Early-age concrete anti-cracking reverse design method

Abstract

The invention discloses an anti-cracking reverse design method for early-age concrete. Comprising the following steps: acquiring material performance parameters; constructing a chemical-thermal-force-phase field damage model of the early-age concrete; constructing a constitutive relationship between early-age behaviors and stress-strain of the concrete; carrying out shrinkage cracking simulation calculation on the early-age concrete, and quantitatively evaluating the damage cracking risk of the early-age concrete by using a damage development index Idd. And performing crack resistance optimization design on the early-age performance of the concrete by using an orthogonal design method to obtain a crack resistance optimization result in the performance combination and development process of the concrete. According to the method, the damage development index peak value is taken as a control target, the shrinkage cracking risk of the early-age concrete structure under different material properties, development processes, constraints and curing temperatures can be quantitatively evaluated, the optimal design of the concrete mix proportion is guided, a large amount of test exploration is avoided, the manpower and material resource cost is reduced, and the method is suitable for large-scale popularization and application. The method has important significance on early crack control of a concrete structure and has remarkable progress.

Description

Technical Field

[0001] The invention belongs to the technical field of concrete mix optimization and particularly relates to a reverse design method for early-age concrete anti-cracking. Background Art

[0002] Concrete has become one of the most widely used building materials due to its excellent mechanical properties and economy. The durability of concrete is essential to ensure the safety and service life of the structure. However, concrete cracks, especially early cracks, will seriously affect the durability of the structure. Therefore, the study of early crack control in concrete has become one of the important research directions in concrete engineering.

[0003] The early-age behavior of concrete includes heat release from hydration reaction, heat conduction and thermal deformation, self-volume deformation, age effect, and performance degradation due to damage. These behaviors are affected by the complex coupling of multiple physical fields such as hydration field, temperature field, stress field, and damage field. In addition, the evolution of early-age behavior of concrete does not exist independently, but has an intrinsic connection, and each variable is difficult to control independently, which makes it impossible for traditional test-based concrete mix optimization to take into account the coupling of multiple physical fields.

[0004] In addition, traditional early crack control methods mainly focus on reducing shrinkage. However, this is only a local optimal solution considering the driving force. The existing technology cannot obtain the global optimal solution and perform global optimal solution optimization design.

[0005] Therefore, it is necessary to invent a reverse design method for early-age concrete crack resistance to solve the above problems. Summary of the invention

[0006] The present invention provides a reverse design method for early-age concrete crack resistance. Based on a chemical-thermal-mechanical-phase field damage coupling model, the damage development index is used as a control target, and a concrete mix ratio optimization design with global optimization of crack resistance under the coupling of multiple physical fields is achieved. The method is more consistent with actual working conditions, and the global optimal solution can be considered in terms of early-age crack control of concrete. This provides strong support for the design of concrete structures and overcomes the technical defects actually existing in use as proposed in the background technology.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A reverse design method for early-age concrete crack resistance comprises the following steps:

[0009] S1. Obtain material performance parameters, sample concrete, and obtain its composition and mix ratio. Determine the sample's deformation stress field simulation parameters, temperature field simulation parameters, and hydration field simulation parameters through uniaxial tension, three-point bending, autogenous volume deformation, and isothermal calorimetry tests, and further obtain elastic modulus development curve, tensile strength development curve, fracture energy development curve, chemical potential evolution curve, and autogenous volume deformation;

[0010] Among them, the obtained material performance parameters include the final hydration degree α ∞ , chemical potential function A(α), final heat release of hydration Q ∞ , thermal conductivity k0, final autogenous shrinkage value η a , hydration threshold α for elastic modulus development E , Hydration threshold for tensile strength development Hydration threshold for fracture energy development Elastic modulus E at final hydration degree ∞ , tensile strength f t∞ , fracture energy G c∞ , elastic modulus development index nE, tensile strength development index nf t and fracture energy development index nG c ;

