Mass concrete structure temperature stress cracking prevention and control method

Abstract

The invention discloses a method for preventing and controlling temperature stress cracking of a mass concrete structure. The method comprises the following steps: preparing low-heat high-toughness concrete; low-heat high-toughness concrete is adopted for pouring a mass concrete structure, and simulation calculation is conducted on a transient temperature field and time-varying temperature stress in the mass concrete construction process; and according to a simulation calculation result, in order to avoid temperature stress cracking in the mass concrete structure, the concrete mold entering temperature, the pouring procedure and the construction maintenance are adjusted in good time. The method can be used for preparing the material capable of reducing the adiabatic temperature rise peak value of the mass concrete and improving the tensile strength of the concrete, reasonably determining the pouring construction maintenance system of the mass concrete, and preventing the mass concrete structure from cracking due to the fact that the temperature stress exceeds the allowable tensile strength of the concrete in the construction process.

Description

Technical Field

[0001] The present invention relates to the field of construction engineering, and in particular to a method for preventing and controlling temperature stress cracking of a large-volume concrete structure. Background Art

[0002] With the continuous construction of large-scale infrastructure such as high-rise buildings, long-span bridges, and traffic passages, mass concrete has become an important component of its structure. In the process of mass concrete application, cracks have occurred in many projects. There are two main reasons for the cracking of mass concrete structures:

[0003] The first is temperature difference cracks. In the early stage of concrete pouring, cement hydration generates a lot of heat, and the concrete itself has poor thermal conductivity. The heat accumulated inside the concrete structure causes the temperature to rise rapidly, which forms a temperature difference between the inside and the outside. When the external temperature is low and the internal temperature is high, tensile stress is formed on the surface of the concrete. Once it exceeds the tensile strength of the concrete, it will cause the concrete to crack. The second is shrinkage cracks. During the hardening process of concrete, the surface moisture loss is faster than the internal part, and the deformation is also greater. The surface shrinkage deformation is constrained by the internal concrete, and the tensile stress generated causes shrinkage cracks on the concrete surface.

[0004] To control the occurrence of cracks in large-volume concrete structures, it is necessary to reduce the hydration temperature rise of concrete and reduce the temperature difference between the inside and outside. At present, the main ways to control the peak adiabatic temperature rise and the temperature difference between the inside and outside of large-volume concrete are: 1. In terms of materials, the hydration heat of cementitious materials is reduced by reducing the amount of cementitious materials, using low-heat cement, phase change materials, and adding mineral admixtures such as fly ash to achieve the effect of temperature control. 2. In terms of construction, large-volume concrete is fully cooled by layered pouring, controlling the thickness of the concrete pouring layer, controlling the concrete pouring construction cycle, and pre-burying cooling water pipes. 3. In terms of maintenance, the surface cooling rate of large-volume concrete is controlled by combining film curing with temperature monitoring to make adjustments so that the surface temperature and internal temperature difference are within the specification range.

[0005] The above-mentioned prior art, although reducing the amount of cementitious materials effectively reduces the peak temperature of concrete, has an adverse effect on the strength and durability of concrete. In addition, in terms of design and construction, due to the lack of accurate judgment of the temperature field and stress field of large-volume concrete, there is no complete design plan and maintenance treatment measures, so it is difficult to accurately master the construction technology of large-volume concrete, and the lack of monitoring means for the temperature changes inside the large-volume concrete makes it difficult to effectively control the temperature and stress field.

[0006] Therefore, a method for preventing and controlling temperature stress cracking of large-volume concrete structures is needed, which can prepare materials that can reduce the peak adiabatic temperature rise of large-volume concrete and improve the tensile strength of concrete, reasonably determine the construction and maintenance system of large-volume concrete pouring, and prevent large-volume concrete structures from cracking due to temperature stress exceeding the allowable tensile strength of concrete during construction. Summary of the invention

[0007] In view of this, the purpose of the present invention is to overcome the defects in the prior art, provide a method for preventing and controlling temperature stress cracking of large-volume concrete structures, be able to prepare materials that can reduce the peak value of the thermal insulation temperature rise of large-volume concrete and improve the tensile strength of concrete, reasonably determine the large-volume concrete pouring construction and maintenance system, and prevent the large-volume concrete structure from cracking due to temperature stress exceeding the allowable tensile strength of concrete during the construction process.

