Prediction model of chloride ion diffusion coefficient of recycled aggregate concrete based on random fractal characteristics of pore structure and construction method and application thereof

Abstract

The application discloses a recycled concrete chloride ion diffusion coefficient prediction model based on pore structure random fractal characteristics and a construction method and application thereof, relates to the technical field of a recycled concrete chloride ion effective diffusion coefficient prediction model construction, and provides the recycled concrete chloride ion diffusion coefficient prediction model, studies transmission channels influencing recycled concrete chloride ion transmission on a mesoscopic scale, and constructs the above model on a macroscopic scale based on the mesoscopic transmission channels. The model analyzes the recycled concrete chloride ion diffusion performance on multiple scales from the mesoscopic scale to the macroscopic scale, can more accurately predict the chloride ion effective diffusion coefficient according to pore structure related data of a cement-based material, and provides reference and guidance for durability research of the recycled concrete.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of concrete chloride ion effective diffusion coefficient prediction model construction, in particular to a recycled concrete chloride ion diffusion coefficient prediction model based on pore structure random fractal characteristics and a construction method thereof. BACKGROUND

[0002] With the acceleration of urbanization process in China, the urban construction and reconstruction speed up, and the construction waste increases dramatically, which causes environmental pollution and garbage surrounding city problems increasingly prominent. The resource utilization of construction waste is an effective measure to solve the environmental and resource problems. Recycled concrete is a new green concrete prepared by crushing construction waste into recycled aggregate, partially or completely replacing natural aggregate, which is the main direction of construction waste resource utilization technology research and promotion at present, and can solve the environmental crisis of construction waste and the resource crisis of natural aggregate.

[0003] During the service of concrete structure, chloride salt erosion is an important factor determining the durability of the structure. Chloride ion Cl - exists in the form of chemical, physical combination and free state in concrete, and constantly migrates, which deteriorates the interface structure of concrete, causes steel corrosion, and further damages the overall structure of reinforced concrete, continuously reduces the strength and durability of concrete. Chloride ion diffusion coefficient is a physical quantity describing the migration of chloride ions in concrete, which is related to the concentration gradient between the two sides of the medium, the total amount of penetrating substances, penetration distance, penetration area and time, etc. How to evaluate the chloride ion diffusion process in concrete and construct the prediction model of chloride ion diffusion coefficient is crucial in concrete technology research and development and actual production.

[0004] Pore structure is the main channel for water and eroded ions to enter the interior of concrete. Concrete contains a porous structure, and the internal pore structure of this porous material is complex, with irregular pore distribution and great randomness. It is necessary to describe the pore structure by a reasonable method. At present, in the aspect of chloride ion transmission, the main method for processing pore structure is to assume the pore structure as a circular pipe model, and to establish a medium transmission channel by assigning different tortuosity to the circular pipe model. This kind of model has applicability within a certain range and is simple to calculate. However, when applied to recycled concrete, the main problem is the complex distribution of pore structure space, and the roughness of pore wall will affect the medium transmission performance. Considering only the tortuosity of the pore channel cannot accurately represent the chloride ion diffusion process of recycled concrete.

[0005] In the existing construction methods of concrete chloride ion diffusion coefficient prediction models, for example, the concrete meso chloride ion diffusion coefficient prediction method provided in Chinese patent application CN110568165A, the micro-pore structure information in the concrete is first obtained, and then the pore rate and pore size distribution of the cement paste are determined according to the pore structure information; then the aggregate volume distribution model is determined by combining the pore rate and pore size distribution through micro-imaging technology, and then the two-dimensional meso structure of the concrete is constructed; then the ion concentration on the surface of the concrete is determined according to the two-dimensional meso structure; finally, a multi-ion transport model based on the Nernst-Planck / Poisson equation set is constructed according to the ion concentration, ion flux, ion apparent diffusion coefficient and ion charge, and then the chloride ion transport concentration distribution in the concrete and the chloride ion diffusion coefficient in the concrete are calculated in turn. However, the technical idea disclosed in the patent application is limited to studying the chloride ion diffusion process of the concrete on the two-dimensional meso structure, and there is a problem that no effective parameters are proposed to establish the connection between the meso and macro scales. The patent cannot solve the chloride ion transport problem of recycled concrete which has a more complex meso structure than concrete. SUMMARY

