The Influence of Cement Grain Structure on Physical and Mechanical Properties of Cement

The paper presents the study of morphological characteristics of cement particles and reveals the influence of the structure of the cement grain composition on the physical and mechanical properties of cement. The following portland cements produced by “Hrazdan Cement Corporation” LLC, which have 52,5 MPa and 42,5 MPa compressive strength limit and hydraulic additives up to 20% and over 20% have been used for the experiment: CEM II/ A-P 42,5N, CEM II/ B-P 42,5N, CEM II/A-Q 42,5N, CEM II/B-Q, CEM II/A-L 42,5N, CEM II/B-L 42,5N, CEM II/A-M 42,5N, CEM II/B-M, CEM III/A-S 42,5N and CEM III/B-S 42,5N. Grain distribution in all the samples has been studied using a CILAS laser analyzer. Microscopic analysis of all the fractions has been carried out with the help of James Swift optical microscope. The given grain compositions have undergone chemical analysis in compliance with the requirements of interstate ISO 5382-2019 and ASTM C114-18 standards. Experimental studies and analyses show that the cements with microsilica have the highest value of water-cement ratio-W/C = 0.7, the highest by volume compression are the cements with volcanic slag-4 mm, the beginning of the bonding period is the longest in case of limestone cements - t = 140 minutes, followed by microsilica cements, and in third place there are artificial slag cements, the results of which are as follows: 130; 124 minutes. The summarized data show that microsilica cements have the highest compressive strength limit among the cements having the same percentage of additives-48.87 MPa.

Ultrafast imaging of terahertz electric waveforms using quantum dots

Microscopic electric fields govern the majority of elementary excitations in condensed matter and drive electronics at frequencies approaching the Terahertz (THz) regime. However, only few imaging schemes are able to resolve sub-wavelength fields in the THz range, such as scanning-probe techniques, electro-optic sampling, and ultrafast electron microscopy. Still, intrinsic constraints on sample geometry, acquisition speed and field strength limit their applicability. Here, we harness the quantum-confined Stark-effect to encode ultrafast electric near-fields into colloidal quantum dot luminescence. Our approach, termed Quantum-probe Field Microscopy (QFIM), combines far-field imaging of visible photons with phase-resolved sampling of electric waveforms. By capturing ultrafast movies, we spatio-temporally resolve a Terahertz resonance inside a bowtie antenna and unveil the propagation of a Terahertz waveguide excitation deeply in the sub-wavelength regime. The demonstrated QFIM approach is compatible with strong-field excitation and sub-micrometer resolution—introducing a direct route towards ultrafast field imaging of complex nanodevices in-operando.

Operational reliability of suspended facade structures with variable thickness of the cladding layer

The article presents the results of laboratory tests of experimental samples for mechanical safety, aimed at obtaining data pertaining to the performance of protective and decorative brick facade structures on metal substructures. The designs of the samples are atypical, with a different arrangement of bricks in the face layer and with ledges evenly distributed over the surface of the sample. Based on the test results, structure behavior under load, the absolute values of the displacement of the cladding layer, and the values of the bond strength between bricks and mortar were established. During the tests, the displacement of the protective and decorative structure in the direction of the applied load and the values of the forces, corresponding to the strength limit, were recorded.The article addresses the issue of the lack of rules for the design and testing of suspended facade systems made of bricks on metal substructures in the regulatory and technical documentation of the Russian Federation.

Analysis of methods for heating the lining of high-temperature aggregates

An analysis of the methods of increasing the temperature of the lining during its drying and heating is given. Three options for increasing the temperature are considered: the maximum possible heating rate at first and its further decrease, the average heating rate, and the minimum heating rate at first and further acceleration to the maximum. It is shown that in order to preserve the resulting temperature stresses, no higher than the strength limit of the material is more efficient, the heating option with the highest possible speed first and its further decline. Ill. 4. Ref. 11.

Measuring and Testing Composite Materials Used in Aircraft Construction

This paper is focused on the use of special composite materials for the construction of aircraft components. It focuses on measuring and testing the strength of reinforced composite materials used in damaged aircraft parts repairs. To determine the layer required to repair a part of the aircraft, it is necessary to know the strength limit of the material and its parts. The article describes experimental measurements of manufactured composite samples that have been subjected to tensile stress. Aim of the performed tensile tests was to determine the maximum tensile stress that the composite materials are able to transmit until they are damaged. Measurement determining the maximum stress level is important to ensure the required safety of the aircraft structure on which the composite structure was repaired.