[0011] S2. Construct a chemical-thermal-mechanical-phase field damage model for early-age concrete. Based on the deformation stress field simulation parameters, temperature field simulation parameters and hydration field simulation parameters of the sample obtained in S1, as well as the elastic modulus development curve, tensile strength development curve, fracture energy development curve, chemical potential evolution curve and autogenous volume deformation, derive the control equations and boundary conditions of the chemical-thermal-mechanical-phase field damage model applicable to early-age concrete, which are used to solve the displacement, temperature, hydration degree and phase field damage in the domain;

[0012] The governing equations for the phase field-displacement problem are as follows:

[0013]

[0014] The boundary conditions of the governing equations for the phase field-displacement problem are

[0015]

[0016] The governing equations for the thermo-chemical problem are as follows:

[0017]

[0018] The boundary conditions of the governing equations for the thermo-chemical problem are

[0019]

[0020] In the formula, is the gradient operator, Δ is the Laplace operator, σ is the stress tensor, b is the body force tensor, n is the unit external normal vector, is the external surface force, g(d) is the energy degradation function, d is the phase field damage variable, is the damage driving history variable, Gc is the critical fracture energy release rate, α(d) is the crack geometry function, is the regularization parameter, l is the crack size parameter, ρc is the volume specific heat capacity of the material, T is the temperature, q is the heat flow, Q is the heat release of cement hydration, Q ∞ is the final hydration exotherm, α is the hydration degree, is the heat flow boundary, k0 is the initial thermal conductivity, A(α) is the chemical potential function, Ea is the chemical activation energy, and R is the ideal gas constant;

[0021] S3. Construct the early age behavior and stress-strain constitutive relationship of concrete. In the process of constructing the constitutive relationship, the hydration degree is used as the internal variable, and the temperature deformation, autogenous volume deformation, creep and age effect are comprehensively considered to obtain:

[0022] The stress-strain constitutive relation expression of concrete is:

[0023] ε e =ε-εr-ε au -ε cr

[0024]

[0025] The relationship between the mechanical properties of early-age concrete and the degree of hydration is expressed as follows:

[0026]

[0027] Where ε is the total strain tensor, ε e is the elastic strain tensor, ε T is the temperature strain tensor, ε au is the autogenous contraction strain tensor, ε cr is the creep strain tensor;

[0028] S4. Early-age concrete shrinkage cracking simulation calculation, establish early-age concrete structure shrinkage cracking model, including generating actual engineering finite element model, generating node and unit information matrix, setting unit material properties and mechanical and thermal boundary conditions. The early-age concrete shrinkage cracking simulation calculation process is as follows:

[0029] S4.1. Use ABAQUS to perform actual engineering finite element modeling and meshing, and export INP files;

[0030] S4.2. Assign displacement, temperature, hydration degree and phase field degrees of freedom to the unit by modifying the INP file keywords;

[0031] S4.3, using user-defined unit UEL to realize the simulation calculation of chemical-thermal-mechanical-phase field coupling model;

[0032] S5. Use the damage development index Idd to quantitatively evaluate the damage and cracking risk of early-age concrete, and perform trial calculations on the samples. When the damage development index Idd ≤ 100%, the concrete structure is judged to be undamaged; otherwise, the concrete structure is judged to be damaged, and in this case, concrete crack resistance optimization design is required;

[0033] S6. Use the orthogonal design method to optimize the crack resistance of concrete at early age, taking the peak value of the damage development index Idd as the evaluation index; determine the test factors, and select multiple test factor levels within the adjustable range of each test factor (about 50% to 150% of the test value) according to the shrinkage cracking law. Different combinations of each test factor will be used as alternative solutions for optimizing the crack resistance of concrete;

[0034] S7. The optimal result of crack resistance of concrete performance combination and development process is obtained. The specific process is as follows:

[0035] S7.1. Design multiple test schemes, repeat the simulation work of S4 and S5 in sequence according to the multiple set schemes, and record the peak value of the damage development index Idd of each test scheme;

[0036] S7.2. Perform range calculations to determine the primary and secondary test factors;

[0037] S7.3. Select an orthogonal table based on the test factors and factor levels, and determine the best parameter combination in the orthogonal table;

[0038] S7.4. Based on the value of the damage development index Idd in the orthogonal table, compare the optimization results of the crack resistance of the concrete performance combination and the development process to guide the optimization design of the concrete mix ratio.

