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8УДК 693
МИГРАЦИЯ ВЛАГИ ПРИ БЕТОНИРОВАНИИ КОНСТРУКЦИЙ НА МЕРЗЛОМ БЕТОННОМ ОСНОВАНИИ
Иванов Данил Андреевич.
Новосибирский государственный архитектурно-строительный университет (Сибстрин), магистр, andreevich@gmail.com.
Молодин Владимир Викторович.
Новосибирский государственный архитектурно-строительный университет (Сибстрин), molodin@sibstrin.ru
Макарихина Инна Михайловна.
Новосибирский государственный архитектурно-строительный университет (Сибстрин), кандидат педагогический наук,
Аннотация. Представлено описание методики и результатов экспериментального исследования процесса внутреннего массопереноса и степени его влияния на прочность в твердеющем при отрицательных температурах бетоне каркасных конструкций, бетонируемых на мерзлом бетонном основании по традиционной технологии - с обогревом твердеющего бетона греющим проводом. Результаты позволили доказать наличие и оценить основные параметры данного процесса. Установлено, что свежеуложенный бетон, находящийся в контакте с промерзшим бетонным основанием переувлажнился, а влажность бетонного основания увеличилась. Приведена прямая зависимость уменьшения прочности бетона от увеличения его влажности.
Ключевые слова: массоперенос, миграция влаги, переувлажнение бетона, мерзлое основание, стык.
MIGRATION OF MOISTURE AT CONCRETINGCONSTRUCTIONS ON
FROZEN CONCRETEBASE
Ivanov Danil Andreevich.
Novosibirsk State University of Architecture and Civil Engineering, graduate student, ivanov.danil. andreevich@gmail. com.
Molodin VladimirVictorovich.
Novosibirsk State University of Architecture and Civil Engineering, Doctor of tech. sciences, molodin@sibstrin.ru.
Makarihina Inna Mikhailovna.
Novosibirsk State University of Architecture and Civil Engineering, PhD (pedagogic), michmacha@mail.ru.
Abstract. This article describes the methodology and results of the experiment to study the process of internal mass transfer in concrete. This concrete hardened at negative temperatures and was located on a frozen concrete base. In addition,in the concrete installed a heating wire. The results proved the presence and evaluation of the main parameters of this process. The fresh concrete has been overmoistened. The
moisture of the frozen concrete base increased. The direct dependence of the decrease in the strength of concrete on moisture increase is given.
Keywords: mass transfer, migration, moisture, overmoistening, concrete, base, contact.
1. Introduction.
Technological interruptions often occur on the construction of concrete structures in winter conditions. They appear because of the end of the shift, interruptions in the delivery of concrete, installation of formwork and fittings. A temperature gradient appears on the contact of "old" chilled concrete with a "new" one after the resumption of concreting. Then a moisture gradient appears due to the action of the temperature gradient. Moisture migrates from warm to cooler layers of concrete, due to the appearance of a reduced partial pressure in low temperature zones [1, 2].
The transfer of moisture leads to its excess in the cold zone. The moisture is unevenly distributed throughout the structure. The water-cement ratio increases in the cooled zones. This leads to a decrease in density, increased porosity and deterioration in the quality of concrete [3]. This can lead to dangerous consequences [4].
Objective:
1. To investigate the process of internal mass transfer and the degree of its influence on the strength of concrete located on a frozen base and including a heating wire.
2. Carry out a concreting of the frame structure in the laboratory and simulate the conditions for concreting the column on the frozen roof.
3. Investigate the migration of moisture and determine the effect of this process on the strength of concrete in the contact area of structures.
2. Methodsof experiment.
We applied a symmetric thermal problem to optimize the experiment. We have concerted half the column on half the floor. The side of the missing part of the structure (Fig. 1) satisfied the thermal symmetry condition.
The role of a frozen ceiling from a notebook of heavy concrete of grade B15 with a size of 0.5 x 0.5 x 0.3 m. A wooden formwork for concreting the column was made 0.5 m high and 0.4 x 0.2 m in cross-section. The heater was 15 cm thick and installed on the back of the structure. Reinforcing bars were mounted in a column to simulate real concreting. The heating wire PNSV-2 with a diameter of 2 mm was installed on the reinforcement frame in 50 mm increments (Fig. 2). The wire was supplied with an alternating electrical current of 16 A with a voltage of 0.8 V. This released energy of 12.8 W in a volume of concrete of 0.04 m3.
