Mass concrete temperature field and crack resistance prediction method

Abstract

The invention discloses a mass concrete temperature field and crack resistance prediction method. According to the method, firstly, a heat source function calculation method combining reaction heat release of a cementing material based on isothermal calorimetry and an Arrhenius equation is established, and the mass concrete temperature field calculation process is improved; in addition, the previous research is mostly based on a strain field solved based on the hydration degree, but the hydration degree cannot be accurately calculated, and the problem can be solved by adopting a maturity theory. Compared with a traditional method, the temperature and strain development of the mass concrete can be predicted more accurately, the temperature prediction precision is improved by 5%-52%, and the strain prediction precision is improved by 18%-43%.

Description

Technical Field

[0001] The present invention relates to a method for predicting the temperature field and crack resistance of mass concrete, and belongs to the technical field of building structure analysis and calculation. Background Art

[0002] With the continuous investment in large-scale infrastructure such as water conservancy, construction, and transportation in China, the application of mass concrete is becoming increasingly widespread. In the past, a large number of studies have been carried out on the problem that a large amount of hydration heat is generated due to the hydration reaction of early-age concrete, and the violent temperature changes during the internal temperature rise and fall processes are extremely likely to cause temperature shrinkage deformation, resulting in cracking. Many successful experiences in controlling cracks have been accumulated. However, with the construction requirements of ultra-large-scale infrastructure such as deep sea, deep earth, and urban underground space in China, and the application of various general raw materials, there is an urgent need for a more accurate concrete crack control technology to avoid resource waste caused by excessive control due to cost reduction and environmental protection. However, currently, the heat release amount converted from the adiabatic temperature rise test is usually used as the heat source function in the temperature control equation, resulting in a large deviation between the simulation calculation results and the actual situation. In fact, after the concrete is poured, due to the heat dissipation effect, the temperatures of different parts are different, and the temperature change will 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, the present invention first establishes a calculation method for the heat source function by combining the reaction heat release of cementitious materials based on isothermal calorimetry and the Arrhenius equation, and improves the calculation process of the temperature field of mass concrete. Further, most of the previous studies only involve the calculation of the temperature field, and the calculation of parameters such as tensile strength and elastic modulus does not involve the influence of different temperature histories and the development of maturity at different spatial positions of mass concrete. In addition, currently, the "cracking risk" is usually used to evaluate whether mass concrete cracks, but in fact, the cracking risk is the probability density of several cracking indices. Therefore, in this paper, the "cracking index" is uniformly used to evaluate the cracking possibility of mass concrete. Based on the above analysis, the following improved calculation framework for the temperature field and cracking index of mass concrete is proposed, as shown in the abstract drawing. It mainly includes: ① improving the heat conduction equation to be applicable to the temperature history difference at any spatial position (x, y, z) of mass concrete; ② changing the adiabatic temperature rise of the heat source function to a new calculation method based on isothermal calorimetry and the Arrhenius equation; ③ proposing a calculation method for strength and elastic modulus based on the maturity theory and the actual temperature development history at a certain position (x, y, z) in mass concrete, and considering the damage to the mechanical properties of concrete under the condition of relatively high internal temperature. Summary of the Invention

[0004] Technical problem: The technical problem to be solved by the present invention is to provide a method for predicting the temperature field and crack resistance of mass concrete, which is more accurate and reliable, aiming at the problems existing in the prediction of the temperature field and crack resistance of mass concrete at present.

