The usage of industrial and agricultural wastes for building materials production plays an important role to improve the environment and economy by preserving nature materials and land resources, reducing land, water and air pollution as well as organizing and storing waste costs. This study mainly focuses on mathematical modeling dependence of the compressive strength of high performance concrete (HPC) at the ages of 3, 7 and 28 days on the amount of rice husk ash (RHA) and fly ash (FA), which are added to the concrete mixtures by using the Central composite rotatable design. The result of this study provides the second-order regression equation of objective function, the images of the surface expression and the corresponding contours of the objective function of the regression equation, as the optimal points of HPC compressive strength. These objective functions, which are the compressive strength values of HPC at the ages of 3, 7 and 28 days, depend on two input variables as: x1 (amount of RHA) and x2 (amount of FA). The Maple 13 program, solving the second-order regression equation, determines the optimum composition of the concrete mixture for obtaining high performance concrete and calculates the maximum value of the HPC compressive strength at the ages of 28 days. The results containMaxR28HPC = 76.716 MPa when RHA = 0.1251 and FA = 0.3119 by mass of Portland cement.
Abstract. The dense development of high-rise construction in urban areas requires a creation of new concretes with essential properties and innovative technologies for preparing concrete mixtures. Besides, it is necessary to develop new ways of presenting concrete mixture and keeping their mobility. This research uses the mathematical method of two-factors rotatable central compositional planning to imitate the effect of amount of rice husk (RHA) and fly ash of thermal power plants (FA) on the workability of high-mobility concrete mixtures. The results of this study displays regression equation of the second order dependence of the objective functions -slump conе and loss of concrete mixture mobility due to the input factors -the amounts RHA (x1) and FA (x2), as well as the surface expression image of these regression equations. An analysis of the regression equations also shows that the amount of RHA and FA had a significant influence on the concrete mixtures mobility. In fact, the particles of RHA and FA will play the role as peculiar "sliding bearings" between the grains of cement leading to the dispersion of cement in the concrete mixture. Therefore, it is possible to regulate the concrete mixture mobility when transporting fresh concrete to the formwork during the high-rise buildings construction in the hot and humid climate of Vietnam. Although the average value of slump test of freshly mixed concrete, measured 60 minutes later after the mixing completion, decreased from 18.2 to 10.52 cm, this value still remained within the allowable range to maintain the mixing and and the delivery of concrete mixture by pumping.
This paper reports the results of a research study aimed at identifying the compositional features of fly ash (FA) TPP “Pha Lai” (Vietnam), silica fume SF-90 (SF90) and superplasticizer SilkRoad SR 5000F (SR5000) contents that influence the sulfate resistance of corrosion-resistant concrete. The effect of FA, SF90 and SR5000 on the strength of corrosion-resistant concrete is obtained quickly, high early-strength concrete. In details, the compressive strength at the age of 1 and 3 days are respectively 29.6% and 61.13% in comparison to 28 days period. The deformation of corrosion-resistant concrete prisms in 5% sodium sulfate solution after 28 days of testing were determined by Russian standard GOST P 56687-2015. Using the mathematical planning method for four factors and the Matlab computer programs was obtained the mathematical model, which was adequately described the influence ofthe water-binding ratio, FA, SF90 and SR5000 contents on the concrete deformation. It was also noted that the effect of FA content is more pronounced than the water-binding ratio and content SF90. In addition, the effect of SR5000 content on the deformation of concrete is negligible, so it was discarded.
The paper considers methods of physical and mathematical modeling that can be used to describe the processes of non-stationary mass transfer of free calcium hydroxide in buried concrete and reinforced concrete structures of coastal zones. The authors proposed a mathematical model of mass transfer in an unrestricted two-layer plate in the form of a system of parabolic partial differential equations with boundary conditions of the second kind at the "concrete-liquid" interface and of the fourth kind at the "concrete-soil" interface. In terms of the results of experimental studies of the mechanisms for reducing the quality of concrete due to exposure to sulfate and acidic environments, using the obtained solutions of the mathematical model, it becomes possible to select the optimal composition of concrete with high anticorrosion properties.
The proportion of people living with HIV (PLWH) in need of antiretroviral therapy (ART) is growing rapidly in Russia. Successful treatment outcomes reduces disease progression and contributes to HIV epidemic control. We conducted a pilot study following 100 PLWH newly found eligible for ART in St. Petersburg, Russia. We determined the proportion of PLWH who initiated ART, remained in treatment, and achieved an undetectable VL during 6-month follow up. Semi-structured interviews were conducted prior the initiation of ART and progress along the cascade was assessed through medical chart review. Individual characteristics associated with successful ART outcomes were assessed as part of efforts to generate hypotheses. Almost all (96%) participants initiated ART, full retention was demonstrated by 80%, among whom 71% achieved undetectable VL. Optimal retention was associated with older age and higher education (p < 0.05). There was no significant difference in ART outcomes between those who used illicit drugs and those had not. Interventions to improve treatment effectiveness should emphasize that initiation, optimal retention and achieving an undetectable VL are independent of drug abuse status. However, our pilot study highlights the need for the further research in the examining links between individual and structural factors and ART effectiveness.
This study built theoretical and practical models to evaluate the corrosion resistance of concrete for coastal offshore structures in Vietnam. A mathematical model was developed in the form of a system of nonlinear partial differential equations characterizing the diffusion “free calcium hydroxide” in a solid of a concrete structure. The model describes the process of non-stationary mass conductivity observed in the “concrete structure—marine environment” system under non-uniform arbitrary initial conditions, as well as combined boundary conditions of the second and third kind, taking into account the nonlinear nature of the coefficients of mass conductivity k and mass transfer β. It was shown that the solution of the boundary value problem of non-stationary mass conductivity allows us to conclude about the duration of the service life of a concrete structure, which will be determined by the processes occurring at the interface: in concrete—mass conductivity, depending on the structural and mechanical characteristics of hydraulic structures, and in the liquid phase—mass transfer, determined by the conditions of interaction at the interface of the indicated phases.
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