A method for predicting the temperature field and crack resistance of large-volume concrete

Abstract

This invention discloses a method for predicting the temperature field and crack resistance of large-volume concrete. Firstly, this invention establishes a method for calculating the heat source function based on isothermal calorimetry combined with the exothermic reaction of cementitious materials and the Arrhenius equation, thus improving the calculation process for the temperature field of large-volume concrete. Furthermore, previous studies mostly relied on solving the strain field based on the degree of hydration, but the degree of hydration cannot be accurately calculated; this problem can be solved by using maturity theory. Compared with traditional methods, this invention can more accurately predict the temperature and strain development of large-volume concrete, improving temperature prediction accuracy by 5%–52% and strain prediction accuracy by 18%–43%.

Description

Technical Field

[0001] This invention relates to a method for predicting the temperature field and crack resistance of large-volume concrete, belonging to the field of building structure analysis and calculation technology. Background Technology

[0002] With my country's continuous investment in large-scale infrastructure projects such as water conservancy, construction, and transportation, the application of mass concrete is becoming increasingly widespread. In the past, extensive research has been conducted on the problem of cracking in early-age concrete due to the large amount of heat generated during hydration and the drastic temperature changes during internal temperature rise and fall, which easily lead to thermal shrinkage deformation. Much successful experience in crack control has been accumulated. However, with the increasing demand for ultra-large-scale infrastructure projects in my country, such as deep-sea, deep-earth, and urban underground spaces, coupled with the use of various general-purpose raw materials, there is an urgent need for a more precise concrete crack control technology to reduce costs and protect the environment, avoiding resource waste caused by excessive control. Currently, the temperature control equation typically uses the heat release function derived from adiabatic temperature rise tests, leading to significant deviations between simulation results and actual conditions. In fact, after concrete pouring, heat dissipation causes different temperatures in different parts, and temperature changes affect the hydration heat release rate and the development of strength and elastic modulus. Therefore, accurately predicting the temperature field and crack resistance of mass concrete is of great significance for the design and construction of building structures.

[0003] Therefore, this invention first establishes a heat source function calculation method based on isothermal calorimetry combining the exothermic reaction of cementitious materials and the Arrhenius equation, thus improving the calculation process of the temperature field for mass concrete. Furthermore, previous studies have mostly focused on temperature field calculations, neglecting the influence of different temperature histories and maturity development at different spatial locations of mass concrete in their calculations of parameters such as tensile strength and elastic modulus. In addition, the "cracking risk" is currently commonly used to assess whether mass concrete will crack, but in reality, cracking risk is a probability density of several cracking indices. Therefore, this paper uniformly adopts the "cracking index" to assess the cracking probability of mass concrete. Based on the above analysis, this paper proposes the following improved calculation framework for the temperature field and cracking index of mass concrete, as shown in the abstract figure. The main improvements include: ① an improved heat conduction equation that can account for temperature history differences at any spatial location (x, y, z) in large-volume concrete; ② a new calculation method based on isothermal calorimetry and the Arrhenius equation for the adiabatic temperature rise of the heat source function; and ③ a proposed method for calculating strength and modulus of elasticity based on maturity theory and the actual temperature development history at a certain location (x, y, z) in large-volume concrete, while also considering the damage to the mechanical properties of concrete under higher internal temperatures. Summary of the Invention

[0004] Technical Problem: The technical problem to be solved by this invention is to provide a method for predicting the temperature field and crack resistance of large-volume concrete, which provides more accurate and reliable results, addressing the existing problems in the prediction of temperature field and crack resistance of large-volume concrete.