[0008] The method for preventing and controlling temperature stress cracking of a large-volume concrete structure of the present invention comprises:

[0009] Preparation of low heat and high toughness concrete;

[0010] Use low-heat and high-toughness concrete to cast large-volume concrete structures, and simulate and calculate the transient temperature field and time-varying temperature stress during the construction process of large-volume concrete;

[0011] According to the simulation calculation results, with the goal of preventing temperature stress cracking inside the large-volume concrete structure, the concrete mold temperature, pouring process, and construction maintenance are adjusted in a timely manner.

[0012] Furthermore, low-hydration thermal cementitious materials, steel fibers, polymers, coarse and fine aggregates, high-efficiency water reducing agents and defoaming agents are used to prepare low-heat and high-toughness concrete; wherein the low-hydration thermal cementitious materials include low-heat Portland cement or low-heat Portland cement mixed with mineral admixtures; the mineral admixtures include fly ash.

[0013] Further, preparing low-heat high-toughness concrete specifically includes:

[0014] Dissolve 5% to 15% of the polymer of the cementitious material in 60% of the designed water volume, stir evenly and set aside; dissolve 0.5% to 2.0% of the high-efficiency water reducer of the cementitious material and 2‰ to 4‰ of the defoamer of the cementitious material in 40% of the designed water volume, stir evenly and set aside;

[0015] According to the designed mix ratio, put the coarse and fine aggregates and steel fibers into the mixer for dry mixing for 2 minutes, and the volumetric dosage of steel fibers is 0.75% to 1%. The storage environment temperature of the coarse and fine aggregates and steel fibers should be controlled before use to ensure that the temperature of the coarse and fine aggregates and steel fibers is between 5 and 25°C.

[0016] Add low-hydration thermal gelling material and stir for another 2 minutes. During the stirring process, evenly add mixing water containing polymer; after stirring evenly, slowly add mixing water containing water reducer and defoamer, and stir for another 3 minutes; after the mixture is evenly stirred, prepare concrete specimens according to the specifications, vibrate and compact them, and then put them into a standard curing room for curing.

[0017] Furthermore, low-heat and high-toughness concrete is used to cast large-volume concrete structures, including:

[0018] In summer, when the outdoor temperature is low, the temperature of concrete entering the mold should not exceed 30°C; in winter, the temperature of concrete entering the mold should not be lower than 5°C;

[0019] Reasonably design the construction process, and complete the continuous pouring in layers and sections. The layer thickness is determined according to the depth of the vibrator and the workability of the concrete, and shall not exceed 500mm. The interval between layer pouring shall not be longer than the initial setting time of the concrete.

[0020] When vibrating concrete, the distance between the vibrator insertion points should not be greater than 1.5 times the radius of action of the vibrator. The vibration time should be based on the appearance of floating slurry on the concrete surface and no longer sinking. The depth of the vibrator inserted into the lower layer of concrete should not be less than 50mm.

[0021] Large volume concrete also undergoes secondary finishing.

[0022] Furthermore, the transient temperature field and time-varying temperature stress of the mass concrete construction process are simulated and calculated, including:

[0023] Defining time-varying characteristic parameters of materials, including density, thermal conductivity and specific heat of concrete; constructing a finite element model for transient temperature field and time-varying temperature stress calculation and analysis;

[0024] The transient temperature field is used to analyze the temperature field and temperature stress of large-volume concrete. The load step end time is 14d, the time step is 0.5d, the number of loop iterations is 28 times, the convergence error is 0.01, and the initial temperature of the concrete is the actual temperature of the concrete slab when it is put into the mold. When performing finite element simulation, the time-dependent hydration heat is converted into a heat source through a functional relationship and loaded onto the nodes or units of the finite element model to simulate the heat generated by the hydration heat. The ambient temperature is set according to the change of the external atmospheric temperature over time. The heat release coefficient of the concrete surface is determined according to the type and thickness of the formwork used for pouring large-volume concrete and the maintenance measures after pouring.

