Self-compacting concrete is a highly flowing concrete that compact under self-weight without any vibration effort. The mortar phase of self-compacting concrete determines its rheological, strength characteristics, and durability. This paper discusses the effect of superplasticizer and silica fume on the properties of self-compacting mortars. Mini-slump flow diameter tests were conducted to determine the rheological properties of mortar mixtures, and compressive strength, density, water absorption, porosity experiments to assess the hardened properties of mortar specimens. Analysis of regression models shows that the use of superplasticizer reduces the water demand of mortar mixtures by up to 40 %, and the addition of silica fume leads to the densification of cement stone structure and reduces the porosity of concrete to 24 %. The maximum compressive strength at 2 days can be achieved with a superplasticizer content of 0.8-1.0 % and 0-5 % silica fume by binder weight, and at 28 days – with 1.0-1.2 % superplasticizer and 10-15 % silica fume content. According to X-ray phase analysis in a cement stone containing superplasticizer and silica fume, the degree of cement hydration increases and reduces the amount of portlandite, which leads to an increase in the strength and durability of the concrete.

Heat-moisture treatment (HMT) is the main way to speed up the strength of reinforced concrete products. But this technology is the cause of high energy and resource costs. The use of traditional plasticizing additives in concrete is difficult in terms of early strength reduction. The modern industry of chemical additives allows regulating superplasticizers (SP) molecular structure. The use of such modifiers in concrete leads to early strength increase. The combination of HMT and SP using in concrete hardening makes the optimization of steaming parameters possible. HMT variable modes of concrete modified with polycarboxylate SP were investigated for early strength. The stripping concrete strength with an increase in SP dosage from 0.4 % to 2.0 % by cement mass decreases as follows: at the maximum curing temperature of 40 °C – on 32.8 %; at the temperature of 60 °C – on 27.2 %, at the temperature of 80 °C – on 23.8 %. The concrete compressive strength after HMT without the presteaming period is lower by only 5.4-14.3 % in comparison with concrete steamed with one for 1-2 hours. The research was carried out on concrete using sand with a low size modulus of 1.70. The authors show the superplasticizer effectiveness in terms of reducing the water-cement ratio and joint action with HMT. Mathematical models of the early concrete strength were obtained depending on curing temperature, presteaming period duration, and the SP dosage. The obtained results can be used in the reinforced concrete products production, subject to additional research.

The article is devoted to the issues of ontology, research of the effectiveness and prospects of large-panel and monolithic reinforced concrete construction of residential objects. Increasing demands on the effectiveness of multi-story housing construction determine the relevance of this study. Methodological foundations of substantiation and a comparative analysis of the effectiveness of monolithic and large-panel reinforced concrete multi-story construction are considered; prospects for the development of these housing technologies are justified. The article presents an analysis of the pros and cons of monolithic and large-panel reinforced concrete multi-storey construction of residential buildings, a comparative analysis of the processes of organization and technology of construction works. The problems of large-panel and monolithic construction are ranked according to the level of their materiality, measures to improve construction technologies are proposed. The comparative analysis of cost indicators of monolithic and panel construction on the basis of the data presented in collections of standards of the price of construction is presented. The need to introduce and update the normative documentation having legal force for regulation of the basic provisions of the modern improved large-panel construction is proved. Attention is paid to the design conditions for the manufacture and installation of the monolithic and large-panel multi-storey construction of residential buildings used in the methodological approach. Cases of expediency of comparison on criterion "duration of construction" are analyzed. The article proposes an original methodological approach for evaluating the effectiveness of monolithic and large-panel construction, taking into account the conditions and nature of the uncertainty of the initial data, which will justify the choice of the most effective options for the construction of multi-storey housing construction.

Self-compacting concrete is a highly flowing concrete that compact under self-weight without any vibration effort. The high workability of concrete mixtures is achieved by the reduction in coarse aggregate content while increasing fines aggregate volume. Usually, coarse quartz sands with fineness modulus more than 2.5 are used as fine aggregate. The use of low fineness modulus sands, characterized by high intergranular porosity and high specific surface area, as a fine aggregate, leads to an increase in the concrete mixture water demand and a decrease in its flowability. In such conditions, to obtain high-workability and high-strength concrete, increased cement consumption is required. The purpose of this study was to develop high-strength self-compacting concrete with reduced cement consumption and using low fineness modulus sand. To achieve this purpose, we optimized aggregates consumption in concrete composition, which achieves minimum aggregate mixture intergranular porosity, best concrete mixture workability without segregation, and maximum concrete strength. It has been established that silica fume and superplasticizer using allows obtaining high-strength self-compacting concrete of strength class B60 with low-quality fine sand and reduced cement consumption of 360 kg/m3. With increasing mixtures retention time before concrete placing, there was an insignificant increase in the flowability of obtained self-compacting concrete mixture and an improvement in the physical and mechanical properties of concrete. Remixing of concrete mixtures and concrete placing after 90 minutes from concrete mix preparation moment allowed to increase concrete strength to 17%, increase its density to 15 kg/m3, and reduce porosity to 25%.

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