[0005] Technical solution: A method for predicting the temperature field and crack resistance of mass concrete according to the present invention includes the following steps:

[0006] Step 1, improve the heat conduction equation, and the original equation should be rewritten as: where m is the amount of cementitious materials per unit volume of concrete, kg / m 3 ; q is the heat of hydration released per unit mass of cementitious materials per unit time under isothermal conditions, J / (kg·h); c is the specific heat of concrete, kJ / (kg·°C); ρ is the density of concrete, kg / m 3 ; λ is the thermal conductivity of concrete, W / (m·°C); T is the temperature of concrete, °C; and are the diffusion rates of the concrete temperature T in the x, y, and z directions respectively; Q is the heat generated per unit time per unit volume, J / (m 3 ·h);

[0007] Step 2, in Step 1, q adopts the heat release result of the cementitious material reaction in the isothermal calorimetry test, where q = dQ' / dt, Q' is the heat release of the cementitious material per unit mass, J / g;

[0008] Step 3, establish the calculation equation of the heat source:

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

[0010] Step 4, first, measure the heat of reaction of the cementitious materials in the concrete under the isothermal calorimetry conditions of 20°C and 40°C, and then based on the non-linear least squares method, Q' max , A and E a can be obtained;

[0011] Step 5, the thermal conductivity and specific heat capacity of the concrete are measured by the flat plate steady state method and the DSC sapphire method with a thermal conductivity tester respectively;

[0012] Step 6, the maturity of the concrete is calculated by the following formula:

[0013] S=(t e -τ)T 0

[0014] Wherein, S is the maturity, °C·h; t e is the total equivalent age under the actual temperature history, h; τ is the final setting time of the concrete, h; T 0 is the reference temperature, °C;

[0015] Step Seven: According to the calculation formula of maturity, the elastic modulus, tensile strength and autogenous shrinkage value of the concrete at different times and different positions (x, y, z) can be obtained;

[0016] Step Eight: The elastic strain of the concrete considering the effect of steel bars is calculated by the following formula:

[0017]

[0018] Wherein, the first term is the tensile strain generated by the autogenous shrinkage of the concrete, and the second term is the strain generated when the temperature of the concrete changes; among them, n = E s / E c and k = R’ / E c , ρ’ is the reinforcement ratio of the reinforced concrete, %; R’ is the degree of restraint, approximately 1 for the fixed plate structure, E c is the elastic modulus of the concrete, GPa; E s is the elastic modulus of the steel bar, GPa; α c is the thermal expansion coefficient of the concrete, and the measured value is 13.56×10 -6 / °C; α s is the thermal expansion coefficient of the steel bar, taking 1.2×10 -5 / °C;

[0019] Step Nine: The strain of the concrete under the creep effect can be obtained by the following formula:

[0020] Wherein, ε cr (t) is the creep strain; ε ce (t) is the elastic strain; is the creep coefficient;

[0021] Among them, the creep coefficient of the concrete is calculated by the following formula:

[0022]

[0023] Wherein, t e is the equivalent age of the concrete at the calculation consideration time, d; τ is the final setting time of the concrete, d; f cm is the compressive strength of the concrete, MPa; β H is the coefficient related to the relative humidity, %; RH is the relative humidity of the concrete, %; h is the thickness of the concrete, mm;

[0024] Step 10, the calculation formula for the concrete cracking index is as follows:

[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) is the creep strain of any point inside the concrete affected by temperature at time t; f t (t,T,x,y,z) is the self-tensile strength - cracking resistance of any point inside the concrete affected by temperature at time t, in MPa.

[0027] Among them:

[0028] In Step 7, the elastic modulus and tensile strength values of concrete at different times and different positions (x,y,z) are calculated through the temperature history combined with the maturity theory. First, the development curves of the elastic modulus and tensile strength of concrete at 20°C are measured in the laboratory. Taking 20°C as the 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 different positions (x,y,z) can be obtained.

[0029] In Step 7, the autogenous shrinkage values of concrete at different times and different positions (x,y,z) are calculated through the temperature history combined with the maturity theory; first, the autogenous shrinkage development curve of concrete at 20°C is measured in the laboratory. Taking 20°C as the reference, when the maturity is the same, the autogenous shrinkage of concrete is also the same, and the autogenous shrinkage values of concrete at different times and different positions (x,y,z) are obtained.

[0030] For the self-tensile strength - cracking resistance η(t,T,x,y,z) of any point inside the concrete affected by temperature at time t, 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, there is a possibility of concrete cracking; when η(t,T,x,y,z)≤0.7, the concrete basically will not crack.