[0025] The temperature field distribution cloud map of large-volume concrete pouring construction at different stages is calculated. According to the temperature field distribution cloud map, the calculation results of transient temperature peak at different ages after pouring are obtained; the maximum temperature stress at different periods and the concrete temperature stress time history curve at different periods after pouring are obtained through finite element calculation.

[0026] Furthermore, the following method is used to determine whether temperature stress cracking occurs inside the massive concrete structure:

[0027] If the temperature stress of large-volume concrete in a certain period of time is greater than the allowable tensile stress of concrete in that period, it indicates that temperature stress cracking of concrete will occur; if the temperature stress of large-volume concrete is less than the allowable tensile stress of concrete in that period of time, it indicates that temperature stress cracking of concrete will not occur.

[0028] Furthermore, the construction and maintenance specifically includes:

[0029] The temperature control of concrete curing is carried out in stages. Before the concrete reaches the peak temperature, it is not kept warm for curing. At this time, the concrete surface must be moist, and the curing water must be prevented from evaporating too quickly.

[0030] When the temperature inside the concrete reaches the peak, cover it with straw bags, sacks, cotton felt and a layer of plastic sheeting for insulation. If the insulation effect is not satisfactory, add another layer of insulation blanket and then cover it with a layer of plastic sheeting.

[0031] During the curing process, the temperature difference between the inside and outside of the concrete is controlled within 25°C through temperature monitoring, the cooling rate is controlled within 2.0°C / d, the difference between the concrete surface temperature and the atmospheric environment temperature is no more than 20°C, and the internal temperature of the concrete does not exceed 75°C;

[0032] When the insulation time is controlled until the difference between the internal temperature and the surface temperature of the concrete is less than 25℃, the insulation measures are terminated; the insulation cover layer is removed layer by layer, and when the difference between the surface temperature of the concrete and the maximum temperature of the environment is less than 20℃, all of it is removed; the difference between the curing water temperature and the surface temperature of the concrete is not greater than 15℃.

[0033] The beneficial effects of the present invention are as follows: a method for preventing and controlling temperature stress cracking of a large-volume concrete structure disclosed in the present invention develops low-heat and high-toughness concrete by adopting a variety of methods such as low-hydration thermal gelling materials, adding polymers, adding fibers, etc., to reduce the early hydration temperature rise and cooling rate after the pouring construction of large-volume concrete, reduce the temperature stress generated during the hydration process of concrete, and improve the crack resistance during the construction of large-volume concrete. Through the finite element numerical calculation of the temperature field during the pouring construction of large-volume concrete, the temperature field distribution characteristics of large-volume concrete at different construction stages are mastered, and the temperature field and temperature stress at different points inside the large-volume concrete are analyzed. During the pouring construction process, the temperature of different points and different time periods of large-volume concrete is monitored in real time, and the thickness of the large-volume concrete pouring construction layer, the construction process arrangement, the construction maintenance system, etc. are reasonably determined to prevent the large-volume concrete structure from cracking due to temperature stress exceeding the allowable tensile strength of concrete during the construction process. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0035] Figure 1 It is a schematic diagram of the process of the cracking prevention and control method of the present invention;

[0036] Figure 2 It is a schematic diagram of the peak heat release rate curve of the embodiment;

[0037] Figure 3 It is a schematic diagram of the total heat release curve of the embodiment;

[0038] Figure 4 The temperature field distribution cloud diagram of ordinary concrete NC-50 and steel fiber concrete SC-50;

