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15Tashkent state transport university
THE USE OF CHEMICAL ADDITIVES IN COLD CLIMATES AND THEIR EFFECT ON THE CONCRETE MIXTURE
Shakhirov Tuygun Turgunovich, Usmonova Durdona Asatilloyevna, Samadova Dilso'z Asatullo qizi
Tashkent institute of architecture and civil engineering (Tashkent, Uzbekistan)
Annotation: The article presents information about the effect of cold chemical additives on the concrete mixture. Chemical additives are the easiest and most accessible technological way to improve the properties of concrete. There are several types of chemical additives depending on their effect on the concrete mixture. One of them is frost-resistant, that is, additives that increase the strength of concrete by resisting freezing in cold conditions. We compared the types of additives and their ratios for the concrete mixture depending on the air temperature.
Keywords: concrete, antifreeze additives, efficiency, evaluation methodology
ИСПОЛЬЗОВАНИЕ ХИМИЧЕСКИХ ДОБАВОК В ХОЛОДНОМ КЛИМАТЕ И ИХ ВЛИЯНИЕ НА БЕТОННУЮ СМЕСЬ
Шакиров Туйгун Тургунович, Усмонова Дурдона Асатиллоевна, Самадова Дилсуз Асатулло кизи
Ташкентский архитектурно-строительный институт (Ташкент, Узбекистан)
Аннотатция В статье представлена информация о влиянии холодной химической
добавки на бетонную смесь. Химические добавки являются наиболее простым и доступным технологическим способом улучшения свойств бетона. Различают несколько видов химических добавок в зависимости от их воздействия на бетонную смесь. Один из них - морозостойкий, то есть добавки, повышающие прочность бетона за счет сопротивления замерзанию в холодных условиях. Сравнивали виды добавок и их соотношения для бетонной смеси в зависимости от температуры воздуха.
Ключевые слова: бетон, противоморозные добавки, эффективность, методика оценки.
Introduction. Usage of chemical additives while executing concrete works at negative temperatures is considered as a convenient and economical method. Range of the used antifreeze additives is rather wide. A great number of new additives are advertised but their characteristics have not been practically studied. Evaluation of the antifreeze additive efficiency is unfortunately rather long process and it does not provide comprehensive data on concrete structure formation processes. Due to this development of rapid and comprehensive methodology for construction companies is urgently required. Freezing processes of antifreeze additive aqueous solutions and hardening of cement paste with them have been investigated in the paper. The paper proposes a methodology for determination of freezing point for aqueous solutions of chemical additives of various applications. Identity of freezing point for a chemical additive aqueous solution and cement
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paste with an equal concentration of the additive in the paste pore fluid has been determined while taking calcium nitrate and sodium formate additives as an example. The paper demonstrates the possibility to evaluate efficiency of antifreeze additive action on the basis of kinetics in temperature changes of the cement paste with additives by its consecutive freezing and defrosting. A methodology for operational evaluation in the field of chemical additive application for concreting items at negative temperatures has been offered in the paper. The methodology does not require deficient and expensive test-equipment. It can be applied at ordinary construction companies and it is comprehensible for personnel of low-qualification. The paper shows the possibility to develop an original methodologyfor designing concrete structure which is based on operating efficiency determinations for single and integrated antifreeze additives.
Object of research: cement paste and concretes prepared using superplasticizer.
Subject of research: the subject of the dissertation is the strengthening of the concrete mix with the superplasticizer "Beton-strong17", which is one of the modern superplasticizers.
The purpose of the research: to study the effect of superplasticizer "Beton-strong17"on the concrete mix and the properties of concrete, a comprehensive experimental-theoretical study of the technology of preparation of concrete with the addition of modern superplasticizers.
Objectives of the research: To study the effect of superplasticizer "Beton-strong17"on the properties of Portland cement and concrete
Main part. Application of chemical additives in the production technology of concrete works at low temperatures is one of one of the most widespread methods of winter concreting [1-2]. "Introduction Antifreeze additives -technologically the easiest, most convenient and cost-effective cost-effective way of winter concreting. this method is 1.2-1.4 times more economical than the method steam heating and concreting with preceding fencing of the structure and its insulation from inside and 1.3-1.5 times more economical than electric heating and electric heating. Unheated winter concreting through the use of antifreeze additives can save heat and power at more flexible technology of works". [3]
It is well known that the hardening of cement concrete slows down as the temperature decreases and practically ceases when the liquid phase freezes. Therefore, in order to ensure curing in winter conditions is necessary water in the concrete must be prevented from freezing - this can be achieved either by keeping the water in the concrete at a positive temperature or by reducing the temperature to a moderate level curing in winter conditions, it is therefore necessary to prevent water from freezing in the concrete. This can be achieved either by keeping the concrete at a positive temperature during the curing period until it reaches critical strength, or by reducing the freezing point
The freezing point of the liquid phase can either be reduced by introducing chemical admixtures various chemical additives.In principle, the main purpose of antifreeze additives - to reduce the freezing point of the concrete mixture (anti-
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freezing effect), ie, to ensure the possibility of conducting Concrete works at subzero temperatures. In turn, the freezing point concrete mixture is determined by the freezing temperature freezing temperature of the pore fluid (indirectly - the freezing temperature of the aqueous solution of additives used for mixing the dry concrete components). The characteristics of such additives are presented as in the normative documents, For example, in TCP 45-5.03-21-2006 "Concrete work at negative air temperatures. Rules of production" as well as in numerous literary sources [1,4].
