Dustiness assessment at reinforced concrete articles production in LLC "Monolit-Yug" shop

. The aim of the paper is to determine fine fractions content of mineral particles formed during the production of building materials. The production of concrete and reinforced concrete products is associated with the use of dispersed mineral substances in large quantities as aggregates. This is accompanied by dust emissions and, accordingly, air pollution. The efficiency of aspiration systems at enterprises is low due to the specifics of the location and operation of the equipment complex in space and time. Therefore, an attempt was made to develop a method for separating dust fractions when assessing the working area dustiness in the production of concrete and reinforced concrete products. Since the main method of assessing dust content is gravimetric, it is proposed to isolate the most dangerous dust particles for the human body when taking air samples with blocks by means of three filters from various filter materials. The performed calibration in laboratory conditions and measurements directly at the workplaces of LLC "Monolit-Yug" showed relatively small errors in air sampling and made it possible to estimate the total dust content and the proportion of fine dust fractions. The conducted research will be used for recommendations development to improve the efficiency of aspiration systems.


Introduction
The globalization of the economy, caused by growing people's needs in food, energy, housing, industrial goods, etc., leads to a sharp increase in the burden on the environment, causing environmental problems [1]. The deterioration of the ecological situation is especially acute in large and medium-sized industrial cities due to the ever growing pollution [2]. Of course, many factors affect the ecology of cities, but the greatest contribution is made by industrial enterprises. It is impossible to exclude them from economic activity since they provide the people's and state needs. [3]. Any production activity is accompanied in one way or another by the use of natural resources, their processing, transportation and emissions. This requires improving measures to minimize the production impact on the environment.
Production growth requires an increase in the use of atmospheric air, which is one of the most important natural resources. This, accordingly, causes its intensive pollution with highly dispersed solid particles [4][5]. Since atmospheric air is an important factor in human life, dust formation is a very common hazardous and harmful production aspect that determines the environmental risk to human health [6][7]. Despite the ability of nature to self-healing, human activity in most cases negatively affects the ecological state of nature.

Justification of relevance
The analysis shows that despite the constant improvement of technological processes and dust and gas cleaning systems, which capture hundreds of thousands of tons of dust [7][8], a huge amount of dust is still emitted into the atmosphere in all branches of production. Unfortunately, the production of building materials occupies one of the leading places of air pollution.
In the production of concrete and reinforced concrete products, bulk materials are used, which in the loading and unloading zones usually form a large amount of highly dispersed mineral particles. Vibrating platforms are also the source of dust formation, since when vibrating, excess concrete mixture falls on the platform and then is further crushed to a dusty state by formwork elements [9].
Since the vertical scheme is mainly used for the production of concrete mix, the technological equipment in the shop is located on a large area and at different heights. This does not allow for sufficiently effective air aspiration in the working area. The concrete mix ingredients form an aerosol with particles of various sizes; therefore, their distribution in the work area will differ significantly. Fine dust pollutes not only the working area of production, but also the surrounding area, since small dust fractions can be suspended for a long time.
According to the degree of impact on the human body, dust particles with a size of 2.5 to 10 microns are considered the most dangerous, since they are able to penetrate deeply into the human lungs and they can be hardly removed from there [9][10]. Such dust fractions are dangerous not only for employees of the enterprises, but also for people outside of production, since small particles can stay in the air for a long time and, accordingly, they can be transported to distances of up to several kilometers [8]. Therefore, the assessment of pollution in the people's work area and the determination of the content of dangerous fractions in the general dustiness of the working area will allow improving measures to ensure labor safety directly at the enterprise and increase environmental safety in the adjacent territories.

