Creep prediction method of steel-concrete composite columns based on moisture transfer paths

Abstract

The present invention proposes a creep prediction method for steel-concrete composite columns based on moisture transfer paths, which belongs to the field of concrete technology and includes the steps of determining basic performance parameters, calculating the length of concrete moisture transfer paths at the moisture transfer level, calculating the creep deformation of concrete at the material level, and calculating the creep deformation of steel-concrete composite columns at the structural level. This model establishes a creep prediction method suitable for various steel-concrete composite columns such as steel tube concrete columns, steel tube-constrained reinforced concrete columns, and steel-concrete columns based on the moisture transfer mechanism that affects concrete creep. The model considers the influence of steel components blocking the transfer of internal humidity of concrete to the external environment, and the uneven distribution of internal humidity of concrete in the horizontal and vertical directions on creep, while also considering the physical filling effect of fly ash and mineral powder and the influence of chemical volcanic ash reaction. This method can accurately predict the creep deformation of various steel-concrete composite columns over time.

Description

Technical Field

[0001] The invention belongs to the technical field of concrete, and in particular relates to a creep prediction method for steel-concrete composite columns based on moisture transfer paths. Background Art

[0002] As a new type of green high-performance composite component, steel-concrete composite columns have the advantages of high compressive bearing capacity, excellent seismic performance, simple joint structure with reinforced concrete beams, and convenient construction. Various components have been developed, such as steel tube concrete columns, steel tube confined reinforced concrete columns, and steel concrete columns. Used as structural columns of (super) high-rise buildings and key columns of large-span structures, they can effectively solve the contradiction between the low shear span ratio or high axial compression ratio of column components under heavy loads and the seismic performance requirements of the structure.

[0003] In high-rise buildings, it is crucial to accurately predict the creep deformation of vertical load-bearing components. Otherwise, the difference in the cumulative vertical deformation between different load-bearing components will cause additional internal forces in the horizontal components and affect non-structural components such as glass curtain walls and pipes. Creep deformation is the main factor affecting the vertical deformation of column components. The creep deformation of steel-concrete composite columns is about 79% to 120% of the short-term deformation. Therefore, it is necessary to accurately predict creep deformation to ensure structural safety and building use function.

[0004] The barrier effect of steel components (steel pipes, steel sections, etc.) on the transfer of internal humidity of concrete to the external environment causes different creep deformations of different steel-concrete composite columns. The column end concrete of steel tube-constrained reinforced concrete columns is generally in an open state. This longitudinal partial exposure condition causes the creep deformation of concrete to be 46% lower than that of reinforced concrete columns under fully exposed conditions, and 52% higher than that of reinforced concrete columns under fully enclosed conditions (the internal humidity of steel tube concrete cannot be transferred to the outside). The addition of mineral admixtures such as fly ash and mineral powder into concrete affects the creep deformation of concrete over time. At the same time, the stress redistribution between the core concrete and steel components (steel pipes, steel sections, etc.) makes the creep deformation of steel-concrete composite columns more complicated over time. In order to further clarify the creep deformation development of different steel-concrete composite columns, it is urgent to propose a creep deformation prediction method for steel-concrete composite columns that considers the barrier effect of steel components (steel pipes, steel sections, etc.) on the transfer of internal humidity of concrete to the external environment.

[0005] Essentially, the difference in creep deformation of different steel-concrete composite columns lies in the fact that the barrier effect of steel components on the transfer of moisture from the concrete to the external environment changes the moisture transfer path of the core concrete. The path for the moisture inside the concrete to dissipate to the environment is called the concrete moisture transfer path. The longer the moisture transfer path, the less likely the moisture inside the concrete is to be transferred to the outside, and the smaller the creep deformation of the concrete. Therefore, it is urgent to establish a creep prediction model for steel-concrete composite columns based on the moisture transfer path. Compared with the traditional concrete prediction model, this model should take into account the influence of the difference in the length of the concrete moisture transfer path on the creep deformation. This creep deformation prediction model will provide a calculation basis for the creep deformation of steel-concrete composite columns such as steel tube concrete columns, steel tube confined reinforced concrete columns and steel concrete columns, and further promote the academic research and engineering application of various steel-concrete composite columns. Summary of the invention

[0006] In view of this, in order to solve the technical problems mentioned in the above background technology, the present invention provides a creep prediction method for steel-concrete composite columns based on moisture transfer paths, that is, firstly, at the moisture transfer level, a method for calculating the length of the concrete moisture transfer path is proposed; at the material level, a calculation method considering the physical filling effect of fly ash and mineral powder and the influence of chemical volcanic ash reaction on concrete creep is proposed; at the component level, the trapezoidal integral rule, the force equilibrium equation and the deformation coordination equation between the core concrete and the longitudinal steel bars are adopted, and the stress redistribution between the core concrete and the longitudinal steel bars is considered, and then the creep deformation of the steel-concrete composite column is calculated.