At the same time the range of recommended additives the number of chemical additives, both really effective and "miracle", is growing every year and belonging to the category of "miraculous". The question is logical: how can the construction industry can choose and use exactly those additives they need based on the experimental testing of their properties and not on advertising? To do this, there are standardized Methods for assessing the effectiveness of antifreeze additives, presented in GOST 28084-89 "Low-Freezing Cooling Liquids. General technical conditions" and GOST 30459-96 "Concrete admixtures. Methods of Determination efficiency".
Antifreeze effect of antifreeze additives. Consider the technique of the experiment, which allows almost any building organization promptly, within several days, to define the efficiency of an antifreeze additive, which consists of the possibility of reducing the freezing temperature and the effect of the additive (or a set of additives) on the kinetics of structure formation of cement dough (cement stone), concrete mixture (concrete).
As for determining the freezing temperature of the aqueous solution of the antifreeze additive (or other, since this value is a characteristic, determining the field of application of the additive), the temperature of the aqueous antifreeze additive (or any other) can be determined with the following procedure for determining this temperature, which is described in GOST 28084-89. It assumes that the tested liquid placed in a refrigerator and cooled with until the appearance of ice crystals ice crystals appear. This moment is determined visually, when with the naked eye a turbidity is noted in the liquid as a sign of the beginning of crystallization.
The temperature at which at which clouding is noticed is recorded as the temperature at which crystallization begins. Quite The reverse methodology is also quite workable, whereby the additive solution is frozen first where the additive solution is first frozen, and then (again visually) already at a positive temperature to fix the temperature thawing of the solution. Naturally, the described technique for determining the freezing temperature is imperfect, as it is subject to the influence of the human factor and can lead to significant errors in the results.
The authors of the article propose a method based on the known effect of invariability temperature when reaching the freezing temperature of the liquid (in this case, the chemical additive) . That is, on the curve "time - liquid temperature" there is a clearly that is due to formation of new crystals during freezing of liquid in contact with the cooling medium and, naturally, heat extraction
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The total temperature of the freezing liquid does not change. Such freezing pattern is typical for liquids with the presence of crystallization centers. Otherwise, there is some short-term decrease in temperature followed by This pattern of freezing is characteristic of liquids with crystallization centers. (horizontal section). In this case, when estimate of freezing temperature should be ignored. The setup for investigation of freezing kinetics is simple enough.
The temperature sensor (DS 1921) is placed in an aluminum pouch covered by a thermal insulating plug, and the pouch, in turn, in a float floating on the surface of the liquid to be tested. A vessel with a solution of the additive is placed into a freezer (in this case, a common household freezer providing a medium temperature of minus 18 C) and frozen at a temperature well below the freezing point of the liquid. The DS 1921 records the ambient temperature intermittently (in accordance with a preset frequency). The computer then outputs the information in the form of a table, plots the temperature variations and records the freezing point of the additive. Examples of such graphs for relatively poorly studied antifreeze additives - calcium nitrate (CN) and sodium formate (SF) - are shown in Fig. 1, 2. The presence of the above-described slight temperature drop can be seen, For example, on Fig. 1, 2 for high concentrations of NC and FN (30 and 14 % accordingly) after 3.5-4.0 h after the beginning of freezing
Fig. 1. Changes in the temperature of the NK additive solution during freezing, 2. Changes in the temperature of the FN additive solution during
freezing
The obtained numerical values of the "steps" corresponding to the freezing temperatures of the solutions are shown in Table 1.
