Dual-time-variable Diffusion Analysis Method of Chloride in Concrete Based on ANSYS APDL

Abstract

The present invention provides a method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL, which includes the following steps: Step S1, enter the thermal analysis module and set the element type; Step S2, establish a two-dimensional array according to the time-varying function relationship of the diffusion coefficient, with one column being the load step and serving as the material property number, and the other column being the corresponding time-varying diffusion coefficient, and define the time-varying diffusion coefficient by cycling according to the material property number; Step S3, cycle to define the time-varying surface chloride ion concentration of the concrete structural member as the time-varying boundary condition; Step S4, set the geometric model parameters and non-time-varying material properties of the concrete structural member, and construct the geometric model; Step S5, perform mesh division; Step S6, set the analysis type and solution method; Step S7, set the initial chloride ion concentration value in the concrete member, cycle to load the time-varying boundary condition according to the load step, and change the material property of the element between the load steps, and cycle through continuous load steps for calculation and solution; Step S8, post-process the calculation results.

Description

Technical Field

[0001] The present invention relates to a method for analyzing chloride ion diffusion in concrete, and more particularly to a method for analyzing the two-time-variable diffusion of chloride salts in concrete based on ANSYS APDL. Background Art

[0002] The durability of concrete structures has always been a hot issue widely concerned in the field of civil engineering. The reasons affecting the durability of concrete are complex and intricate, and the main factors include freeze-thaw cycles, alkali-aggregate reactions, and steel corrosion caused by carbonation and chloride salts. Among them, for concrete structures located in non-cold regions, the destructive effect of freeze-thaw cycles on concrete can be ignored; the alkali-aggregate reaction can be prevented by adding admixtures that inhibit the alkali-aggregate reaction; carbonation has a certain impact on the durability of concrete but can be controlled by strengthening construction quality. However, the steel corrosion caused by chloride ion erosion is the main reason for the deterioration of the durability of reinforced concrete structures. Therefore, establishing an appropriate simulation method to predict the distribution of chloride ion concentration in concrete structural members has important theoretical significance and engineering application value for the durability design and evaluation of reinforced concrete structures.

[0003] The transport of external chloride ions into concrete is a complex physico-chemical process involving many mechanisms, mainly diffusion, capillary action, seepage action, and electromigration, and is also affected by the binding of free chloride ions by the cementitious materials in concrete. The intrusion of chloride ions into the interior of concrete is a combination of the above several actions, and under certain specific environmental conditions, a certain transport mode will be dominant. For concrete structures in the coastal environment, in the specified thickness of the protective layer, the water content of the concrete may be very high, and in most cases, the transport mechanism of chloride ion erosion of concrete is mainly diffusion. Therefore, most of the research on the intrusion of chloride ions into concrete is based on this transport mode.

[0004] At present, the analytical solutions of Fick's second law and its one-dimensional diffusion mathematical model are mostly used to predict the chloride ion concentration in concrete and evaluate its durability. The above-mentioned Fick diffusion law assumes that the diffusion coefficient is a constant value, that is, it describes a steady-state diffusion process. In fact, the diffusion coefficient of chloride ions in concrete is time-varying. It is reflected that the longer the age of concrete, the smaller the chloride ion diffusion coefficient. Especially in the first few years after the concrete is exposed, the decline rate of the chloride ion diffusion rate is particularly obvious. Therefore, the time-variability of the chloride ion diffusion coefficient must be considered in the chloride ion diffusion analysis. Moreover, the analytical solutions of conventional diffusion prediction models are all based on a constant concentration surface. In fact, when concrete is exposed to a chloride salt environment, the chloride ions in the environment do not act on the concrete surface step by step with the final concentration value directly, but accumulate gradually over time. Therefore, it is obvious that the analytical solutions of Fick's second law and its one-dimensional diffusion mathematical model are currently used more to predict the chloride ion concentration distribution and durability evaluation in concrete, which is quite different from the actual situation of chloride salt diffusion in concrete.