[0006] The chloride ion diffusion performance of the concrete on the macro scale is caused by the complex characteristics of the material on the meso scale. Establishing the connection between different scales not only can reveal the chloride ion diffusion performance of the recycled concrete from the nature, but also can make the development of the chloride ion diffusion theory get rid of the bondage of experience. The "scale-free" and "self-similarity" of the fractal theory provide the theoretical possibility for establishing the connection and quantitative description between different scales, and open up a new idea for solving the chloride ion diffusion problem on multiple scales. Therefore, based on the pore structure random fractal theory, the present application provides a recycled concrete chloride ion diffusion coefficient prediction model and a construction method thereof, and proposes a chloride ion diffusion coefficient of the recycled concrete, which has important significance for perfecting the chloride ion transport and durability theory of the recycled concrete. The specific implementation is as follows.

[0007] The construction method of the recycled concrete chloride ion diffusion coefficient prediction model based on the pore structure random fractal characteristics is based on the random fractal theory, and starts from the meso scale of the recycled concrete. First, a three-dimensional pore volume fractal model and a two-dimensional pore channel tortuosity fractal model are established, and then an effective chloride ion diffusion coefficient model is established based on the pore structure fractal characteristics. Finally, the recycled concrete chloride ion diffusion coefficient prediction model under the saturation condition is established.

[0008] Preferably, the construction method comprises the following steps:

[0009] S1, based on the principle of Menger sponge model, a three-dimensional pore volume fractal model in the recycled concrete is established;

[0010] S2, based on the principle of Von Koch model, a two-dimensional pore tortuosity fractal model in recycled concrete is established;

[0011] S3, an effective chloride ion diffusion coefficient model based on the fractal characteristics of pore structure is established;

[0012] S4, combining the Fick's second law and the effective chloride ion diffusion coefficient model based on the fractal characteristics of pore structure in step S3, a recycled concrete chloride ion diffusion coefficient prediction model under saturated conditions is established.

[0013] More preferably, in step S1 of the above construction method, the three-dimensional pore volume fractal model in recycled concrete established is

[0014]

[0015] In formula (1), p is the relative porosity of the material; R is the side length of the cube; r k is the side length of the small cube after k iterations;

[0016] The two-dimensional pore tortuosity fractal model in recycled concrete established in step S2 is

[0017]

[0018] In formula (2), L0 is the straight line length of the effective pore along the flow direction; L t is the actual length of the effective pore; λ is the pore diameter; D t is the fractal dimension describing the tortuosity of the flow tube;

[0019] The effective chloride ion diffusion coefficient model based on the fractal characteristics of pore structure established in step S3 is

[0020]

[0021] In formula (3), D cA is the effective diffusion coefficient of the ion in the porous material considering the pore structure, is the porosity of the porous medium, D c is the diffusion coefficient, D t is the fractal dimension describing the tortuosity of the flow tube;

[0022] The recycled concrete chloride ion diffusion coefficient prediction model under saturated conditions established in step S4 is

[0023]

[0024] The initial conditions and boundary conditions of the model are:

[0025]

[0026] In equation (4), t0 and t are the soaking times, respectively; m is the attenuation coefficient considering the time-varying effect of chloride ion diffusion; D c，app The apparent diffusion coefficient is expressed in m³. 2 / s; In equation (5), C smax The surface free chloride ion content after stabilization is expressed in %; r is a dimensionless coefficient characterizing the chloride ion accumulation rate.

[0027] More preferably, step S1 of the above construction method specifically includes:

[0028] S11. Based on the Menger sponge model principle, the recycled concrete is first considered as a cube with side length R, and the first iteration is performed. Specifically, the cube is divided into m equal parts to obtain m... 3 After removing n small cubes from the first iteration, we obtain the result containing (m) small cubes. 3 A figure consisting of (n) small cubes;

[0029] S12. Perform the iterative operation k times according to step S11, and finally obtain the result containing (m 3 -n) k A solid figure of the small cube, wherein the side length of the small cube is r. k The volume V of the final solid figure is then obtained. k for

[0030]

[0031] The final equation (6) is converted to equation (1).