[0039] Preferably, in S1, the elastic modulus development process, tensile strength development process and fracture energy development process are set corresponding to the horizontal axis age when the hydration degree development process is plotted, and the starting hydration degree of performance development at an ultra-early age, i.e., the permeation threshold, is given.

[0040] Preferably, in S2, the phase field model used is set to a unified cohesive force phase field model;

[0041] Among them, the energy degradation function g(d) and the crack geometry function α(d) are as follows:

[0042]

[0043] In the formula, a1, a2, a3 and m are parameters, and their values ​​correspond to different softening curves. Here, the Comelissen softening curve, which is generally applicable to concrete, is adopted, and its corresponding parameter value is

[0044]

[0045] a2=1.386, a3=0.9106, m=2.

[0046] Preferably, in S2, the coupling effect of the damage field, temperature field and hydration field is considered, which is specifically manifested in that the occurrence of damage will lead to the degradation of thermal conductivity and chemical potential, and the hydration field, temperature field, displacement field and phase field are fully coupled to simulate, and the interleaved algorithm and Newton-Raphson solver are used to solve the coupling problem.

[0047] Preferably, in S3, the creep strain in the stress-strain constitutive relation of concrete consists of transient thermal creep and foundation creep;

[0048] Among them, transient thermal creep is defined as:

[0049]

[0050] The foundation creep adopts a rheological model that considers the influence of temperature and hydration degree. It consists of three Kelvin-Voigt units connected in series to form a Kelvin-Voigt chain. A single Kelvin-Voigt unit includes a spring and a damper in parallel, and the evolution relationship of a single Kelvin-Voigt unit is as follows:

[0051]

[0052] in, For time, the basic creep of the Kelvin-Voigt chain is expressed as:

[0053]

[0054] Considering the influence of temperature and hydration degree, the spring stiffness and damper viscosity They are:

[0055]

[0056] In the formula, E ac is the creep activation energy, which can be determined based on experiments, and T0=293K is the standard temperature.

[0057] Preferably, in S3, a simplified assumption is made for the initial hydration degree, and the hydration degrees corresponding to the beginning development of tensile strength, elastic modulus and fracture energy are set to be the same, which is expressed as:

[0058]

[0059] The development speed of tensile strength, elastic modulus and fracture energy is determined by different nE, nf t , and nG c The value of is adjusted;

[0060] Generally, there are significant differences in the development speed of tensile strength, elastic modulus and fracture energy. The elastic modulus develops faster than the tensile strength, while the fracture energy develops faster than the elastic modulus. It can be expressed as:

[0061] nG c ≤nE≤nf t .

[0062] Preferably, in S5, unlike the previous local optimization design method with minimum shrinkage as the goal, the damage development index Idd is used as the design control index; it is defined as the ratio of damage driving force to damage resistance, representing a global control index from the perspective of damage driving force and damage resistance game, and its expression is as follows:

[0063]

[0064] In the formula, Driven by damage, is damage resistance, and is the effective stress tensor The corresponding maximum principal stress.

[0065] Preferably, in S5, anti-cracking optimization measures are adopted to adjust the concrete mix ratio, and the anti-cracking optimization measures include but are not limited to adjusting the type of cement, adding the content of mineral admixtures (such as fly ash, slag), adjusting the amount of aggregate, and adding admixtures (such as auxiliary cementitious materials, temperature rise inhibitors, expansion agents, shrinkage reducers, etc.), which are used to change the physical properties and chemical reaction processes of concrete.

[0066] Preferably, in S6, the test factors are key control factors of the early shrinkage cracking process, including the permeation threshold α0, the final value of autogenous shrinkage η a , Final value of elastic modulus E ∞ , Final value of tensile strength f t∞ , Final value of fracture energy G c∞ , elastic modulus development index nE, tensile strength development index nf t , fracture energy development index nG c And chemical potential function A(α). By analyzing and adjusting the above factors, the purpose of optimizing the crack resistance of concrete is achieved.

