EA017610B1 - A method of tube confined concrete structure strength enhancement - Google Patents

Abstract

The invention relates to the field of building, in particular to high rise buildings and heavy duty bridges constructing, namely to the methods of tube confined concrete structure strength enhancement by achieving the effect of hoop which results from constraint of transverse extension of concrete under compression. The essence of the method is in the fact that when the tube is filled with concrete and a concrete core (1) is formed longitudinal deformation gap sufficient for free ranging lateral distortion if loaded initially to the strength limit under unconfined compression is developed between a concrete core (1) and a bore surface (3). To provide separate account of possible longitudinal distortion of a concrete core and a steel tube compressive longitudinal load begins with loading a concrete pole and after achieving its shrinkage by longitudinal distortion gap longitudinal compression load is spread onto a steel tube as well.

Description

The invention relates to the field of construction, in particular high-rise buildings and load-bearing bridges, and in particular to methods for increasing the strength of a concrete structure based on the achievement of the well-known cage effect resulting from the constraint (restriction) of transverse expansion of concrete during compression.

It is known [1, p.5] an increase of 25% in the strength of metal pipes filled with concrete under axial compression, compared with the total strength of the individually tested unfilled pipe and concrete core. This method establishes the fundamental possibility of increasing the overall strength of the concrete structure compared to the total strength of the individual load-bearing elements, but does not make it possible to choose the best concrete and pipe parameters for the given conditions.

A known method of calculating and loading a pipeline element [1, p. 27, Fig. 1.5], in which the force interaction of concrete with a steel pipe begins simultaneously with the loading of the concrete structure. This method is adopted as a prototype.

The disadvantage of this method is only the partial use of the pipe-concrete effect associated with overloading the outer steel pipe and underloading of concrete due to a sharp (about 10 times) difference in the elastic moduli of steel and concrete with a much smaller difference in longitudinal deformations. So the experimental values of increasing the strength of concrete with various parameters of concrete structures [1, p. 9, table. 1.1] are significantly (2.08-3.23 times) less than those when testing concrete in laboratory conditions (3-5 times).

The task of the invention is to enhance the concrete effect to the theoretically possible, which further increases its economic and technical attractiveness, and also expands the scope of the concrete effect.

The problem is solved in that a method for increasing the strength of a concrete structure is proposed, including preparing concrete and testing it for strength and deformability under uniaxial and volume compression, which consists in the fact that according to the invention, with axial loading of the concrete structure for the concrete core in the pipe, the possibility of lateral deformation without significant reduction to a load value corresponding to the strength of concrete under uniaxial compression, while through initial separation in of load to the concrete core and steel pipe possible longitudinal deformation of the concrete core and steel pipe to provide separately calculated limit their strength with the pipe-concrete effect, taking into account differences in required strains in accordance with the expression

where Δ1 is the longitudinal deformation gap between the impact of the end compressive load on the concrete core and pipe;

- longitudinal length of the concrete structure, cm;

E _b and E _T are the elastic moduli of concrete and steel, respectively, MPa;

σι, _ο - ultimate tensile strength of concrete for uniaxial compression, MPa;

σ _τ - ultimate strength of pipe steel, MPa;

μ is the Poisson's ratio; δ is the pipe wall thickness, cm;

b _vn - inner diameter of the pipe, see

The longitudinal deformation gap is created through the use of an easily deformable special layer on the inner surface of the pipe, which allows some movement of the concrete core along the pipe during load perception until the longitudinal loading of the pipe with a compressive load begins.

In order to ensure separate accounting of possible longitudinal deformations of the concrete core and steel pipe, the longitudinal compressive load on the concrete structure begins with loading the concrete pillar and, after reaching its shrinkage by the value of the longitudinal deformation gap, the longitudinal compressive load is also applied to the steel pipe.

In the implementation of the invention, the practical conditions for the use of concrete structures can approach theoretical (laboratory).

The way to solve the technical problem is explained by the principle diagram of the work of the concrete structure shown in the drawing, which shows the following. A column of concrete, presented in the form of a core (1) of a cylindrical shape, through a lateral compliant spacer (2) interacts with the inner surface of the steel pipe filled by it (3). The design also contains a malleable end gasket (4) and an upper end plate (5) for transferring the vertical load (shown by the arrow in Fig. 1) to the pipe concrete structure, as well as a solid base (6). As is known [2, p. 99-100], the destruction of a concrete sample with longitudinal compression in the absence of the influence of the ends occurs by cracking parallel to the direction of compressive forces. As follows from the generalized Hooke's law [2, p. 173-175] when the volumetric stress state of the material and the equality of lateral stresses (σ ₂ = σ ₃ ) axial stress with localization (virtually no) of lateral

- 1 017610 formations (ε ₂ = ε ₃ »0), which is typical for pipe concrete, the axial stress is

ΙςΜ ...