Fig. 1. Experimental design a - general view, b - the process of installing sensors.
The air temperature in the certified climatic chamber was -30 °C during the cooling of the base and the experiment itself.
Chromel-Copel thermocouples were temperature sensors. Their readings were taken with the help of a multichannel temperature gauge Thermodot-22M5. It was connected to a personal computer. We used the software TermodatTools.
The moisture content of concrete must be calculated according to the standard procedure in accordance with GOST 12730.2-78. However, this technique is not applicable in this work. Because we are interested in the moisture parameters inside the concrete. Therefore, the moisture parameters and the nature of its change were determined using the electrical properties of the concrete. We used the conductometric method [5].
Fig. 2. The scheme of the experimental design: 1 - heater, 2 - column, 3 - base, 4 - reinforcing bars, 5 - heating wire, 6 - formwork.
The process of moisture migration was investigated by measuring the electrical resistance of concrete. Sensors from a two-wire copper wire PRPPM (BM) 2 x 0.9, 0.9 mm in diameter and a wire spacing of 3.5 mm (Fig. 3), measured the electrical resistance. The sensors were installed in the center and on the periphery of the structure along its entire height (Fig. 4, lines A and B). The readings were taken with a universal digital voltmeter V7-38.
1
3
4
—f— -r ->j
1- 10 . -20-
Fig. 3. The design of the sensor for measuring electrical resistance: 1 - wire PRPPM (BM) 2 x 0.9, 2 - insulated copper cores, 3 - copper wires without insulation
(electrodes), 4 - insulation.
We obtained moisture values using the formula [6]. The formula shows the dependence of moisture on electrical resistance.
W
rel
Rexp * (1 + P * ('exp - tn ))
(2)
exp in-
rgeWrel- relative moisture of concrete, %;
R in,R exp - initial and experimental electrical resistance of the concrete mixture, Ohm; tin, t exp- initial and experimental temperature of the concrete mixture, 0C; P = 0,02 - temperature coefficient of electrical resistance of concrete, 1/°C;
The coefficient k = 10 was introduced when calculating the local moisture content of concrete [6]. Because the liquid phase of concrete is more conductive than at the crystallization stage (about 10 times).
Î-1 2-2
Fig. 4. Location of moisture sensors and sensor placement lines (A, B)
We took the sensor readings every hour for the first 11 hours. Then every 5 hours for 75 hours. The total time of the experiment was 86 hours.
3. Discussion of experimental results. 3.1. Initial period
The moisture content of the concrete column was relatively uniform at the initial time (Figure 5, 6). The moisture was distributed as follows:
-on the contact of structures «3%;
- in the middle layer «2-2,5%;
- in the upper layer« 1,5-2,5%.
The rate of change in moisture was different at different times and in different sections of the column in height. The maximum change (increase) in concrete moisture was observed after 6 hours in the center and after 9-11 hours at the periphery of the column section at a distance of 25 mm to 75 mm of the column height. The moisture content of the concrete increased at the contact of the structures and at the height of the column from 75 mm to 350 mm. Because the electrical conductivity of the liquid phase has increased due to the dissolution of alkalis from the cement composition [5]. The moisture content of the concrete slightly decreased at the top of the column. Because the upper part was covered with a plastic film and a layer of insulation. This action helped to reduce the influence of the process of external mass rivalry.
a)
0 1 2 3 4 5 6 7 S 9 10 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 T, h
b)
W, % 20
-W12 —•—W13 —•—W14 —•—W15 —•—W16
>
/* L /
i \
i j pi s £ s s s
w
0 1 2 3 4 5 6 7 8 9 10 11 16 21 26 31 36 41 46 51 56 61 66 71 76 SI 86 T, h
Fig. 5. Distribution of moisture over time in the lines of sensors A (a) and B (b): W7-W16 -moisture at points 7-16 (Fig. 4).
■WA WB ■TA TB ■T
-35-30-25-20-15-10 -5
350
0 5 b)
10 15 20 25 30 35
T, °C W,%
5 250
a
o
B
o
ID
c_>
a
150
50
-50
-35 -30 -25 -20 -15-10 -5
15 20 25 30 35
W, %
Fig. 6. Distribution of temperature and moisture along the height of the structure at the beginning (a)
and at the end (b) of the experiment: WA, WB, TA, TB - moisture and temperature along the lines of sensors A and B, T - temperature in the climatic chamber.
