A method for determining the timing of pre-tensioned box girder release by combining hydration heat and mechanical properties

Abstract

The present application relates to the field of bridge construction and discloses a method for determining the timing of releasing prestressed box girders by combining hydration heat and mechanical properties. Temperature sensors and strain gauges are placed in the first beam of each batch of prefabricated prestressed box girders to monitor the hydration heat effect and changes in the stress of the steel strands. Concrete test blocks are tested for compressive strength and elastic modulus at different ages during beam curing, and the release age is preliminarily determined based on the mechanical properties at early ages. The release timing is controlled based on the hydration heat effect by analyzing the temperature difference between the core and the surface of the prestressed box girder, the temperature difference between the surface and the environment, and the temperature difference between the core and the mold over time. Finally, the hydration heat effect of the box girder and the mechanical properties of concrete are combined to determine the timing of prestress release based on the temperature-stress dual control. This method avoids releasing prestress when the hydration heat is still significant, prevents cracking of the beam body and loss of additional prestress due to excessive stress, and ensures the bearing capacity and durability of the prestressed prestressed box girder.

Description

Technical Field

[0001] The present invention relates to the technical field of bridge construction, and in particular to a method for determining the timing of releasing a pre-tensioned box girder by combining hydration heat and mechanical properties. Background Art

[0002] At present, the timing of tensioning in engineering projects is only determined based on the concrete strength and elastic modulus required by the specifications. For example, the "JTG / T3650-2020 Technical Specifications for Highway Bridge and Culvert Construction" stipulates that "the strength and elastic modulus (or age) of the component concrete when prestressed tendons are released should comply with the design requirements; when the design does not specify, the strength of the concrete should not be less than 80% of the design strength grade value; the elastic modulus should not be less than 80% of the 28d elastic modulus of concrete. When the concrete age is used instead of the elastic modulus control, it should be no less than 5d." The "TB / T3433-2016 Precast Prestressed Concrete Simply Supported Beams for High-Speed ​​Railways" stipulates that "prestressed tendons should be released when the concrete strength and elastic modulus of the beam body meet the design requirements and the concrete age is not less than 72h." Such regulations are too simple and lack clear and feasible operating procedures, which makes it difficult to accurately control the construction quality.

[0003] As the most critical step in prestressed concrete construction, prestressing release should not only consider the mechanical properties of the concrete material, but also the temperature field generated by the heat of hydration at an early age and the resulting prestress loss. In actual construction, release is often performed while the heat of hydration is still significant. Large temperature differences between the core and surface temperatures, and between the surface and ambient temperature, during release can lead to significant temperature stresses. Combined with the stresses generated by prestressing release, these can cause excessive local tensile stresses and cracking in the concrete, weakening the beam's load-bearing capacity and durability. Furthermore, if the temperature difference between the core and the mold entry temperature during release is too large, additional prestress loss can occur, reducing the prestressing strength of the box girder during the final stage. This can make it difficult to maintain the beam's linear shape and crack resistance over the long term.

[0004] In view of the above problems, it is necessary to introduce a more scientific and reasonable method to determine the timing of release. Summary of the Invention

[0005] In response to the shortcomings of the existing technology, the present invention provides a method for determining the timing of releasing the pre-tensioned box girder by combining the hydration heat and mechanical properties. By combining the hydration heat effect and mechanical properties, the timing of releasing the pre-tensioned prestressed concrete box girder is determined to avoid the problems of concrete cracking and additional prestress loss.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solutions: a method for determining the timing of releasing a pre-tensioned box girder by combining hydration heat and mechanical properties, comprising the following steps:

[0007] S1. Temperature sensors and strain gauges are placed at predetermined locations on the prefabricated pre-tensioned box beam to monitor the hydration heat effect of concrete and stress changes in the prestressed steel strands.

