SHRINKAGE DEFORMATIONS OF CONCRETE IN NATURAL CONDITIONS OF THE REPUBLIC OF UZBEKISTAN

ABSTRACT

The article provides an analysis of the deformability of concrete in a dry and hot climate. The coefficients for taking into account the influence of a dry hot climate are given. Shrinkage deformations of concrete in a dry hot climate has a pronounced periodic character, depending on seasonal fluctuations in temperature and air humidity.

АННОТАЦИЯ

В статье приводится анализ деформативности бетона в условиях сухого и жаркого климата. Приводятся коэффициенты учета влияния сухого жаркого климата. Деформации усадки бетона в условиях сухого жаркого климата имеет ярко выраженный периодический характер в зависимости от сезонного колебания температуры и влажности воздуха.

Keywords: shrinkage, stresses, deformation, humidity, temperature, thermal expansion, wet storage, solar radiation, moisture loss of concrete, seasonal change, normal condition

Ключевые слова: усадка, напряжения, деформация, влажность, температура, температурное расширение, влажное хранение, солнечная радиация, влагоотдача бетона, сезонное изменение, нормальное условие

The climatic conditions of the Republic of Uzbekistan are characterized by sharp continentality. In summer, the air temperature can exceed +40⁰ C, while the relative humidity drops to 10-15% or lower. In such climatic conditions, from direct exposure to solar radiation, the surface of reinforced concrete and concrete structures heats up to 70-80⁰ C. At the same time, significant concrete shrinkage deformations appear, leading to the formation and opening of cracks on the surface of reinforced concrete and concrete structures [1,2, 3,4,5].

One of the most important factors in improving the reliability and durability of structures of buildings and structures, especially for the Republic of Uzbekistan, is the further improvement of methods for their calculation, taking into account real operating conditions [6,7,8,9,10,11,12].

When concrete hardens in a dry hot climate, two opposite processes, constructive and destructive, interact. The more structural processes prevail, the deeper and denser the hydration of cement will be, the more intense the physical and chemical processes of hardening will be, the concrete will gain strength faster, and the concrete will be more resistant in hot climates. If the concrete is not properly maintained, fresh concrete will dehydrate. Concrete in dry weather during the first day loses 50 ... 70% of the mixing water.

Intensive evaporation of moisture from the surface of freshly laid concrete causes plastic and moisture shrinkage of concrete. Plastic shrinkage of concrete occurs immediately after the formation of the concrete mixture, when it has not yet fully hardened. Plastic shrinkage of concrete causes the formation of surface cracks. Therefore, in order to prevent the evaporation of water from concrete, immediately after molding, concrete should be wet-treated. Any delay from the beginning of concrete care over 20...30 minutes already contributes to the development of plastic shrinkage of concrete. Moist care of concrete immediately after the completion of the molding of a product or structure reduces the possibility of plastic shrinkage and cracking of exposed surfaces of concrete. The minimum duration of the initial care of freshly laid concrete in order to obtain the least plastic shrinkage in hot dry weather is 6 ... 7 hours [13,14,15,16,17,18,19,20].

Further care of concrete does not significantly affect the subsequent development of concrete plastic shrinkage deformation, but it is necessary for the formation of a dense concrete structure and a set of 50...70% compressive strength. Concrete is carefully covered with moisture-proof or damp materials for 8...10 days, the concrete is constantly kept in wet conditions, preventing it from drying out. Under natural conditions of a dry hot climate, humidity deformations of concrete shrinkage develop along a certain cyclic curve damped by seasonal changes in the humidity of the outside air [21,22,23,24,25,26,27,28,29].

In the warm, dry season, the greatest development of shrinkage deformations is observed in concrete, which gradually stops its development in the cold, wet season and turns into concrete swelling deformations. However, the swelling deformation is less than the shrinkage deformation of concrete. The amplitude of the cycle of moisture shrinkage and swelling deformations decreases with time, but eventually moisture shrinkage deformations remain in the concrete. The deformations of moisture shrinkage of concrete are the greater, the smaller the section of the element and the lower the relative humidity of the air. The influence of the dimensions of the section of elements on the deformation of concrete shrinkage is most pronounced in the initial periods of operation of the structure. The maximum values ​​of the deformation of moisture shrinkage of concrete are observed during the manufacture of the element in the warm dry season.

Figure. 1 Time development of concrete shrinkage deformations in prisms of cross section 7x7 and 20x20 cm in a dry hot climate

The calculated values ​​of the concrete shrinkage deformation for a given operating time are calculated using a hyperbolic dependence.

                                                (1.1)

where ∆τ is the time in a day. from the end of the wet storage of concrete to the specified service life. Parameter α_cs - growth rate of concrete shrinkage strains, the value of which is taken from Table. 1 depending on the season of manufacture and the reduced section height.

Table 1

The value

Note. In the manufacture of structures other than those indicated in Table. 1. The values ​​of the parameter α_cs are taken by linear interpolation.

The calculated limit values ​​of concrete shrinkage deformations are calculated based on the relative seasonal (average monthly) air humidity during the construction period.The limiting design values ​​of concrete shrinkage deformations, corresponding to the flow rate of water for mixing the concrete mixture and the actual operating conditions of the structures, are calculated by the formula.

                                                     (1.2)

The value of the coefficient φ_n is found in Table.3 depending on the season of manufacture of the structure and the reduced sectional height.

The value of the calculated shrinkage deformation of concrete ε_cs for concrete of compressive strength class B25...B65 and standard cone draft up to 7 cm is taken equal to (270...400).10^-5. The values ​​of the coefficient φ_w, which takes into account the relative humidity of the outside air by the beginning of concrete drying, are determined in the same way as when calculating the creep deformations of concrete.

The values ​​of concrete shrinkage deformations are calculated by the formula

                                         (1.3)

Humidity deformation of concrete shrinkage in a cold, more humid season, taking into account the reversible moisture swelling deformation, if necessary, can be considered as the difference between concrete shrinkage deformation ε_cs determined by formula (1.3) and swelling deformations calculated by the formula:

                                                  (1.4)

where ∝_w - seasonal moisture deformations of concrete swelling, equal to 

 the amplitude of annual changes in the seasonal relative humidity of the air,which for the IV climatic region is allowed to be taken on average equal to 40%.

φ - coefficient, taking into account the scale factor, for seasonal moisture deformations of concrete swelling, is taken according to Table 2

Table 2

The limiting values ​​of deformations of moisture shrinkage of concrete can also be taken according to Table 3, depending on the relative humidity of the outside air and the reduced section height.

Table 3

Note.

1.h_red is the reduced height of the section of the element, which characterizes the massiveness of the structures and is equal to the area of ​​the section divided by 1/2 of its diameter in contact with air.

2. Shrinkage deformations should be multiplied by: 0.85 - for structures made of concrete of a class below B 25.

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