[0005] In addition, it is difficult for the analytical solutions of Fick's second law and its one-dimensional diffusion mathematical model to accurately predict the chloride ion diffusion distribution at different times and positions in concrete, especially in areas with complex geometric structures such as the corners of components. The finite element method can adapt to the solution of problems with complex geometric shapes and boundary conditions. However, due to the complexity of the geometric model, material properties and boundary conditions of actual engineering concrete structural members, if the time-varying effects are considered at the same time, the finite element programs written by individuals often consider insufficiently comprehensively, have low computing efficiency, the correctness of the programs needs to be verified, and the post-processing function is also relatively weak. If a general finite element software can be used to simulate the time-varying diffusion problem of chloride ions, it is undoubtedly an important analysis method for evaluating the durability of concrete structures.

[0006] At present, some scholars have proposed methods for simulating the chloride ion diffusion in concrete based on the general finite element software ANSYS, but none of them can consider the double time-variability of the diffusion coefficient and the boundary conditions at the same time. There are large differences between the analysis results obtained by these existing simulation methods and the actual chloride ion diffusion distribution. Summary of the Invention

[0007] The present invention is proposed to solve the above problems, and the purpose is to provide a method for analyzing the double time-varying diffusion of chloride salts in concrete based on ANSYS APDL.

[0008] The present invention provides a method for analyzing the double time-varying diffusion of chloride salts in concrete based on ANSYS APDL, which analyzes the chloride ion diffusion in concrete structural members through the finite element analysis software ANSYS, including the following steps:

[0009] Step S1: Simulate the two-time-variable diffusion of chloride ions in the concrete structural member based on the thermal analysis module of ANSYS. Enter the thermal analysis module and set the element type for analysis.

[0010] Step S2: Establish a two-dimensional array according to the time-variable function relationship of the diffusion coefficient. In this two-dimensional array, one column is the load step and serves as the material property number, and the other column is the time-variable diffusion coefficient in the material property corresponding to the load step. Define the time-variable diffusion coefficient by cycling according to the material property number.

[0011] Step S3: Define the time-variable surface chloride concentration of the time-variable concrete structural member by cycling according to the load step as the time-variable boundary condition.

[0012] Step S4: Set the geometric model parameters of the concrete structural member, and set the material properties other than the time-variable diffusion coefficient and the time-variable surface chloride concentration, and construct the geometric model of the concrete structural member.

[0013] Step S5: Mesh the geometric model of the concrete structural member.

[0014] Step S6: Set the analysis type and solution method.

[0015] Step S7: Set the initial chloride concentration value in the concrete structural member, cycle and load the time-variable boundary condition according to the load step, and change the material properties of the element between load steps, and perform calculation and solution by cycling through continuous load steps.

[0016] Step S8: Post-process the calculation results to obtain the chloride concentration value that changes with time at any position in the concrete structural member under the given parameters.

[0017] In the method for analyzing the two-time-variable diffusion of concrete chloride salts based on ANSYS APDL provided by the present invention, it may also have the following characteristics: Among them, in step S7, the material properties of the element are changed between load steps through the MPCHG command of the ANSYS APDL program; or after meshing the geometric model in step S5, the EGEN command of the ANSYS APDL program is used to copy and generate units that are multiples of the total number of load steps, and the corresponding command parameter MINC is taken as 1, specifying that the increment of the material property number of each newly generated unit is 1. At this time, the specific operation of step S7 is: set the initial chloride concentration value in the concrete structural member, cycle and load the time-variable boundary condition according to the load step, only activate the units with the material properties corresponding to the corresponding load step through the element birth and death commands EALIVE and EKILL of the ANSYS APDL program, and then perform calculation and solution by cycling through continuous load steps.