[0008] S2. Monitor the hydration heat effect and strand stress of the pre-tensioned box girder, continuously monitoring from the pouring of the girder concrete to obtain the temperature and stress development curve over time;

[0009] S3. Preliminarily determine the age of the pre-tensioned box girder based on the mechanical properties of early-stage concrete. Test the concrete specimens cured with the beams to obtain the compressive strength and elastic modulus at different ages, and preliminarily determine the timing of tensioning.

[0010] S4. Combine hydration heat and mechanical properties, determine the timing of prestressing release based on temperature-stress dual control, and release according to design requirements.

[0011] Preferably, the step S1 includes:

[0012] Temperature sensors are placed at the core of the bottom plate and web junction, the core of the top plate and web junction, the core of 1 / 2 height of the web, the core of 1 / 2 width of the bottom plate, the core of 1 / 2 width of the top plate, and the shallow surface of the two sections at the mid-span and beam ends;

[0013] Near the mid-span of the pre-tensioned box girder, install temperature sensors inside the box girder mold cavity and outside the box girder respectively;

[0014] Strain gauges are arranged on at least three bundles of prestressed steel strands, at least two of which are broken line bars, and strain gauges are symmetrically arranged at both ends of the same bundle of steel strands.

[0015] Preferably, the step S2 includes:

[0016] The frequency of collecting data on hydration heat effect and steel strand strain shall not be less than once every 5 minutes;

[0017] Monitor and record the temperature data of each temperature measuring point, and calculate the temperature difference between the core and the surface, the surface and the ambient temperature, and the core and the mold temperature over time;

[0018] Monitor and record the stress data of each strain gauge and draw the stress development curve of the steel strand over time.

[0019] Preferably, when drawing the stress development curve of the steel strand over time, a stress control line lower than the initial tensile stress σ02%, i.e., the 0.98σ0 line, is marked to determine the timing of tensioning.

[0020] Preferably, in step S3, the test of the mechanical properties of concrete and the setting age T based on the mechanical properties of concrete are p The determination includes the following steps:

[0021] S3-1: Randomly select a group of test blocks and test the compressive strength and elastic modulus of concrete at 3 days of age;

[0022] If the measured values ​​all reach 80% of the design values, the initial setting age period is no less than 3 days;

[0023] S3-2: If the measured values ​​do not all reach 80% of the design values, but are all above 70% of the design values, a group of test blocks shall be randomly selected to test the compressive strength and elastic modulus of the concrete at 4 days of age;

[0024] If the measured values ​​all reach 80% of the design values, the initial setting age period is no less than 4 days;

[0025] If there are still items that do not meet the standards, the linear extrapolation method is used to calculate the age T corresponding to the time when the design value is 80%. p1 , and T p1 Not less than 5d;

[0026] S3-2-1: Randomly select a group of test blocks and test the concrete age T p1 Compressive strength and elastic modulus at 3000 nm;

[0027] If the measured values ​​all reach 80% of the design values, the initial release age period is to be no less than T p1 ;

[0028] If the measured values ​​still do not reach 80% of the design values, the linear extrapolation method is used to calculate the corresponding age T when the unreached items reach 80% of the design values. p2 , and T p2 Not less than 7 days;

[0029] S3-2-2: Preliminary plan is to set the stocking age to be no less than T p2 , and test the concrete age T p2 Compressive strength and elastic modulus at 3000 nm;

[0030] S3-3: If the measured mechanical properties of concrete at 3 days are not all higher than 70% of the design values, a group of test blocks shall be randomly selected to test the compressive strength and elastic modulus of concrete at 5 days;

[0031] If the measured values ​​all reach 80% of the design values, the initial setting age period is no less than 5 days;

[0032] If there are still items that do not meet the standards, test the mechanical properties of the concrete at 7 days of age;

[0033] If the measured values ​​all reach 80% of the design values, the initial setting age period is no less than 7 days;

[0034] If the measured values ​​still do not reach 80% of the design values, it is necessary to adjust the concrete mix ratio or curing measures to improve the early mechanical properties.

