The article deals with the influence of complex mineral additive "chemical water treatment sludge + flask or fly ash" on the frost resistance of building mortars depending on the redispersible polymer powder dosage from 0 to 3%. According to the results of research after 100 freeze-thaw cycles, the condition RF/R0 > 1.2 is met, and the k value according to clause 5.2.4 of GOST 10060-2012 after 100–125 freeze-thaw cycles ranged from 0.94 to 1.73, the minimum value of the relative modulus of elasticity EF/E0 = 0.97, and the residual expansion deformation did not exceed 0.03%. Building mortars with fly ash have higher frost resistance according to the bending strength criterion in comparison with mortars with flask. The frost resistance criterion for bending strength is more “strict” than the criteria according to GOST 10060-2012. It is not advisable to use a dosage of RPP greater than 2% for the production of building solutions with increased frost resistance requirements.

Keywords: frost resistance of building mortars, frost resistance criteria, redispersible polymer powders, compressive and flexural strength, elastic modulus, permanent expansion deformation

The article deals with the results of research on the influence of the dosage and composition of the complex mineral additive containing chemical water treatment sludge with gaize, burnt rock or fly ash. The result of investigation is the equation for the change in shrinkage deformations over the time for mortars containing a complex mineral additive in comparison with a standard mixture without additives. There is evaluating of the influence of changes in the mortar moisture on the amount of shrinkage deformation. On the base of the research, we astablished the dependences of shrinkage deformations on the moisture content of the mortar, according to the composition and dosage of the complex mineral additive. The study revaels the influence of a complex mineral additive on shrinkage deformations. The equation has been obtained that describes the change in shrinkage deformations depending on the relative humidity and it has been shown that the tested mortars with a rational composition and dosage of a complex mineral additive with an average air humidity of the driest month in Rostov-on-Don of 47% will not exceed 0.55 mm/m . Shrinkage deformation with the ratio of the complex additive with fly ash or burnt rock 20-30% are 0.42-0.79 relative to the additive-free standard.

Keywords: dry construction mixture, shrinkage, mineral additive, mortar, chemical water treatment sludge, burnt rock, fly ash

Layered reinforced concrete beams made of multi-modulus of elasticity concretes are increasingly used in engineering practice. The step-by-step iterative method in combination with numerical integration in the calculation of such structures using a nonlinear deformation model and real deformation diagrams is a rational solution, despite the currently lack of a generally recognized approach to assessing the stress-strain state of sections of layered beams.

Keywords: double-layer reinforced concrete beams, normal cross-sections, nonlinear deformation model, high-modulus concrete, neutral axis coordinate

The work deals with the results of research on the influence of complex mineral additive: chemical water treatment sludge with burnt rock or fly ash or gaize. The rational content of chemical water treatment sludge in the complex additive was determined to be 50%. The relationship between compressive strength, flexural strength and W/C ratio, bettwen the flexural strength, E-modulus and the compressive strength were obtained. Complex mineral additive into fine grain concrete does not lead to a decrease in flexural strength and does not increase the E-modulus of fine grain concrete. The average range of the E-modulus of fine grain concrete with complex mineral additive practically coincide with those in Russian National Standart 56378 - 2015. The use of chemical water treatment sludge for fine grain concrete at amount of 20% of complex additive of a rateable composition instead of part of the cement provided to increasing of compressive strength at an equal W/C ratio relatively to the non-additive standard by 7 - 20%, and the flexural strength by 12 - 22% .

Keywords: miniral additive, tensile strength, modulus of elasticity, gaize, fly ash, burnt rock, chemical water treatment sludge

The influence of the type and dosage of some redispersible polymer powders on the shrinkage deformation of building mortars obtained from dry building mixes, including for underfloor heating, after exposure at a temperature of 70 ° C in accordance with GOST R 56387-2018 was investigated. The introduction of admixtures 4042N, E06RA, 5603 into the mixtures did not lead to an increase in shrinkage deformations when maintained according to GOST R 56387-2018 relative to the additive-free standard, regardless of the type and dosage of the admixture, while with these admixtures, regardless of their dosage, within 1-3%, the values of shrinkage deformations were less than 1.5 mm / m. The greatest influence of the type and dosage of the redispersed polymer powders on the shrinkage change is manifested at the heating stage, which is due to their influence on the kinetics of dehydration and the magnitude of the E-modulus. Depending on the type of cement, type and dosage of the admixture, the moisture loss values by the end of heating to 70oC ranged from 0.114 to 0.629 relative to the total moisture loss by the end of holding. Naturally, a change in shrinkage by the end of heating from 0.027 to 0.595 relative to complete shrinkage by the end of aging was noted, while a proportional relationship between moisture loss and shrinkage was revealed. In some formulations, an expansion of up to 0.469 mm/m was recorded by the end of heating. For a comprehensive assessment of the degree of influence of admixtures on changes in the deformation and strength properties of mortar at a holding temperature of 70 ° C according to GOST R 56387-2018, an indicator of the conditional stress level is proposed, varying depending on the holding stage, type of cement, type and dosage of the additive in the range from 0.489 to 3.05.