01- μ 02.3 (1) where μ is the Poisson's ratio (for concrete μ ₃ = 0.15-0.20 [1, p. 15], for pipe steel μ = 0.33);

σ ₂ , ₃ - lateral stresses perceived by a steel pipe.

Then, with μ6 = 0.15, σ ₁ = 5.7 σ ₂ , ₃ , with μ = 0.20, σ ₁ = 4 σ ₂ , ₃ , that is, with full force interaction of concrete with the shell, the concrete strength under axial load at least in the area of elastic reaction of a steel pipe, it increases from 4 to 5.7 times depending on the properties of concrete.

As follows from the results of experiments prof. A.A. Dolzhenko given in [1, tab. 1.1], and the calculations of the authors of the claimed invention (table. 1), with a joint load of concrete aggregate and pipe, the increase in concrete strength due to the pipe-concrete effect is

Δ№ ₆

----- = 2.08-3.23 times (column 9), which is significantly less than theoretically possible

For

ΔΝ ₆ ΔΝ ^ρ _β ______ = 2.08-6.27 (column 10), and 1 + ------ = 3.08-7.27 (column 11)

Ν _β0 Ν _6ο

Table 1

No. 4 cm δ, cm 0 _T , MPa 0®, MPa L / t, kN L / b., KN L / o, kN L / ³ i.e., kN Δ№ _β Νβ, ο ΔΝ ^ρ ₅ Ν _β0 and t Νθ.ο σ% MPa No. _in one 2 3 4 5 6 7 8 nine 10 eleven 12 thirteen one 25.4 0.86 304 16.1 2150 820 2970 4800 3.23 4.7 5.7 92 1.5 2 25 6 0 92 345 16.1 2650 830 3480 4900 270 6.27 7 27 117 178 3 255 0.84 304 16.1 2J0 §twenty 2920 4740 3.22 4.9 5.9 95 1.48 4 254 084 304 151 2100 820 2020 4740 3.22 4 9 5.9 95 1.5 5 20.6 0.25 340 15.1 500 500 1060 1600 2.08 2.08 3.08 46.5 1.32 6 203 0 33 330 15.1 710 490 1200 1800 2.22 2 82 3.82 57.5 1.43 7 30.5 0.36 334 15.1 1170 1100 2270 3700 2.31 2.24 3.24 49 1.25 8 30.5 0.56 334 15.1 1820 1100 2920 5000 2.90 3.24 4.24 64 1.31

Note: ά - pipe diameter; δ, is the pipe wall thickness; σ _τ is the tensile strength of the pipe material; σ _δ is the tensile strength of concrete under uniaxial compression; Ν _{τ is the} ultimate load on the pipe; N _{b O} - the same for concrete with uniaxial compression; Ν _{ο -} ultimate load on the pipe and concrete without taking into account the pipe-concrete effect; N ^E _Tb is the same obtained experimentally with allowance for the pipe-concrete effect while loading the concrete core and steel shell.

As can be seen from the table. 1 of the calculation results, with μ, ₃ = 0.20 for concrete, the theoretically possible use of the total pipe-concrete effect significantly (by 25-73%, column 13) exceeds that achieved in the experiment. It should be noted that the initial use of the strength of concrete, which is almost freely deformable in the transverse direction inside the pipe, increases the total pipe-concrete effect at δ _δ = 15.1-16.1 MPa by 10-30% depending on the ratio of the initial pipe strength (column 5) and concrete (column 6). On the fundamental correctness presented in table. 1 of the calculation results is evidenced by the fact that the theoretical calculations of the pipe-concrete effect with simultaneous perception of the load by the pipe and concrete and a slight difference in their initial strength (Table 1, the intersection of lines 5 and 7 with columns 9 and 10) are approximately the same.

The technical result is achieved by the fact that under axial loading of the concrete structure for the concrete core (1) in the pipe, the possibility of lateral deformation without significant compression to the value of the load corresponding to the strength of the concrete under uniaxial compression is created (see drawing). In practice, this is achieved by the fact that when filling the pipe with concrete and forming a concrete core (1) between it and the inner surface of the steel pipe (3), a longitudinal deformation gap is created that is sufficient for free lateral deformation of the concrete core during its initial loading to the ultimate strength under uniaxial compression . Such a gap can be created through the use of an easily deformable special layer in the form of a lateral flexible strip (2) on the inner surface of the pipe. The value of the initial deformation of this layer in the radial direction of the pipe should correspond to the value of the lateral deformation of the concrete core during its initial loading to the ultimate strength under uniaxial compression. At the same time, this layer should provide the possibility of some movement of the concrete column along the pipe during load perception until the longitudinal loading of the pipe with a compressive load begins. As a material for creating gaskets for such a layer, for example, multilayer and highly porous plastics can be used.