[0039] Figure 5 The temperature field distribution cloud diagram of steel fiber concrete SC-50 and low heat high toughness concrete SPC-50;

[0040] Figure 6 The concrete temperature stress time history curves at different time periods after pouring of the embodiment are shown in the figure;

[0041] Figure 7 This is a diagram showing the arrangement of temperature measurement points for large-volume concrete in a vehicle passage of an embodiment. DETAILED DESCRIPTION

[0042] The present invention is further described below in conjunction with the accompanying drawings, as shown in the drawings:

[0043] This embodiment discloses a method for preventing and controlling temperature stress cracking of a large concrete structure, comprising the following steps:

[0044] Preparation of low heat and high toughness concrete;

[0045] Use low-heat and high-toughness concrete to cast large-volume concrete structures, and simulate and calculate the transient temperature field and time-varying temperature stress during the construction process of large-volume concrete;

[0046] According to the simulation calculation results, with the goal of preventing temperature stress cracking inside the large-volume concrete structure, the concrete mold temperature, pouring process, and construction maintenance are adjusted in a timely manner.

[0047] In this embodiment, low-heat high-toughness concrete is prepared by using low-hydration thermal gelling material, steel fiber, polymer, coarse and fine aggregate, high-efficiency water reducing agent and defoaming agent; wherein the low-hydration thermal gelling material includes low-heat Portland cement or low-heat Portland cement mixed with mineral admixtures; the mineral admixtures include fly ash. The preparation of low-heat high-toughness concrete specifically includes:

[0048] Dissolve 5% to 15% of the polymer of the cementitious material in 60% of the designed water volume, stir evenly and set aside; dissolve 0.5% to 2.0% of the high-efficiency water reducer of the cementitious material and 2‰ to 4‰ of the defoamer of the cementitious material in 40% of the designed water volume, stir evenly and set aside;

[0049] According to the designed mix ratio, put the coarse and fine aggregates and steel fibers into the mixer for dry mixing for 2 minutes, and the volumetric dosage of steel fibers is 0.75% to 1%. The storage environment temperature of the coarse and fine aggregates and steel fibers should be controlled before use to ensure that the temperature of the coarse and fine aggregates and steel fibers is between 5 and 25°C.

[0050] Add low-hydration thermal gelling material and stir for another 2 minutes. During the stirring process, evenly add mixing water containing polymer; after stirring evenly, slowly add mixing water containing water reducer and defoamer, and stir for another 3 minutes; after the mixture is evenly stirred, prepare concrete specimens according to the specifications, vibrate and compact them, and then put them into a standard curing room for curing.

[0051] Take the construction of large-volume concrete for a vehicle passage under a municipal road as an example. The top and bottom plates of the vehicle passage are 1.5 meters thick, the middle wall is 2 meters thick, and the side wall is 1.5 meters thick. The design uses C50 large-volume concrete with low heat, high toughness and excellent crack resistance. Each segment is poured vertically in three times. The first pouring is the bottom plate. After the bottom plate concrete has been cured for 7 days, the second concrete pouring begins. The second pouring is the middle wall and side wall concrete to the top plate position. After the side wall concrete has been cured for 7 days, the third top plate concrete pouring begins. The heat preservation and moisture maintenance is carried out for 7 days. Among them, the preparation of low-heat, high-toughness and excellent crack resistance concrete: the cementitious material uses low-heat silicate cement 42.5 cement, or 10% to 30% of grade II and above fly ash is added to the low-heat silicate cement; the crushed stone is 5-25mm limestone crushed stone, the sand is machine-made sand, the fineness modulus is 2.8, which is medium sand; the polymer is Styrofan SD 623ap carboxylated styrene butadiene latex, with a solid content of 50.0% and a viscosity of 8.98m 2 / s, surface tension is 100mN / m; steel fiber is cold-drawn steel wire cut type steel fiber, length 30mm, aspect ratio 60, tensile strength 980MPa; water reducer is polycarboxylic acid high-efficiency water reducer, water reduction rate is 25% to 35%, defoamer is low-viscosity BASF defoamer.