[0018] In the method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL provided by the present invention, it may also have the following characteristics: Among them, the specific process of simulating the time-varying diffusion of chloride ions in the concrete structural member through the thermal analysis module is as follows:

[0019] In the thermal analysis module, the three-dimensional heat conduction differential equation under unsteady conditions is:

[0020]

[0021] In formula (1), k x , k y and k z respectively represent the thermal conductivity in the x, y, and z directions, represents the heat generation per unit volume, ρ and c respectively represent the density and specific heat capacity of the medium, and T represents the temperature.

[0022] Substitute the temperature T in formula (1) with the concentration C, and let k x = k y = k z = D(t), ρ = c = 1, to simulate the time-varying diffusion of chloride ions through the thermal analysis module, and obtain the control differential equation for the time-varying diffusion of chloride ions under three-dimensional unsteady conditions as follows:

[0023]

[0024] In formula (2), D(t) is the time-varying diffusion coefficient, and C is the concentration.

[0025] In the method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL provided by the present invention, it may also have the following characteristics: Among them, in step S1, when applying the thermal analysis module for three-dimensional diffusion analysis, set the element type of the thermal analysis module to SOLID90 element, and the SOLID90 element is a 20-node hexahedral thermal solid element; or in step S1, when applying the thermal analysis module for two-dimensional diffusion analysis, set the element type of the thermal analysis module to PLANE35 element or PLANE77 element, the PLANE35 element is a 6-node triangular thermal plane element, and the PLANE77 element is an 8-node quadrilateral thermal plane element.

[0026] In the method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL provided by the present invention, it may also have the following characteristics: Among them, step S2 includes the following sub-steps:

[0027] Step S2-1, establish a two-dimensional array using the time-varying function relationship of the diffusion coefficient, and the time-varying function relationship of the diffusion coefficient is as shown in formula (3):

[0028]

[0029] Step S2-2: Calculate the time-varying diffusion coefficient according to formula (4), and define the time-varying diffusion coefficient corresponding to the load step in a loop according to the material property number. Formula (4) is as follows:

[0030]

[0031] In formulas (3) and (4), t0 is the reference exposure age of concrete, D0 is the chloride ion diffusion coefficient of concrete at the reference age t0, m is the time-dependent constant, t max is the maximum influence time considering the attenuation of the diffusion coefficient, and Δ is the load step size.

[0032] In the concrete chloride double time-varying diffusion analysis method based on ANSYS APDL provided by the present invention, it may also have the following characteristics: Among them, in step S3, the specific process of defining the time-varying surface chloride ion concentration in a loop according to the load step is as follows:

[0033] Obtain the functional relationship between the surface chloride ion concentration and time by fitting the measured results of the surface chloride ion concentration of the actual concrete structural member, and establish an array vector to obtain the time-varying surface chloride ion concentration corresponding to the load step.

[0034] In the concrete chloride double time-varying diffusion analysis method based on ANSYS APDL provided by the present invention, it may also have the following characteristics: Among them, in step S5, when performing mesh division, the mesh elements within the range from the exposed surface of the concrete structural member to the concrete protective layer or to the outer layer of steel bar surface are encrypted, and the mesh element sizes in other regions are gradually enlarged.

[0035] In the concrete chloride double time-varying diffusion analysis method based on ANSYS APDL provided by the present invention, it may also have the following characteristics: Among them, in step S6, the analysis type is set to transient analysis, and the solution method is set to adopt the complete N-R nonlinear solution algorithm.

[0036] In the concrete chloride double time-varying diffusion analysis method based on ANSYS APDL provided by the present invention, it may also have the following characteristics: Among them, in step S7, the initial chloride ion concentration value in the concrete structural member is set through the TUNIF command of the ANSYS APDL program.