[0005] Technical solution: The present invention provides a method for predicting the temperature field and crack resistance of large-volume concrete, comprising the following steps:

[0006] Step 1: Improve the heat conduction equation. The original equation... It should be rewritten as: Where m is the amount of cementitious material per unit volume of concrete, kg / m³ 3 q represents the heat of hydration of a unit mass of cementitious material per unit time under constant temperature conditions, in J / (kg·h); c represents the specific heat of concrete, in kJ / (kg·℃); ρ represents the density of concrete, in kg / m³. 3 λ is the thermal conductivity of concrete, W / (m·℃); T is the temperature of concrete, ℃. and These represent the diffusion rates of concrete temperature T in the x, y, and z directions, respectively; Q is the heat emitted per unit volume per unit time, in J / (m³). 3 ·h);

[0007] Step 2: In Step 1, q is the result of the exothermic reaction of the cementitious material from the isothermal calorimetry test, where q = dQ' / dt, and Q' is the heat release per unit mass of cementitious material, J / g.

[0008] Step 3: Establish the calculation equation for the heat source:

[0009] Where Q'(t,T') is the heat release per unit mass of cementitious material at time t due to temperature; A refers to the pre-factor; R is the gas constant, 8.314 J / (mol·K); T' is the isothermal temperature, °C; E a Activation energy, J / mol;

[0010] Step four: First, measure the heat of reaction of cementitious materials in concrete under isothermal calorimetry conditions of 20℃ and 40℃. Then, based on the nonlinear least squares method, Q' can be calculated. max , A and E a ;

[0011] Step 5: The thermal conductivity and specific heat capacity of the concrete were measured using a thermal conductivity meter with the steady-state plate method and the DSC sapphire method, respectively.

[0012] Step six, the maturity of the concrete is calculated using the following formula:

[0013] S=(t e -τ)T0

[0014] In the formula, S represents maturity, in °C·h; t e τ is the sum of equivalent ages under actual temperature history, h; T0 is the final setting time of concrete, h; T0 is the reference temperature, ℃.

[0015] Step 7: Based on the maturity calculation formula, the elastic modulus, tensile strength, and autogenous shrinkage value of concrete at different times and locations (x, y, z) can be obtained.

[0016] Step 8: Calculate the elastic strain of concrete under the action of reinforcing steel using the following formula:

[0017]

[0018] In the formula, the first term represents the tensile strain caused by the shrinkage of the concrete itself, and the second term represents the strain caused by the temperature change of the concrete; where n = E s / E c k = R' / E c ρ' is the reinforcement ratio of reinforced concrete, %; R' is the degree of restraint, approximately 1 for embedded slab structures, E c E represents the elastic modulus of concrete, expressed in GB / T 100; E represents the elastic modulus of concrete. s α represents the elastic modulus of the reinforcing steel, in GPa; c The coefficient of thermal expansion of concrete is 13.56 × 10⁻⁶. -6 / ℃;α s The coefficient of thermal expansion of the reinforcing steel is taken as 1.2 × 10⁻⁶. -5 / ℃;

[0019] Step nine, the concrete strain under creep effect can be obtained by the following formula:

[0020] In the formula, ε cr (t) represents creep strain; ε ce (t) represents elastic strain; The creep coefficient;

[0021] Among them, the creep coefficient of concrete Calculated using the following formula:

[0022]

[0023] In the formula, t e To calculate the equivalent age of concrete at the time considered, d; τ is the final setting time of concrete, d; f cm β represents the compressive strength of concrete, in MPa; H , is the coefficient related to relative humidity, %; RH is the relative humidity of the concrete, %; h is the concrete thickness, mm;

[0024] Step 10, the formula for calculating the concrete cracking index is:

[0025]

[0026] In the formula, E(t,T,x,y,z) is the elastic modulus of any point inside the concrete affected by temperature at time t, in GPa; ε cr (t,T,x,y,z) represents the creep strain at any point inside the concrete at time t due to temperature influence; f t (t,T,x,y,z) represents the tensile strength-cracking resistance of any point inside the concrete at time t under the influence of temperature, in MPa.