[0031] In Step 8, when the temperature rises, ΔT>0, which is compressive strain; when the temperature drops, ΔT<0, which is tensile strain.

[0032] Beneficial effects: The present invention discloses a method for predicting the temperature field and crack resistance of mass concrete. Compared with the existing methods, the method proposed by the present invention provides a method for accurately predicting the performance and deformation of concrete at any temperature history and any age, improves the prediction accuracy of the temperature field and cracking index of mass concrete, and saves a large amount of time and labor costs. Description of the Drawings

[0033] Figure 1 is 1 in the 1 / 4 bottom plate concrete U (0, 0, 5), 1 M (0, 0, 2.75), 3 U (34, 0, 5) and 3 M (34, 0, 2.75) location schematic diagram;

[0034] Figure 2 is the elastic modulus - time relationship of OPC concrete at 20°C;

[0035] Figure 3 is the tensile strength - time relationship of OPC concrete at 20°C;

[0036] Figure 4 is the elastic modulus - maturity relationship of OPC concrete at 20°C;

[0037] Figure 5 is the tensile strength - maturity relationship of OPC concrete at 20°C;

[0038] Figure 6 is the comparison between the temperature simulation and the measured results of mass concrete; Figure 6 In (a), it is point 1 U (0, 0, 5) and 1 M (0, 0, 2.75) traditional method, Figure 6 In (b), it is point 1 U (0, 0, 5) and 1 M (0, 0, 2.75) new method; Figure 6 In (c), it is point 3 U (34, 0, 5) and 3 M (34, 0, 2.75) traditional method, Figure 6 In (d), it is point 3 U (34, 0, 5) and 3 M (34, 0, 2.75) new method;

[0039] Figure 7 is the comparison between the strain simulation and the measured results of mass concrete, Figure 7 In (a), it is point 1 U (0, 0, 5) and 1 M (0, 0, 2.75) traditional method, Figure 7 In (b), point 1 U (0, 0, 5) and 1 M (0, 0, 2.75) new method; Figure 7 In (c), it is point 3 U (34, 0, 5) and 3 M (34, 0, 2.75) traditional method,Figure 7 Point 3 in (d) U (34, 0, 5) and 3 M (34, 0, 2.75) New method;

[0040] Figure 8 It is the calculation result of the temperature evolution of OPC concrete at point 1 under different initial pouring temperatures U (0, 0, 5) and 1 M (0, 0, 2.75);

[0041] Figure 9 It is the development of the cracking index of OPC concrete at point 1 under different initial pouring temperatures U (0, 0, 5) and 1 M (0, 0, 2.75); Figure 10 It is the calculation result of the temperature evolution of OPC concrete at point 1 under different cementitious material dosages U (0, 0, 5) and 1 M (0, 0, 2.75);

[0042] Figure 11 It is the development of the cracking index of OPC concrete at point 1 under different cementitious material dosages U (0, 0, 5) and 1 M (0, 0, 2.75).

[0043] Figure 12 It is the calculation framework of the present invention. Detailed implementation mode

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

[0045] The method for predicting the temperature field and crack resistance of mass concrete based on isothermal calorimetry and maturity theory of the present invention comprises the following steps:

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

[0047] (2) Conduct mesh division. Select DC3D8 heat conduction cubic elements. Due to a large number of mesh division elements in the calculation process, reduced integration is selected.

[0048] (3) Establish a heat conduction equation, where m is the cementitious material dosage per unit volume of concrete, kg / m 3; q adopts the heat release result of the cementitious material reaction in the isothermal calorimetry test. Among them, q = dQ’ / dt, Q’ is the heat release of the cementitious material per unit mass, J / g; c is the specific heat of the concrete, kJ / (kg·℃); ρ is the density of the concrete, kg / m 3 ; λ is the thermal conductivity of the concrete, W / (m·℃).

