Научная статья на тему 'Experimental study of thermal strained state of reinforced concrete elements in natural conditions'

Experimental study of thermal strained state of reinforced concrete elements in natural conditions Текст научной статьи по специальности «Строительство и архитектура»

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Ключевые слова
DESTRUCTIVE PROCESSES / REINFORCED CONCRETE / UNINTENDED DEFORMATION

Аннотация научной статьи по строительству и архитектуре, автор научной работы — Khodjaev Abbos Agzamovich, Khodjaeva Zulfiya Shuhratovna

Uneven distrubution of temperature to RC-elements is cause to apperance of stress and deformations which are non-linear character. Such cyclical changing of temperature leads to appearing of micro-rifts on internal side of RC-elements. By time such rift creating prosses reachs external side of RC-elements, which means contrary event to present methodics of calculating of rift-creating procces under thermal stress. Суclical growth and reduce the stress on the cross section of the element inevitably leads to residual deformations in the concrete. Thus accumulating residual strains give rise to microfractures. The process of formation of micro-cracks occur in this case the outer surfaces of the structure, as is customary in the classical theory of reinforced concrete, and inside it. With time microcracks formation process reaches the outer layers of concrete, which already leads to the appearance of visible cracks on its surface in spite of calculation according to conventional techniques, according to which the fracture design must be provided.

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Текст научной работы на тему «Experimental study of thermal strained state of reinforced concrete elements in natural conditions»

2,0 4,0 6,0 8,0 10,0 Mni, 10 Nm

Fig. 4. Graphic changes of scope of fluctuations of angular speeds and the moments on shaft milling and winging drums from a variation of loading from processed soil: 1 - A<m ; 2 - A<jw ; 3 - AM ; 4 - AM

M, Nnip. 1/s

120 21,0

Fig. 5. Dependences of change¡¡ ,<¡>m, Mwand Mm from variation M,

Fig. 6. Graphic dependences of change of average values of angular speeds and the moments on shaft milling and winging drums at a variation of loading from sowing seeds with the crushed soil:1 - <jm; 2 - <jw; 3 - Mm ; 4 - Mw

References:

1. Djuraev A., Turdaliyev V., MuhammedovJ. The combined unit. Patent of Republic of Uzbekistan, №FAP20150012. - 2015.

2. Djuraev A. Dynamics of working mechanisms clap of the processing cars. - Tashkent, 1987. - 168 p.

Khodjaev Abbos Agzamovich, Prof. of Building construction Department, Tashkent Architecture and building Institute, Uzbekistan

Khodjaeva Zulfiya Shuhratovna, Senior teacher of Bridges and transport tunnels Department, Tashkent automobile and Road institute, Uzbekistan E-mail: sarvarxoja97@mail.ru

Experimental study of thermal strained state of reinforced concrete elements in natural conditions

Abstract: Uneven distrubution of temperature to RC-elements is cause to apperance of stress and deformations which are non-linear character. Such cyclical changing of temperature leads to appearing of micro-rifts on internal side of RC-elements.

Experimental study of thermal strained state of reinforced concrete elements in natural conditions

By time such rift creating presses reachs external side of RC-elements, which means contrary event to present methodics of calculating of rift-creating procces under thermal stress.

Cyclical growth and reduce the stress on the cross section of the element inevitably leads to residual deformations in the concrete. Thus accumulating residual strains give rise to microfractures. The process of formation of micro-cracks occur in this case the outer surfaces of the structure, as is customary in the classical theory of reinforced concrete, and inside it. With time microcracks formation process reaches the outer layers of concrete, which already leads to the appearance of visible cracks on its surface in spite of calculation according to conventional techniques, according to which the fracture design must be provided.

Keywords: destructive processes, reinforced concrete, unintended deformation.