[0027] in:

[0028] In step seven, the elastic modulus and tensile strength values ​​of concrete at different times and locations (x, y, z) are calculated using temperature history combined with maturity theory. First, the development curves of elastic modulus and tensile strength of concrete at 20℃ are measured in the laboratory. Using 20℃ as a reference, when the maturity is the same, the elastic modulus and tensile strength of concrete are also the same; thus, the elastic modulus and tensile strength values ​​of concrete at different times and locations (x, y, z) can be obtained.

[0029] In step seven, the autogenous shrinkage values ​​of concrete at different times and locations (x, y, z) are calculated by combining temperature history with maturity theory. First, the autogenous shrinkage development curve of concrete at 20℃ is measured in the laboratory. Taking 20℃ as a reference, when the maturity is the same, the autogenous shrinkage of concrete is also the same, thus obtaining the autogenous shrinkage values ​​of concrete at different times and locations (x, y, z).

[0030] The tensile strength-cracking resistance η(t,T,x,y,z) of any point inside the concrete affected by temperature at time t is defined as follows: when η(t,T,x,y,z)>1.0, the concrete will definitely crack; when 0.7<η(t,T,x,y,z)≤1.0, the concrete may crack; when η(t,T,x,y,z)≤0.7, the concrete will basically not crack.

[0031] In step eight, when the temperature rises, ΔT>0, which is compressive strain; when the temperature drops, ΔT<0, which is tensile strain.

[0032] Beneficial effects: This invention discloses a method for predicting the temperature field and crack resistance of large-volume concrete. Compared with existing methods, the method proposed in this invention provides an accurate way to predict the performance and deformation of concrete at any temperature history and at any age, which improves the prediction accuracy of the temperature field and cracking index of large-volume concrete, and saves a lot of time and labor costs. Attached Figure Description

[0033] Figure 1 It is 1 / 4 of the bottom slab concrete. U (0,0,5), 1 M (0,0,2.75), 3 U (34,0,5) and 3 M (34,0,2.75) Location diagram;

[0034] Figure 2 This is the relationship between the elastic modulus of OPC concrete at 20℃ and time.

[0035] Figure 3 This is the tensile strength-time relationship of OPC concrete at 20℃.

[0036] Figure 4 This is the relationship between the elastic modulus and maturity of OPC concrete at 20℃.

[0037] Figure 5 This is the relationship between the tensile strength and maturity of OPC concrete at 20℃.

[0038] Figure 6 It is a comparison of temperature simulation and actual measurement results for large-volume concrete. Figure 6 (a) in the text is point 1. U (0,0,5) and 1 M (0,0,2.75) Traditional method, Figure 6 (b) in the text is point 1. U (0,0,5) and 1 M (0,0,2.75) New method; Figure 6 (c) in the text is point 3. U (34,0,5) and 3 M (34,0,2.75) Traditional method, Figure 6 (d) in the text is point 3. U (34,0,5) and 3 M (34,0,2.75) New method;

[0039] Figure 7 This is a comparison of simulated and measured results of strain in large-volume concrete. Figure 7 (a) in the text is point 1. U (0,0,5) and 1 M (0,0,2.75) Traditional method, Figure 7 Point (b) 1 in U (0,0,5) and 1 M (0,0,2.75) New method; Figure 7 (c) in the text is point 3. U (34,0,5) and 3 M (34,0,2.75) Traditional method, Figure 7 Point (d) 3 inU (34,0,5) and 3 M (34,0,2.75) New method;

[0040] Figure 8 OPC concrete at different pouring temperatures at point 1 U (0,0,5) and 1 M Calculation results of temperature evolution for (0,0,2.75);

[0041] Figure 9 OPC concrete at different pouring temperatures at point 1 U (0,0,5) and 1 M The development of the cracking index (0,0,2.75); Figure 10 OPC concrete at point 1 with different amounts of cementitious materials U (0,0,5) and 1 M Calculation results of temperature evolution for (0,0,2.75);

[0042] Figure 11 OPC concrete at point 1 with different amounts of cementitious materials U (0,0,5) and 1 M The development of the cracking index (0,0,2.75).