[0052] In this embodiment, low-heat high-toughness concrete is used to cast a large-volume concrete structure, which specifically includes:

[0053] In summer, when the outdoor temperature is low, the temperature of concrete entering the mold should not exceed 30°C; in winter, the temperature of concrete entering the mold should not be lower than 5°C;

[0054] Reasonably design the construction process, and complete the continuous pouring in layers and sections. The layer thickness is determined according to the depth of the vibrator and the workability of the concrete, and shall not exceed 500mm. The interval between layer pouring shall not be longer than the initial setting time of the concrete.

[0055] When vibrating concrete, the distance between the vibrator insertion points should not be greater than 1.5 times the radius of action of the vibrator. The vibration time should be based on the appearance of floating slurry on the concrete surface and no longer sinking. The depth of the vibrator inserted into the lower layer of concrete should not be less than 50mm.

[0056] Large-volume concrete is also subjected to secondary finishing and polishing to reduce surface shrinkage cracks.

[0057] In this embodiment, the transient temperature field and time-varying temperature stress of the mass concrete construction process are simulated and calculated, specifically including:

[0058] Define time-varying material characteristic parameters, which include density, thermal conductivity and specific heat of concrete; wherein, density, thermal conductivity and specific heat of concrete are all derived from test results, for example, density is taken as the measured value of concrete at 28 days, thermal conductivity and specific heat of concrete at different ages are calculated by fitting functions, and thermal conductivity and specific heat of ordinary concrete and low-heat high-toughness concrete vary with time; characteristic parameters such as thermal conductivity, specific heat and density of foundation are taken with reference to existing literature.

[0059] The test results of thermal conductivity of ordinary concrete and low-heat high-toughness concrete at different ages are shown in Table 1:

[0060] Table 1

[0061]

[0062] The specific heat test results of ordinary concrete and low-heat high-toughness concrete at different ages are shown in Table 2:

[0063] Table 2

[0064]

[0065] Finite element analysis software is used, such as Solid70 unit, to construct a finite element model for transient temperature field and time-varying temperature stress calculation and analysis; wherein, the transient temperature field is used to perform large-volume concrete temperature field and temperature stress analysis, the load step end time is 14d, the time step is 0.5d, the number of cycle iterations is 28 times, the convergence error is 0.01, and the initial temperature of the concrete is the actual measured mold temperature of the concrete slab; when performing finite element simulation, the time-dependent hydration heat is converted into a heat source through a functional relationship and loaded onto the nodes or units of the finite element model to simulate the heat generated by the hydration heat, thereby being able to more accurately analyze the transient temperature field of the concrete slab.

[0066] Among them, the hydration heat release rate and hydration heat release curve of the ordinary silicate cement system (JC), the silicate cement system mixed with fly ash (F10, F15, F20), and the low-heat silicate cement system mixed with polymer (L6, L9, L12, L15) are as follows: Figure 2 , Figure 3 As shown, based on the test results, the peak heat release rate and the total heat release are calculated.

[0067] The ambient temperature is set according to the change of the external atmospheric temperature over time. If the ambient temperature changes greatly during the day and night, and the temperature difference between day and night is large, it will have an adverse effect on the temperature field of large-volume concrete, resulting in an increase in the temperature gradient during the concrete curing process, generating temperature stress, and even causing concrete cracking. Therefore, finite element calculation and analysis should be used to predict the temperature field during concrete pouring and combine the monitoring results, and then reasonable measures should be taken for targeted protection to prevent the early temperature cracks in the concrete. The heat release coefficient of the concrete surface is determined according to the type and thickness of the formwork used for mass concrete pouring and the curing measures after pouring.