[0032] More preferably, step S2 of the above construction method specifically includes:

[0033] S21. According to Tyler's fluid mechanics theorem, a random and complex porous medium can be represented as:

[0034]

[0035] In equation (7), ε is the scale of the relative measurement;

[0036] S22. Substituting the pore diameter λ as ε into equation (7), the fractal scaling relation for pores is:

[0037]

[0038] In a two-dimensional plane, satisfying 1 <D t <2, when D t When the value is 2, it means that the pores have become so tortuous that they have filled the entire plane.

[0039] S23, based on the principle of Von Koch model, the two-dimensional pore channel tortuosity fractal model in recycled concrete is

[0040]

[0041] Substitute formula (8) into formula (9), after conversion, the two-dimensional pore channel tortuosity fractal model in recycled concrete, that is, formula (2).

[0042] Further preferably, step S3 of the above construction method is specifically:

[0043] S31, in the pore medium in recycled concrete, the pore axial diffusion flux J Ai , the effective diffusion flux J AS And the expansion flux J Ac In the capillary after considering the capillary bundle model is expressed as:

[0044]

[0045]

[0046]

[0047] In formula (11), The porosity of porous medium. In formula (12), k is a constant; D c The diffusion coefficient; from formula (10) and formula (11)

[0048]

[0049] Substitute formula (13) into formula (12):

[0050]

[0051] S32, in the pore medium in recycled concrete, the effective diffusion flux J AS Also expressed as

[0052]

[0053] In the formula, ΔC A The change of molar concentration in the pore, D cA The effective diffusion coefficient of ions in the pore structure inside the porous material;

[0054] From formula (14) and formula (15)

[0055]

[0056] S33, define the pore tortuosity index as tortuosity τ

[0057]

[0058] Substitute formula (17) into formula (16) to obtain

[0059]

[0060] S34, based on the principle of Katz and Menger model, the lower limit of relative scale epsilon is pore diameter lambda, and the upper limit is L0;The total volume of the porous medium measured by the lower limit measurement scale is A (L0 / lambda) 3 , then the pore volume can be expressed as A (L0 / lambda) Dv , wherein A is a constant;Then the porosity The calculation formula is:

[0061]

[0062] The pore tortuosity is represented as

[0063]

[0064] Substitute formula (20) into formula (18) to establish the effective chloride ion diffusion coefficient model based on the pore structure fractal model, as shown in formula (3) above.

[0065] The application also provides a recycled concrete chloride ion diffusion coefficient prediction model based on the random fractal characteristics of pore structure, which is constructed by the above construction method.

[0066] The recycled concrete chloride ion diffusion coefficient prediction model provided by the application can be widely applied in the research of the durability of recycled concrete structure by reacting the replacement rate of different recycled aggregates to the mesoscopic pore structure and reacting the mesoscopic pore structure to the chloride ion corrosion performance.

[0067] The application establishes a recycled concrete chloride ion diffusion channel model based on the random fractal theory and proposes a chloride ion diffusion coefficient of recycled concrete.

[0068] Compared with the prior art, the recycled concrete chloride ion diffusion coefficient prediction model provided by the application has the advantages that, compared with the existing recycled concrete chloride ion diffusion coefficient determination method, the recycled concrete chloride ion diffusion coefficient prediction model provided by the application studies the transmission channel affecting the chloride ion transmission of recycled concrete on the mesoscopic scale, and the above-mentioned model on the macroscopic scale is constructed based on the mesoscopic transmission channel. BRIEF DESCRIPTION OF DRAWINGS

[0069] Figure 1 are open and closed pore schematic diagrams;

[0070] Figure 2 is a pore size distribution differential curve;

[0071] Figure 3 is a logr-log(dv / dr) curve of the test piece NC;

[0072] Figure 4 is a logr-log(dv / dr) curve of the test piece RC50;