[0007] To achieve the above object, the present invention adopts the following technical solution: a creep prediction method for steel-concrete composite columns based on moisture transfer paths, specifically comprising the following steps:

[0008] (1) Determine the basic performance parameters: fly ash content and mineral powder content of concrete; compressive strength and elastic modulus of concrete; elastic modulus, yield strength, ultimate strength and Poisson's ratio of steel components (steel pipes, steel sections, longitudinal reinforcement, etc.); cross-sectional concrete and steel component area; column height of steel pipe; start and end ages of steel-concrete composite columns bearing constant loads and the loads borne; ambient temperature and humidity;

[0009] (2) At the moisture transfer level, calculate the moisture transfer path length of concrete: Use the moisture transfer path length to replace the nominal size of the BS EN1992 model to consider the effect of steel components (steel pipes, steel sections, longitudinal reinforcement, etc.) blocking the transfer of moisture inside the concrete to the external environment, and the resulting uneven distribution of moisture inside the concrete in the transverse and longitudinal directions on creep. According to the cross-sectional and elevation dimensions of the component and the dimensions of the steel components (steel pipes, steel sections, longitudinal reinforcement, etc.), calculate the shortest distance from the center point of the steel-concrete composite column to the exposed concrete surface of the component. This distance is the effective moisture transfer path length of the steel-concrete composite column.

[0010] (3) At the material level, calculate the creep deformation of concrete: Considering the physical filling effect of fly ash and mineral powder and the influence of chemical pozzolanic reaction on the creep of concrete, calculate the creep deformation of concrete under constant load according to the fly ash and mineral powder dosages;

[0011] (4) At the component level, the creep deformation of steel-concrete composite columns is calculated: the influence of creep deformation that develops rapidly in the early stage and slowly in the later stage is considered, and the load holding time is divided into 100 steps; the influence of stress redistribution between concrete and steel components (steel pipes, steel sections, longitudinal reinforcements, etc.) is considered when the steel-concrete composite column is subjected to a constant load, and the stress-strain relationship of concrete is established according to the trapezoidal integral rule; the creep deformation of concrete is calculated according to the force equilibrium equation and the deformation coordination equation when the steel-concrete composite column is subjected to a constant load, and the concrete and steel components jointly bear the vertical load, and the deformation of the two parts is coordinated. That is, the creep of the steel-concrete composite column.

[0012] Furthermore, in step (2), the moisture transfer aspect includes the following situations:

[0013] For steel tube concrete columns, the effective moisture transfer path length is taken as infinity; for steel-concrete columns, the effective moisture transfer path length is taken as half of the length of the cross-section diagonal; for steel tube confined reinforced concrete columns, the effective moisture transfer path length is calculated according to the formula Calculation; For reinforced concrete columns, the effective moisture transfer path is taken as the nominal size in the BS EN1992 model;

[0014] Where h0 is the effective moisture transfer path length of the longitudinally partially exposed steel tube confined reinforced concrete column (mm); H is the distance between the steel tube slits of the composite member (mm); A c and u are the areas of the core concrete (mm 2 ) and circumference (mm).

[0015] Furthermore, step (3) includes the following formula at the material level:

[0016]

[0017] Where t0 is the time when the constant load begins (d); t is any time during the constant load process (d); It is the creep coefficient of concrete from time t0 when it is subjected to a constant load to time t, that is, the ratio of the creep deformation of concrete at time t to the instantaneous deformation of concrete at the end of loading (time t0); is the nominal creep coefficient of concrete, calculated according to the BS EN 1992 model; β HIt is the influence coefficient of ambient humidity and specimen size on creep development trend, calculated according to BSEN 1992 model; k1 and k2 are the parameters of the influence of fly ash and mineral powder on the final value and development trend of concrete creep, respectively.

[0018] Furthermore, in step (4), at the component level, in calculating the creep deformation of the steel-concrete composite column: considering the influence of the rapid development of creep deformation in the early stage and the slow development in the later stage, the load holding time is divided into 100 steps, and according to the formula calculate;

[0019] Where, t end is the end time (d) when the steel-concrete composite column bears the load; Δt k is any time t k With the previous moment t k-1 The time step length (d); Δt i is any time t i With the previous moment t i-1 is the time step length (d); k is the time step number.

[0020] Furthermore, in step (4), in calculating the creep deformation of the steel-concrete composite column at the component level, the stress-strain relationship of the concrete under the continuously decreasing load is established according to the trapezoidal integration rule, which includes the following formula:

[0021]

[0022]

[0023]

[0024] Where, t k-1 With t k are the start and end time of the kth step (d); σ ck and σ cj They are t k With t j The concrete stress at the moment (MPa); ε totk and ε shk They are t k Total deformation and shrinkage of concrete at time (×10- 6 );M c1k and M c2kj They are creep influence coefficient; J(t k ,t j ) is the concrete subjected to a constant load from time tj to time t k The creep function of time (×10 -6 / MPa) and creep coefficient were calculated by substituting formula (1) and formula (2) into the BS EN 1992 model, taking into account the influence of steel components (steel pipes, steel sections, longitudinal reinforcements, etc.) blocking the transfer of internal humidity of concrete to the external environment, and the subsequent uneven distribution of internal humidity of concrete in the horizontal and vertical directions on creep, and the influence of physical filling effect and chemical pozzolanic reaction of fly ash and mineral powder on concrete creep.

