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This paper presents the technological aspects of increasing the thermal stability of polymers, with epoxy binder used to form the polymer materials. Polyethylene polyamine was used to crosslink the epoxy binder. To ensure the thermal stability of the polymer, nanodispersed condensed carbon with a dispersion of 10–16 nm was used. The research into nanocomposites under the influence of elevated temperatures was carried out using the "Thermoscan-2" derivatograph. Complex studies of thermophysical properties were carried out, according to the results of which the optimal content of nanofiller (0.050 pts.wt.) was determined. At the same time, this particular polymer was characterized by the following properties: temperature of the beginning of mass loss—T0 = 624.9 K; final temperature of mass loss—Tf = 718.7 K; relative mass loss—εm = 60.3%. Research into the activation energy of thermal destruction was performed to determine the resistance to the destruction of chemical bonds. It was proved that the maximum value of activation energy (170.1 kJ/mol) is characterized by nanocomposites with a content of nanodispersed condensed carbon of 0.050 pts.wt., which indicates the thermal stability of the polymer. [ABSTRACT FROM AUTHOR]

Corrosive destruction and hydroabrasive wear is a serious problem in the operation of machine parts and water transport mechanisms. It is promising to develop new composite materials with improved properties to increase the reliability of transport vehicles. In this regard, the use of new polymer-based materials, which are characterized by improved anticorrosion properties and wear resistance, is promising. In this work, therefore, for the formation of multifunctional protective coatings, epoxy dian oligomer brand ED-20, polyethylene polyamine (PEPA) hardener, a mixture of nanodispersed compounds with a dispersion of 30–90 nm, fillers Agocel S-2000 and Waltrop with a dispersion of 8–12 μm, and particles of iron slag with a dispersion of 60–63 μm are used for the formation of multifunctional protective coatings. Using the method of mathematically planning the experiment, the content of additives of different physico-chemical natures in the epoxy binder is optimized to obtain fireproof coatings with improved operational characteristics. A mathematical model is developed for optimizing the content of components in the formation of protective anticorrosion and wear-resistant coatings for means of transport as a result of the complex effect of a mixture of nanodispersed compounds, iron scale, and Waltrop. Based on the mathematical planning of the experiment, new regularities of increasing the corrosion resistance and resources of the means of transport are established through the formation of four different protective coatings, which are tested for resistance to aggressive environments (technical water—CAS No. 7732-18-5, gasoline—CAS No. 64742-82-1, acetone—CAS No. 67-64-1, I-20A lubricant—CAS No. 64742-62-7, sodium solutions—CAS No. 1310-73-2, and sulfuric acid—CAS No. 7664-93-9) and hydroabrasive wear resistances. A study of the change in the permeability index in aggressive environments is additionally carried out, taking into account the rational ratio of dispersive fillers in the epoxy binder, which made it possible to create an effective barrier to the penetration of aggressive water molecules into the base. A decrease in the permeability of protective coatings by 2.0–3.3 times relative to the epoxy matrix is achieved. In addition, the wear resistance of the developed materials under the action of hydroabrasion is investigated. The relative resistance of the CM to the action of hydroabrasion was found by the method of materials and coatings testing on the gas-abrasive wear with a centrifugal accelerator. This method enables one to model the real process of the wear of mechanism parts under the hydroabrasive action. It is shown that the coefficient of the wear resistance of the developed materials is 1.3 times higher than that of the polymer matrix, which indicates the resistance of the composites to the influence of hydroabrasive environment. As a result, modified epoxy composite protective coatings with improved anticorrosion properties and wear resistance under hydroabrasive conditions are developed. It is established that the protective coating filled with particles of a mixture of nanodispersed compounds (30–90 nm), iron scale (60–63 μm), and Waltrop (8–12 μm) has the lowest permeability indicators. The permeability in natural conditions of such a coating during the time t = 300 days of the study is χ = 0.5%, which is 3.6 times less than the similar indicators of the epoxy matrix. It is substantiated that the protective coating filled with particles of a mixture of nanodispersed compounds (30–90 nm), iron scale (60–63 μm), and Agocel S-2000 (8–12 μm) is characterized by the highest indicators of wear resistance. The coefficient of wear resistance under the action of hydroabrasion of such a coating is K = 1.75, which is 1.3 times higher than the similar indicators of the original epoxy matrix. [ABSTRACT FROM AUTHOR]