[0035] Preferably, the linear extrapolation method is used to calculate the concrete age T p1 The formula is:

[0036]

[0037] Among them, f d is the design value of compressive strength or elastic modulus, f1 and f2 are the measured mechanical property values ​​at ages T1 and T2 respectively, T1 and T2 are two known ages, T p1 is the estimated age, and T p1 Not less than 5d;

[0038] The linear extrapolation method is used to calculate the concrete age T p2 The formula is:

[0039]

[0040] Among them, T3 and T4 are age 4d and T p1 The concrete age at that time, f3 and f4 are the measured mechanical properties values ​​at the corresponding ages, T p2 is the calculated concrete age, and T p2 Should not be less than 7d.

[0041] Preferably, the step S4 includes:

[0042] Determine the concrete age T h1 , the corresponding age when the temperature difference between the core and the surface of the box girder does not exceed 10℃;

[0043] Determine the concrete age T h2 , the corresponding age when the temperature difference between the shallow surface of the box girder and the ambient temperature does not exceed 10℃;

[0044] Determine the concrete age T h3 , the age corresponding to when the difference between the box girder core temperature and the concrete mold entry temperature does not exceed 10℃;

[0045] Concrete age T controlled by concrete hydration heat effect during tensioning h Take T h1 、T h2 and T h3 The maximum value in .

[0046] Preferably, in step S4, the timing of placing the sheet is T r The following conditions are met:

[0047] Time to release T r The value should be greater than 24 times the tensioning age T calculated based on the mechanical properties of concrete. pand the expansion age T based on the control of concrete hydration thermal effect h The larger of

[0048] The stress value of the steel strand during tensioning should not be less than 98% of the initial tensile stress.

[0049] Preferably, in the said release step, the release is carried out in the order and steps required by the design, first the broken line reinforcement is released, and then the straight line reinforcement is released, and the overall synchronous release process of the jack is adopted to ensure that the oil circuit unloading speed at both ends of the beam is synchronized, and the release is carried out in stages, evenly, symmetrically and staggeredly.

[0050] The present invention also provides a device for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties, comprising:

[0051] Temperature monitoring module, used to monitor the hydration heat effect of concrete and ambient temperature;

[0052] Stress monitoring module, used to monitor stress changes in steel strands;

[0053] Data acquisition and processing module, used to collect and process temperature and stress data in real time, and draw temperature and stress development curves over time;

[0054] The release timing calculation module is used to comprehensively determine the prestress release timing based on the mechanical properties of concrete and the hydration heat effect, based on the temperature-stress dual control principle;

[0055] The control module is used to control the sheet-releasing device to perform the sheet-releasing operation according to the calculated sheet-releasing timing and sequence.

[0056] The present invention provides a method for determining the timing of releasing a pre-tensioned box girder by combining hydration heat and mechanical properties.

[0057] It has the following beneficial effects:

[0058] 1. The present invention combines dual monitoring of hydration heat effect and concrete mechanical properties to scientifically determine the optimal time for prestressing release, avoiding the errors caused by relying solely on age or experience, and significantly improving the accuracy of release timing.

[0059] 2. The present invention can effectively avoid local temperature stress caused by temperature gradient by real-time monitoring of the temperature difference between the core and the surface of the concrete, as well as the difference between the surface temperature and the ambient temperature, thereby reducing the risk of concrete cracking and ensuring structural integrity.

[0060] 3. The present invention ensures that the stress value of the steel strands during tensioning is not less than 98% of the initial stress by real-time monitoring of the temperature difference between the core temperature of the box girder and the temperature of the concrete entering the mold, as well as real-time monitoring and control of the stress of the steel strands, thereby avoiding unnecessary additional prestress loss and ensuring the long-term prestressing strength of the structure.

[0061] 4. By testing and calculating the early mechanical properties of concrete, the present invention can accurately calculate the timing of tensioning, avoid quality problems or construction delays caused by too early or too late tensioning, and optimize the efficiency and progress control of the entire construction process.