[0043] Figure 12 This is the computational framework of the present invention. Detailed Implementation

[0044] The present invention will be further described with reference to the following embodiments. However, it should be understood that these embodiments are for illustrative purposes only and should not be construed as limiting the implementation of the present invention. The present invention will now be described in further detail with reference to specific embodiments.

[0045] This invention relates to a method for predicting the temperature field and crack resistance of large-volume concrete based on isothermal calorimetry and maturity theory. The steps are as follows:

[0046] (1) First, a solid model of a large volume concrete is created in ABAQUS. The concrete size is 76.1m×74.1m×5.5m and the concrete grade is C50.

[0047] (2) Mesh generation was performed, and DC3D8 thermal conductivity cube elements were selected. Due to the large number of meshes, reduced integral was used during the calculation process.

[0048] (3) Establish the heat conduction equation. Where m is the amount of cementitious material per unit volume of concrete, kg / m³ 3;q represents the exothermic reaction result of the cementitious material using isothermal calorimetry. Where q = dQ' / dt, Q' is the exothermic heat per unit mass of cementitious material, J / g; c is the specific heat of concrete, kJ / (kg·℃); ρ is the density of concrete, kg / m³. 3 λ is the thermal conductivity of concrete, W / (m·℃).

[0049]

[0050] Where Q'(t,T') is the heat release per unit mass of cementitious material at time t due to temperature; A refers to the pre-factor; R is the gas constant, 8.314 J / (mol·K); T' is the isothermal temperature, °C; E a Activation energy, J / mol;

[0051] (5) First, measure the reaction heat of cementitious materials in concrete under isothermal calorimetry conditions of 20℃ and 40℃, and then calculate Q' based on the nonlinear least squares method. max , A and E a .

[0052] (6) The thermal conductivity and specific heat capacity of concrete were measured by a thermal conductivity tester (plate steady-state method) and DSC sapphire method, respectively.

[0053] (7) Further, the temperature field boundary conditions and initial conditions are set. At the initial instant, the temperature field distribution of the large-volume concrete is set to a constant of 35℃ or 30℃. The third type of boundary condition is adopted, assuming that the heat flow through the concrete surface is related to the concrete surface temperature T and the ambient temperature T. a The difference is directly proportional, that is In the formula, β is the thermal convection coefficient, W / (m²). 2 •K). The bottom of the concrete is subject to Type IV boundary conditions. λ1 and λ2 are the thermal conductivity coefficients of concrete and bedrock, respectively.

[0054] (8) Furthermore, the maturity of concrete can be calculated using the following formula:

[0055] S=(t e -τ)T0

[0056] In the formula, S represents maturity, in °C·h; t e τ is the sum of equivalent ages under actual temperature history, h; T0 is the final setting time of concrete, h; T0 is the reference temperature, ℃.

[0057] Based on the maturity calculation formula, the elastic modulus, tensile strength, and autogenous shrinkage development curves of concrete at 20℃ are first measured in the laboratory. Using 20℃ as a reference, when the maturity is the same, the elastic modulus, tensile strength, and autogenous shrinkage of the concrete are also the same. Therefore, the elastic modulus, tensile strength, and autogenous shrinkage values ​​of concrete at different times and locations (x, y, z) can be obtained.

[0058] (9) Furthermore, the elastic strain of concrete under the action of reinforcement can be calculated by the following formula:

[0059]

[0060] In the formula, the first term represents the tensile strain caused by the shrinkage of the concrete itself, and the second term represents the strain caused by changes in the concrete temperature (ΔT>0 during heating, representing compressive strain; ΔT<0 during cooling, representing tensile strain). Where n=E s / E c k = R' / E c ρ' is the reinforcement ratio of reinforced concrete, %; R' is the degree of restraint, which can be approximated as 1 for embedded slab structures, E c E represents the elastic modulus of concrete, expressed in GB / T 100; E represents the elastic modulus of concrete. s α represents the elastic modulus of the reinforcing steel, in GPa; c The coefficient of thermal expansion of concrete is 13.56 × 10⁻⁶. -6 / ℃;α s The coefficient of thermal expansion of the reinforcing steel is taken as 1.2 × 10⁻⁶. -5 / ℃.