[0067] Preferably, in S6, the shrinkage cracking law is determined by single factor analysis. In the single factor analysis, it is assumed that other factors remain unchanged and only the control factor is changed for calculation to obtain the shrinkage cracking law without considering the coupling effect of multiple factors.

[0068] Compared with the prior art, the present invention has the following beneficial effects:

[0069] By taking the peak value of the damage development index as the control target, it is possible to quantitatively evaluate the shrinkage cracking risk of early-age concrete structures under different material properties, development processes, constraints and curing temperatures, and provide the material property combination and development process that optimizes cracking resistance, thereby guiding the optimal design of concrete mix, avoiding a large amount of experimental exploration, and reducing manpower and material costs. This is of great significance for the control of early cracks in concrete structures and is a significant improvement compared to existing technologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Figure 1 It is a schematic diagram of the overall process of the present invention;

[0071] Figure 2 This is a game demonstration diagram of the damage driving force and damage resistance of the present invention;

[0072] Figure 3 It is a diagram of the elastic modulus development process of the present invention;

[0073] Figure 4 This is a graph showing the variation of the damage development index with the permeation threshold of the present invention. DETAILED DESCRIPTION

[0074] In conjunction with the drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0075] like Figure 1-4 As shown, a reverse design method for early-age concrete crack resistance includes the following steps:

[0076] S1. Obtain material performance parameters, sample concrete, and obtain its composition and mix ratio. Determine the sample's deformation stress field simulation parameters, temperature field simulation parameters, and hydration field simulation parameters through uniaxial tension, three-point bending, autogenous volume deformation, and isothermal calorimetry tests, and further obtain elastic modulus development curve, tensile strength development curve, fracture energy development curve, chemical potential evolution curve, and autogenous volume deformation;

[0077] Among them, the obtained material performance parameters include the final hydration degree α ∞, chemical potential function A(α), final heat release of hydration Q ∞ , thermal conductivity k0, final autogenous shrinkage value η a , hydration threshold α for elastic modulus development E , Hydration threshold for tensile strength development Hydration threshold for fracture energy development Elastic modulus E at final hydration degree ∞ , tensile strength f t∞ , fracture energy G c∞ , elastic modulus development index nE, tensile strength development index nf t and fracture energy development index nG c ;

[0078] When the elastic modulus development process, tensile strength development process and fracture energy development process are plotted against the hydration degree development process, the horizontal axis age is set correspondingly, and the initial hydration degree of performance development at an extremely early age, i.e., the permeation threshold, is given;

[0079] Preferably, in S2, the phase field model used is set to a unified cohesive force phase field model;

[0080] Among them, the energy degradation function g(d) and the crack geometry function a(d) are as follows:

[0081]

[0082] In the formula, a1, a2, a3 and m are parameters, and their values ​​correspond to different softening curves. Here, the Comelissen softening curve, which is generally applicable to concrete, is adopted, and its corresponding parameter value is

[0083]

[0084] a2=1.386, a3=0.9106, m=2;

[0085] S2. Construct a chemical-thermal-mechanical-phase field damage model for early-age concrete. The model is constructed according to the second law of continuum mechanics and thermodynamics. Based on the deformation stress field simulation parameters, temperature field simulation parameters and hydration field simulation parameters of the sample obtained in S1, as well as the elastic modulus development curve, tensile strength development curve, fracture energy development curve, chemical potential evolution curve and autogenous volume deformation, the control equations and boundary conditions of the chemical-thermal-mechanical-phase field damage model for early-age concrete are derived to solve the displacement, temperature, hydration degree and phase field damage in the domain.