3.2. Curing period of concrete
Moisture was stable during the experiment (Fig. 5). The values of moisture changes were the same and amounted to about 4% of the height of the column. The moisture drop was -0.5% relative to the sensor placement lines in the last 35 hours of the experiment. The values of the differences were different, because the moisture migrated more to the permafrost base.
The maximum moisture values were observed at the periphery of the section and the contact of the structures. Because the process of internal mass transfer has transferred moisture to the colder areas of concrete (formwork panels and base).
The regular thermal regime was established in the experimental design, due to the action of a constant distributed heat source. He was the guarantor of the stability of moisture fields.
3.3. The final period of the experiment.
The greatest changes (increase) in the moisture content of concrete were at the height of the column from 0 to 75 mm after the end of the experiment (Fig. 5, 6). In this region, moisture migrated from the upper and middle parts due to mass transfer. The moisture was distributed as follows:
-in the upper part« 1,5-2,5%;
- in the middle part «1,5-3%;
-at an altitude of 25 to 75 mm «2,5-4,5%;
- on the contact of structures «6%.
Low moisture of the concrete was in the upper and middle part of the column. Because the transfer of moisture was in the concrete of the middle and lower layers. In the concrete of the middle part of the column, slight changes in moisture are observed. Essential changes in moisture are observed in the lower part. This reflects the intensive mass transfer to this zone.
The moisture in the center of the column section was less than at the periphery. Because the moisture migrated to the cold formwork boards throughout the height of the column.
The moisture content of the base changed from 1.97% to 2.8% over the last 67 hours of the experiment.
3.4. The effect of changing the moisture content of concrete on its strength
According to most researchers, the reduction in strength is due to the increase in the water-cement ratio of the cooled concrete layers [7].
Gusakov A.M. The dependence of the strength of concrete on the water-cement ratio and the distance from the cooled end of the experimental setup was cited [3]. From this dependence it follows that the strength of overmoistened concrete is reduced by 30% when the water-cement ratio is increased by 1.4 times. The layers of concrete were overmoistened 2 times in the center and 2.5 times in the periphery of the section of the column during our experiment. It follows from this that the strength of the overmoistened layers has decreased by approximately -45-55% relative to the expected strength. This value is not accurate, is of an evaluation nature and requires verification by a specific theoretical and experimental study. Strength can be reduced by half in overmoistened concrete concatenation of frame structures that are concreted in winter conditions. This does not guarantee the provision of spatial rigidity of the frame of the entire building [8].
4.Conclusions.
1. The results of the experiment proved the existence of a process of internal mass transfer. The results of the experiment made it possible to estimate the main parameters of this process (temperature and moisture), it was established that the concrete was re-moistened 2-2.5 times at a distance from the contact boundary to 75 mm at the height of the column. The moisture transfer was longitudinal and transverse The moisture content difference was 0.5% at the center and at the periphery near the end of the experiment, and the moisture content of the concrete base increased by 0.8%.
2. The results of the experiment indirectly confirm the studies of other scientists. They confirm the direct dependence of the reduction in the strength of concrete from the increase in its moisture content. The strength of concrete can be reduced by 45-55%
with increasing moisture by 2-2.5 times due to the effect of internal mass transfer. This can lead to dangerous consequences.
3. The process of internal mass transfer should be taken into account in technological design and in the process of performing the work. It is necessary to provide technical solutions to reduce the effect of moisture transfer on the strength of concrete.
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2. Brzhanov R.T. Problems of choosing methods of winter concreting / Vestnik PGU № 2, 2009 - 14-33p.
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4. Dauzhanov N.T. and Aruova L.B. Heat and mass transfer at various technologies of solar thermal processing of reinforced concrete/Vestnik MGSU №4, 2011 - 288-292 p.
5. Shcherba V.V. Technology of concreting buildings of monolithic buildings with pre-storage of concrete from dehydration by using film-forming materials:dissertation-Moscow, 2005.
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7. Nedelya N.N. Effect of concrete moisture on its strength / Beton i zhelezobeton, izbrannye stat'i, 1983.
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