Perforated cold-formed steel (PCFS) beams are increasingly used in constructional industry and become popular due to their advantages and economic benefit. Apart from the light weight, the openings in PCFS beams can also be used for services such as pipe utilities to release more building space. However, beams with web perforations may collapse as the result of web-distortional buckling or flange/web distortional buckling due to the web weakness caused by perforations. In this paper a study on the flange/web distortional buckling of PCFS beams is presented. An analytical solution to determine the critical stress of flange/web distortional buckling of PCFS beams is derived by using energy method. To demonstrate the analytical solution, finite element analysis is also carried out. The finite element analysis results show that the analytical solution provided can give good and reliable prediction for the flange/web distortional buckling of PCFS beams. [ABSTRACT FROM AUTHOR]

This article investigates the punching shear behavior of recycled aggregate concrete (RAC) two-way slabs. Ten 1500 mm × 1500 mm × 100 mm slabs were tested monotonically. Eight slabs were cast with RAC, whereas two control slabs were cast with natural aggregate concrete (NAC). The RAC incorporated coarse recycled concrete aggregate (RCA) at replacement levels of 25%, 50%, 75% and 100%. Two flexural reinforcement ratios (0.8% and 1.5%) were examined. The results show that the normalized punching shear strength of 100% RAC slabs decreased by 6.5% and 9% compared to NAC slabs for ρ = 1.5% and ρ = 0.8%, respectively. Doubling the amount of flexural reinforcement can increase the punching shear capacity of 100% RAC slabs by up to 45%. A punching shear database of 44 RAC slabs from literature and the 8 RAC slabs presented in this study revealed that the punching shear strength of RAC slabs predicted by ACI 318 was conservative, except for slabs with low reinforcement ratios (<0.6%). The punching shear strength predicted by Eurocode 2 gave more conservative results for all levels of RCA replacement and all flexural reinforcement ratios. A yield-line analysis also showed that the failure mode of the RAC slabs was controlled by punching shear. [ABSTRACT FROM AUTHOR]

The majority of the existing literature on the lateral stability of castellated beams deals with experimental and/or numerical studies. This paper presents a comprehensive analytical study of the lateral-torsional buckling of simply supported castellated beams subject to pure bending and/or a uniformly distributed load. Using the principle of total potential energy, analytical expressions for the critical buckling moments and loads are derived and applied for various beam lengths. The three different locations of the applied load are used: At the top flange, shear center and bottom flange. The results show that the influence of web openings on the critical buckling moments and loads are mainly due to the reduction of the torsional constant caused by the web openings. Web shear effects and web shear buckling become important only when the beam is short and the flange is wide. The critical moments and loads will be overestimated or underestimated if the full or reduced section properties are used. The accurate critical moment or load should be calculated based on the average torsional constant of the full and reduced sections rather than simply taking the average of the critical moments or loads calculated from the full and reduced section properties. The present analytical solutions are verified using 3D finite element analysis results. [ABSTRACT FROM AUTHOR]

This paper presents an analytical study on the thermal buckling analysis of axially loaded columns of thin-walled open section with nonuniform sectional properties. Obtained herein are critical loads related to flexural, torsional and flexural-torsional buckling of an I-section column subjected to an axial compressive load applied at the geometric centroid, and under linearly varied non-uniform temperature distribution scenarios. The analysis is accomplished using traditional energy methods. The influences of thermal strain, nonuniform distribution of pre-buckling stresses, and variation of pre-buckling stresses along the longitudinal axis of the column on critical buckling loads are examined. The present results highlight the importance of nonuniform sectional properties in the buckling analysis of columns of doubly symmetric section. [ABSTRACT FROM AUTHOR]

This paper presents a two-dimensional, two-phase ionic transport model with a surface charge at solid–liquid interfaces. The present model is applied to investigate the effect of surface charges at the solid–liquid interface on the ionic transport in a cement paste when it is subjected to an externally applied electric field. The surface charge in the present model is considered by modifying the Nernst–Planck equation in which the electrostatic potential is dependent not only on the externally applied electric field but also on the dissimilar diffusivity of different ionic species including the surface charges. The coupled transport equations of individual ionic species are solved numerically using a finite element method built in commercial software COMSOL. Some important features about the effect of surface charge on the concentration distribution, migration speed and flux of individual ionic species are discussed. [ABSTRACT FROM AUTHOR]