[0086] The governing equations for the phase field-displacement problem are as follows:

[0087]

[0088] The boundary conditions of the governing equations for the phase field-displacement problem are

[0089]

[0090] The governing equations for the thermo-chemical problem are as follows:

[0091]

[0092] The boundary conditions of the governing equations for the thermo-chemical problem are

[0093]

[0094] In the formula, is the gradient operator, Δ is the Laplace operator, σ is the stress tensor, b is the body force tensor, n is the unit external normal vector, is the external surface force, g(d) is the energy degradation function, d is the phase field damage variable, is the damage driving history variable, Gc is the critical fracture energy release rate, a(d) is the crack geometry function, is the regularization parameter, l is the crack size parameter, ρc is the material specific heat, T is the temperature, q is the heat flow, Q is the heat release of cement hydration, Q ∞ is the final hydration exotherm, α is the hydration degree, is the heat flow boundary, k0 is the initial thermal conductivity, A(α) is the chemical potential function, Ea is the chemical activation energy, and R is the ideal gas constant. In this embodiment, R is 8.314 J / (K*mol);

[0095] Considering the coupling effect of damage field, temperature field and hydration field, the occurrence of damage will lead to the degradation of thermal conductivity and chemical potential, and the hydration field, temperature field, displacement field and phase field are fully coupled for simulation. The staggered algorithm and Newton-Raphson solver are used to solve the coupling problem.

[0096] S3. Construct the early age behavior and stress-strain constitutive relationship of concrete. In the process of constructing the constitutive relationship, the hydration degree is used as the internal variable, and the temperature deformation, autogenous volume deformation, creep and age effect are comprehensively considered to obtain:

[0097] The stress-strain constitutive relation expression of concrete is:

[0098] ε e =ε-ε T -ε au -ε cr

[0099]

[0100] The relationship between the mechanical properties of early-age concrete and the degree of hydration is expressed as follows:

[0101]

[0102] Where ε is the total strain tensor, ε e is the elastic strain tensor, ε T is the temperature strain tensor, ε au is the autogenous contraction strain tensor, ε cr is the creep strain tensor;

[0103] The creep strain in the stress-strain constitutive relation of concrete consists of transient thermal creep and foundation creep;

[0104] Among them, transient thermal creep is defined as:

[0105]

[0106] The foundation creep adopts a rheological model that considers the influence of temperature and hydration degree. It consists of three Kelvin-Voigt units connected in series to form a Kelvin-Voigt chain. A single Kelvin-Voigt unit includes a spring and a damper in parallel, and the evolution relationship of a single Kelvin-Voigt unit is as follows:

[0107]

[0108] in, For time, the basic creep of the Kelvin-Voigt chain is expressed as:

[0109]

[0110] Considering the influence of temperature and hydration degree, the spring stiffness and damper viscosity They are:

[0111]

[0112] In the formula, E ac is the creep activation energy, which can be determined by experiment, and T0 = 293K is the standard temperature;

[0113] The initial hydration degree is simplified by assuming that the tensile strength, elastic modulus and fracture energy begin to develop at the same hydration degree, which can be expressed as:

[0114]

[0115] The development speed of tensile strength, elastic modulus and fracture energy is determined by different nE, nf t , and nG c The value of is adjusted;

[0116] Generally, there are significant differences in the development speed of tensile strength, elastic modulus and fracture energy. The elastic modulus develops faster than the tensile strength, while the fracture energy develops faster than the elastic modulus. It can be expressed as:

[0117] nG c ≤nE≤nf t

[0118] S4. Early-age concrete shrinkage cracking simulation calculation, establish early-age concrete structure shrinkage cracking model, including generating actual engineering finite element model, generating node and unit information matrix, setting unit material properties and mechanical and thermal boundary conditions. The early-age concrete shrinkage cracking simulation calculation process is as follows:

[0119] S4.1. Use ABAQUS to perform actual engineering finite element modeling and meshing, and export INP files;

[0120] S4.2. Assign displacement, temperature, hydration degree and phase field degrees of freedom to the unit by modifying the INP file keywords;

[0121] S4.3, using user-defined unit UEL to realize the simulation calculation of chemical-thermal-mechanical-phase field coupling model;

[0122] S5. Use the damage development index Idd to quantitatively evaluate the damage and cracking risk of early-age concrete, and perform trial calculations on the samples. When the damage development index Idd ≤ 100%, the concrete structure is judged to be undamaged; otherwise, the concrete structure is judged to be damaged, and in this case, concrete crack resistance optimization design is required;