Introduction numerical methods. In this, the initial condition calculation is given

As you know the reliability and durability of reinforced con- constant volume and temperature of the circuit design crete structures is connected directly not only with the stresses and T = t (x,y, z, T = 0). (1)

force action on them, but with the conditions in which they are produced and operated. The most typical in this context for Central Asia and other regions with a predominance of dry hot climate are temperature and climate impacts that are complex and ambiguous, and raises a number of destructive processes in reinforced concrete. As a result of these effects in reinforced concrete structures often have unintended deformation, premature cracking and other negative phenomena, reducing their performance. The reason for these phenomena is an intensive dewatering of concrete, sudden changes in temperature and humidity during the day and throughout the year, which can lead to internal strains and stresses of concrete. In connection with this, important task is the objective consideration of these impacts during the design and calculation of reinforced concrete structures. To solve this problem it is necessary to have a real picture of the behavior of reinforced concrete elements under the influence of the ever-changing flow of the temperature, the nature of changes in external and internal temperature fields, taking into account their daily and seasonal fluctuations.

Statement of the problem.

To this end, the department "Building construction" of Architecture and building Institute of Tashkent held special experimental studies. The object of research was the size of the fragments of beams 120 x 25 x 18 that were blind bore varying depth of normal section and under the influence of natural temperature and humidity in the climatic zone of Tashkent. Changes in temperature over the cross section of the concrete beams were fixed in two ways: using thermocouples of chromel-drops are laid in the beam during the concreting and mercury thermometers. The measurements were within 1.5 years, several times a day.

Calculation of stress on the across of the sections

The investigations managed to get a real picture of the changes experienced by the normal temperature fields of concrete cross-section of the element caused by their thermal deformation and stress. Experiments have shown that under real conditions the temperature field of concrete and reinforced concrete structures over the cross section changes continuously over time, is non-stationary and nonlinear.

The uneven distribution of temperature causes in concrete and reinforced concrete elements of the thermal deformation and stress. Temperature deformation with small changes of stress are generally considered as elastic. However, our experiments have shown that with intensive changes in temperature and solar radiation there is non-uniform stress state of the cross section of the element and its own considerable strain exceeds the elastic limit.

The values of strain and stress on the cross section of the element depending on the measured values of temperature fields determined by the following procedure. The temperature distribution was determined by solving the heat equation for given initial boundary conditions. Calculation of temperature fields can be performed by

The effects of temperature dependence between strains and stresses established by Hooke's law as the fundamental equation of thermoelasticity in this case,

= 1 ^ E L

at.

(2)

<x,y,z) E L <W0 ,x) ' ^<z,z,y)>

Due to the temperature difference over the cross section design can determine changes strains and deformations, in turn, the thermal stresses.

Thermal stresses in the concrete structures are considered as a factor in the elastic stress-strain state. The element of the linear stress state, the voltage determined by the formula by volume stressed state.

= E„ ■ <t2 -1) (3)

or:

^ = . (4)

1 - C- ^

Here ^ — Poisson's ratio. On the basis of this technique calculated the thermal stress state of the experienced elements of lightweight concrete in extreme and mean cross-sectional plane. The most significant changes in these parameters as expected, occur in the summer, when the outside temperature reaches +40.. .45 °C, and humidity — average annual minimum as shown on Fig. 1.

In general, the three-dimensional picture of the stress- strain state of the concrete element under the influence of natural temperature and humidity of the environment is shown on Fig. 2.

These charts (diagrams) allow visually assess the dynamics of changes in temperature, strain and stress on the cross section of the element in the three-dimensional coordinates.

Thus, even a relatively small temperature difference in the range of 10 °C - section element leads to a substantial alternating strains and stresses. The values of strains and stresses in the edge zones (ebt = 0.07-10-3; cbt = 1.68 MPa) greater than the average cross-sectional plane (ebt = 0.05-10-3; cbt = 1.2 MPa). However, the maximum values of strain and stress of the central and edge areas have a gradient along the cross section member.