[0037] Functions and effects of the invention

[0038] According to the method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL involved in the present invention, the double-time-varying nature of chloride ion accumulation on the concrete surface and the diffusion rate is considered simultaneously during the diffusion analysis process. Therefore, it can more accurately predict the distribution of chloride ion concentration in concrete structural members. In addition, the method of the present invention is based on the general finite element software ANSYS. By using the thermal analysis module of ANSYS and writing an APDL program to simulate the double-time-varying diffusion of chloride ions in concrete structural members, it can adapt to complex geometric structure models, greatly improving the solution efficiency compared to finite element programs written personally, and having a more powerful post-processing function and universality. Therefore, the simulation results are more real and reliable. Description of the Drawings

[0039] Figure 1 is the flow chart of the method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL in the embodiment of the present invention;

[0040] Figure 2 is the contour map of chloride ion content when the water-binder ratio of the concrete member in the test example of the present invention is 0.45;

[0041] Figure 3 is the comparison chart of chloride ion content distribution results when the water-binder ratio of the concrete member in the test example of the present invention is 0.45;

[0042] Figure 4 is the contour map of chloride ion content when the water-binder ratio of the concrete member in the test example of the present invention is 0.55;

[0043] Figure 5 is the comparison chart of chloride ion content distribution results when the water-binder ratio of the concrete member in the test example of the present invention is 0.55;

[0044] Figure 6 is the contour map of chloride ion content when the water-binder ratio of the concrete member in the test example of the present invention is 0.65;

[0045] Figure 7 is the comparison chart of chloride ion content distribution results when the water-binder ratio of the concrete member in the test example of the present invention is 0.65. Detailed Embodiments

[0046] In order to make the technical means, innovative features, achieved purposes and effects realized by the present invention easy to understand, the following embodiments will specifically elaborate on the method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL of the present invention in conjunction with the drawings.

[0047] <Embodiment>

[0048] Figure 1It is the flow chart of the concrete chloride double-time-varying diffusion analysis method based on ANSYS APDL in the embodiments of the present invention.

[0049] As Figure 1 shown, the concrete chloride double-time-varying diffusion analysis method based on ANSYS APDL (ANSYS Parametric Design Language) in this embodiment analyzes the double-time-varying diffusion of chloride ions in concrete structural members through the thermal analysis module of the finite element analysis software ANSYS and by writing an APDL program, including the following steps:

[0050] Step S1, simulate the double-time-varying diffusion of chloride ions in the concrete structural member based on the thermal analysis module of ANSYS, enter the thermal analysis (or multi-physics field) module of ANSYS and set the element type for analysis.

[0051] The specific process of simulating the time-varying diffusion of chloride ions in the concrete structural member through the thermal analysis module of ANSYS is as follows:

[0052] ANSYS itself does not have a diffusion analysis module, but it can be solved by means of a heat conduction problem similar in form to the time-varying diffusion control differential equation of chloride ions. In the thermal analysis module, according to the law of conservation of energy, the three-dimensional unsteady-state heat conduction differential equation is:

[0053]

[0054] In formula (1), k x , k y and k z respectively represent the thermal conductivity coefficients in the x, y, and z directions, represents the heat generation per unit volume, ρ and c respectively represent the density and specific heat capacity of the medium, and T represents the temperature.

[0055] Substitute the temperature T in formula (1) with the concentration C, and let k x = k y = k z = D(t), ρ = c = 1, to simulate the time-varying diffusion of chloride ions through the thermal analysis module, and obtain the time-varying diffusion control differential equation of chloride ions under three-dimensional unsteady-state conditions as follows:

[0056]

[0057] In formula (2), D(t) is the time-varying diffusion coefficient and C is the concentration.

[0058] In step S1, when performing three-dimensional diffusion analysis using the thermal analysis module, set the element type of the thermal analysis module to SOLID90 element, and the SOLID90 element is a 20-node hexahedral thermal solid element; or

[0059] In step S1, when performing two-dimensional diffusion analysis using the thermal analysis module, set the element type of the thermal analysis module to PLANE35 element or PLANE77 element. The PLANE35 element is a 6-node triangular thermal plane element, and the PLANE77 element is an 8-node quadrilateral thermal plane element.