[0123] Different from the previous local optimization design method with the minimum shrinkage as the goal, the damage development index Idd is used as the design control index; it is defined as the ratio of damage driving force to damage resistance, which represents the global control index from the perspective of damage driving force and damage resistance game. Its expression is as follows:

[0124]

[0125] In the formula, Driven by damage, is damage resistance, and is the effective stress tensor The corresponding maximum principal stress;

[0126] Adopt anti-cracking optimization measures to adjust the concrete mix ratio. Anti-cracking optimization measures include but are not limited to adjusting the type of cement, adding the content of mineral admixtures (such as fly ash, slag), adjusting the amount of aggregates, and adding admixtures (such as auxiliary cementitious materials, temperature rise inhibitors, expansion agents, shrinkage reducers, etc.) to change the physical properties and chemical reaction process of concrete;

[0127] S6. Use the orthogonal design method to optimize the crack resistance of concrete at early age, taking the peak value of the damage development index Idd as the evaluation index; determine the test factors, and select multiple test factor levels within the adjustable range of each test factor (about 50% to 150% of the test value) according to the shrinkage cracking law, that is, each test factor takes several values, and different combinations of each test factor will be used as alternative solutions for optimizing the crack resistance of concrete;

[0128] The test factors are the key control factors of the early shrinkage cracking process, including the permeation threshold α0, the final value of autogenous shrinkage η a , Final value of elastic modulus E ∞ , Final value of tensile strength f t∞ , Final value of fracture energy G c∞ , elastic modulus development index nE, tensile strength development index nf t , fracture energy development index nG c And chemical potential function A(α). By analyzing and adjusting the above factors, the purpose of optimizing the crack resistance of concrete is achieved;

[0129] The shrinkage cracking law is determined by single factor analysis, that is, the change law when a single factor changes and other factors remain unchanged. In the single factor analysis, it is assumed that other factors remain unchanged, and only the control factor is changed for calculation, and the shrinkage cracking law without considering the coupling effect of multiple factors is obtained;

[0130] S7. The optimal result of crack resistance of concrete performance combination and development process is obtained. The specific process is as follows:

[0131] S7.1. Design multiple test schemes, repeat the simulation work of S4 and S5 in sequence according to the multiple set schemes, record the peak value of the damage development index Idd of each test scheme, and the peak value reaching 100% is considered damaged, and the maximum value is used for judgment, and no attention is needed for the undamaged parts;

[0132] S7.2. Calculate the range and determine the main test factors and the secondary test factors. The main and secondary test factors are determined based on the range. As the range decreases, the influence of the test factors decreases.

[0133] S7.3. Select an orthogonal table based on the test factors and factor levels, and determine the best parameter combination in the orthogonal table; the orthogonal design table and intuitive analysis results are shown in the following table:

[0134] Table 1L 27 (3 13 )Orthogonal design table

[0135]

[0136]

[0137] Note: A is the tensile strength development index, B is the elastic modulus development index, C is the permeation threshold, Idd is the damage development index, Idd-k 1j , Idd-k 2j , Idd-k 3j are the average values ​​of the first, second and third levels of the factors, and Idd-R is Idd-k 1j , Idd-k 2j , Idd-k 3j Very bad.

[0138] S7.4. Based on the value of the damage development index Idd in the orthogonal table, compare the optimization results of the crack resistance of the concrete performance combination and the development process to guide the optimization design of the concrete mix ratio.