[0073] Figure 5 is a logr-log(dv / dr) curve of the test piece RC100;

[0074] Figure 6 is a comparison of the predicted value and the measured value of the effective chloride ion diffusion coefficient of the test piece NC in the example of the present application;

[0075] Figure 7 is a comparison of the predicted value and the measured value of the effective chloride ion diffusion coefficient of the test piece RC50 in the example of the present application;

[0076] Figure 8 is a comparison of the predicted value and the measured value of the effective chloride ion diffusion coefficient of the test piece RC100 in the example of the present application. DETAILED DESCRIPTION

[0077] The technical solutions of the present application will be described clearly and completely below. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0078] The prediction model of the chloride ion diffusion coefficient of the recycled concrete provided in the following experimental example is constructed in the following method:

[0079] S1, based on the principle of Menger sponge model, a three-dimensional pore volume fractal model in recycled concrete is established;

[0080] S11, based on the principle of Menger sponge model, first, the recycled concrete is regarded as a cube with side length R and the first iteration is carried out, that is, the cube is divided into m equal parts to obtain m 3 small cubes, and after removing n small cubes, a graph containing (m 3 -n) small cubes after the first iteration is obtained;

[0081] S12, the iteration operation is carried out k times according to the manner of step S11, and finally a graph containing (m 3 -n)k a perspective view of one of the small cubes, the edge length of the small cube being r k ; then the volume V of the final perspective view is k

[0082]

[0083] The final formula (6) is converted to formula (1). In formula (1), p is the relative porosity of the material; R is the edge length of the cube; r k is the edge length of the small cube after k iterations.

[0084] S2, based on the principle of Von Koch model, a two-dimensional pore tortuosity fractal model in recycled concrete is established;

[0085] S21, according to Tyler fluid mechanics theorem, through random complex porous medium can be expressed as

[0086]

[0087] In formula (7), ε is the relative measurement scale;

[0088] S22, the pore diameter λ is regarded as ε and brought into formula (7), then the fractal scaling relationship of the pore is

[0089]

[0090] In the two-dimensional plane, 1<D t <2, when D t =2, it means that the pore tortuosity has filled the entire plane;

[0091] S23, based on the principle of Von Koch model, the two-dimensional pore tortuosity fractal model in recycled concrete is

[0092]

[0093] Substitute formula (8) into formula (9), and after conversion, the two-dimensional pore tortuosity fractal model in recycled concrete is formula (2) as follows

[0094]

[0095] In formula (2), L0 is the straight line length of the effective pore along the flow direction; L t is the actual length of the effective pore; λ is the pore diameter; D t is the fractal dimension describing the degree of flow pipe tortuosity;

[0096] S3, an effective chloride ion diffusion coefficient model based on the fractal characteristics of pore structure is established; ​

[0097] S31, in the pore medium in recycled concrete, the pore axial diffusion flux J Ai , the effective diffusion flux J AS and the expansion flux J in the capillary after considering the capillary bundle model Ac is expressed as:

[0098]

[0099]

[0100]

[0101] In formula (11), is the porosity of the porous medium. In formula (12), k is a constant; D c is the diffusion coefficient; obtained from formula (10) and formula (11)

[0102]

[0103] Substituting formula (13) into formula (12) has:

[0104]

[0105] S32, in the pore medium in recycled concrete, the effective diffusion flux J AS is also expressed as

[0106]

[0107] In the formula, ΔC A is the change of molar concentration in the pore, D cA is the effective diffusion coefficient of ions in the interior of the porous material considering the pore structure;

[0108] Obtained from formula (14) and formula (15)

[0109]

[0110] S33, define the pore tortuosity index as tortuosity τ

[0111]

[0112] Substituting formula (17) into formula (16) has:

[0113]

[0114] S34, based on the principles of Katz and Menger model, the lower limit of the relative scale ε is the pore diameter λ, and the upper limit is L0; The total volume of the porous medium measured by the lower limit measurement scale is A(L0 / λ) 3, then the pore volume can be expressed as A (L0 / λ) Dv , where A is a constant; then the porosity The calculation formula is:

[0115]

[0116] The pore tortuosity is expressed as

[0117]

[0118] Substitute formula (20) into formula (18) to establish the effective chloride ion diffusion coefficient model based on the pore structure fractal model, that is, formula (3).