[0049]

[0050] Among them, Q’(t,T’) is the heat release of the cementitious material per unit mass affected by temperature at time t; A is the pre-exponential factor; R is the gas constant, 8.314 J / (mol·K); T’ is the constant temperature, ℃; E a is the activation energy, J / mol;

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

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

[0053] (7) Further, set the temperature field boundary conditions and initial conditions. At the initial moment, the temperature field distribution of the mass concrete is set to a constant of 35℃ or 30℃; adopt the third type of boundary condition, assuming that the heat flux passing through the concrete surface is proportional to the difference between the concrete surface temperature T and the ambient temperature T a , that is In the formula, β is the heat convection coefficient, W / (m 2 ·K). The fourth type of boundary condition is adopted at the bottom of the concrete, λ 1 and λ 2 are the thermal conductivities of the concrete and the bedrock respectively.

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

[0055] S = (t e -τ)T 0

[0056] In the formula, S is the maturity, ℃·h; t e is the total equivalent age under the actual temperature history, h; τ is the final setting time of the concrete, h; T 0 is the reference temperature, ℃.

[0057] According to the maturity calculation formula, first measure the elastic modulus, tensile strength and autogenous shrinkage development curve of concrete at 20°C in the laboratory. Taking 20°C as the reference, when the maturity is the same, the elastic modulus, tensile strength and autogenous shrinkage of concrete are also the same. Therefore, the values of the elastic modulus, tensile strength and autogenous shrinkage of concrete at different times and different positions (x, y, z) can be obtained.

[0058] (9) Further, the elastic strain of concrete considering the effect of steel bars can be calculated by the following formula:

[0059]

[0060] In the formula, the first term is the tensile strain generated by the autogenous shrinkage of concrete, and the second term is the strain generated when the temperature of concrete changes (when the temperature rises, ΔT>0, it is compressive strain; when the temperature drops, ΔT<0, it is tensile strain). Among them, n = E s / E c , k = R’ / E c , ρ’ is the reinforcement ratio of reinforced concrete, %; R’ is the degree of restraint, and the embedded plate structure can be approximated as 1, E c is the elastic modulus of concrete, GPa; E s is the elastic modulus of steel bars, GPa; α c is the thermal expansion coefficient of concrete, and the measured value is 13.56×10 -6 / °C; α s is the thermal expansion coefficient of steel bars, taking 1.2×10 -5 / °C.

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

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

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

[0064]

[0065]

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

[0067] (11) Further, the concrete cracking index calculation formula 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, GPa; ε cr (t, T, x, y, z) is the creep strain of any point inside the concrete affected by temperature at time t; f t (t, T, x, y, z) is the self-tensile strength (cracking resistance) of any point inside the concrete affected by temperature at time t, MPa.

[0070] Example

[0071] I. According to the results of the 20°C isothermal calorimetry test, Q’(24h, 293.15K), Q’(72h, 293.15K), and Q’(168h, 293.15K) at t = 1d, 3d, and 7d can be obtained; according to the results of the 40°C isothermal calorimetry test, Q’(24h, 313.15K), Q’(72h, 313.15K), and Q’(168h, 313.15K) at t = 1d, 3d, and 7d can be obtained;

[0072] II. Measure the elastic modulus and tensile strength of the concrete at 3d, 7d, 28d, and 90d at 20°C in the laboratory, and obtain the mathematical relationship between the elastic modulus and time through fitting. Then, combined with the maturity formula, the relationship between the elastic modulus and tensile strength of the concrete and maturity can be obtained. According to the temperature development history of the concrete at different positions (x, y, z) calculated by the ABAQUS finite element software, with 20°C as the reference, when the maturity is the same, the elastic modulus and tensile strength of the concrete are also the same. Therefore, the elastic modulus and tensile strength values of the concrete at different times and different positions (x, y, z) can be obtained.

[0073] III. Use a corrugated pipe shrinkage measuring instrument to test the time-dependent evolution of the autogenous shrinkage of the 20°C concrete under the condition of a water bath constant temperature environment. Similarly, combined with the temperature history and maturity theory, the autogenous shrinkage values of the concrete at different times and different positions (x, y, z) can be obtained.