Keywords: Shrinkage, building mortars, dry building mixes, redispersible polymer powders, underfloor heating

The analysis of the dependences of the "flexural strength – compressive strength" of ordinary concretes obtained as a result of processing numerous experimental data of the authors and borrowed from published works, as well as data presented in various regulatory documents is presented. Based on the obtained regression equations describing the dependence of the flexural strength of concrete on the compressive strength in the compressive strength range from 30 to 100 MPa, it is concluded that it is advisable to normalize the classes of flexural strength without taking into account the prescription features of concrete. Some inconsistency of the ratios of classes for flexural strength and compression in the analyzed regulatory documents has been revealed. The high reliability of the previously obtained dependence of the flexural strength of concrete on the compressive strength has been confirmed. A formula is proposed that takes into account the effect of the addition of silica fume in the composition of the concrete mixture on the ratio of flexural strength and compression. The role of the quality of aggregates in ensuring high flexural strength of concrete is noted.

Keywords: the flexural strength of concrete, the compressive strength of concrete, concrete strength classes, concrete for road and airfield coatings

An approach is proposed to determine the parameters of concreting flat massive foundation slabs - the operational performance of the concrete mix supplier, vehicles and concrete pump, taking into account the limiting factors - the intensity of the supply and placement of the concrete mix and the time of overlapping layers. A scheme for determining the overall coefficient of operational performance is proposed. As a result of timing, the values ​​of the transition coefficient from technical to operational productivity, respectively, for a concrete mixer truck with a volume of 10 m3 from 0.54 to 0.65 and 0.41 for a concrete pump truck with a technical productivity of 120 m3 / h, respectively, were determined when concreting a massive flat foundation slab with a volume of 1500 m3 using concrete mix with grade P4 for workability at a delivery distance of up to 50 m. It is proposed to consider a coefficient that takes into account the decrease in the productivity of an autoconcrete pump due to the variability of the concrete mix supply mode as the main reserve for increasing operational productivity. Schemes are given for determining the duration of concreting a structure without technological interruptions, both with an equal thickness of the stacked layers, and with different schemes for assigning the thicknesses of the stacked layers. The inexpediency of erecting such structures with a layer overlapping time of less than 4 hours is shown. The approach considered in the work can be used to select rational methods for concreting such structures in a continuous pattern or with division into temperature-shrinkage blocks and the construction of working joints. The latter scheme, in connection with the problem of ensuring the quality of the seams and the solidity of the structure, seems to be less preferable.

Keywords: concreting intensity, technical productivity, operational productivity, massive flat foundation slabs, layer overlap time, layer thickness

The results of studies of two-layer reinforced concrete beams with a layer of various thicknesses of high-modulus concrete of the frame structure in the compressed zone and traditional heavy concrete in the rest of the section are presented. The stress-strain state of normal cross sections along the height of the beam was estimated by the values of relative deformations at the level of stretched and compressed reinforcement using the hypothesis of flat sections and actual compression and calculated tensile deformation diagrams of concrete. The influence of the thickness of a layer of high-modulus concrete on the nature of the stress-strain state between the layers is shown. The possibility of destruction of the beam on the concrete of the compressed zone was revealed due to the abrupt decrease in the actual height of the compressed zone due to the "shooting" of a layer of high-modulus concrete with its thickness less than 0.1 h0. The dependence of the relative actual thickness of the compressed concrete, i.e. the distance from the edge fiber of the compressed concrete to the neutral axis, in the studied beams on the magnitude of the bending moment is obtained. It is established that for single-layer beams, with a difference in the elastic modulus of concrete up to 20%, the dependences of the relative actual thickness of the compressed concrete layer on the bending moment are almost identical. In the studied beams, the limiting state of which is caused by the fluidity of the reinforcement, the thickness of the compressed concrete layer was 0.37 ...0.4 with a relative moment value of 0.5, and 0.33 ...0.36 with a maximum bending moment value.

Keywords: double-layer reinforced concrete beams, normal cross-sections, nonlinear deformation model, high-modulus concrete, neutral axis coordinate

The influence of the properties of the base and the holding conditions on the adhesion strength to the base of modified building mortars obtained from dry building mixes with a content of 1-3% of redispersible polymer powders has been investigated. A decrease in the adhesion strength of the modified mortar with a standard concrete base was revealed to an average of 0.45-0.8 relative to normal hardening conditions when maintained at elevated temperatures according to GOST R 56387-2018 and to 0.4-0.55 when cyclically heated to 60 ° C, depending on the properties of cement, type and dosage of RPP. The relative strength of adhesion to granite under the same conditions was 0.3-0.65 and 0.2-0.8, respectively. The adhesion strength of modified mortars during hardening under normal conditions relative to the adhesion strength with a standard concrete base is approximately: with silicate brick from 30 to 50%, with ceramic brick from 65 to 80%, with aerated concrete about 50%, with granite from 9 to 18%. The recommended initial dosage of redispersible polymer powders for dry building mixes for heated floors is 1.5%.