Methods and materials
The analysis shows that most modern high-precision means of monitoring the concentration of dust in the air and estimating particle sizes are usually stationary, very expensive and require special training of personnel. Therefore, the main method of periodic monitoring of dustiness at work places remains the weight, regulated by GOST, based on determining the amount of deposited dust on the filter material [11]. However, this method also has disadvantages: high labor intensity, the need to take into account the speed of air movement, the dependence of the accuracy of research results on the quality of filters and the qualifications of researchers; inability to assess the content of individual dust fractions [11].
The process of preparing a concrete mixture is characterized by a cyclical nature of the work and the movement of technological equipment around the shop. In addition, it is necessary to take into account the following factors: the position of the building in space, wind direction, temperature and humidity of the air, as they determine the stability of the aerosol and its spread both indoors and outside of the production. In this case, an objective assessment of dustiness is possible by means of using statistical methods That is, when performing a series of measurements in different places and at different times, with a minimum interval between measurements. If we take into account the need to assess the content of hazardous dust fractions of 2.5-10 microns, which requires the separation of pollution into fractions, then the time spent on conducting research increases many times.
Hence it follows that for periodic monitoring of the dust content, portable measuring equipment is needed, which allows repeated measurements at short intervals. Moreover, it must have stable characteristics and the ability to determine hazardous fractions. That is, it is necessary to first filter out dust particles larger than 10 microns, and then retain particles up to 2.0 microns on the next filter.
According to the results of microscopic studies of filtering materials, which are often used today, it was found out that the chaotic arrangement of synthetic fibers in the materials does not allow the separation of particles of strictly defined sizes [12]. In addition, as the pollution increases, the resistance to air movement changes due to the low dust holding capacity. Nevertheless, it was possible to determine two materials that best meet the accepted requirements. This is a filter material F5 with a density of 200 g/m 2 for coarse cleaning and Petryanov's FPP 15 material for fine cleaning [13]. Thus, it is required to pump air through several filters that retain particles of a certain size. In addition, it is necessary to use metal retaining meshes on which filters are placed to prevent deformation of the synthetic material.
The fractional composition of dust in sand and crushed stone is very heterogeneous, and the process of preparing a concrete mixture is characterized by a cyclical nature, and the factors affecting dust formation can change over time. Therefore, during long-term sampling of air, the obtained measurement results will be incorrect. That is, it is necessary to take samples with the shortest possible time interval.
In order to ensure the required accuracy and reduce the test time, it is advisable to place the filters in special blocks previously prepared in the laboratory, which will make it possible to perform measurements at short intervals and thereby increase the accuracy of determining the dust content.
During preliminary tests, it was established that when taking air samples, the unit should consist of three filter elements and two meshes to prevent deformation of filters. It was also noted that such a block is characterized by significant aerodynamic resistance, which reduces the air consumption during measurements, and, accordingly, requires an increase in the sampling time.
Therefore, to reduce the measurement time when working with a filter unit, a sampling capacity of up to 300 dm 3 /min is required with a depression exceeding the limit values for the filters used. Moreover, productivity and depression should remain as stable as possible throughout the measurement period. At the same time, the quality of filter materials plays an important role, that is, their ability to filter out or pass particles of certain sizes.
In the process of using microscopic analysis during experimental studies, the most effective filtering materials were established to separate particles larger than 10 microns -a filter of the F5 class from thin perchlorovinyl fibers, and particles up to 0,5 microns -analytical filters AFA-VP from Petryanov's cloth. A mesh with a mesh size of 80 microns allows filtering out large particles and prevent rapid contamination of the F5 material and, accordingly, an increase in depression.
Despite the large assortment of sampling devices for the gravimetric method of studying air pollution with a flow rate of up to 300 dm 3 /min of air, it is possible to select an apparatus with a depression of up to 0,120 kPa. Therefore, a decision was made to manufacture a measuring kit based on a 100 W car vacuum cleaner, which has the required performance and depression.
An important positive factor is the use of a constant voltage of 12 V to power the motor of the vacuum cleaner, which makes it possible to use small-sized batteries as a power source. The small dimensions of the vacuum cleaner and the power of the electric motor from the rechargeable battery ensure the mobility and autonomy of the kit, and the use of the engine shutdown timer ensures the stability of the sampling parameters. A special attachment (Figure 1) for attaching the filter unit allows for 15 -20 seconds changing the filter units and significantly simplifying and shortening the sampling process at various points of the workplace. An increase in the number of sampling together with a statistical analysis of the results ensures the adequacy of research. To assess the adequacy of measurements and select the optimal sampling period, the volume of pumped air was calibrated for certain time intervals. The set of measurements was carried out in the laboratory with an electronic counter TG 4. When processing the measurement results with a statistical assessment of the accuracy, the density and humidity of the air were taken into account.
As a result, it was noted that the accuracy of the volume of air pumping in 1 min. the operation of the blower was 4.33%, and in 4 min. 1.36%. A further increase in the air sampling time does not lead to a significant decrease in errors. The connection of filter blocks during air sampling for 4 minutes provides a reduction in the measurement error to 7.9-1.2 dm 3 /min up to 1.19%, which indicates the adequacy of the research.
The pressure drop with the connected filter blocks is on average 0.124 kPa and does not exceed the permissible value for the filter materials used.