[0068] Calculate the temperature distribution cloud diagram at different stages of mass concrete pouring construction. For example, Figure 4 and Figure 5 The temperature field distribution cloud diagram shown in the figure is used to obtain the calculation results of the transient temperature peak values ​​at different ages after pouring, for example, the peak temperatures shown in Table 3 are obtained.

[0069] Table 3

[0070]

[0071] When steel fiber concrete (SC-50) and low heat high toughness concrete (SPC-50) are used to cast the vehicle passage, the maximum temperature stress at different times and the concrete temperature stress time history curves at different times after casting are obtained through finite element calculation according to the casting process, such as Figure 6 shown.

[0072] In this embodiment, whether temperature stress cracking occurs inside the massive concrete structure is determined according to the following method:

[0073] If the temperature stress of large-volume concrete in a certain period of time is greater than the allowable tensile stress of concrete in that period, it indicates that temperature stress cracking of concrete will occur; if the temperature stress of large-volume concrete is less than the allowable tensile stress of concrete in that period of time, it indicates that temperature stress cracking of concrete will not occur.

[0074] Among them, the tensile strength of concrete at each age is taken as the allowable tensile stress:

[0075] R f (t) = ab × l n (t+c);

[0076] Where: R f (t) is the tensile strength of concrete at different ages (MPa); l n is the splitting tensile strength; t is the concrete age (d); a, b, c are the fitting parameters of concrete tensile strength.

[0077] The splitting tensile strength of ordinary concrete and low-heat high-toughness concrete at different ages is shown in Table 4:

[0078] Table 4

[0079]

[0080] The fitting parameter results of the early-age tensile strength of different types of concrete are shown in Table 5:

[0081] Table 5

[0082]

[0083] The fitting results of tensile strength of concrete at different ages can be obtained as shown in Table 6:

[0084]

[0085] In this embodiment, the construction and maintenance specifically includes:

[0086] The temperature control of concrete curing is carried out in stages. Before the concrete reaches the peak temperature, it is not kept warm for curing. At this time, the concrete surface must be moist, and the curing water must be prevented from evaporating too quickly.

[0087] When the temperature inside the concrete reaches the peak, cover it with straw bags, sacks, cotton felt and a layer of plastic sheeting for insulation. If the insulation effect is not satisfactory, add another layer of insulation blanket and then cover it with a layer of plastic sheeting.

[0088] During the curing process, the temperature difference between the inside and outside of the concrete is controlled within 25°C through temperature monitoring, the cooling rate is controlled within 2.0°C / d, the temperature difference between the concrete surface and the atmospheric environment is no more than 20°C, and the internal temperature of the concrete does not exceed 75°C; for the underpass of a municipal road, temperature monitoring is carried out, specifically including: the temperature measurement points are arranged at the center and edge of the bottom plate, top plate, side wall, and middle wall of the underpass:

[0089] Temperature measuring point 1 is arranged at the center and edge of the bottom plate (0.1m away from the top surface of the bottom plate); temperature measuring point 2 is arranged at the center and edge of the middle wall (0.1m away from the edge of the middle wall); temperature measuring point 3 is arranged at the center and edge of the top plate (0.1m away from the top surface of the top plate); temperature measuring point 4 is arranged at the center and edge of the side wall (0.1m away from the outer edge of the side wall). The specific arrangement of the temperature measuring points is as follows Figure 7 shown.

[0090] Among them, the peak values ​​of the internal and external temperature differences at different measuring points after construction are shown in Table 7:

[0091] Table 7

[0092]

[0093] When the insulation time is controlled until the difference between the internal temperature and the surface temperature of the concrete is less than 25℃, the insulation measures are terminated; the insulation cover layer is removed layer by layer, and when the difference between the surface temperature of the concrete and the maximum temperature of the environment is less than 20℃, all of it is removed; the difference between the curing water temperature and the surface temperature of the concrete is not greater than 15℃.