[0025] Furthermore, in step (4), in the calculation of the creep deformation of the steel-concrete composite column at the component level, it is considered that the steel-concrete composite column is subjected to a constant load, the concrete and the steel component jointly bear the vertical load, and the deformation of the two parts is coordinated. The creep of the concrete under the continuously decreasing load is calculated according to the force equilibrium equation and the deformation coordination equation, that is, the creep of the steel-concrete composite column, which includes the following formula:

[0026]

[0027] ε ck =ε totk -ε shk (8)

[0028] Where N L A is the load (N) that the composite column bears after construction is completed; c is the cross-sectional area of ​​the concrete (mm 2 );E s and A s They are the elastic modulus (MPa) and cross-sectional area (mm 2 );σ cj Yes j The concrete stress at the moment (MPa); ε totk and ε ck They are t k The total deformation and creep deformation of concrete at the moment (×10 -6 ); shk Yes k The shrinkage deformation of concrete at the moment (×10 -6 ), calculated according to the BS EN 1992 model.

[0029] Compared with the prior art, the present invention proposes a creep prediction method for steel-concrete composite columns based on the moisture transfer path. The model takes into account the effect of steel components (steel pipes, steel sections, longitudinal reinforcements, etc.) blocking the transfer of moisture inside the concrete to the external environment, and the resulting uneven distribution of moisture inside the concrete in the horizontal and vertical directions on creep. The physical filling effect of fly ash and mineral powder and the chemical volcanic ash reaction are also considered. On this basis, the trapezoidal integral rule, the force balance equation and the deformation coordination equation between the core concrete and the longitudinal reinforcement are adopted, and the stress redistribution between the core concrete and the steel components (steel pipes, steel sections, longitudinal reinforcements, etc.) is considered, and then the creep deformation of the steel-concrete composite column is calculated over time. Overall, this prediction method has the following advantages:

[0030] (1) Based on the concrete moisture transfer mechanism, a concrete creep prediction method is established that takes into account the influence of the uneven distribution of concrete moisture along the horizontal and vertical directions caused by the steel components (steel pipes, steel sections, longitudinal reinforcements, etc.) blocking the transfer of concrete moisture to the external environment. The creep of concrete is caused by the extrusion of water between the CSH gel layers, a hydration product of concrete, into the capillaries and macropores, as well as the relative slip or shear between the CSH gels. The self-drying effect of concrete and the outward transfer of moisture will lead to a decrease in the moisture content in the concrete capillaries and an increase in the pore volume, that is, a decrease in relative humidity. The increase in pores will intensify the extrusion of water between the CSH gel layers into the capillaries and macropores, thereby intensifying the generation of creep deformation. The shorter the moisture transfer path, the easier it is for the moisture inside the concrete to transfer outward, and the greater the creep deformation. Previous models did not consider the influence of the difference in the moisture transfer path. The present invention starts with the concrete moisture transfer path and develops a concrete creep prediction method that takes into account the barrier effect of steel components (steel pipes, steel sections, longitudinal reinforcements, etc.) based on the creep mechanism.

[0031] (2) The influence of the physical filling effect of fly ash and mineral powder and the chemical pozzolanic reaction on the creep of concrete is considered. The creep of concrete is significantly affected by fly ash and mineral powder. The physical filling effect of fly ash and mineral powder increases the content of capillary pores inside the concrete, making the concrete interface transition zone denser, thereby hindering the relative slip or shear between CSH gels and reducing the creep of concrete; the chemical pozzolanic reaction consumes less water than the cement hydration reaction, and the pore volume is reduced, thereby inhibiting the extrusion of water between the CSH gel layers into the capillaries and macropores, thereby reducing creep deformation. The present invention comprehensively considers the above-mentioned physical filling effect and chemical pozzolanic reaction, and provides a calculation formula that considers the influence of fly ash and mineral powder.

[0032] (3) Considering the influence of stress redistribution between core concrete and steel components (steel pipes, steel sections, longitudinal reinforcement, etc.) when the core concrete and steel components jointly bear vertical loads, a creep deformation prediction model for steel tube-confined reinforced concrete columns was established. The stress-strain relationship of the core concrete was established according to the trapezoidal integral rule; combined with the force equilibrium equation and deformation coordination equation of the core concrete and steel components, a creep deformation prediction method for steel-concrete composite columns considering the influence of stress redistribution between the core concrete and longitudinal reinforcement was established. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme in the embodiments of the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0034] In the attached picture:

[0035] Figure 1 It is a calculation flow chart of the creep prediction method of steel-concrete composite columns based on moisture transfer paths according to the present invention;

[0036] Figure 2 A front view of a steel tube-confined reinforced concrete column for experimental verification of the creep prediction method of steel-concrete composite columns based on moisture transfer paths according to the present invention;