[0060] In step S2, establish a two-dimensional array according to the time-varying function relationship of the diffusion coefficient. One column in this two-dimensional array is the load step and serves as the material property number, and the other column is the time-varying diffusion coefficient in the material property corresponding to the load step. Define the time-varying diffusion coefficient in a loop according to the material property number.

[0061] Step S2 includes the following sub-steps:

[0062] In step S2-1, establish a two-dimensional array using the time-varying function relationship of the diffusion coefficient. The time-varying function relationship of the diffusion coefficient is as shown in formula (3):

[0063]

[0064] In step S2-2, calculate the time-varying diffusion coefficient according to formula (4), and define the time-varying diffusion coefficient corresponding to the load step in a loop according to the material property number. Formula (4) is as follows:

[0065]

[0066] In formula (3) and formula (4), t0 is the reference age of concrete exposure, D0 is the chloride ion diffusion coefficient of concrete at the reference age t0, m is the time-dependent constant, t max is the maximum influence time considering the attenuation of the diffusion coefficient, and Δ is the load step size.

[0067] In step S3, define the time-varying surface chloride ion concentration of the time-varying concrete structural member in a loop as the time-varying boundary condition.

[0068] In step S3, the specific process of defining the time-varying surface chloride ion concentration in a loop is as follows:

[0069] According to the measured results of the surface chloride ion concentration of the actual concrete structural member, obtain the function relationship of the surface chloride ion concentration varying with time through fitting and establish an array vector, or directly refer to the function relationship of the concrete surface concentration growth model proposed in the relevant literature to establish an array vector, and obtain the time-varying surface chloride ion concentration corresponding to the load step.

[0070] Step S4: Set the geometric model parameters of the concrete structural member, and set other non-time-varying material properties except for the time-varying diffusion coefficient and the time-varying surface chloride concentration, and construct the geometric model of the concrete structural member.

[0071] Step S5: Perform mesh division on the geometric model of the concrete structural member.

[0072] When performing mesh division in Step S5, the mesh elements within the range from the exposed surface of the concrete structural member to at least the concrete cover or to the surface of the outer layer of steel bars are refined, and the mesh element sizes in other areas are gradually enlarged.

[0073] Step S6: Set the analysis type and solution method.

[0074] In Step S6, to be consistent with the transient heat transfer process, the analysis type is set to transient analysis, and since the analysis is a nonlinear heat analysis for only a single physical field, the solution method is set to use the full N-R nonlinear solution algorithm.

[0075] Step S7: Set the initial chloride concentration value in the concrete structural member, cyclically apply the time-varying boundary conditions in load steps, and change the material properties of the elements between load steps through the MPCHG command of the ANSYS APDL program, and perform calculation and solution for continuous load steps in a loop.

[0076] In Step S7, set the initial chloride concentration value in the concrete structural member through the TUNIF command of the ANSYS APDL program.

[0077] Step S8: Post-process the calculation results to obtain the chloride concentration values varying with time at any position within the concrete structural member under given parameters.

[0078] The above steps are the variable-parameter continuous load step analysis method. In this embodiment, an overlapping - birth and death element continuous load step analysis method and its steps are also proposed. The overlapping elements refer to generating multiple independent co-node elements between the same nodes, and different material properties can be assigned to each co-node element respectively. At the same time, the element birth and death function is used to simulate the time-varying change process of the material, so as to achieve the purpose of the time-varying characteristics of the material with time inheritance.

[0079] When using the overlapping - birth and death element continuous load step analysis method, after meshing the geometric model in step S5, the EGEN command of the ANSYS APDL program is used to copy and generate elements that are multiples of the total number of load steps. The corresponding command parameter MINC is taken as 1, specifying that the increment of the material property number of each newly generated element is 1. At this time, the specific operation of step S7 is as follows: set the initial chloride ion concentration value in the concrete structural member, apply the time - varying boundary conditions in a load step cycle, and through the element birth and death commands EALIVE and EKILL of the ANSYS APDL program, only activate the elements with material properties corresponding to the respective load steps, and then perform the calculation and solution through the cyclic continuous load steps. The remaining steps are the same as those of the aforementioned continuous load step analysis method with variable parameters.