In addition to the possibility of transverse expansion of concrete in the pipe, it is necessary to create conditions for the sequential perception of the compressive load first by concrete, and at the end of the loading process by a steel pipe in accordance with the shrink characteristics (elastic moduli) of concrete (E _b = 0.1460.232) -10 ⁵ MPa c depending on the tensile strength [2, p.662, Appendix 9]) and steel (E = (2.0-2.1) -10 ⁵

- 2 017610

MPa), which differ by an order of magnitude.

Due to the large difference in the shrinkage and strength characteristics of concrete and steel, their joint work on compression should begin under conditions when the full use of strength characteristics is consistent in space and time. Therefore, the initial separation of the effect of the load on the concrete core and the steel pipe consists in the separation of the spatiotemporal characteristics of the loading of concrete (1) and the pipe (3) and is carried out due to structural elements, such as a compliant end gasket (4), an upper end gasket (5) . In this case, the entire pipe-concrete structure abuts against a solid base (6).

For such an implementation of the method, the above version of the generalized Hooke law can be used with the symbol σ ₂ , ₃ = σ ₂ . Then the procedure for calculating the operation parameters of the concrete structure for the full use of the concrete effect can be described as follows:

1. Lateral deformation of the concrete core and the gap between it and the inner surface of the pipe where r _b . _about - the initial radius of the concrete cylinder, cm;

σ _{1> ο} - concrete strength under uniaxial compression, MPa;

E _b - Young's modulus of elasticity for concrete, MPa.

2. The limit reaction of the inner surface of the pipe to the lateral expansion of concrete 25 with ₂ = —_ σ _τ , (3) where σ _τ is the elastic limit of steel, MPa;

3. Maximum allowable longitudinal stress of concrete

1-μ ²⁵ σι.δ = σι, _ο + - --- σ _τ , (4) μ 6 _βΗ

4. The total longitudinal deformation of the concrete core / / 1-M 26.

Δ / _σ = (σ _1ο + - - σ _τ ) (5)

Fuck μ s! Int

5. Complete longitudinal deformation of the steel pipe taking into account Δ1 << 1, when the difference in the lengths of the concrete core and steel pipe can be ignored (1 _b ~ 1 _t ~ 1)

6. The excess of the shrinkage of the concrete core over the shrinkage of the steel pipe

where K ₁ = σ _τ / σ ₁ . _ο and K ₂ = E _T / E _b .

Consequently, the possible longitudinal deformations of the concrete core and steel pipe are provided separately up to the calculated ultimate strength, taking into account the pipe-concrete effect and taking into account the necessary differences in deformations, by means of which the longitudinal compressive load on the concrete structure begins with loading of the concrete column and only after it has shrunk to Δ1, the longitudinal compressive load should be extended to the steel pipe.

As can be seen from the above calculation method, the quantity Δ1 depends on many parameters, of which the most influential is the Poisson's ratio μ. When analyzing the efficiency coefficients of concrete and Poisson in [1, pp. 32-33], the values μ = 0.1 and μ = 0.15-0.5 (according to O. Ya Berg) are given, and it is also said that μ of concrete varies nonlinearly depending on the current stresses. Apparently, the value of E _b also depends on the current stresses, which is also confirmed by the data [2, p.662, Appendix 9] for concrete at σ ₁ . _ο = 10.15 and 20 MPa, characterized by Е _б = (1.46-1.96) -10 ⁴ , (1,642.14) -10 ⁴ and (1.82-2.32) -10 ⁴ MPa. Here, with an increase in σ ₁ . _ο 2 times the limit values of E _b increased by 18%.

An example is the calculation of the excess shrinkage of a concrete cylinder core over a steel pipe. Using the data table. 1, we accept for the thinnest pipe: b _vn = 20.6 cm, δ = 0.25 cm; σ ₁ . _ο = 15.1 MPa; E _b = 2-10 ⁴ MPa [2, p.662]; μ = 0.17 [2, p.662]; σ _τ = 330 MPa; E _T = 2-10 ⁵ MPa; 1 = 8 m. Then _r 1, 1-0.17 20.25. 330

Δ / = 8Οθ [____, (15.1 + ______ _____ 330) -____, 1 = 0.85 cm.

¹ ΓΪ0 ⁴ 0.17 20.6 ΣΪΟ ⁵ - ¹

Thus, the deformation gap between the load on the concrete cylinder and the pipe is 8.5 mm.

Initial lateral clearance between the concrete core and the inner surface of the pipe

- 3 017610

0.17 15.1

Δγ = ______ 10.3 ______ = 1.32 10 ' ³ cm = 0.013 mm,

1-0.17 2-10 ⁴ which is enough only for a thin film coating. Moreover, σ ₂₂ = 8 MPa.

An elementary calculation of the possible loads for this concrete structure gives, taking into account the use of the invention, N ^P T. _b = 2320 kN (experimental value N ^E T. _b = 1600 kN), which is almost 1.45 times the experiment.