[0119]

[0120] In formula (3), D cA is the effective diffusion coefficient of the ion in the porous material, is the porosity of the porous medium, D c is the diffusion coefficient, and D t is the fractal dimension describing the tortuosity of the flow tube.

[0121] S4, combined with the Fick second law and the effective chloride ion diffusion coefficient model based on the pore structure fractal characteristics in step S3, the saturated condition under the recycled concrete chloride ion diffusion coefficient prediction model is established.

[0122]

[0123] The initial condition and boundary condition of the model are:

[0124]

[0125] In formula (4), t0 and t are the soaking time, respectively; m is the attenuation coefficient considering the time-varying effect of chloride ion diffusion; D c，app is the apparent diffusion coefficient, with the unit of m 2 / s; in formula (5), C smax is the stable surface free chloride ion content, with the unit of %; r is a dimensionless coefficient representing the accumulation rate of chloride ion.

[0126] Experimental example:

[0127] The pore structure of the recycled concrete mentioned in the embodiments of the present application includes gel pores and capillary pores. Both the gel pores and the capillary pores are effective chloride ion transmission channels, and are open pores in the pore structure. Open pores and closed pores are shown as follows. Figure 1 The pores in the cement-based material exhibit complex geometric morphology and random spatial distribution, which conforms to the fractal characteristics.

[0128] To verify the reliability of the recycled concrete chloride ion diffusion coefficient prediction model of the application, preselect relevant raw materials, give relevant mix proportion design, then calculate the relevant chloride ion diffusion coefficient according to the prediction model established in steps 1-4, at the same time, prepare recycled concrete materials according to the selected raw materials and mix proportion, and perform chloride ion content detection to obtain the chloride ion diffusion coefficient test value, and compare it with the model design value. Including the following steps:

[0129] 1. Selection of relevant test raw materials:

[0130] Cement: P.O 42.5 ordinary portland cement. Fine aggregate: natural river sand, fineness modulus 2.7. Natural coarse aggregate: graded 5-20mm, water absorption rate 1.12%, crushing index 6.4%. Recycled coarse aggregate: crushed by an e-type crusher after original concrete with strength grade C40 ordinary concrete, graded 5-20mm, water absorption rate 4.18%, crushing index 17%.

[0131] 2. Mix proportion design:

[0132] Select the proposed materials to design the mix proportion of recycled concrete, and the recycled aggregate replacement rate is 0%, 50% and 100%. The raw material mix proportion of recycled concrete is shown in Table 1 below.

[0133] Table 1 Mix proportion of recycled concrete

[0134]

[0135] After the mixture is fully stirred, it is placed in a mold with a size of 100mmx100mmx100mm for molding. After the test piece is poured and cured for 48h, it is removed from the mold and placed in a standard curing room for curing. The curing temperature is 20±2℃, the humidity is 95%, and the curing age is 28d.

[0136] 3. Calculation of fractal dimension of recycled concrete pore structure:

[0137] The pore structure of recycled concrete is determined by mercury intrusion test. The mercury porosimeter uses low pressure pore measurement method, and the maximum pressure value is 228MPa. The measurable pore size range is 5nm to 360000nm.

[0138] The production process of the mercury intrusion test piece is as follows: the standard cube test piece cured to 28d age is broken with a sharp hammer, and a sample with a side length of about 5mm is taken at the center of the test piece. The sample is placed in a wide-mouth bottle and hydrated with anhydrous ethanol. The sample is dried in an oven at 105℃ for 24h before the mercury intrusion test, and then cooled to room temperature and placed in a dry box for standby.

[0139] The measured value of the pore structure is obtained by the mercury intrusion test, and the pore size differential curve is obtained as shown in Figure 2 .