[0080] In this embodiment, when the mesh volume of the finite element model is large and the number of load steps is large, using the overlapping - birth and death element continuous load step solution method will consume a large amount of computing and storage resources. The purpose of this embodiment in proposing the overlapping - birth and death element solution method is, firstly, to propose another numerical simulation method suitable for simulating the double - time - varying diffusion of chloride ions in concrete structural members in ANSYS APDL; secondly, it is to verify the time inheritance of the solution obtained by the continuous load step analysis method with variable parameters. In addition, the above two finite element numerical simulation methods are not only applicable to the analysis of double - time - varying diffusion problems of three - dimensional solid elements, but also applicable to the analysis of simplified two - dimensional diffusion problems.

[0081] <Test Example>

[0082] In this test example, by using the concrete chloride double-time-varying diffusion analysis method based on ANSYS APDL of the present invention (the variable-parameter continuous load step analysis method and the overlapping-live-dead element continuous load step analysis method), three-dimensional solid and two-dimensional plane finite element models are respectively established to conduct chloride double-time-varying diffusion analysis on the test concrete specimens. The size of the specimen model is 0.5m×0.5m (×0.5m). The element types are respectively the 20-node hexahedral thermal solid element SOLID90 element and the 8-node quadrilateral thermal plane element PLANE77 element. The mesh size is uniformly taken as 5mm, and the calculation step length is 1a. The chloride ion diffusion situation of the specimen after 5a of exposure in the chloride salt environment is simulated. At the same time, the analysis results of the present invention are compared with the actual test data of the test specimens exposed in the bay tidal zone for 5a and the analytical results using the Petcherdchoo correction model (Petcherdchoo A. Time dependent models of apparent diffusion coefficient and surface chloride for chloride transport in fly ash concrete[J]. Construction and Building Materials, 2013, 38: 497-507.) to verify the effectiveness of the method of the present invention.

[0083] Figure 2 It is the chloride ion content distribution nephogram of the concrete member with a water-binder ratio of 0.45 in the test example of the present invention; Figure 3 It is the comparison chart of the chloride ion content distribution results of the concrete member with a water-binder ratio of 0.45 in the test example of the present invention; Figure 4 It is the chloride ion content distribution nephogram of the concrete member with a water-binder ratio of 0.55 in the test example of the present invention; Figure 5 It is the comparison chart of the chloride ion content distribution results of the concrete member with a water-binder ratio of 0.55 in the test example of the present invention; Figure 6 It is the chloride ion content distribution nephogram of the concrete member with a water-binder ratio of 0.65 in the test example of the present invention; Figure 7 It is the comparison chart of the chloride ion content distribution results of the concrete member with a water-binder ratio of 0.65 in the test example of the present invention.

[0084] Figure 2 In (a), (b), and (c), they are respectively the chloride ion content distribution nephograms of the concrete member with a water-binder ratio of 0.45 when the fly ash content is 0%, 25%, and 50%.

[0085] Figure 4Among them, (a), (b), and (c) are respectively the contour maps of chloride ion content when the fly ash content in the concrete members with a water-binder ratio of 0.55 is 0%, 25%, and 50%.

[0086] Figure 6 Among them, (a), (b), and (c) are respectively the contour maps of chloride ion content when the fly ash content in the concrete members with a water-binder ratio of 0.65 is 0%, 25%, and 50%.

[0087] As Figure 2 、 Figure 4 and Figure 6 shown, through the concrete chloride salt two-time-variable diffusion analysis method based on ANSYS APDL of the present invention, the chloride ion diffusion process of concrete structural members with different material mix ratios can be successfully simulated and analyzed. According to the comparison and analysis of the contour maps of chloride ion content obtained by post-processing, it can be concluded that with the increase of fly ash content in the concrete structural members, the diffusion of chloride ions into the concrete can be effectively inhibited.