[0037] Figure 3 1-1 is a cross-sectional view of a steel tube-confined reinforced concrete column for experimental verification of the creep prediction method of steel-concrete composite columns based on moisture transfer paths according to the present invention;

[0038] Among them, 1-column end plate, 2-steel pipe slit, 3-core concrete, 4-moisture transmission path, 5-outer steel pipe, 6-composite column center point, 7-longitudinal steel bar, 8-stirrup;

[0039] Figure 4 This is a comparison chart of creep deformation prediction results of steel-concrete composite columns based on moisture transfer path of the present invention for steel tube-confined reinforced concrete columns filled with C70 concrete and loaded for 28 days with experimental measured results;

[0040] Figure 5 It is a front view of a steel-concrete column for experimental verification of the creep prediction method of steel-concrete composite columns based on moisture transfer paths according to the present invention;

[0041] Figure 61-1 is a cross-sectional view of a steel-concrete column for experimental verification of the creep prediction method of steel-concrete composite columns based on moisture transfer paths according to the present invention;

[0042] Among them, 1-column end plate, 9-concrete, 3-moisture transmission path, 10-steel, 11-center point of composite column;

[0043] Figure 7 This is a comparison chart of creep deformation prediction results of steel-concrete composite columns filled with C40 concrete and loaded for 14 days using the moisture transfer path-based creep prediction method of the present invention and the experimental measured results;

[0044] Figure 8 The front view of a steel tube concrete column is a test verification of the creep prediction method of steel-concrete composite columns based on moisture transfer paths according to the present invention;

[0045] Fig. 9 A cross-sectional view of a steel tube concrete column 1-1 for experimental verification of the creep prediction method of steel-concrete composite columns based on moisture transfer paths according to the present invention;

[0046] Among them, 1-column end plate, 12-steel tube, 3-core concrete, 11-center point of composite column;

[0047] Fig.10 This is a comparison chart of the creep deformation prediction results of steel-concrete composite columns filled with C40 concrete and loaded for 29 days using the moisture transfer path-based creep prediction method for the present invention and the experimental measured results. DETAILED DESCRIPTION

[0048] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely explain the technical solutions in the embodiments of the present invention. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict, and the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0049] See also Figure 1-10 The present embodiment is described as a creep prediction method for steel-concrete composite columns based on moisture transfer paths, which specifically includes the following steps:

[0050] (1) Determine the basic performance parameters: fly ash content and mineral powder content of concrete; compressive strength and elastic modulus of concrete; elastic modulus, yield strength, ultimate strength and Poisson's ratio of steel components (steel pipes, steel sections, longitudinal reinforcement, etc.); cross-sectional concrete and steel component area; column height of steel pipe; start and end ages of steel-concrete composite columns bearing constant loads and the loads borne; ambient temperature and humidity;

[0051] (2) At the moisture transfer level, calculate the moisture transfer path length of concrete: Use the moisture transfer path length to replace the nominal size of the BS EN1992 model to consider the effect of steel components (steel pipes, steel sections, longitudinal reinforcement, etc.) blocking the transfer of moisture inside the concrete to the external environment, and the resulting uneven distribution of moisture inside the concrete in the transverse and longitudinal directions on creep. According to the cross-sectional and elevation dimensions of the component and the dimensions of the steel components (steel pipes, steel sections, longitudinal reinforcement, etc.), calculate the shortest distance from the center point of the steel-concrete composite column to the exposed concrete surface of the component. This distance is the effective moisture transfer path length of the steel-concrete composite column.

[0052] (3) At the material level, calculate the creep deformation of concrete: Considering the physical filling effect of fly ash and mineral powder and the influence of chemical pozzolanic reaction on the creep of concrete, calculate the creep deformation of concrete under constant load according to the fly ash and mineral powder dosages;

[0053] (4) At the component level, the creep deformation of steel-concrete composite columns is calculated: the influence of creep deformation that develops rapidly in the early stage and slowly in the later stage is considered, and the load holding time is divided into 100 steps; the influence of stress redistribution between concrete and steel components (steel pipes, steel sections, longitudinal reinforcements, etc.) is considered when the steel-concrete composite column is subjected to a constant load, and the stress-strain relationship of concrete is established according to the trapezoidal integral rule; the creep deformation of concrete is calculated according to the force equilibrium equation and the deformation coordination equation when the steel-concrete composite column is subjected to a constant load, and the concrete and steel components jointly bear the vertical load, and the deformation of the two parts is coordinated. That is, the creep of the steel-concrete composite column.