[0139] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A reverse design method for early age concrete crack resistance, characterized in that: The steps include: S1. Obtain material performance parameters, sample concrete, and obtain its composition and mix ratio. Determine the sample's deformation stress field simulation parameters, temperature field simulation parameters, and hydration field simulation parameters through uniaxial tension, three-point bending, autogenous volume deformation, and isothermal calorimetry tests, and further obtain elastic modulus development curve, tensile strength development curve, fracture energy development curve, chemical potential evolution curve, and autogenous volume deformation; Among them, the obtained material performance parameters include the final hydration degree α ∞ , chemical potential function A(a), final heat release of hydration Q ∞ , thermal conductivity k0, final value of autogenous shrinkage η a , hydration threshold α for elastic modulus development E , Hydration threshold for tensile strength development Hydration threshold for fracture energy development Elastic modulus E at final hydration degree ∞ , tensile strength f t∞ , fracture energy G c∞ , elastic modulus development index nE, tensile strength development index nf t and fracture energy development index nG c ; S2. Construct a chemical-thermal-mechanical-phase field damage model for early-age concrete. Based on the deformation stress field simulation parameters, temperature field simulation parameters and hydration field simulation parameters of the sample obtained in S1, as well as the elastic modulus development curve, tensile strength development curve, fracture energy development curve, chemical potential evolution curve and autogenous volume deformation, derive the control equations and boundary conditions of the chemical-thermal-mechanical-phase field damage model applicable to early-age concrete, which are used to solve the displacement, temperature, hydration degree and phase field damage in the domain; The governing equations for the phase field-displacement problem are as follows: The boundary conditions of the governing equations for the phase field-displacement problem are The governing equations for the thermo-chemical problem are as follows: The boundary conditions of the governing equations for the thermo-chemical problem are In the formula, is the gradient operator, Δ is the Laplace operator, σ is the stress tensor, b is the body force tensor, n is the unit external normal vector, is the external surface force, g(d) is the energy degradation function, d is the phase field damage variable, is the damage driving history variable, Gc is the critical fracture energy release rate, α(d) is the crack geometry function, is the regularization parameter, l is the crack size parameter, ρc is the volume specific heat capacity of the material, T is the temperature, q is the heat flow, Q is the heat release of cement hydration, Q ∞ is the final hydration exotherm, α is the hydration degree, is the heat flow boundary, k0 is the initial thermal conductivity, k is the thermal conductivity after damage degradation, A(α) is the chemical potential function, E a is the chemical activation energy, R is the ideal gas constant; the above parameters are taken according to the test results; S3. Construct the early age behavior and stress-strain constitutive relationship of concrete. In the process of constructing the constitutive relationship, the hydration degree is used as the internal variable, and the temperature deformation, autogenous volume deformation, creep and age effect are comprehensively considered to obtain: The stress-strain constitutive relation expression of concrete is: e e =e-e T -e au -e cr The relationship between the mechanical properties of early-age concrete and the degree of hydration is expressed as follows: Where ε is the total strain tensor, ε e is the elastic strain tensor, ε T is the temperature strain tensor, ε au is the autogenous contraction strain tensor, ε cr is the creep strain tensor; S4. Early-age concrete shrinkage cracking simulation calculation, establish early-age concrete structure shrinkage cracking model, including generating actual engineering finite element model, generating node and unit information matrix, setting unit material properties and mechanical and thermal boundary conditions. The early-age concrete shrinkage cracking simulation calculation process is as follows: S4.

1. Use ABAQUS to perform actual engineering finite element modeling and meshing, and export INP files; S4.

2. Assign displacement, temperature, hydration degree and phase field degrees of freedom to the unit by modifying the INP file keywords; S4.3, using user-defined unit UEL to realize the simulation calculation of chemical-thermal-mechanical-phase field coupling model; S5. Use the damage development index Idd to quantitatively evaluate the damage and cracking risk of early-age concrete, and perform trial calculations on the samples. When the damage development index Idd ≤ 100%, the concrete structure is judged to be undamaged; otherwise, the concrete structure is judged to be damaged, and in this case, concrete crack resistance optimization design is required; S6. Use the orthogonal design method to optimize the crack resistance of early-age concrete performance, taking the peak value of the damage development index Idd as the evaluation index; determine the test factors, and select multiple test factor levels within the controllable range of each test factor according to the shrinkage cracking law. Different combinations of each test factor will be used as alternative solutions for optimizing the crack resistance of concrete; S7. The optimal result of crack resistance of concrete performance combination and development process is obtained. The specific process is as follows: S7.

1. Design multiple test schemes, repeat the simulation work of S4 and S5 in sequence according to the multiple set schemes, and record the peak value of the damage development index Idd of each test scheme; S7.

2. Calculate the range and determine the main test factors and secondary test factors based on the range; S7.

3. Select an orthogonal table based on the test factors and factor levels, and determine the best parameter combination in the orthogonal table; S7.