Discussion of research results
The study of dustiness was carried out in November-December, which led to a high moisture content of aggregates for concrete, therefore, when loading the mixer, dust emission was insignificant. During this period, the main source of dust formation is the vibrating platform, since the excess concrete mixture when vibrating the next product fell under the steel mold and was crushed, and when the temperature increased, it dried and formed dust. This allowed us to limit sampling only during the molding of products in the immediate vicinity of the vibrating platform. Also, the reduction in contamination provided a better microscopic analysis to assess the separation of dust into fractions.
Preparation for the research consisted in preparing filters and blocks. For this purpose, discs with an area of 10 cm 2 were made from F5 class filtering material. The weighing of clean filters was carried out in the laboratory on analytical scales with an accuracy of ± 0.1 mg. To simplify the work with analytical scales, the filters were pre-weighed on the VT-500 torsion scales with an accuracy of 1 mg. Then the filters were installed in the boxes in a certain sequence and fixed with special rings. Filter boxes were marked and placed in sealed bags to prevent dusting prior to testing (Figure 2).

Fig. 2. Filter blocks for measurements.
Air sampling in the workshop by each filter unit was carried out within two minutes during the operation of the vibrating platform 45 and the preparation of the mold for transportation to the steaming chamber. An increase in the sampling time, as shown by preliminary studies, led to significant contamination of all filters and to a change in the depression and air volume. In each cycle of operation of the vibrating platform, one sample was selected. Immediately after sampling and extraction from the nozzle, the boxes were closed with paper disks, which were fixed with tape. Then they were placed in bags and delivered to the laboratory for weighing.
In the laboratory, the filters are alternately very carefully removed from the box without pushing and shaking and placed on the scale cup. Since the AFA-VP analytical filters retain a static charge for a long time, when they are removed from the box, they are placed with tweezers in the middle of the scale cup so that they do not touch parts of the scale body anywhere, since an error may occur.
The results were recorded in an Excel spreadsheet for calculations. The calculation algorithm is as follows. The initial data are entered: the average volume of air pumped through the filter blocks in 2 minutes; the mass of clean filters; the mass of filters with dust; atmospheric pressure; temperature and humidity of the air in the shop. Further, taking into account the equation of the dependence of the saturated steam pressure on temperature, the reduced volume of air is calculated for the conditions of measuring dustiness in the workshop. After determining the mass of dust on each filter of the block, the mass of fractions in 1 m 3 of the reduced volume is determined.
The content of dust fractions is defined as the ratio of the mass of dust on the analytical filter to the sum of the masses of particles on the three filters. The results of measurements of dust content in the area of operation of the vibrating platform on November 12, 2020 and November 19, 2020 are shown in Tables 1 and 2, respectively.
The analysis shows that during the operation of the vibrating platform, the dust concentration can reach 145 mg/m 3 . This is many times higher than the MPC for mineral dust. We can also note the low content of hazardous dust fractions of 14.5%, which, in our opinion, is due to the high humidity of concrete aggregates and atmospheric air. The amount of dust deposited on the block walls, O-rings and retaining screens did not exceed 3.7% of the total weight on the three filters.

Сonclusions
To isolate the most dangerous fraction of dust particles 2.5 -10 microns, three filters are required: a mesh with a mesh size of 0.08 mm to separate fractions over 80 microns; the second filter is made of F5 class material, which allows retaining fractions of more than 10 microns, and the third one is made of Petryanov's material from 10 to 1 microns. The placement of filters in special blocks allows them to be changed in 10 -15 seconds and to reduce the sampling time, however, they have significant aerodynamic resistance. Therefore, the use of commercially available aspirators in research is not possible.
A number of experiments made it possible to develop a measuring kit for determining dust content, consisting of a car vacuum cleaner; battery; a time relay and an attachment for placing filter units.
During the calibration of the complex with 6 filter blocks, it was established: the accuracy of the volume count for one filter is 4.77-3.44%, with a variation of 7.1-4.2 dm 3 /min; the average error value of 6 filters is 1.9%, with a variation of 2.45 dm 3 /min; when air is sampled for 2-3 minutes, the volume count error does not exceed 5%, and at 4 minutes it decreases to 2%.
Investigation of dust content during the operation of the vibration platform in the shop of LLC "Monolit-Yug" proves that dust concentration can reach 145 mg/m 3 , which is many times higher than the maximum permissible concentration of mineral dust, but after 5-7 minutes, at the time of the next mold, the dust content decreases almost 5 times.
At high humidity of concrete aggregates and atmospheric air, there is a low content of dangerous dust fractions of 14.5%.
Depending on the intensity of dust formation, the sampling time of the developed complex is determined by the dust capacity of the filtering materials and should be from 2 to 10 minutes.
Currently carried out research on the above methodology will allow developing measures to optimize the operation of the aspiration system and reduce dust formation in the summer, both at workplaces and surrounding areas.