[0094] After the present invention uses low-heat and high-toughness concrete to cast a large-volume concrete structure, the temperature stress of the concrete is significantly reduced, and the peak value of the temperature stress is reduced by 34.3% compared with the temperature stress of ordinary concrete of the same age. After casting ordinary concrete, the temperature stress exceeds the allowable tensile stress by 18.3% to 90.3% at different construction stages, especially 3 to 5 days after construction, the temperature stress exceeds the allowable tensile stress by the largest amplitude. After casting low-heat and high-toughness concrete, the temperature stress of the concrete exceeds the allowable tensile stress by 0.9% 3 to 4 days after construction, and the temperature stress of the concrete in the remaining construction stages does not exceed the allowable tensile stress, and there is no risk of cracking of the concrete. The reason is that low-heat and high-toughness concrete delays the hydration heat of the cementitious material, reduces the peak temperature inside the concrete, and reduces the temperature gradient; at the same time, the polymer added to the concrete significantly reduces its elastic modulus, improves the deformability of the concrete, and enables the concrete structure to better adapt to temperature changes, thereby reducing the temperature stress. That is, low-heat and high-toughness concrete can significantly reduce the internal temperature rise peak and cooling rate of large-volume concrete, reduce the thermal stress of large-volume concrete, and improve crack resistance.

[0095] The present invention combines finite element calculation of the temperature field of large-volume concrete to obtain parameters such as peak temperature, peak value of internal and external temperature difference and cooling rate, and further predict the risk of cracking of large-volume concrete, so as to reasonably determine the temperature of concrete entering the mold, timely adjust the construction process, and accurately set the insulation and moisture preservation curing period for the large-volume concrete after pouring, so as to ensure that the peak temperature, internal and external temperature difference and cooling rate do not exceed the allowable value, and finally ensure that temperature stress cracking does not occur in large-volume concrete.

[0096] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A method for preventing and controlling temperature stress cracking of a large concrete structure, characterized in that: include: Preparation of low heat and high toughness concrete; Low-heat and high-toughness concrete is used to cast large-volume concrete structures, and the transient temperature field and time-varying temperature stress of the large-volume concrete construction process are simulated and calculated; According to the simulation calculation results, with the goal of preventing temperature stress cracking inside the large-volume concrete structure, the concrete mold temperature, pouring process, and construction maintenance are adjusted in a timely manner.

2. The method for preventing and controlling temperature stress cracking of a large concrete structure according to claim 1, characterized in that: Low-heat high-toughness concrete is prepared by using low-hydration thermal gelling materials, steel fibers, polymers, coarse and fine aggregates, high-efficiency water reducing agents and defoaming agents; wherein the low-hydration thermal gelling materials include low-heat silicate cement or low-heat silicate cement mixed with mineral admixtures; the mineral admixtures include fly ash.

3. The method for preventing and controlling temperature stress cracking of a large volume concrete structure according to claim 2 is characterized in that: Preparation of low heat and high toughness concrete, specifically including: Dissolve 5% to 15% of the polymer of the cementitious material in 60% of the designed water volume, stir evenly and set aside; dissolve 0.5% to 2.0% of the high-efficiency water reducer of the cementitious material and 2‰ to 4‰ of the defoamer of the cementitious material in 40% of the designed water volume, stir evenly and set aside; According to the designed mix ratio, put the coarse and fine aggregates and steel fibers into the mixer for dry mixing for 2 minutes, and the volumetric dosage of steel fibers is 0.75% to 1%. The storage environment temperature of the coarse and fine aggregates and steel fibers should be controlled before use to ensure that the temperature of the coarse and fine aggregates and steel fibers is between 5 and 25°C. Add low-hydration thermal gelling material and stir for another 2 minutes. During the stirring process, evenly add mixing water containing polymer; after stirring evenly, slowly add mixing water containing water reducer and defoamer, and stir for another 3 minutes; after the mixture is evenly stirred, prepare concrete specimens according to the specifications, vibrate and compact them, and then put them into a standard curing room for curing.