[0061] (10) Furthermore, the concrete strain under creep effect can be obtained by the following formula:

[0062] In the formula, ε cr (t) represents creep strain; ε ce (t) represents elastic strain; This is the creep coefficient.

[0063] Among them, the creep coefficient of concrete It can be calculated using the following formula:

[0064]

[0065]

[0066] In the formula, t e To calculate the equivalent age of concrete at the time considered, d; τ is the final setting time of concrete, d; f cm β represents the compressive strength of concrete, in MPa; His a coefficient related to relative humidity, %; RH is the relative humidity of the concrete, %; h is the concrete thickness, mm.

[0067] (11) Further, the formula for calculating the concrete cracking index is:

[0068]

[0069] In the formula, E(t,T,x,y,z) is the elastic modulus of any point inside the concrete affected by temperature at time t, in GPa; ε cr (t,T,x,y,z) represents the creep strain at any point inside the concrete at time t due to temperature influence; f t (t,T,x,y,z) represents the tensile strength (cracking resistance) of any point inside the concrete at time t under the influence of temperature, in MPa.

[0070] Example

[0071] I. Based on the results of the 20℃ isothermal calorimetry experiment, Q'(24h, 293.15K), Q'(72h, 293.15K), and Q'(168h, 293.15K) can be obtained for t=1d, 3d, and 7d; based on the results of the 40℃ isothermal calorimetry experiment, Q'(24h, 313.15K), Q'(72h, 313.15K), and Q'(168h, 313.15K) can be obtained for t=1d, 3d, and 7d.

[0072] II. The elastic modulus and tensile strength of concrete at 20℃ for 3 days, 7 days, 28 days, and 90 days were measured in the laboratory. A mathematical relationship between the elastic modulus and time was obtained through fitting. Then, combined with the maturity formula, the relationship between the elastic modulus and tensile strength of concrete and its maturity level was obtained. Based on the temperature development history of concrete at different locations (x, y, z) calculated using ABAQUS finite element software, with 20℃ as a reference, the elastic modulus and tensile strength of concrete are the same when the maturity level is the same. Therefore, the elastic modulus and tensile strength values ​​of concrete at different times and locations (x, y, z) can be obtained.

[0073] Third, the autogenous shrinkage of 20℃ concrete over time was tested using a corrugated pipe shrinkage tester under constant temperature conditions in a water bath. Similarly, by combining temperature history and maturity theory, the autogenous shrinkage values ​​of concrete at different times and locations (x, y, z) can be obtained.

[0074] IV. Solving Q' based on the nonlinear least squares method max M smax , A and E a

[0075] V. Maturity S is calculated using the following formula: S = (te -τ)T0

[0076] VI. The elastic strain of concrete under the action of reinforcing steel can be calculated using the following formula:

[0077]

[0078] In the formula, the first term represents the tensile strain caused by the shrinkage of the concrete itself, and the second term represents the strain caused by changes in the concrete temperature (ΔT>0 during heating, representing compressive strain; ΔT<0 during cooling, representing tensile strain). Where n=E s / E c k = R' / E c ρ' is the reinforcement ratio of reinforced concrete, %; R' is the degree of restraint, which can be approximated as 1 for embedded slab structures, E c E represents the elastic modulus of concrete, expressed in GB / T 100; E represents the elastic modulus of concrete. s α represents the elastic modulus of the reinforcing steel, in GPa; c The coefficient of thermal expansion of concrete is 13.56 × 10⁻⁶. -6 / ℃;α s The coefficient of thermal expansion of the reinforcing steel is taken as 1.2 × 10⁻⁶. -5 / ℃.

[0079] VII. The concrete strain under creep effect can be obtained by the following formula:

[0080] In the formula, ε cr (t) represents creep strain; ε ce (t) represents elastic strain; This is the creep coefficient.