It should also be noted that the values obtained by the stress experienced demonstrate dependence on the difference (difference) cross section temperature by element, i. e. the greater the temperature difference across the section element, the higher the value of the internal stress. Thus, when a temperature difference in section I-I on 13 °C (55 ° - 42 °) stress values in the middle section reaches 1.68 MPa, and in section III-III at a differential temperature of 10-11 °C for several lower voltage and represent only 1.08-1.32 MPa [3, 106-108].

It is important to note the effect of stress gradient that occurs under conditions of temperature flow. As seen from the stress diagram (Fig. 1) at the same temperature at the surface of the reinforced concrete element (55 °C) received voltage values are different from each other. For sections II, II-II and III-III, they are respectively

1.44, 1.20 and 1.68 MPa. A similar effect is observed in other sections. There is an effect due to the difference in temperature at the edge and interior areas sectional element. The greater the temperature difference, the further removed values from the midline. Accordingly, these points more compared to displacement and therefore stresses. Therefore, in the sections I-I and III—III and voltages in the edge zones and inner layers of the reinforced concrete element is higher than the average cross section II-II.

This Thermo-strained state element is precisely the reason for the significant destructive processes as early cracking, education to operational deformations, etc.

Summary

Analysis of the stress values shows that they are at a maximum temperature close to that of the tensile strength of concrete in tension. Most importantly, with the passage of time within a day mark maximum stresses during cooling or heating is changed from positive to negative and vice versa. That is, the fibers in the middle part of the concrete section of the element during the day undergoing tensile stress, and then compressing and vice versa, depending on the time of day (morning, evening). Such a voltage drop in the body of the reinforced concrete element takes place during the whole

period of its operation, but with varying intensity depending on the time of year.

Such cyclical growth and reduce the stress on the cross section of the element inevitably leads to residual deformations in the concrete. Thus accumulating residual strains give rise to microfractures. The process of formation of micro-cracks occur in this case the outer surfaces of the structure, as is customary in the classical theory of reinforced concrete, and inside it. With time microcracks formation process reaches the outer layers of concrete, which already leads to the appearance of visible cracks on its surface in spite of calculation according to conventional techniques, according to which the fracture design must be provided.

This contradiction arises precisely because these methods do not take into account the dynamics of the effects of temperature on the concrete and reinforced concrete elements Thermo-stressed state explicitly. The most significant impact is when specified, as shown by our study, is not the presence ofhigh (or low) temperature per se, but the dynamics of the passage of the temperature field in the cross section reinforced concrete element, which leads to destructive processes detected. If in the design process to objectively take into account the working conditions of the design, such events can be avoided [1, 6-7].

Fig. 1. Changing the temperature of the concrete - t, °C and temperature voltage - a the cross section of the structure (vertical)

Providing with uniform soil covering of the scattered seeds of wheat

Fig. 2. Changing the temperature of the concrete - t, °C and temperature voltage - ct the cross section of the structure (on teffect = 40 °C and time 16 00 hours)

References:

1. Ashrabov A. A., Zaytzev Yu. V. Elements of fracture mechanics of concrete. Tashkent: O'qituvchi, 1981. - 238 p.

2. Khodjaeva Z. Sh., Khodjaev A. A. Thermo-stressed condition of flexural reinforced concrete elements under complex thermal-climatic impactions allowing for nonlinear deformation of concrete. - Tashkent, 2015. - P. 205.

Xudoyberdiyev Tolibjon, professor of the Department

Muradov Rahimjon, PhD in Technique, the Faculty of Agro-engineering, Agricultural Institute of Andijan, the Republic of Uzbekistan

E-mail: mrahimjon@bk.ru

Providing with uniform soil covering of the scattered seeds of wheat

Abstract: In article the theoretical analysis of possibility of maintenance seal the scattered seeds of wheat is resulted by uniform thickness of the soil received as a result of formation of furrows.

Keywords: the open space, the special installation, the new technology, an irrigated flute, crest soils, hiller with wings, a surface, a return proportion.

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