[0074] IV. Based on the nonlinear least squares method, solve for Q’ max 、M smax 、A and E a

[0075] V. The maturity S is calculated by the following formula: S = (te -τ)T 0

[0076] VI. The elastic strain of concrete considering the effect of steel bars can be calculated by the following formula:

[0077]

[0078] In the formula, the first term is the tensile strain generated by the autogenous shrinkage of concrete, and the second term is the strain generated when the temperature of concrete changes (when the temperature rises, ΔT>0, it is compressive strain; when the temperature drops, ΔT<0, it is tensile strain). Among them, n = E s / E c , k = R’ / E c , ρ’ is the reinforcement ratio of reinforced concrete, %; R’ is the degree of restraint, and the embedded plate structure can be approximated as 1, E c is the elastic modulus of concrete, GPa; E s is the elastic modulus of steel bars, GPa; α c is the thermal expansion coefficient of concrete, and the measured value is 13.56×10 -6 / °C; α s is the thermal expansion coefficient of steel bars, taking 1.2×10 -5 / °C.

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

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

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

[0082]

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

[0084] VIII. The calculation formula for the concrete cracking index is:

[0085]

[0086] where \(E(t,T,x,y,z)\) is the elastic modulus of any point inside the concrete affected by temperature at time \(t\), in GPa; \(\varepsilon\) cr (t,T,x,y,z) is the creep strain of any point inside the concrete affected by temperature at time \(t\); \(f\) t (t,T,x,y,z) is the self-tensile strength (cracking resistance) of any point inside the concrete affected by temperature at time \(t\), in MPa.

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

[0088] The size of a certain mass concrete is \(76.1m\times74.1m\times5.5m\), the concrete grade is C50, the concrete mix ratio is shown in Table 1, it is poured in summer, the average temperature is \(35^{\circ}C\), and it is in a sealed and water-lossless environment. Before pouring the concrete, the sensors are tied to the bottom of the steel bars in advance. For the convenience of observation and comparative analysis, the tying positions are marked in detail, with a total of 4 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 shown.

[0089] First, measure the elastic modulus and tensile strength of the concrete at 3d, 7d, 28d, and 90d at \(20^{\circ}C\) in the laboratory, and obtain the mathematical relationships between the elastic modulus, tensile strength and time through fitting, as Figure 2 and Figure 3 shown. Combining with the maturity formula, the relationships between the elastic modulus, tensile strength of the concrete and maturity can be obtained, as Figure 4 and Figure 5 shown.

[0090] Next, under the given initial conditions and boundary conditions, according to the provided heat conduction equation, heat source functions (traditional method and new method) and model parameters, the temperature field of the mass concrete can be obtained, and the measured temperature values and predicted temperature values are compared, as Figure 6 shown. Further, the measured strain values and predicted strain values are compared, as Figure 7 shown. When calculating the temperature field, the traditional method uses the adiabatic temperature rise as the heat source; when solving the strain field, the traditional method is calculated based on the degree of hydration. In order to evaluate the calculation accuracy of the traditional method and the new method and characterize the correlation between the predicted value and the true value, it can be calculated by the following formula:

[0091]

[0092] where \(y\) iis the true value; f i is the predicted value; is the average value of the true values; R 2 is the prediction accuracy, R 2 The closer it is to 1, the higher the accuracy of the model prediction.

[0093] The calculation results are shown in Table 1 and Table 2. Compared with the traditional temperature field calculation method using adiabatic temperature rise as the heat source function, the temperature prediction accuracy of the new method at the 1# measurement point position can be improved by 5% - 52%, and the temperature prediction accuracy at the 3# measurement point position can be improved by 4% - 30%.

[0094] Table 1 Statistical table of the correlation between the measured and predicted temperatures at the 1# measurement point

[0095]

[0096] Note: "1 U Full" represents the full curve; "1 U Up" represents the temperature rise section curve; "1 U Down" represents the temperature drop section curve; the rest are similar.