[0081] Among them, the creep coefficient of concrete It can be calculated using the following formula:

[0082]

[0083] In the formula, t e To calculate the equivalent age of concrete at the time considered, d; τ is the final setting time of concrete, d; f cm β represents the compressive strength of concrete, in MPa; H is a coefficient related to relative humidity, %; RH is the relative humidity of the concrete, %; h is the concrete thickness, mm.

[0084] 8. The formula for calculating the concrete cracking index is:

[0085]

[0086] In the formula, E(t,T,x,y,z) is the elastic modulus of any point inside the concrete affected by temperature at time t, in GPa; εcr (t,T,x,y,z) represents the creep strain at any point inside the concrete at time t due to temperature influence; f t (t,T,x,y,z) represents the tensile strength (cracking resistance) of any point inside the concrete at time t under the influence of temperature, in MPa.

[0087] The following calculation examines the temperature field and crack resistance of mass concrete based on isothermal calorimetry and maturity theory:

[0088] A large-volume concrete structure measures 76.1m × 74.1m × 5.5m, with a concrete grade of C50. The concrete mix proportions are shown in Table 1. The concrete was poured in summer, with an average temperature of 35℃, in a sealed environment preventing water loss. Before pouring the concrete, sensors were pre-attached to the bottom of the reinforcing steel bars. For ease of observation and comparative analysis, the attachment points were clearly marked, totaling four points: 1 U (0,0,5), 1 M (0,0,2.75), 3 U (34,0,5) and 3 M (34,0,2.75), as Figure 1 As shown.

[0089] First, the elastic modulus and tensile strength of concrete at 20℃ for 3 days, 7 days, 28 days, and 90 days were measured in the laboratory. Then, the mathematical relationship between the elastic modulus and tensile strength and time was obtained through fitting, as follows: Figure 2 and Figure 3 As shown. Combining the maturity formula, the relationship between the elastic modulus and tensile strength of concrete and its maturity can be obtained, as follows: Figure 4 and Figure 5 As shown.

[0090] Next, under given initial and boundary conditions, the temperature field of the large-volume concrete can be calculated based on the provided heat conduction equation, heat source function (traditional and new methods), and model parameters. The measured and predicted temperature values ​​are then compared. Figure 6 As shown. Further, the measured strain values ​​and predicted strain values ​​are compared, as follows... Figure 7 As shown. In calculating the temperature field, traditional methods use adiabatic temperature rise as the heat source; in solving the strain field, traditional methods are based on hydration degree. To evaluate the calculation accuracy of traditional and new methods, and to characterize the correlation between predicted and actual values, the following formula can be used for calculation:

[0091]

[0092] In the formula, y i It is the true value; f i It is a predicted value; It is the average of the true values; R 2 For prediction accuracy, R 2 The closer the value is to 1, the higher the accuracy of the model's predictions.

[0093] The calculation results are shown in Tables 1 and 2. Compared with the traditional temperature field calculation method that uses adiabatic temperature rise as the heat source function, the new method can improve the temperature prediction accuracy at measuring point 1 by 5% to 52% and at measuring point 3 by 4% to 30%.

[0094] Table 1. Correlation Statistics between Measured and Predicted Temperature Values ​​at Measurement Point #1

[0095]

[0096] Note: 1 U "All" represents the entire curve; "1" represents the entire curve. U "Up" represents the temperature rise section of the curve; "1" U "Down" represents the cooling section of the curve; the rest are similar.

[0097] Table 2. Correlation Statistics of Measured and Predicted Temperature Values ​​at Measurement Point #3

[0098]

[0099] Similarly, compared with the traditional method, the new method has improved the prediction accuracy of the full strain development curve, the descending segment curve and the ascending segment curve at the 1# and 3# measuring points, as shown in Table 3.