4. Based on the value of the damage development index Idd in the orthogonal table, compare the optimization results of the crack resistance of the concrete performance combination and the development process to guide the optimization design of the concrete mix ratio.

2. The method for reverse design of early age concrete crack resistance according to claim 1, characterized in that: In S1, the elastic modulus development process, tensile strength development process and fracture energy development process are set corresponding to the horizontal axis age when the hydration degree development process is plotted, and the starting hydration degree of performance development at an ultra-early age is given.

3. The method for reverse design of early age concrete crack resistance according to claim 1, characterized in that: In S2, the phase field model used is set to the unified cohesive phase field model; Among them, the energy degradation function g(d) and the crack geometry function α(d) are as follows: In the formula, a1, a2, a3 and m are parameters, and their values ​​correspond to different softening curves. The corresponding parameter values ​​are a2=1.386, a3=0.9106, m=2 Where E is the elastic modulus.

4. The method for reverse design of early age concrete crack resistance according to claim 1, characterized in that: In S2, the coupling effects of damage field, temperature field and hydration field are considered. Specifically, the occurrence of damage will lead to the degradation of thermal conductivity and chemical potential, and the hydration field, temperature field, displacement field and phase field are fully coupled for simulation. The interleaved algorithm and Newton-Raphson solver are used to solve the coupling problem.

5. The method for reverse design of early age concrete crack resistance according to claim 1, characterized in that: In S3, the creep strain in the stress-strain constitutive relation of concrete consists of transient thermal creep and foundation creep; Among them, transient thermal creep is defined as: The foundation creep adopts a rheological model that considers the influence of temperature and hydration degree. It consists of three Kelvin-Voigt units connected in series to form a Kelvin-Voigt chain. A single Kelvin-Voigt unit includes a spring and a damper in parallel, and the evolution relationship of a single Kelvin-Voigt unit is as follows: in, For time, the basic creep of the Kelvin-Voigt chain is expressed as: Considering the influence of temperature and hydration degree, the spring stiffness and damper viscosity They are: In the formula, E ac is the creep activation energy, which can be determined based on experiments, and T0=293K is the standard temperature.

6. The method for reverse design of early age concrete crack resistance according to claim 1, characterized in that: In S3, the tensile strength, elastic modulus and fracture energy start to develop corresponding to the same hydration degree, which is expressed as: The development speed of tensile strength, elastic modulus and fracture energy is determined by different nE, nf t , and nG c The value of is adjusted; The elastic modulus develops faster than the tensile strength, while the fracture energy develops faster than the elastic modulus, which is expressed as: n-G c ≤n-E≤n-f t 。 7. The method for reverse design of early age concrete crack resistance according to claim 1, characterized in that: In S5, the damage development index Idd is used as the design control index, and its expression is as follows: In the formula, Driven by damage, is damage resistance, and is the effective stress tensor The corresponding maximum principal stress.

8. The method for reverse design of early age concrete crack resistance according to claim 1, characterized in that: In S5, anti-cracking optimization measures are adopted to adjust the concrete mix ratio. The anti-cracking optimization measures include but are not limited to adjusting the type of cement, the content of mineral admixtures, adjusting the amount of aggregates, and adding admixtures to change the physical properties and chemical reaction processes of concrete.

9. The method for reverse design of early age concrete crack resistance according to claim 1, characterized in that: In S6, the test factors are the key control factors of the early shrinkage cracking process, including the permeation threshold α0, the final value of autogenous shrinkage η a , final value of elastic modulus E ∞ , Final value of tensile strength f t∞ , Final value of fracture energy G c∞ , elastic modulus development index nE, tensile strength development index nf t , fracture energy development index nG c and chemical potential function A(α).

10. The method for reverse design of early age concrete crack resistance according to claim 1, characterized in that: In S6, the shrinkage cracking law is determined by single factor analysis. In the single factor analysis, it is assumed that other factors remain unchanged and only the control factor is changed for calculation, so as to obtain the shrinkage cracking law without considering the coupling effect of multiple factors.

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