Table 1
Concentration of solution, % Solution freezing temperature, oC
NK FN
0 -0,5 —0,5
2 — —1,5
5 -1,5 —3,5
10 — —6,0
14 — —9,0
15 —4,0 —
24 —6,5 —
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30
-9,0
t z NK = - 0,0037C 2nk - 0.1679C NK - 0.5396;
t z FN = -0,0064C
FN -
0.5078C fn-0.534
Concentration of additive solution, % Solution freezing temperature, °C, according to the method
TCP 45-5.03-21-2006 | proposed
CN
2 —1,2 —2,0
8 —5,2 —6,0
14 —10,1 —11,0
19 —15,3 —16,5
— —21,1 —23,0
CC
4 —2,0 —2,5
10 —5,7 —6,0
14 —9,5 —9,5
19 —15,9 —15,5
SN
4 —1,8 —2,0
12 —5,8 —5,0
19 —10,0 —8,5
25 —15,7 —11,5
We observe a fairly stable dependence of the freezing temperature of NK solutions (tz NK) and FN (tzFN) solutions on their concentrations (SNK) and (SFN)
n lAHOn n SlCtA- (J)
(1)
By antifreeze effect the addition of sodium formate is more preferable, because at equal concentrations the freezing temperature of the aqueous solution of FN is significantly higher than that of NK.
According to the described methodology, experiments were experiments with commonly known additives,some of which are presented in Table 2. In the same table 2 presents the comparative characteristics for the freezing point of antifreeze antifreeze additives, taken from TCP 45-5.03-21-2006 and duplicated in a practically unchanged other regulatory documents and literary sources. As can be seen from Table 2, there is CK practically complete convergence. There are some insignificant differences for CH.
Table 2
Freezing temperature of additive solutions sodium chloride (CN), calcium
As for SN, which is quite widely used in the practice of winter concreting, in the area of high concentrations is observed a significant deviation. It should be It should be noted that there is a difference in the definitions freezing point and for other additives, not listed in Table 2. If we assume that the proposed methodology
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is more accurate (and this still than that described in GOST 28084-89 the areas of their application in winter concreting should be corrected for some additives.Structure formation of cement dough at a positive temperature. However, the purpose of antifreeze additives is not limited to the function of reducing the temperature of its aqueous solution. There are substances that are very effective from this point of view from this point of view, but not always promising for the modification of concrete, because, in particular, they reduce some of the characteristics of concrete. A fairly convincing example is the addition of potash (K2CO3), which, at a 40% concentration, the freezing point of the aqueous solution to minus 36.5 ° C, but significantly reduces the frost resistance of concrete (the most important characteristic determining its durability) [5].
It is desirable that antifreeze additives are also gas pedals of concrete hardening, which provides a short time to achieve the critical strength of concrete. Even more effective are antifreeze additives with an additional plasticizing effect (as well as multi-component), allowing to reduce the water-cement ratio of the concrete mixture and intensify the structure formation of cement dough and the corresponding increase in the strength of concrete. In order to evaluate the effect of chemical additives on structural forming processes of concrete, engineers-technologists use a variety of [1, 2, 5, ], the main of which is the study of the kinetics of strength changes of cement stone or concrete. Although this method is quite attractive (most importantly, it is a reference method), it allows one to obtain results only in sufficiently long periods of hardening.
For example, when evaluating the effectiveness of Antifreeze additive test period 28 days. Consequently, it is necessary to resort to methods related to the operational and at the same time which take into account a variety of influencing factors: the type and dosage of chemical additives, type and rate of binder, water-cement water-cement ratio, etc. In the light of the requirements outlined, the method of evaluating the effect of In the light of the stated requirements the method of assessing the effect of additives on the kinetics of heat release of cement, which received a fairly wide development [7], looks attractive in the light of the stated requirements. However, there is an obstacle to a wide use of this method - For its implementation specialized equipment (calorimeters) is necessary, which is available only for only for reputable scientific organizations. Conclusion:
1. A technique for determining the freezing temperature of aqueous solutions of chemical additives for various purposes, excluding the influence of the human factor and thereby increasing the reliability of the results obtained, is proposed. The paper proposes a method for determining the freezing temperature of water solutions of chemical additives for various purposes.
2. The freezing temperatures of antifreeze additives widely used in winter construction practice have been studied. It is shown, that for the solution of the sodium nitrite additive of high concentration the established freezing temperature of sodium nitrite additive of high concentration is considerably higher than the one indicated in the literature sources
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3. Using calcium nitrate and sodium formate additives as an example and sodium formate the identity of the freezing temperature of the aqueous solution of the chemical additive and cement dough with an equal concentration of the additive in the pore fluid of the dough has been revealed.The possibility of estimating the antifreezing of antifreeze additives according to the kinetics of temperature changes in the cement dough with additives by means of its sequential freezing and thawing.
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