[0100] Table 3. Correlation statistics between measured and predicted strain values ​​at measuring points #1 and #3

[0101]

[0102] Note: 1 U "All" represents the entire curve; "1" represents the entire curve. U "Down" represents the descending segment of the curve; "1" U "Up" represents the rising segment of the curve; the rest are similar.

[0103] according to Figure 6 Temperature field analysis results show that the highest temperature at the center of the mass concrete exceeds 80℃, which does not comply with the relevant provisions of GB51028 "Technical Specification for Temperature Measurement and Control of Mass Concrete," which states that the center temperature of mass concrete should not exceed 80℃. Furthermore, according to GB 50496 "Standard for Construction of Mass Concrete," the concrete placement temperature should be controlled between 5 and 30℃. Therefore, this invention predicts the temperature field and cracking index of mass concrete by reducing the placement temperature (35℃→30℃). The calculation results are as follows... Figure 8As shown, when the placement temperature decreased from 35℃ to 30℃, the highest temperature at the center of the OPC concrete decreased by approximately 10℃, which complies with the requirement in GB 51028 "Technical Specification for Temperature Control of Mass Concrete" that the center temperature should not exceed 80℃. Furthermore, the cracking index was also significantly reduced, such as... Figure 9 As shown. Therefore, reducing temperature rise is key to suppressing cracking in mass concrete. By lowering the temperature of the concrete mixture before it is poured into the formwork, the internal temperature and cracking index of the concrete can be effectively reduced.

[0104] Similarly, besides lowering the pouring temperature to reduce the maximum temperature at the center of large-volume concrete, the heat release from hydration can also be reduced by adjusting the amount of cementitious materials in the concrete. For example... Figure 10 and Figure 11 As shown, when the amount of cementitious material is reduced by 10%, the maximum temperature at the center of the concrete can be reduced by about 8°C, and the cracking index also decreases to some extent. This indicates that reducing the amount of cementitious material has a certain effect on controlling the maximum temperature at the center and the cracking index of large-volume concrete, but it is still somewhat inferior to reducing the temperature at the time of placement.

[0105] The above embodiments describe the present invention and its implementation methods. This description is not restrictive; the examples listed are only some embodiments of the present invention, and actual implementation methods are far from limited to these. Therefore, any manufacturing methods and embodiments similar to the technical solutions of the present invention, without departing from the spirit of the invention, and without creative design, shall fall within the protection scope of the present invention.