4. The method for preventing and controlling temperature stress cracking of a large concrete structure according to claim 1, characterized in that: Use low-heat and high-toughness concrete to cast large-volume concrete structures, including: In summer, when the outdoor temperature is low, the temperature of concrete entering the mold should not exceed 30°C; in winter, the temperature of concrete entering the mold should not be lower than 5°C; Reasonably design the construction process, and complete the continuous pouring in layers and sections. The layer thickness is determined according to the depth of the vibrator and the workability of the concrete, and shall not exceed 500mm. The interval between layer pouring shall not be longer than the initial setting time of the concrete. When vibrating concrete, the distance between the vibrator insertion points should not be greater than 1.5 times the radius of action of the vibrator. The vibration time should be based on the appearance of floating slurry on the concrete surface and no longer sinking. The depth of the vibrator inserted into the lower layer of concrete should not be less than 50mm. Large volume concrete also undergoes secondary finishing.

5. The method for preventing and controlling temperature stress cracking of a large concrete structure according to claim 1, characterized in that: Simulate the transient temperature field and time-varying temperature stress of large-volume concrete construction process, including: Defining time-varying characteristic parameters of materials, including density, thermal conductivity and specific heat of concrete; constructing a finite element model for transient temperature field and time-varying temperature stress calculation and analysis; The transient temperature field is used to analyze the temperature field and temperature stress of large-volume concrete. The load step end time is 14d, the time step is 0.5d, the number of loop iterations is 28 times, the convergence error is 0.01, and the initial temperature of the concrete is the actual temperature of the concrete slab when it is put into the mold. When performing finite element simulation, the time-dependent hydration heat is converted into a heat source through a functional relationship and loaded onto the nodes or units of the finite element model to simulate the heat generated by the hydration heat. The ambient temperature is set according to the change of the external atmospheric temperature over time. The heat release coefficient of the concrete surface is determined according to the type and thickness of the formwork used for pouring large-volume concrete and the maintenance measures after pouring. The temperature field distribution cloud map of large-volume concrete pouring construction at different stages is calculated. According to the temperature field distribution cloud map, the calculation results of transient temperature peak at different ages after pouring are obtained; the maximum temperature stress at different periods and the concrete temperature stress time history curve at different periods after pouring are obtained through finite element calculation.

6. The method for preventing and controlling temperature stress cracking of a large concrete structure according to claim 5, characterized in that: Determine whether temperature stress cracking occurs inside a large concrete structure by the following method: If the temperature stress of a large volume of concrete in a certain period of time is greater than the allowable tensile stress of the concrete in that period of time, it indicates that the concrete has temperature stress cracking; If the temperature stress of large-volume concrete is less than the allowable tensile stress of concrete during this period, it indicates that temperature stress cracking will not occur in the concrete.

7. The method for preventing and controlling temperature stress cracking of a large concrete structure according to claim 1, characterized in that: The construction and maintenance specifically include: The temperature control of concrete curing is carried out in stages. Before the concrete reaches the peak temperature, it is not kept warm for curing. At this time, the concrete surface must be moist, and the curing water must be prevented from evaporating too quickly. When the temperature inside the concrete reaches the peak, cover it with straw bags, sacks, cotton felt and a layer of plastic sheeting for insulation. If the insulation effect is not satisfactory, add another layer of insulation blanket and then cover it with a layer of plastic sheeting. During the curing process, the temperature difference between the inside and outside of the concrete is controlled within 25°C through temperature monitoring, the cooling rate is controlled within 2.0°C / d, the difference between the concrete surface temperature and the atmospheric environment temperature is no more than 20°C, and the internal temperature of the concrete does not exceed 75°C; When the insulation time is controlled until the difference between the internal temperature and the surface temperature of the concrete is less than 25℃, the insulation measures are terminated; the insulation cover layer is removed layer by layer, and when the difference between the surface temperature of the concrete and the maximum temperature of the environment is less than 20℃, all of it is removed; the difference between the curing water temperature and the surface temperature of the concrete is not greater than 15℃.

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