[0054] In step (2), the moisture transfer path length is used to replace the nominal size of the BS EN 1992 model in terms of moisture transfer, so as to consider the effect of the uneven distribution of moisture inside the concrete in the horizontal and vertical directions on creep caused by the steel components (steel pipes, steel sections, longitudinal reinforcements, etc.) blocking the transfer of moisture inside the concrete to the external environment. According to the cross-sectional and elevational dimensions of the component and the dimensions of the steel components (steel pipes, steel sections, longitudinal reinforcements, etc.), the shortest distance from the center point of the steel-concrete composite column to the exposed concrete surface of the component is calculated. This distance is the effective moisture transfer path length of the steel-concrete composite column, which includes the following situations:

[0055] For steel tube concrete columns, the effective moisture transfer path length is taken as infinity; for steel-concrete columns, the effective moisture transfer path length is taken as half of the length of the cross-section diagonal; for steel tube confined reinforced concrete columns, the effective moisture transfer path length is calculated according to the formula Calculation; For reinforced concrete columns, the effective moisture transfer path is taken as the nominal size in the BS EN1992 model;

[0056] Where h0 is the effective moisture transfer path length of the longitudinally partially exposed steel tube confined reinforced concrete column (mm); H is the distance between the steel tube slits of the composite member (mm); A c and u are the areas of the core concrete (mm 2 ) and circumference (mm).

[0057] In step (3), at the material level, the physical filling effect of fly ash and mineral powder and the influence of chemical pozzolanic reaction on the creep of concrete are considered. The creep deformation of concrete is calculated according to the fly ash content and mineral powder content, including the following formula:

[0058]

[0059] Where t0 is the time when the constant load begins (d); t is any time during the constant load process (d); It is the creep coefficient of concrete from time t0 when it is subjected to a constant load to time t, that is, the ratio of the creep deformation of concrete at time t to the instantaneous deformation of concrete at the end of loading (time t0); is the nominal creep coefficient of concrete, calculated according to the BS EN 1992 model; β H It is the influence coefficient of ambient humidity and specimen size on creep development trend, calculated according to BSEN 1992 model; k1 and k2 are the parameters of the influence of fly ash and mineral powder on the final value and development trend of concrete creep, respectively.

[0060] In step (4), at the component level, the creep deformation of the steel-concrete composite column is calculated by considering the influence of the rapid development of creep deformation in the early stage and the slow development in the later stage. The holding time is divided into 100 steps, and the formula is used:

[0061]

[0062] In the formula, t end is the end time (d) when the steel-concrete composite column bears the load; Δt k is any time t k With the previous moment t k-1 The time step length (d); Δt i is any time t i With the previous moment t i-1 is the time step length (d); k is the time step number.

[0063] In step (4), at the component level, in calculating the creep deformation of the steel-concrete composite column, the stress-strain relationship of the concrete under the continuously decreasing load is established according to the trapezoidal integration rule, which includes the following formula:

[0064]

[0065]

[0066]

[0067] In the formula, t k-1 With t k are the start and end time of the kth step (d); σ ck and σ cj They are t k With t j The concrete stress at the moment (MPa); ε totk and ε shk They are t k Total deformation and shrinkage of concrete at time (×10- 6 );M c1k and Mc2kj are creep influence coefficients respectively; J(t k ,t j ) is the concrete subjected to a constant load from time tj to time t k The creep function of time (×10 -6 / MPa) and creep coefficient were calculated by substituting formula (1) and formula (2) into the BS EN 1992 model, taking into account the influence of steel components (steel pipes, steel sections, longitudinal reinforcements, etc.) blocking the transfer of internal humidity of concrete to the external environment, and the subsequent uneven distribution of internal humidity of concrete in the horizontal and vertical directions on creep, and the influence of physical filling effect and chemical pozzolanic reaction of fly ash and mineral powder on concrete creep.

[0068] In step (4), at the component level, the creep deformation of the steel-concrete composite column is calculated: considering that the steel-concrete composite column is subjected to a constant load, the concrete and the steel components jointly bear the vertical load, and the deformation of the two parts is coordinated. The creep of the concrete under the continuously decreasing load is calculated according to the force equilibrium equation and the deformation coordination equation, that is, the creep of the steel-concrete composite column, which includes the following formula:

[0069]

[0070] ε ck =ε totk -ε shk (8)

[0071] Where N L A is the load (N) that the composite column bears after construction is completed; c is the cross-sectional area of ​​the concrete (mm 2 );E s and A s They are the elastic modulus (MPa) and cross-sectional area (mm 2);σ cj Yes j The concrete stress at the moment (MPa); ε totk and ε ck They are t k The total deformation and creep deformation of concrete at the moment (×10 -6 ); shk Yes k The shrinkage deformation of concrete at the moment (×10 -6 ), calculated according to the BS EN 1992 model.

[0072] Specific Example 1: According to the C70 high-strength concrete mix ratio in Table 1, 19% fly ash and 19% mineral powder were added to the concrete to make a steel pipe-constrained reinforced concrete column with a height of 600 mm, a diameter of 217 mm, a wall thickness of 2.1 mm, and a slit spacing of 480 mm. Six longitudinal steel bars with a diameter of 18 mm and stirrups of φ8@200 were set inside the concrete. The test piece was constructed as follows Figure 2-3 As shown. The concrete at the joint remains exposed and is exposed to a constant temperature and humidity environment with a relative humidity of 58.6% and a temperature of 21.3 degrees Celsius. The average 28-day cylindrical compressive strength of the core concrete was measured to be 66.6MPa, and the 28-day elastic modulus was 42.4GPa; the elastic moduli of the longitudinal reinforcement and the outer steel pipe were 201GPa and 183GPa, respectively, the yield strengths were 480.3MPa and 373.5MPa, respectively, the ultimate strengths were 571.5MPa and 469.7MPa, respectively, and the Poisson's ratios were 0.295 and 0.248, respectively; the loading start age and end age were 28 days and 760 days, respectively; the load borne was 1307kN.