[0088] As Figure 3 、 Figure 5 and Figure 7 shown, by comparing the analytical results of the Petcherdchoo correction model with the actual test data, it can be seen that under different material mix ratios, the two sets of data can fit well, indicating that the calculation formula and assumptions adopted by the Petcherdchoo correction model conform to the test rules. And the chloride ion concentration distribution calculated by the concrete chloride salt two-time-variable diffusion analysis method based on ANSYS APDL proposed by the present invention is very close to the analytical results of the Petcherdchoo correction model, and the correlation coefficient is above 0.99, indicating that the method proposed by the present invention effectively simulates the diffusion process. By writing the ANSYS APDL program for the two-time-variable analysis of boundary conditions and diffusion coefficients, it can truly reflect the actual law of chloride ion diffusion in concrete over time.

[0089] In addition, according to the calculation results of this test example, the calculation results of the variable-parameter continuous load step analysis method are exactly the same as those of the overlapping-live-dead element continuous load step solution method. Therefore, the calculation results of the two simulation methods proposed by the present invention are represented by the same result curve in Figure 3 、 Figure 5 and Figure 7 , which also verifies that the calculation results of the variable-parameter continuous load step analysis method have time inheritance.

[0090] Functions and effects of the embodiment

[0091] ​According to the ANSYS-based concrete chloride double-time-varying diffusion analysis method involved in this embodiment, the double-time-varying properties of chloride ion accumulation and diffusion rate on the concrete surface are considered simultaneously during the diffusion analysis process. Therefore, it can more accurately predict the distribution of chloride ion concentration in concrete structural members. Furthermore, the method of this embodiment is completed based on the finite element software ANSYS. By using the thermal analysis module of ANSYS and writing the APDL program to simulate the double-time-varying diffusion of chloride ions in concrete structural members, it can adapt to complex geometric structure models. Compared with the finite element program written by individuals, the solution efficiency is greatly improved, and the post-processing function is more powerful, with universality. Therefore, the simulation results are more real and reliable.

Claims

1. A method for analyzing the two-time-variable diffusion of chloride salts in concrete based on ANSYS APDL, which analyzes the chloride ion diffusion in concrete structural members through the finite element analysis software ANSYS, is characterized in that Including the following steps: Step S1: Simulate the two-time-variable diffusion of chloride ions in the concrete structural member based on the thermal analysis module of ANSYS, enter the thermal analysis module and set the element type for analysis; Step S2: Establish a two-dimensional array according to the time-variable function relationship of the diffusion coefficient. One column in this two-dimensional array is the load step and serves as the material property number, and the other column is the time-variable diffusion coefficient in the material property corresponding to the load step. Define the time-variable diffusion coefficient by cycling according to the material property number; Step S3: Define the time-variable surface chloride concentration of the time-variable concrete structural member by cycling according to the load step as the time-variable boundary condition; Step S4: Set the geometric model parameters of the concrete structural member, and set the material properties other than the time-variable diffusion coefficient and the time-variable surface chloride concentration, and construct the geometric model of the concrete structural member; Step S5: Mesh the geometric model of the concrete structural member; Step S6: Set the analysis type and solution method; Step S7: Set the initial chloride concentration value in the concrete structural member, load the time-variable boundary condition by cycling according to the load step, and change the material property of the element between the load steps, and cycle continuously through the load steps for calculation and solution; Step S8: Post-process the calculation results to obtain the chloride concentration value varying with time at any position in the concrete structural member under given parameters.