Claims

1. A method for predicting the temperature field and crack resistance of large-volume concrete, characterized in that, Includes the following steps: Step 1: Improve the heat conduction equation. The original equation... It should be rewritten as: Where m is the amount of cementitious material per unit volume of concrete, kg / m³ 3 q represents the heat of hydration of a unit mass of cementitious material per unit time under constant temperature conditions, in J / (kg·h); c represents the specific heat of concrete, in kJ / (kg·℃); ρ represents the density of concrete, in kg / m³. 3 ; λ is the thermal conductivity of concrete, W / (m·℃); T is the temperature of concrete, ℃; and These represent the diffusion rates of concrete temperature T in the x, y, and z directions, respectively; Q is the heat emitted per unit volume per unit time, in J / (m³). 3 ·h); Step 2: In Step 1, q is the result of the exothermic reaction of the cementitious material from the isothermal calorimetry test, where q = dQ' / dt, and Q' is the heat release per unit mass of cementitious material, J / g. Step 3: Establish the calculation equation for the heat source: Where Q'(t,T') is the heat release per unit mass of cementitious material at time t due to temperature; A refers to the pre-factor; R is the gas constant, 8.314 J / (mol·K); T' is the isothermal temperature, °C; E a Activation energy, J / mol; Step four: First, measure the heat of reaction of cementitious materials in concrete under isothermal calorimetry conditions of 20℃ and 40℃. Then, based on the nonlinear least squares method, Q' can be calculated. max , A and E a ; Step 5: The thermal conductivity and specific heat capacity of the concrete were measured using a thermal conductivity meter with the steady-state plate method and the DSC sapphire method, respectively. Step six, the maturity of the concrete is calculated using the following formula: S=(t e -τ)T0 In the formula, S represents maturity, in °C·h; t e τ is the sum of equivalent ages under actual temperature history, h; T0 is the final setting time of concrete, h; T0 is the reference temperature, ℃. Step 7: Based on the maturity calculation formula, the elastic modulus, tensile strength, and autogenous shrinkage value of concrete at different times and locations (x, y, z) can be obtained. Step 8: Calculate the elastic strain of concrete under the action of reinforcing steel using the following formula: In the formula, the first term represents the tensile strain caused by the shrinkage of the concrete itself, and the second term represents the strain caused by the temperature change of the concrete; where n = E s / E c k = R' / E c ρ' is the reinforcement ratio of reinforced concrete, %; R' is the degree of restraint, approximately 1 for embedded slab structures, E c E represents the elastic modulus of concrete, expressed in GB / T 100; E represents the elastic modulus of concrete. s α represents the elastic modulus of the reinforcing steel, in GPa; c The coefficient of thermal expansion of concrete is 13.56 × 10⁻⁶. -6 / ℃;α s The coefficient of thermal expansion of the reinforcing steel is taken as 1.2 × 10⁻⁶. -5 / ℃; Step nine, the concrete strain under creep effect can be obtained by the following formula: In the formula, ε cr (t) represents creep strain; ε ce (t) represents elastic strain; The creep coefficient; Among them, the creep coefficient of concrete Calculated using the following formula: In the formula, t e To calculate the equivalent age of concrete at the time considered, d; τ is the final setting time of concrete, d; f cm β represents the compressive strength of concrete, in MPa; H , is the coefficient related to relative humidity, %; RH is the relative humidity of the concrete, %; h is the concrete thickness, mm; Step 10, the formula for calculating the concrete cracking index is: In the formula, E(t,T,x,y,z) is the elastic modulus of any point inside the concrete affected by temperature at time t, in GPa; ε cr (t,T,x,y,z) represents the creep strain at any point inside the concrete at time t due to temperature influence; f t (t,T,x,y,z) represents the tensile strength-cracking resistance of any point inside the concrete at time t under the influence of temperature, in MPa.

2. The method for predicting the temperature field and crack resistance of large-volume concrete according to claim 1, characterized in that, In step seven, the elastic modulus and tensile strength values ​​of concrete at different times and locations (x, y, z) are calculated using temperature history combined with maturity theory. First, the development curves of elastic modulus and tensile strength of concrete at 20℃ are measured in the laboratory. Using 20℃ as a reference, when the maturity is the same, the elastic modulus and tensile strength of concrete are also the same; thus, the elastic modulus and tensile strength values ​​of concrete at different times and locations (x, y, z) can be obtained.

3. The method for predicting the temperature field and crack resistance of large-volume concrete according to claim 1, characterized in that, In step seven, the autogenous shrinkage values ​​of concrete at different times and locations (x, y, z) are calculated by combining temperature history with maturity theory. First, the autogenous shrinkage development curve of concrete at 20℃ is measured in the laboratory. Taking 20℃ as a reference, when the maturity is the same, the autogenous shrinkage of concrete is also the same, thus obtaining the autogenous shrinkage values ​​of concrete at different times and locations (x, y, z).

4. The method for predicting the temperature field and crack resistance of large-volume concrete according to claim 1, characterized in that, The tensile strength-cracking resistance η(t,T,x,y,z) of any point inside the concrete affected by temperature at time t is defined as follows: when η(t,T,x,y,z)>1.0, the concrete will definitely crack; when 0.7<η(t,T,x,y,z)≤1.0, the concrete may crack; when η(t,T,x,y,z)≤0.7, the concrete will basically not crack.

5. The method for predicting the temperature field and crack resistance of large-volume concrete according to claim 1, characterized in that, In step eight, when the temperature rises, ΔT>0, which is compressive strain; when the temperature drops, ΔT<0, which is tensile strain.

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