On Providing an Assessment Monitoring System for Especially Essential Structures Viktor Kolokhov1,a*, Liliya Kushnerova2,b, Lina Moroz3,c and Tetiana Pavlenko1,d 1SHEE «Prydniprovska State Academy of Civil Engineering and Architecture», 24а, Chernyshevsky str., Dnipro, Ukraine, 49600 2Kyiv National University of Construction and Architecture, 31, Povitroflotsky Avenue, Kyiv, Ukraine, 03037 3Dnipro State Agrarian and Economic University, 25, Serhii Efremov str., Dnipro, Ukraine, 49600 akolokhov.viktor@pgasa.dp.ua, bliliyakushnerova@gmail.com, clinysek-slv@i.ua, dtmj@ukr.net Keywords: concrete, construction, structure, emergency situations, condition, assessment, properties, electroconductivity. Abstract. The article deals with the assessment problems of especially essential structures. Increased demands on prevention of emergency situations and minimizing the consequences in the event of their occurrence require constant determination of especially essential structures condition. Achieving the goal of reliability and continuity of information is possible by coating the structure surface by a layer of electroconductive concrete, working as a monitoring system sensor. The study of the electrical properties of concrete was performed using the voltmeter – ammeter scheme. After the measurements had been made, the conditional electrical resistance of the electrode pair was calculated. The analysis of the above dependencies found that the change in the electrical resistance of the material from its stress approaches the linear law at lower values of W/C over a larger section of the studied interval. Processing of the obtained data showed that the measurement results were significantly affected by the shape and size of the electrodes used during the experiments. 1 Introduction Increased demands on prevention of emergency situations and minimizing the consequences in the event of their occurrence require constant determination of especially essential structures condition. Most of these structures are massive reinforced concrete structures that take significant loads during operation or when project accidents are localized. These are primarily protective and localizing systems of nuclear power plants, hydroelectric dams and hydroelectric power plants, destruction of which leads to heavy-duty technogenic impacts on the environment and humans. According to the current regulations [1-3], the technical condition assessment is presented as a discrete process that is implemented using a specific algorithm. Monitoring the condition of such buildings and structures was provided by systems that were installed during construction. But over time, these systems partially failed [4] or became obsolete. This situation reduces the monitoring system reliability and may lead to delayed response during emergencies. Restoration of the necessary level of reliability of monitoring systems is provided by current means on the modernized base of control devices. An example of such a solution can be a system that uses modern means of automated strain monitoring [5]. As a part of such systems, the vast majority of devices are high-precision electronic total stations, GPS, levels, inclinometers, accelerometers, and so on. The structure assessment is based on calculations that use data about the properties of materials of construction, level of impacts and loads, as well as the results of the identified damage and defects. Determination of the properties of the structure materials is based on the use of regulated non-destructive testing methods [6-8]. Damage detection is performed by visual inspection [9] and using modern research methods and tools [10-12]. However, the reliability of the results of determining the state of structures is also affected by changes in the properties of materials, which is associated with various factors. The influence of technological and operational factors on the results of determining the properties of Materials Science Forum Submitted: 2020-03-23 ISSN: 1662-9752, Vol. 1006, pp 130-135 Revised: 2020-03-29 © 2020 Trans Tech Publications Ltd, Switzerland Accepted: 2020-03-30 Online: 2020-08-07 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (#541009602-19/07/20,16:31:43) concrete in structures using non-destructive methods is given in [13]. Usually, calculation systems are used to determine the technical condition of complex buildings and structures. One of these is the "Lira–Windows" software package (PC), which relies on the finite element method in moving parts when determining the technical condition of a building [14]. This software package allows creating a system for monitoring the condition of buildings and structures. The formation of the system is provided by a combination of a PC with a system for automatic determining the properties of materials, loads and impacts. The methodology of the state enterprise "Ukrainian Research and Design Institute of Building Materials and Products" requires the establishment of supervision over the behavior of local zones of construction [9]. However, the existing common tools and methods for determining the properties of concrete in operated structures do not allow correct reflecting possible changes in its characteristics. Other tools and methods are too expensive. 