[0097] Table 2 Statistical table of the correlation between the measured and predicted temperatures at the 3# measurement point

[0098]

[0099] Similarly, compared with the traditional method, the prediction accuracy of the new method on the full curve, descending section curve and ascending section curve of the strain development at the 1# and 3# measurement point positions has been improved, as shown in Table 3.

[0100] Table 3 Statistical table of the correlation between the measured and predicted strains at the 1# and 3# measurement points

[0101]

[0102] Note: "1 U Full" represents the full curve; "1 U Down" represents the descending section curve; "1 U Up" represents the ascending section curve; the rest are similar.

[0103] According to Figure 6From the analysis results of the temperature field, it can be seen that the highest temperature at the center of the mass concrete exceeds 80 °C, which does not conform to the relevant provisions in GB51028 "Technical Specification for Temperature Measurement and Control of Mass Concrete": the center temperature of mass concrete should not exceed 80 °C. At the same time, according to the requirements in GB 50496 "Construction Standard for Mass Concrete": the temperature of the concrete entering the formwork should be controlled between 5 and 30 °C. Therefore, in the present invention, by reducing the temperature of the concrete entering the formwork (35 °C → 30 °C), the temperature field and cracking index of the mass concrete are predicted. The calculation results are as Figure 8 shown. When the temperature of the concrete entering the formwork is reduced from 35 °C to 30 °C, the highest temperature at the center of the OPC concrete is reduced by about 10 °C, which conforms to the requirement in GB 51028 "Technical Specification for Temperature Measurement and Control of Mass Concrete" that the center temperature should not be higher than 80 °C. In addition, the cracking index is also significantly reduced, as Figure 9 shown. Therefore, reducing the temperature rise is the key to suppressing cracking in mass concrete. By reducing the temperature of the concrete mixture entering the formwork, the internal temperature and cracking index of the concrete can be effectively reduced.

[0104] Similarly, in addition to reducing the highest temperature at the center of the mass concrete by reducing the temperature of the concrete entering the formwork, the heat of hydration can also be reduced by adjusting the amount of cementitious materials in the concrete. As Figure 10 and Figure 11 shown, when the amount of cementitious materials is reduced by 10%, the highest temperature at the center of the concrete can be reduced by about 8 °C, and the cracking index is also reduced to a certain extent. This shows that reducing the amount of cementitious materials has a certain effect on controlling the highest temperature at the center of the mass concrete and the cracking index, but there is still a certain gap compared with the method of reducing the temperature of the concrete entering the formwork.

[0105] The above embodiments describe the present invention and its implementation manners. This description is not restrictive. The listed ones are only some implementation manners of the present invention, and the actual implementation manners are far from limited to this. Therefore, without departing from the purpose of the present invention, without creative design, the manufacturing methods and embodiments similar to the technical solutions of the present invention all belong to the protection scope of the present invention.