[0062] 5. The present invention avoids the problems of concrete cracking or prestress loss caused by improper tensioning by precisely controlling the timing of tensioning, thereby improving the overall structural durability and service life of the pre-tensioned box girder.

[0063] 6. The method of the present invention is applicable to the construction of prestressed concrete box girders in different environments, can be flexibly adjusted according to different engineering conditions, has strong adaptability, and is easy to promote and apply in actual construction, and has good practical value.

[0064] 7. The present invention combines temperature and stress monitoring data with mechanical property test results to form a method for determining the timing of tensioning based on temperature-stress dual control, which effectively improves the scientific nature of quality control during the construction process and ensures the overall quality of the project. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 Schematic diagram of the method flow of the present invention;

[0066] Figure 2 This is a schematic diagram of the arrangement of temperature sensors in the mid-span section of a box girder according to an embodiment of the present invention;

[0067] Figure 3 This is a schematic diagram of the arrangement of temperature sensors at the end sections of a box girder according to an embodiment of the present invention;

[0068] Figure 4 This is a schematic diagram of the arrangement of steel strand strain gauges according to an embodiment of the present invention;

[0069] Figure 5 This is a curve diagram showing the development of the hydration heat effect of the concrete in the mid-span section of the box girder according to an embodiment of the present invention over time;

[0070] Figure 6 This is a curve diagram showing the development of the hydration heat effect of concrete at the end section of a box girder over time according to an embodiment of the present invention;

[0071] Figure 7 A graph showing the development of stress values ​​of steel strands over time according to an embodiment of the present invention;

[0072] Figure 8 A curve diagram showing the development of concrete compressive strength with age according to an embodiment of the present invention;

[0073] Figure 9 is a curve diagram of the development of the elastic modulus of concrete with age according to an embodiment of the present invention;

[0074] Figure 10 Schematic diagram of the device structure of the present invention.

[0075] In the figure: 1. strain gauge; 2. tensioning device; 3. steel strand; 4. diverter; 5. pedestal; 6. box girder body; 100. temperature monitoring module; 200. stress monitoring module; 300. data acquisition and processing module; 400. tensioning timing calculation module; 500. control module. DETAILED DESCRIPTION

[0076] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the present specification. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0077] Please see the attached Figure 1 The embodiment of the present invention provides a method for determining the timing of releasing pre-tensioned box girders by combining hydration heat and mechanical properties. The method aims to optimize the method for determining the timing of releasing pre-tensioned prestressed concrete box girders during bridge construction, avoid releasing concrete when the hydration heat effect is still intense, solve the problems of cracking of the box girder structure and loss of additional prestress, and thus improve the bearing capacity and durability of the box girder. By real-time monitoring of the concrete hydration heat effect and mechanical property changes, and dual control of temperature and stress, the accurate timing of releasing is ultimately determined. The specific implementation steps are as follows:

[0078] Step S1: Install the temperature sensor and strain gauge:

[0079] During the construction of prefabricated pre-tensioned box girders, after completing the formwork reinforcement work and the threading of the prestressed steel strands, temperature sensors and strain gauges are arranged at predetermined positions on the beam body to monitor the hydration heat effect of the concrete and the stress changes of the prestressed steel strands in real time.

[0080] Arrangement of temperature sensors:

[0081] Temperature sensors are placed at the following key locations in the mid-span and beam end sections:

[0082] The core part of the joint of the bottom plate and the web, the core part of the joint of the top plate and the web, the core part of 1 / 2 height of the web, the core part of 1 / 2 width of the bottom plate, the core part of 1 / 2 width of the top plate, and the shallow surface.

[0083] These temperature sensors are used to measure the temperature at different locations of the beam, with particular attention paid to the temperature difference between the core and the surface, the difference between the surface and the ambient temperature, and the difference between the core temperature and the concrete mold temperature, to ensure a comprehensive understanding of the temperature changes inside the beam.