In the case of using the existing technology of the prototype, providing for the simultaneous loading of concrete and pipe (without using σ _{1. Ο} ) N ^P _{T. b} = 2320-520 = 1800 kN, which is only 12% more than obtained in the experiment.

For the thickest-walled pipe: b _vn = 25.6 cm, δ = 0.92 cm; σ ₁ . _ο = 16.1 MPa; E _b = 2-10 ⁴ MPa [2, p.662]; μ = 0.17 [2, p.662]; σ _τ = 345 MPa; E _T = 2-10 ⁵ MPa; 1 = 8 m.

Then r 1, 1-0.17

Δ / = 800 [----, 116.1 + ----- 2T0 ⁴ 0.17

20.92. 345, ______ 345) -_____ 1 = 5.05 cm.

25.6 U 2 ^b

Thus, the deformation gap between the effect of the load on the concrete core and simultaneously on the concrete core and pipe is 50.5 mm, which is almost 6 times greater than in the previous case with a thin-walled pipe.

Initial lateral clearance between the concrete core and the inner surface of the pipe

0.17 16.1

Δγ = _____ 12.8 ____ = 1.89 × ³ cm = 0.019 mm,

1-0.17 210 ⁴ which is approximately 1.5 times more compared to the previous case. Moreover, σ _2.3 = 24.8 MPa.

Elementary calculation of possible load for a thick-walled pipe-concrete construction gives considering the use of the invention t.b N ^p = 9,400 kN (experimental value N ^E m. _B = 4900 kN), which is almost 1.9 times more experiment.

In the case of using the existing technology for simultaneous loading of concrete and pipe (without using σ1.ο), but with the creation of the possibility of separate longitudinal deformation of concrete and steel, the possible design load is N ^E _{T. b} = 8580 kN, which is 1.75 times the experiment.

Thus, in thin-walled pipe-concrete structures with the simultaneous impact of the load on concrete and pipe, the pipe-concrete effect is used almost completely, and in thick-walled pipes - only partially.

The present invention allows to increase the load on the concrete structure compared with the achieved 1.45-1.90 times.

When using ordinary steel pipes for gas pipelines with a diameter of b _nv = 142 cm with a wall thickness of δ = 2.2 cm and high-strength concrete (σ _δ = 45-50 MPa) in the case of using the present invention, the calculated load on such a single pipe concrete structure can reach 20,000 t, and at the same time the load of concrete and steel is 40% less. With concrete strength σ _δ = 30 MPa, the bearing capacity of this concrete structure is reduced by 20%. Therefore, the present invention can be very useful in the construction of high-rise buildings and load-bearing bridges.

Information sources:

1. Luksha L.K. Durability of pipe concrete. - Mn .: Higher School, 1977. - 96 p.

2. Pisarenko G.S. and others. Resistance of materials. - Kiev: Vishcha school, 1973.672p.

Claims (4)

CLAIM 1. A method of increasing the strength of a pipe-concrete structure, in which an easily deformable side gasket (2) is installed on the inner surface of the steel pipe (3) to create a longitudinal deformation gap sufficient for free deformation of the concrete core (1) in the radial direction of the pipe (3) during its initial loading, to the ultimate strength under uniaxial compression, install a flexible end gasket (4) and fill the steel pipe (3) with concrete, forming a concrete core (1), then install the ends lining (5) and loading onto the concrete core (1) through the end plate (5), while sharing the effect of the load on the concrete core (1) through the end plate (5) and on the steel pipe (3) by means of the end strip (4) ) taking into account the pipe-concrete effect and taking into account the necessary difference in deformations in accordance with the expression where Δ1 is the longitudinal deformation gap between the impact of the end compressive load on the concrete core (1) and the pipe (3); - 4 017610 1 - the longitudinal length of the concrete structure, cm; Еб and Е _т - elastic moduli of concrete and steel, respectively, MPa; σχ.ο - ultimate tensile strength of concrete for uniaxial compression, MPa; σ _τ - ultimate strength of pipe steel, MPa; μ is the Poisson's ratio; δ is the pipe wall thickness, cm; 0 _int - inner diameter of the pipe, see 2. The method according to claim 1, characterized in that the longitudinal deformation gap is created through the use of an easily deformable special layer on the inner surface of the pipe, which allows some movement of the concrete column along the pipe during load perception until the longitudinal loading of the pipe with a compressive load begins. 3. The method according to claim 1, characterized in that the longitudinal compressive load on the concrete structure begins with loading the concrete pillar and after reaching its shrinkage by the magnitude of the longitudinal deformation gap, the longitudinal compressive load is also applied to the steel pipe. 4. The method according to claim 1, characterized in that the entire pipe-concrete structure, when it is loaded, abuts against a solid base (6).