[0140] The pore volume and the pore tortuosity calculation model established according to the present application from the pore size differential curve, according to the formula (1)-(2) and (6)-(8), has log(dv / dr)~(2-D v )log(dr), the curve is drawn after taking the logarithm of dv / dr and dr respectively, is approximately a straight line, and the volume fractal dimension D v can be calculated by calculating the slope. The logr-log(dv / dr) curve of the NC is Figure 3 The logr-log(dv / dr) curve of the RC50 is Figure 4 The logr-log(dv / dr) curve of the RC100 is Figure 5 . Figures 3-5 The process of calculating the pore volume fractal dimension according to the model corresponds to the above steps S1 and S2. Similarly, the pore tortuosity fractal dimension can be calculated. As shown in Table 2 below.

[0141] Table 2 Recycled concrete pore volume D v and pore tortuosity fractal dimension D t

[0142] Test piece Pore volume fractal dimension D v ]] Fractal dimension of pore tortuosity D t ]] NC 2.6983 1.1377 RC50 2.6636 1.0848 RC100 2.6430 1.0323

[0143] 4. Chloride ion content determination of recycled concrete:

[0144] The chloride ion content determination test uses a test block of 100mmx100mmx100mm, and the chloride ion content test is carried out after 28d of standard curing. The long-term immersion test method is used, and the immersion time is 30d, 60d and 90d respectively. The free chloride ion content is determined. The test process is carried out according to the specifications JGJ / T 322-2013 Appendix C and D.

[0145] 5. Comparison of test value and simulation value:

[0146] The model is calculated by using the Crank-Nicolson type difference format. The comparison of the test value and the simulation value is shown in Figures 6-8 . Figures 6-8 The comparison of the test value and the model prediction value of the chloride ion diffusion coefficient is shown in the above step S4, which is used to verify the feasibility of the model.

[0147] From the results of the present experimental example, the chloride ion effective diffusion coefficient prediction value calculated by the prediction model of the present application is in good agreement with the measured value, the theoretical and calculation system is mature, the parameter setting is simple, and the pore structure test experiment is operable. The present application establishes a macro-scale recycled concrete chloride ion effective coefficient prediction model based on the meso-scale transmission channel, analyzes the chloride ion transmission channel, diffusion characteristics and rules of recycled concrete on multiple scales, and has certain theoretical and practical application value.

[0148] The above detailed description describes the embodiments of the present application in detail, but the present application is not limited to the specific details in the above embodiments. Within the scope of the claims and technical concepts of the present application, various simple modifications and changes can be made to the technical solutions of the present application, and these simple modifications all belong to the protection scope of the present application.

Claims

1. A method for constructing a prediction model for the chloride ion diffusion coefficient of recycled concrete based on the random fractal characteristics of pore structure, characterized in that, Based on the theory of random fractals, starting from the microscale of recycled concrete, we first establish a three-dimensional pore volume fractal model and a two-dimensional pore tortuosity fractal model, then establish an effective chloride ion diffusion coefficient model based on the fractal characteristics of pore structure; finally, we establish a prediction model for the chloride ion diffusion coefficient of recycled concrete under saturation conditions. Specifically, the following steps are included: S1. Based on the Menger sponge model principle, establish a three-dimensional pore volume fractal model in recycled concrete; S2. Based on the Von Koch model principle, establish a two-dimensional fractal model of pore tortuosity in recycled concrete; S3. Establish an effective chloride ion diffusion coefficient model based on the fractal characteristics of pore structure; S4. Combining Fick's second law and the effective chloride ion diffusion coefficient model based on the fractal characteristics of pore structure in step S3, a prediction model for the chloride ion diffusion coefficient of recycled concrete under saturation conditions is established.