Claims

1. A method for predicting temperature field and crack resistance of mass concrete, characterized in that: The following steps are involved: Step 1: Improve the heat conduction equation. The original equation Should be rewritten as: Where m is the amount of cementitious material in unit volume of concrete, kg / m 3 ; q is the hydration heat of unit mass of cementitious material per unit time under constant temperature conditions, J / (kg.h); c is the specific heat of concrete, kJ / (kg·℃); ρ is the density of concrete, kg / m 3 ; λ is the thermal conductivity of concrete, W / (m·℃); T is the temperature of concrete, ℃; and are 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, J / (m 3 h); Step 2: q in step 1 is the heat release result of the gelling material reaction in an isothermal calorimetric test, wherein q=dQ' / dt, Q' is the heat release per unit mass of the gelling material, J / g; Step 3: Establish the calculation equation of heat source: Where Q'(t,T') is the heat released by unit mass of cementitious material at time t under the influence of temperature; A is the pre-exponential factor; R is the gas constant, 8.314 J / (mol·K); T' is the constant temperature, ℃; E a is the activation energy, J / mol; Step 4: First, measure the reaction heat of cementitious materials in concrete under isothermal calorimetric conditions at 20°C and 40°C, and then calculate Q' based on the nonlinear least squares method. max , A and E a ; Step 5: The thermal conductivity and specific heat capacity of the concrete are measured by a thermal conductivity tester using a plate steady-state method and a DSC sapphire method, respectively; Step 6: The maturity of concrete is calculated by the following formula: S=(t e -τ)T0 Where S is maturity, ℃·h; t e is the sum of equivalent ages under actual temperature history, h; τ is the final setting time of concrete, h; T0 is the reference temperature, °C; Step 7: According to the maturity calculation formula, the elastic modulus, tensile strength and autogenous shrinkage value of concrete at different locations (x, y, z) at different times can be obtained; Step 8: Calculate the elastic strain of concrete under the action of steel bars using the following formula: In the formula, the first term is the tensile strain caused by the shrinkage of concrete itself, and the second term is the strain caused by the change of concrete temperature; where n = E s / E c , k = R' / E c , ρ' is the reinforcement ratio of reinforced concrete, %; R' is the degree of constraint, which is approximately 1 for the embedded plate structure, E c is the elastic modulus of concrete, GPa; E s is the elastic modulus of the steel bar, GPa; α c is the thermal expansion coefficient of concrete, the measured value is 13.56×10 -6 / ℃;α s is the thermal expansion coefficient of the steel bar, which is 1.2×10 -5 / ℃; Step 9: The concrete strain under creep effect can be obtained by the following formula: In the formula, ε cr (t) is the creep strain; ε ce (t) is the elastic strain; is the creep coefficient; Among them, the creep coefficient of concrete Calculated by the following formula: In the formula, t e is the equivalent age of concrete at the time of calculation, d; τ is the final setting time of concrete, d; f cm is the compressive strength of concrete, MPa; β H is the coefficient related to relative humidity, %; RH is the relative humidity of concrete, %; h is the thickness of concrete, mm; Step 10: The concrete cracking index calculation formula is: Where E(t,T,x,y,z) is the elastic modulus of any point inside the concrete affected by temperature at time t, GPa; ε cr (t,T,x,y,z) is the creep strain of any point inside the concrete affected by temperature at time t; f t (t,T,x,y,z) is the tensile strength-cracking resistance of any point inside the concrete affected by temperature at time t, in MPa.

2. A method for predicting temperature field and crack resistance of mass concrete according to claim 1, characterized in that: In step seven, the elastic modulus and tensile strength values ​​of concrete at different locations (x, y, z) at different times are calculated by combining temperature history with maturity theory. First, the elastic modulus and tensile strength development curve of concrete at 20°C are measured in the laboratory. Taking 20°C as a reference, when the maturity is the same, the elastic modulus and tensile strength of concrete are also the same; the elastic modulus and tensile strength values ​​of concrete at different locations (x, y, z) at different times can be obtained.

3. A method for predicting temperature field and crack resistance of mass concrete according to claim 1, characterized in that: In step seven, the autogenous shrinkage values ​​of concrete at different locations (x, y, z) at different times are calculated by combining temperature history with maturity theory; first, the autogenous shrinkage development curve of concrete at 20°C is measured in the laboratory. Taking 20°C as a reference, when the maturity is the same, the autogenous shrinkage of concrete is also the same, and the autogenous shrinkage values ​​of concrete at different locations (x, y, z) at different times are obtained.

4. A method for predicting temperature field and crack resistance of mass concrete according to claim 1, characterized in that: The tensile strength of any point inside the concrete affected by temperature at time t is the cracking resistance η(t,T,x,y,z). 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. A method for predicting temperature field and crack resistance of mass concrete according to claim 1, characterized in that: In the step eight, when the temperature is increased, ΔT>0, which is compressive strain; when the temperature is decreased, ΔT<0, which is tensile strain.

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