction of the known quantitative regularities of the change in the strength of vibrating concrete from the value of B / W and the porosity, taking into account the heterogeneity of the structure of the centrifuged concrete over the cross section.

Keywords: centrifugation, heavy concrete, concrete mixture, power line supports, additives to concrete, enterprises for the production of reinforced concrete products