2. The construction method according to claim 1, characterized in that, In step S1, the three-dimensional pore volume fractal model established in the recycled concrete is as follows: （1） In equation (1), D v Let fractal dimension be the pore volume of recycled concrete; p The relative porosity of the material; R Let be the side length of the cube; r k For the process k The side length of the small cube after the next iteration; The two-dimensional tortuosity fractal model of the pores in the recycled concrete established in step S2 is as follows: （2） In equation (2), L 0 represents the linear length of the effective pore size along the flow direction; L t The actual length of the effective pore; λ The pore diameter; D t The fractal dimension is used to describe the degree of tortuosity of a flow tube. The effective chloride ion diffusion coefficient model established in step S3 based on the fractal characteristics of the pore structure is as follows: （3） In equation (3), D cA To consider the effective diffusion coefficient of ions within porous materials, φ Porosity is the porosity of the porous medium. D c The diffusion coefficient is... D t The fractal dimension is used to describe the degree of tortuosity of a flow tube. The prediction model for the chloride ion diffusion coefficient of recycled concrete under saturated conditions established in step S4 is as follows: （4） The initial and boundary conditions of the model are as follows: （5） In equation (4); t 0 and t These are the soaking times; m The attenuation coefficient takes into account the time-varying effect of chloride ion diffusion; D c，app The apparent diffusion coefficient is expressed in m³. 2 / s; In equation (5), C smax The content of free chloride ions on the surface after stabilization, expressed in % (%). r This is a dimensionless coefficient characterizing the rate of chloride ion accumulation.

3. The construction method according to claim 2, characterized in that, Step S1 is as follows: S11. Based on the Menger sponge model principle, the recycled concrete is first considered as having a side length of... R The cube is then divided into m equal parts for the first iteration. 3 After removing n small cubes from the first iteration, we obtain the result containing (m) small cubes. 3 A figure consisting of (n) small cubes; S12. Perform the iterative operation k times according to step S11, and finally obtain the result containing (m 3 -n) k A three-dimensional shape of the small cube, wherein the side length of the small cube is... r k The volume of the final solid figure is then determined. V k for （6）； Finally, after conversion, we obtain equation (1).

4. The construction method according to claim 2, characterized in that, Step S2 is as follows: S21. According to Tyler's fluid mechanics theorem, a random and complex porous medium can be represented as: （7）； In equation (7), ε The scale is relative to the measurement. S22, The pore diameter λ See as ε Substituting into equation (7), the fractal scaling relation for porous structures is: （8）； S23. Based on the Von Koch model principle, the fractal model of two-dimensional pore tortuosity in recycled concrete is as follows: （9） Substituting equation (8) into equation (9), after conversion, the fractal model of the tortuosity of two-dimensional pores in recycled concrete is equation (2).

5. The construction method according to claim 2, characterized in that, Step S3 is as follows: S31. Axial diffusion flux through pores in recycled concrete medium. J Ai Effective diffusion flux J AS And the expansion flux within the capillary after considering the capillary bundle model J Ac Represented as: （10） （11） （12） In equation (11), φ Let be the porosity of the porous medium; in equation (12), k It is a constant; D c The diffusion coefficient is obtained from equations (10) and (11). （13） Substituting equation (13) into equation (12), we get: （14） S32. Effective diffusion flux in porous media of recycled concrete. J AS It is also expressed as （15） In the formula, ΔC A This represents the change in molar concentration within the pores. D cA To consider the effective diffusion coefficient of ions within porous materials; From equations (14) and (15), we obtain （16） S33. Define the tortuosity index of pore tortuosity as tortuosity. τ for （17） Substituting equation (17) into equation (16) yields （18）； S34. Based on the principles of the Katz and Menger models, relative scale ε The lower limit is the pore diameter. λ The upper limit is L 0; The total volume of the porous medium measured using the lower limit measurement scale is A ( L 0 / λ ) 3 Then the pore volume can be expressed as A ( L 0 / λ ) Dv ,in A If it is a constant, then the porosity is... φ The formula for calculation is: （19） The tortuosity of the hole is expressed as （20） Substituting equation (20) into equation (18), we establish an effective chloride ion diffusion coefficient model based on the pore structure fractal model, namely equation (3).

6. A model for predicting the chloride ion diffusion coefficient of recycled concrete constructed using the construction method described in any one of claims 1-5.

7. An application of a prediction model for the chloride ion diffusion coefficient of recycled concrete constructed using the construction method described in any one of claims 1-5, characterized in that, Used to study the durability of recycled concrete structures.

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