[0073] Table 1 Mix ratio of C70 high-strength concrete in Example 1 (unit: kg / m 3 )

[0074]

[0075] The creep prediction method of steel-concrete composite columns based on moisture transfer paths in the present invention is used to calculate the basic parameters of core concrete, longitudinal reinforcement, and outer steel pipe measured by experiments. The calculation process is as follows:

[0076] (1) Determine the basic performance parameters: the fly ash and mineral powder content of concrete is 19%; the average 28-day cylindrical compressive strength of the core concrete is 66.6MPa, and the 28-day elastic modulus is 42.4GPa; the elastic modulus of the longitudinal reinforcement and the outer steel pipe are 201GPa and 183GPa, respectively, the yield strength is 480.3MPa and 373.5MPa, respectively, the ultimate strength is 571.5MPa and 469.7MPa, respectively, and the Poisson's ratio is 0.295 and 0.248, respectively; the cross-sectional area of ​​the core concrete, longitudinal reinforcement, and outer steel pipe is 35547.81mm 2 、1526.04mm 2 and 1417.05mm 2 ; The column height and the distance between the slits of the steel pipe are 600mm and 480mm respectively; The predicted starting age and ending age are 28 days and 760 days respectively; The load is 1307kN; The temperature and humidity of the environment are 21.3 degrees Celsius and 58.6% relative humidity respectively;

[0077] (2) In terms of moisture transfer, calculate the effective moisture transfer path length of concrete:

[0078]

[0079] (3) At the material level, calculate the creep deformation of concrete:

[0080] The correction parameters of mineral admixtures for the effect of fly ash and mineral powder on the creep of core concrete are taken according to the following formula:

[0081]

[0082]

[0083] (4) At the component level, the load holding time step is divided into 100 steps:

[0084]

[0085] (5) At the component level, the stress-strain relationship of the core concrete is established according to the trapezoidal integration rule:

[0086]

[0087]

[0088]

[0089] (6) At the component level, the creep deformation of steel tube confined reinforced concrete columns is calculated according to the force balance equation and deformation coordination equation:

[0090]

[0091] ε ck =ε totk -ε shk

[0092]

[0093] The creep deformation calculated by this prediction method is compared with the experimental results. Figure 4 As shown, the results show that the creep deformation prediction method of the steel-concrete composite column of the present invention is relatively accurate.

[0094] Specific Example 2: A steel-concrete column with a height of 500 mm and a diameter of 180 mm was made according to the C40 concrete mix ratio in Table 2 without adding fly ash and mineral powder. A cross-shaped steel with a thickness of 0.5 mm was embedded in the concrete. The test piece was constructed as follows: Figure 5-6 As shown. The concrete remained exposed in a constant temperature and humidity environment with a relative humidity of 52% and a temperature of 20 degrees Celsius. The average 28-day cylindrical compressive strength of the core concrete was 43.9MPa, the 28-day elastic modulus was 26.5GPa, the average 14-day cylindrical compressive strength was 40.3MPa, and the 14-day elastic modulus was 25.3GPa; the elastic modulus of the steel was 206GPa; the loading start age and end age were 14 days and 168 days respectively; the load was 240kN.

[0095] Table 2 Mix ratio of C40 concrete in Example 1 (unit: kg / m 3 )

[0096]

[0097] The creep prediction method of steel-concrete composite columns based on moisture transfer paths in the present invention is used to calculate the basic parameters of concrete and steel measured by experiments. The calculation process is as follows:

[0098] (1) Determine the basic performance parameters of concrete and steel: the content of fly ash and mineral powder in concrete is 0%; the average compressive strength of the cylinder after 28 days is 43.9 MPa, the elastic modulus after 28 days is 26.5 GPa, the average compressive strength of the cylinder after 14 days is 40.3 MPa, the elastic modulus after 14 days is 25.3 GPa; the elastic modulus of the steel is 206 GPa; the cross-sectional area of ​​the concrete and steel is 32400 mm 2 and 240mm 2 ; The column height of the steel pipe is 500mm; The loading start age and end age are 14 days and 168 days respectively; The load is 240kN; The temperature and humidity of the environment are 20 degrees Celsius and 52% relative humidity respectively;

[0099] (2) In terms of moisture transfer, the effective moisture transfer path length of the steel-concrete column is as follows: Figure 5-6 As shown, its value is:

[0100]

[0101] (3) At the material level, calculate the creep deformation of the core concrete under constant load:

[0102] The correction parameters of mineral admixtures affecting the creep of core concrete by fly ash and mineral powder are taken according to the following formula:

[0103]

[0104]

[0105] (4) At the component level, the load holding time step is divided into 100 steps:

[0106]

[0107] (5) At the component level, the stress-strain relationship of the core concrete under a continuously decreasing load is established according to the trapezoidal integration rule:

[0108]

[0109]

[0110]

[0111] (6) At the component level, the creep deformation of the core concrete under a continuously decreasing load is calculated according to the force balance equation and the deformation coordination equation, that is, the creep deformation of the steel tube confined reinforced concrete column:

[0112]

[0113] ε ck =ε totk -ε shk

[0114]

[0115] The creep deformation calculated by this prediction method is compared with the experimental results. Figure 7 As shown, the results show that the creep deformation prediction method of the steel-concrete composite column of the present invention is relatively accurate.