2 Unresolved Issues Changes in the properties of concrete structures are usually determined as the result of measuring deformations in the local area. For this purpose, various types of strain gauges are used, both direct- acting (direct measurements of the movement of rappers fixed on the concrete surface) and indirect- acting (strain gauges, fiber-optic sensors, etc.) with additional converters of the received information [15]. In the first case, the system is more difficult to adapt to the automation of measurements. In the second case, there are problems with preserving the sensors during the structure operation. The purpose of the study is to improve and increase the reliability of the means of reflecting changes in the properties of concrete in structures during their operation. 3 Main Part Achieving the goal of reliability and continuity of information is possible by coating the structure surface by a layer of electroconductive concrete, working as a monitoring system sensor. As an analogue of such concrete, the development of the Research Institute of Concrete and Reinforced Concrete of the USSR Gosstroy was used [16], in which the electrical conductivity of concrete was provided by adding coke to the concrete. In order to improve the electrical properties of electroconductive concrete, only an electroconductive conductive aggregate was used. Soot and graphite crushed in different fractions were used as such aggregate. Concretes with the components ratio providing the densest structure were used for the study. To do this, pieces of graphite were crushed and sifted through a standard set of sieves. Varying the residues on the sieves (0.314; 0.63; 1.25 and 2.5) in different ratios and adding soot, various mixtures for the aggregate of electroconductive concrete were obtained. Changing the ratio between the components was aimed at obtaining the most dense structure. For further research, an aggregate with a weight ratio of Soot: Graphite (0.63): Graphite (2.5) = 1: 2: 4 was adopted. Taking into account the specific rheological properties of concrete with an electroconductive aggregate (a mixture of graphite and soot), experiments were made with concrete at a water-cement ratio of more than 0.6. Only with this W/C it was possible to form both experimental samples and an electroconductive layer on the surface of concrete structures. The use of plasticizing additives was rejected a priori in order to reduce the influence factors on the properties of electroconductive concrete. The concrete compositions, used during the research, are shown in Table 1. Samples with a size of 40x40x160 mm (Fig. 1a) were made. 4 graphite electrodes were installed in each sample to study the properties of electroconductive concrete. The layout of electrodes in the samples is shown in Fig. 1b. The study of the electrical properties of concrete was performed using the voltmeter – ammeter scheme. Materials Science Forum Vol. 1006 131 Table 1. Ratio between components of electroconductive concrete of different compositions Composition № Cement Water Aggregate W/C 1 2.619 1.667 1 0.636 2 3.095 2.143 1 0.692 3 3.095 2.381 1 0.769 4 3.333 2.381 1 0.714 5 3.333 2.619 1 0.786 а) b) Fig. 1. Experimental samples from electroconductive concrete: a) samples; b) layout of electrodes Measurements were performed sequentially for each pair of electrodes. During the measurement, we tried to ensure that the voltage and current parameters were within the middle part of the device measuring range. The measurements were repeated up to 10 times to obtain statistically reliable results. Statistical processing was performed using EXCEL. The calculation results are shown in Table 2. Table 2. Measuring results of electrical resistance for concretes of various compositions Comp. № Electrical resistance between pairs of electrodes, [ohm] 1-2 2-4 4-3 1-3 4-1 3-2 1 3.05±0.18 25.14±1.89 2.33±0.21 23.91±1.70 26.37±2.05 26.33±1.93 2 4.57±0,37 30.19±2.41 2.67±0.26 27.56±2.09 36.97±2.32 35.63±2.58 4 7.84±1.18 36.41±2.19 4.44±0.34 32.09±2.79 39.64±2.74 40.08±2.57 3 25.76±1.87 89.64±5.07 16.30±1.87 83.24±6.70 101.64±8.13 102.90±7.13 5 44.09±3.36 205.36±11.37 28.47±2.18 209.29±15.37 211.18±14.26 217.39±12.12 The distance was measured for each pair of electrodes. After the measurements were made, the ratio of the electrical resistance of concrete between this pair of electrodes R to the distance between them L was calculated. The analysis of the obtained results shows that the dependence of the electrical resistance on the distance for different W/C is almost linear for experimental compositions of electroconductive concretes. Depending on the W/C, each dependency has its own proportionality coefficient. There are deviations in the resistance changes at short distances between the electrodes, which can probably be the result of the influence of the shape of the used electrodes and their size ratio with the distance between them. 132 Problems of Emergency Situations: Materials and Technologies The dependence of the relative electrical resistance (R/L) of electroconductive concrete on the W/C is shown in Fig. 2. This dependence (for the entire definition interval) can be approximated by third- or fourth-degree polynomials with approximation confidence close to 1. In both cases, the dependencies have an inflection point in the range 0.72