Keywords: modified building mortars, redispersible polymer powder, adhesion strength to the base, bending strength, temperature

Based on the research conducted by the authors, data on the stiffness and curvatures of reinforced concrete beams with a layer of various heights of concrete with frame structure are presented. Stiffness, curvature and deflections are given for cases of absence and presence of cracks in the tensile zone of beam. Formulas for describing changes in the stiffness and curvature of sections depending on the magnitude of the moment are obtained. The influence of the thickness of the layer of concrete with frame structure and the diameter of the working reinforcement on the change in the stiffness of the sections is established. The regularity of changing the position of the neutral axis for single and double-layer beams depending on the beam parameters.

Keywords: concrete of frame structure, stiffness, curvature, deflection, layered structures, bending moment, neutral axis

The effect of redispersible polymer powders on the change in the properties of building mortars after 60 cycles of heating to 60 ° C and cooling is investigated. An increase in the compressive strength from 11 to 27% was revealed at a dosage of RPP from 1 to 3%, and the introduction of an air-entrapping additive into the composition does not contribute to an increase in the compressive strength. A significant increase in the ratio of the tensile strength "flexural tension" has been established/compression", while the tensile strength of building mortars during bending practically does not change with an increase in the dosage of RPP, and the increase in tensile strength during bending occurs to a lesser extent than that of building mortars without RPP, and the presence of an air-entrapping additive contributes to an increase in the tensile strength during bending. A decrease in the initial modulus of elasticity was found almost independently of the dosage of the RPP to values of 0.82 – 0.84, and in the presence of an air-entrapping additive to 0.9 relative to normal hardening conditions. A decrease in the adhesion strength to the concrete base to values of 0.59 – 0.75 relative to normal hardening conditions was revealed. The presence of an air-entrapping additive leads to an increase in the adhesion strength after cyclic heating-cooling by more than 4 times.

Keywords: redispersible polymer powder, adhesion strength, modulus of elasticity, compressive and bending strength, cyclic heating-cooling

The overlapping time of the layers during the layer-by-layer laying of the concrete mixture during the construction of massive monolithic reinforced concrete structures is an important technological parameter of concreting, linking the rate of concreting, the thickness of the laid layers and the design parameters. For traditional concrete mixtures compacted by vibration, this parameter is determined by the persistence of the concrete mixture, taking into account its temperature. When using self-compacting concrete mixtures, the time of overlapping layers, in addition to traditional factors, significantly depends on the period of formation of the "elephant skin". The influence of the overlapping time of the layers on the strength of their adhesion, depending on the properties of the concrete mixture and the method of its laying, was studied. The ultimate tensile strength in bending was used as a criterion for adhesion strength. It has been confirmed that when using a traditional concrete mix compacted by vibration without superplasticizing additives based on polycarboxylate esters, the overlapping time of the layers of 2 hours provides a seam of equal strength with the main body. For self-compacting concrete mixes using vibration when laying subsequent layers after the first, the overlapping time corresponds to the storage index and increases to 6 hours, and when layered without vibration, the adhesion strength decreases by about 20%, 70% and more than 90% with an overlapping time of 2.4 and 6 hours, respectively.

Keywords: self-compacting concrete mixes, massive reinforced concrete structures, overcoating time, "elephant skin"

The method of determining crack-forming forces based on the normative approach is considered. Experimental and theoretical studies of reinforced concrete beams made of ordinary heavy concrete, self-compacting concrete with a modifier and concrete of a frame structure have been carried out. Formulas for determining the tensile strength and flexural strength depending on the compressive strength for each of the considered types of concrete are given. The calculated values of the ultimate extensibility of various concretes determined by various methods are given. Coefficients to the formula linking the reduced and initial elastic modulus of various types of concrete are obtained. The influence of the accuracy of the experimental determination of the moment of cracking on the change in the calculated dependencies is analyzed.

Keywords: cracking moment, flexural and tensile strength, frame structure concrete, self-compacting modified concrete

The work of reinforced concrete racks made of concrete frame structure is analyzed. The conclusion is made about the expediency of using concrete frame structure in reinforced concrete columns.

Keywords: concrete frame structure

The results of studies of the effect of mineral additives of natural and man-made origin on some properties of fine-aggregate concrete with the introduction of a mineral additive from 10 to 25% of the cement weight instead of either a part of cement with an increase in the value of W/ C, or a part of sand with a constant value of the value of W / C are presented. A slight, up to 3%, change in the average density of concrete was revealed. A decrease in the E-modulus from 4 to 14% was recorded when replacing a part of cement, while an increase in the flexural strength, depending on the dosage, was up to 26% (marshallite) and 6% (chemical water treatment sludge), and a decrease in the compressive strength was from 10 to 22%. When replacing part of the sand, an increase of up to 10% of the E-modulus, the flexural strength from 19 to 38% (marshallite) and up to 11% (chemical water treatment sludge) was recorded. The increase in the compressive strength, depending on the type and dosage of the additive, ranged from 8 to 20%. No significant effect of additives on the porosity of cement stone was revealed.

Keywords: mineral additive, fine-aggregate concrete, flexural strength, E-modulus, shrinkage, water absorption