[0084] In addition, temperature sensors were installed near the midspan of the box girder to monitor the internal temperature of the girder, and temperature sensors were installed on the outside of the box girder to monitor the external ambient temperature. The data from these sensors was used to assess the temperature difference between the inside and outside of the girder to analyze the extent of the hydration heat effect.

[0085] Strain gauge arrangement:

[0086] Strain gauges are placed on three bundles of prestressed steel strands, at least two of which are zigzag strands. Strain gauges are placed symmetrically at both ends of each strand. These gauges measure changes in tensile stress in the strands, particularly those caused by hydration heat. This arrangement ensures precise monitoring of the strand stress state.

[0087] Step S2: Monitor the hydration heat effect and steel strand stress of the pre-tensioned box girder:

[0088] From the beginning of concrete pouring, an automated monitoring system collects data from temperature sensors and strain gauges. Temperature and stress data are collected every five minutes to ensure real-time monitoring of the beam's temperature field and stress state. Monitoring should continue for at least 120 hours to cover the critical period of hydration heat effects.

[0089] Monitoring of hydration heat effect:

[0090] About 30 hours after concrete pouring, the temperature inside the beam reaches its highest point and the hydration heat reaction reaches its peak. Usually, the highest temperature is located at the part with the thicker cross-section at the end of the beam.

[0091] After this, the temperature will slowly drop. Usually after 80 to 100 hours, the difference between the core temperature and the surface temperature, the surface temperature and the ambient temperature, and the core temperature and the concrete casting temperature will be reduced to within 10°C. The monitoring curve should show the temperature peak and the cooling process over time, which is crucial for determining the timing of the release.

[0092] Monitoring of steel strand stress:

[0093] Strand stress changes with concrete temperature. Hydration heat reactions cause the strand stress to gradually decrease, typically reaching its lowest point at peak temperature. As the temperature gradually decreases, the strand stress recovers, typically returning to over 98% of its original tensile stress around 80 hours. Based on the monitored stress curve, the moment when the strand stress is no less than 0.98σ0 is determined.

[0094] Step S3: Preliminary formulation of the pre-tensioned box girder tensioning age based on the mechanical properties of early-age concrete:

[0095] While pouring concrete, prepare at least five sets of concrete test blocks to test the compressive strength and elastic modulus of concrete at different ages. The test blocks should be cured along with the beam to ensure that their mechanical properties are consistent with those of the actual beam.

[0096] The specific implementation steps are as follows:

[0097] S3-1: Testing the mechanical properties of concrete at 3d age

[0098] A group of test blocks were randomly selected to test the compressive strength and elastic modulus of concrete at 3d age.

[0099] Judging the test results:

[0100] If the measured compressive strength and elastic modulus both reach 80% of the design value, the concrete age T of the pre-tensioned box girder when it is released is preliminarily determined. p Should not be less than 3d.

[0101] The remaining four groups of test blocks were used to test the mechanical properties of concrete at 5, 7, 14 and 28 days of age, respectively.

[0102] If at least one of the measured compressive strength and elastic modulus does not reach 80% of the design value, but both are higher than 70% of the design value, the process proceeds to step S3-2.

[0103] If the measured compressive strength and elastic modulus are lower than 70% of the designed value, the process proceeds to step S3-3.

[0104] S3-2: When the mechanical properties at 3d age do not all reach 80% of the design value, but are all above 70%

[0105] S3-2-1: Testing the mechanical properties of concrete at 4 days of age

[0106] A group of test blocks were randomly selected to test the compressive strength and elastic modulus of concrete at 4 days of age.

[0107] Judging the test results:

[0108] If the measured compressive strength and elastic modulus both reach 80% of the design value, the initial expansion age T is set. p Should not be less than 4d.

[0109] The remaining three groups of test blocks were used to test the mechanical properties of concrete at 7, 14 and 28 days of age.

[0110] If at least one of the measured compressive strength and elastic modulus does not reach 80% of the design value, proceed to step S3-2-2.