Investigation of the relationship between the strength limit and the long time fatigue of steel reinforcements of reinforced concrete structures

The results of experimental studies have shown a strong effect of diffusion hydrogen on the static and cyclic parameters of crack resistance of reinforcing steel. It was found that with increasing flooding, especially when the hydrogen content exceeds 5 cm3/100g, both static strength and long-term strength (fatigue) decrease sharply. Moreover, these areas of hydrogen solution in reinforcing steel are characterized by a viscous nature of fracture, while for heavily flooded reinforcement (from 5 to 12 cm3/100g) is characterized by brittle fracture by the mechanism of microcracking in the hardened (martensite or troostite structure). The analysis of the obtained experimental results allowed to determine the optimal hydrogen content in the reinforcing steel (3…5 cm3/100g), the excess of which can cause a decrease in the crack resistance of the reinforcement during long-term operation, especially in corrosive environments. The mechanism of hydrogen influence on crack resistance of metal at static and alternating loading which consists in diffusion and dislocation movement of hydrogen in structure of a reinforcing core that as a result that causes strong flooding of steel and its embrittlement is offered. It is established that carbon and low-alloy sieves, which are characterized by ferritic-pearlitic and sorbitol structure provide high resistance, especially to long-term fatigue, and the transition to steels with a structure of martensite or tempered (transient structure of bainite) structure of bainite sharply reduces reinforcing steel, which makes it impossible to use in the manufacture of reinforcement involved in reinforced concrete structures designed for long-term operation (more than 50…60 years). Thus, the obtained diagram can be recommended to designers of reinforced concrete structures for hydraulic purposes, as it greatly facilitates the reasonable choice of reinforcement in the development of reinforced concrete structures for responsible and long-term use.

CHANGE IN THE TENSILE STRENGTH OF PRESSED BIRCH WOOD DURING LOCAL CRUSHING

At present, due to the reduction of business wood stocks, the problem of using fast-growing soft hardwoods as industrial raw materials is of particular relevance. This wood has a number of disadvantages – low density, low strength indicators, which limit the scope of its application. It is possible to improve the quality of soft hardwood wood by pressing it (compacting). Compaction of wood has a great influence on the physical and mechanical properties of wood. The wide application of pressed wood is possible in the production of railway sleepers. Pressing wood significantly increases the performance properties of the sleeper beam. One of the advantages of wooden railway sleepers is their high damping properties. The presence of damping properties in wooden sleepers ensures high speed performance and a long service life of the rolling stock. In this connection, research that ensures the rational use of wood raw materials is relevant. The purpose of this study is to determine the nature of the change in the strength limit of pressed birch wood (Betula pendula) during local crumpling.

Conductive materials for connecting tires of multi-element accumulator traction batteries

The article deals with the choice of materials for connecting tires of traction batteries (TB). The optimal parameters of their spot welding with batteries are experimentally established (the first pulse with a current of 7 kA duration of 1 ms, the break between the pulses of 1 ms, the second pulse with a current of 7 kA duration of 2 ms). When operating the traction battery on electric vehicles, the resistance of the connecting tires should not lead to heating of the batteries in order to avoid overheating above 60 °C. In most modern TB, consisting of Li-ion elements, a nickel tape is used for the connection. To ensure the weldability of materials (copper–nickel or nickel–nickel), it is important that the operating temperature is reached at a short-term current pulse in the welding zone. One of the solutions to this problem is the application of a metal coating. Experiments were conducted on the weldability of various materials, including those with applied coatings. The best results in weldability were shown by tires made of tinned copper, which was welded to nickel plates (emitting the battery body). Tear tests of the welded samples were carried out. The tensile strength of the original copper tires was 340–450 MPa. When welding copper–nickel and copper(tinned) – nickel plates, the strength limit values reach 70 % of the strength of the original copper plate. On the basis of the obtained experimental data, a pilot batch of battery TB was manufactured, which successfully passed tests for compliance with the technical requirements for the strength and the value of the transition resistances of the welded joints of connecting buses with batteries.

Influence of titanium and zirconium on structure and heat-resistance of low-carbon iron-aluminium alloys

The paper considers the effect of introducing ferroalloys containing titanium and zirconium on the structure and heat-resistance of low-carbon ferroalloys. Theoretically and experimentally, it has been proven that addition of 1.0 mass. % of titanium and 0.1 mass. % of zirconium to a low-carbon iron-aluminum melt containing 12 – 14 mass. % of aluminum, grinds its structure increasing temporary resistance and heat-melting. Titanium and zirconium are strong carbide-forming elements. When introduced into a low-carbon iron-aluminium alloy, they form a large number of crystallization centers, thus affecting its microstructure, allowing to get shredded and more equal grain compared to an alloy without additive. This in turn increases the strength limit of processed alloy. In addition, the use of titanium as a modifying additive in a low-carbon ferroalloy allows increasing its heatresistance, which exceeds several times the heat-resistance of famous chrome-nickel steel of 20Kh23N18 grade. As a result, a new technology for obtaining titanium and zirconium was developed based on research of the effect of their modifying additives on the structure and heat-resistance of low-carbon iron-aluminum alloys.