[0116] Specific Example 3: According to the C40 high-strength concrete mix ratio in Table 3, 8% fly ash and 0% mineral powder were added to the concrete to produce a steel tube concrete column with a height of 400 mm, a diameter of 140 mm, and a wall thickness of 2.6 mm. The test piece is constructed as follows: Figure 8-9 As shown. In a constant temperature and humidity environment with a relative humidity of 60% and a temperature of 22.4 degrees Celsius. The average 28-day cylindrical compressive strength of the core concrete was measured to be 37.2MPa, and the 28-day elastic modulus was 33.1GPa; the elastic modulus of the steel pipe was 179GPa, the yield strength was 315MPa, and the Poisson's ratio was 0.265; the loading start age and end age were 29 days and 142 days respectively; the load was 515kN.

[0117] Table 1 Mix ratio of C40 concrete in Example 1 (unit: kg / m 3 )

[0118]

[0119] The creep prediction method of steel-concrete composite columns based on moisture transfer paths in the present invention is used to calculate the basic parameters of the core concrete and steel pipe measured by experiments. The calculation process is as follows:

[0120] (1) Determine the basic performance parameters: the fly ash content of concrete is 8%, and the mineral powder content is 0%; the average 28-day cylindrical compressive strength of the core concrete is 37.2MPa, and the 28-day elastic modulus is 33.1GPa; the elastic modulus of the steel pipe is 179GPa, the yield strength is 315MPa, and the Poisson's ratio is 0.265; the cross-sectional area of ​​the core concrete and the steel pipe is 14264.27mm 2 and 1121.73mm 2 ; The column height of the steel pipe is 400mm; The loading start age and end age are 29 days and 142 days respectively; The load is 515kN; The temperature and humidity of the environment are 22.4 degrees Celsius and 60% relative humidity respectively;

[0121] (2) In terms of moisture transfer, the effective moisture transfer path length of concrete is taken as infinite;

[0122] (3) At the material level, calculate the creep deformation of concrete:

[0123] The correction parameters of mineral admixtures for the effect of fly ash and mineral powder on the creep of core concrete are taken according to the following formula:

[0124]

[0125]

[0126] (4) At the component level, the load holding time step is divided into 100 steps:

[0127]

[0128] (5) At the component level, the stress-strain relationship of the core concrete is established according to the trapezoidal integration rule:

[0129]

[0130]

[0131]

[0132] (6) At the component level, the creep deformation of steel tube confined reinforced concrete columns is calculated according to the force balance equation and deformation coordination equation:

[0133]

[0134] ε ck =ε totk -ε shk

[0135]

[0136] The creep deformation calculated by this prediction method is compared with the experimental results. Fig.10 As shown, the results show that the creep deformation prediction method of the steel-concrete composite column of the present invention is relatively accurate.

[0137] Based on the moisture transfer mechanism that affects concrete creep, this model establishes a creep prediction method suitable for various steel-concrete composite columns such as steel tube concrete columns, steel tube confined reinforced concrete columns and steel-concrete columns. The model considers the influence of steel components (steel tubes, steel sections, etc.) blocking the transfer of internal humidity of concrete to the external environment, which in turn causes the uneven distribution of internal humidity of concrete in the horizontal and vertical directions on creep. At the same time, the physical filling effect of fly ash and mineral powder and the influence of chemical volcanic ash reaction are considered. On this basis, the trapezoidal integral rule, the force balance equation and deformation coordination equation between concrete and steel components are adopted, and the stress redistribution between core concrete and steel components is considered, and then the creep deformation of steel-concrete composite columns is calculated. This method can accurately predict the creep deformation development over time of various steel-concrete composite columns such as steel tube concrete columns, steel tube confined reinforced concrete columns and steel-concrete columns.

[0138] The embodiments of the present invention disclosed above are only used to help illustrate the present invention. The embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementation methods described. According to the content of this specification, many modifications and changes can be made. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and use the present invention well.