2. The method for analyzing the two-time-variable diffusion of concrete chloride salts based on ANSYS APDL according to claim 1, wherein: Among them, In step S7, change the material property of the element between the load steps through the MPCHG command of the ANSYS APDL program; or After meshing the geometric model in step S5, use the EGEN command of the ANSYS APDL program to copy and generate elements that are multiples of the total number of load steps. The corresponding command parameter MINC is taken as 1, and the increment of the material property number of each newly generated element is specified as 1. At this time, the specific operation of step S7 is: set the initial chloride concentration value in the concrete structural member, load the time-variable boundary condition by cycling according to the load step, use the element birth and death commands EALIVE and EKILL of the ANSYS APDL program to only activate the elements with the material properties corresponding to the respective load steps, and then cycle continuously through the load steps for calculation and solution.

3. The method for analyzing the two-time-variable diffusion of concrete chloride salts based on ANSYS APDL according to claim 1, wherein: Among them, The specific process of simulating the time-variable diffusion of chloride ions in the concrete structural member through the thermal analysis module is as follows: In the thermal analysis module, the three-dimensional heat conduction differential equation under unsteady conditions is: In formula (1), k x , k y and k z represent the thermal conductivities in the x, y, and z directions respectively, represents the heat generation per unit volume, ρ and c represent the density and specific heat capacity of the medium respectively, and T represents the temperature. Substitute the temperature T in formula (1) with the concentration C, and let k x = k y = k z = D(t), ρ = c = 1, and simulate the time-varying diffusion of chloride ions through the thermal analysis module to obtain the following differential control equation for the time-varying diffusion of chloride ions under three-dimensional unsteady conditions: In formula (2), D(t) is the time-variable diffusion coefficient, and C is the concentration.

4. The method for analyzing the two-time-variable diffusion of chloride salts in concrete based on ANSYS APDL according to claim 1, It is characterized in that: Among them, in step S1, when applying the thermal analysis module for three-dimensional diffusion analysis, set the element type of the thermal analysis module as the SOLID90 element, and the SOLID90 element is a 20-node hexahedral thermal solid element; Or In step S1, when performing two-dimensional diffusion analysis using the thermal analysis module, set the element type of the thermal analysis module to PLANE35 element or PLANE77 element. The PLANE35 element is a 6-node triangular thermal plane element, and the PLANE77 element is an 8-node quadrilateral thermal plane element.

5. The method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL according to claim 1, wherein: Among them, Step S2 includes the following sub-steps: Step S2-1, establish the two-dimensional array using the time-varying function relationship of the diffusion coefficient, and the time-varying function relationship of the diffusion coefficient is as shown in formula (3): Step S2-2, calculate the time-varying diffusion coefficient according to formula (4), and cyclically define the time-varying diffusion coefficient corresponding to the load step according to the material property number. Formula (4) is as follows: In Formulas (3) and (4), t0 is the reference age of concrete exposure, D0 is the chloride ion diffusion coefficient of concrete at the reference age t0, m is the time-dependent constant, and t max is the maximum influence time considering the attenuation of the diffusion coefficient, and Δ is the load step size.

6. The method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL according to claim 1, wherein: Among them, In step S3, the specific process of cyclically defining the time-varying surface chloride ion concentration according to the load step is as follows: Based on the measured results of the surface chloride ion concentration of the actual concrete structural member, obtain the function relationship between the surface chloride ion concentration and time through fitting and establish an array vector to obtain the time-varying surface chloride ion concentration corresponding to the load step.

7. The method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL according to claim 1, wherein: Among them, In step S5, when performing mesh division, encrypt the mesh elements within the range from the exposed surface of the concrete structural member to the concrete cover layer or to the outer layer of steel bar surface, and gradually increase the mesh element size in other areas.

8. The method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL according to claim 1, wherein: Among them, In step S6, set the analysis type to transient analysis, and set the solution method to adopt the full N-R nonlinear solution algorithm.

9. The method for analyzing the double-time-varying diffusion of chloride salts in concrete based on ANSYS APDL according to claim 1, wherein: Among them, In step S7, set the initial chloride ion concentration value in the concrete structural member through the TUNIF command of the ANSYS APDL program.

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