Claims

1. A creep prediction method for steel-concrete composite columns based on moisture transfer paths, characterized in that: The specific steps include: (1) Determine the basic performance parameters: fly ash content and mineral powder content of concrete; compressive strength and elastic modulus of concrete; elastic modulus, yield strength, ultimate strength and Poisson's ratio of steel parts; cross-sectional concrete and steel parts area; column height of steel pipe; starting age and ending age of steel-concrete composite columns bearing constant loads, and loads borne; ambient temperature and humidity; (2) In terms of moisture transfer, the length of the concrete moisture transfer path is calculated: considering the effect of the steel components blocking the transfer of moisture from the concrete interior to the external environment, and thus causing the uneven distribution of moisture inside the concrete in the horizontal and vertical directions on creep, the shortest distance from the center point of the steel-concrete composite column to the exposed concrete surface of the component is calculated according to the cross-sectional and vertical dimensions of the component and the dimensions of the steel components. This distance is the effective moisture transfer path length of the steel-concrete composite column. (3) At the material level, calculate the creep deformation of concrete: Considering the physical filling effect of fly ash and mineral powder and the influence of chemical pozzolanic reaction on the creep of concrete, calculate the creep deformation of concrete under constant load according to the fly ash and mineral powder dosages; (4) At the component level, the creep deformation of the steel-concrete composite column is calculated: the influence of creep deformation that develops rapidly in the early stage and slowly in the later stage is considered, and the load holding time is divided into 100 steps; the influence of stress redistribution between concrete and steel components under constant load is considered, and the stress-strain relationship of concrete is established according to the trapezoidal integral rule; the creep deformation of concrete is calculated according to the force equilibrium equation and the deformation coordination equation, i.e., the creep of the steel-concrete composite column, considering that the concrete and steel components bear the vertical load together under constant load and the deformation of the two parts is coordinated.

2. The creep prediction method for steel-concrete composite columns based on moisture transfer paths according to claim 1 is characterized in that: In step (2), the moisture transfer aspect includes the following situations: For steel tube concrete columns, the effective moisture transfer path length is taken as infinity; for steel-concrete columns, the effective moisture transfer path length is taken as half of the length of the cross-section diagonal; for steel tube confined reinforced concrete columns, the effective moisture transfer path length is calculated according to the formula Calculation; For reinforced concrete columns, the effective moisture transfer path is taken as the nominal size in the BS EN 1992 model; Where h0 is the effective moisture transfer path length of the longitudinally partially exposed steel tube confined reinforced concrete column; H is the distance between the steel tube slits of the composite member; A c and u are the area and perimeter of the core concrete, respectively.

3. The creep prediction method for steel-concrete composite columns based on moisture transfer paths according to claim 2 is characterized in that: Step (3) at the material level includes the following formula: Where t0 is the time when the constant load begins to be applied; t is any time during the process of constant load application; It is the creep coefficient of concrete from time t0 when it is subjected to constant load to time t, that is, the ratio of the creep deformation of concrete at time t to the instantaneous deformation of concrete at the end of loading; is the nominal creep coefficient of concrete, calculated according to the BS EN 1992 model; β H It is the influence coefficient of ambient humidity and specimen size on creep development trend, calculated according to BS EN 1992 model; k1 and k2 are the parameters of the influence of fly ash and mineral powder on the final value and development trend of concrete creep, respectively.

4. The creep prediction method for steel-concrete composite columns based on moisture transfer path length according to claim 3 is characterized in that: In step (4), at the component level, the creep deformation of the steel-concrete composite column is calculated by considering the influence of the rapid development of creep deformation in the early stage and the slow development in the later stage. The holding time is divided into 100 steps, and the formula is used: calculate; Where, t end is the end time of the load bearing of the steel-concrete composite column; Δt k is any time t k With the previous moment t k-1 The time step length between i is any time t i With the previous moment t i-1 The length of the time step between them; k is the time step number.

5. The creep prediction method for steel-concrete composite columns based on moisture transfer paths according to claim 3 is characterized in that: In step (4), at the component level, the creep deformation of the steel-concrete composite column is calculated using the following formula: Where, t k-1 With t k are the start and end time of the kth step respectively; σ ck and σ cj They are t k With t j The stress of concrete at the moment; ε totk and ε shk They are t k The total deformation and shrinkage of concrete at time M c1k and M c2kj They are creep influence coefficient; J(t k ,t j ) is the creep function and creep coefficient of concrete subjected to constant load from time tj to time tk. Formula (1) and formula (2) are substituted into the BS EN 1992 model for calculation, taking into account the influence of the steel components blocking the transfer of internal moisture of the concrete to the external environment, which in turn causes the uneven distribution of internal moisture of the concrete in the horizontal and vertical directions on the creep, and the influence of the physical filling effect of fly ash and mineral powder and the chemical pozzolanic reaction on the creep of concrete.

6. The creep prediction method for steel-concrete composite columns based on moisture transfer paths according to claim 1 is characterized in that: In step (4), at the component level, the creep deformation of the steel-concrete composite column is calculated using the following formula: e ck =e totk -e shk (8) Where N L It is the load that is applied after the construction of the composite column is completed; A c is the cross-sectional area of ​​the concrete; E s and A s are the elastic modulus and cross-sectional area of ​​the steel component respectively; σ cj Yes j The stress of concrete at the moment; ε totk and ε ck They are t k The total deformation and creep deformation of concrete at this moment; ε shk Yes k The shrinkage deformation of concrete at each moment is calculated according to the BS EN 1992 model.

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