[0111] S3-2-2: Linear extrapolation calculation of the age T required to reach 80% of the design value p1

[0112] For the items that do not meet the standards (compressive strength or elastic modulus), the mechanical property data of 3d and 4d age are connected by a straight line, and the slope is used for linear extrapolation to calculate the concrete age T corresponding to 80% of the design value. p1 .

[0113] The calculation formula is as follows:

[0114]

[0115] in:

[0116] f d : Design value (design requirement for compressive strength or elastic modulus);

[0117] f1 and f2: measured mechanical property values ​​at ages T1 (3d) and T2 (4d);

[0118] T p1 : The calculated concrete age, and T p1 Should not be less than 5d.

[0119] S3-2-3: Verification age T p1 Mechanical properties of concrete

[0120] A group of test blocks were randomly selected to test the concrete age T p1 Compressive strength and elastic modulus.

[0121] Judging the test results:

[0122] If the measured compressive strength and elastic modulus both reach 80% of the design value, the initial expansion age T is set. p Should not be less than T p1 .

[0123] The remaining two groups of test blocks were used to test the mechanical properties of concrete at 14d and 28d ages.

[0124] If there are still items that do not meet the standards in the measured compressive strength and elastic modulus, go to step S3-2-4.

[0125] S3-2-4: Linearly extrapolate again to calculate the age T required to reach 80% of the design value p2

[0126] For non-compliant items, connect 4d and T with a straight line. p1 The mechanical properties data of the age are linearly extrapolated using the slope to calculate the concrete age T corresponding to 80% of the design value. p2 .

[0127]

[0128] in:

[0129] T3 and T4 are age 4d and T p1 The age of concrete at the time of

[0130] f3 and f4 are the measured mechanical property values ​​at the corresponding ages;

[0131] T p2 : The calculated concrete age, and T p2 Should not be less than 7d.

[0132] S3-2-5: Determine the final release age T p

[0133] Preliminary plan for the release age T p Should not be less than T p2 .

[0134] Test concrete age T p2 The compressive strength and elastic modulus at 28 days are measured to ensure that the mechanical properties meet the standards.

[0135] S3-3: When the mechanical properties at 3d age are not all higher than 70% of the design value

[0136] S3-3-1: Testing the mechanical properties of concrete at 5 days of age

[0137] A group of test blocks were randomly selected to test the compressive strength and elastic modulus of concrete at 5 days of age.

[0138] Judging the test results:

[0139] If the measured compressive strength and elastic modulus both reach 80% of the design value, the initial expansion age T is set. p Should not be less than 5d.

[0140] The remaining three groups of test blocks were used to test the mechanical properties of concrete at 7, 14 and 28 days of age.

[0141] If at least one of the measured compressive strength and elastic modulus does not reach 80% of the design value, proceed to step S3-3-2.

[0142] S3-3-2: Testing the mechanical properties of concrete at 7 days of age

[0143] A group of test blocks were randomly selected to test the compressive strength and elastic modulus of concrete at 7 days of age.

[0144] Judging the test results:

[0145] If the measured compressive strength and elastic modulus both reach 80% of the design value, the initial expansion age T is set. p Should not be less than 7d.

[0146] The remaining two groups of test blocks were used to test the mechanical properties of concrete at 14d and 28d ages.

[0147] If the measured compressive strength and elastic modulus still do not meet the standards, proceed to step S3-3-3.

[0148] S3-3-3: Adjust concrete mix ratio or curing measures

[0149] Since the mechanical properties at 7 days old still do not meet the design requirements, the following measures need to be taken:

[0150] Improve curing measures: such as adjusting curing temperature, humidity and other conditions to promote early strength growth of concrete.

[0151] Adjust the concrete mix ratio: optimize the water-cement ratio, type and amount of admixtures to improve the early mechanical properties of concrete.

[0152] After adjustment, the timing of releasing the pre-tensioned box girder is re-determined according to this method to ensure that the mechanical properties of the concrete meet the design requirements.

[0153] Through the above detailed test and calculation steps, the pre-tensioning age T of the pre-tensioned box girder can be scientifically and accurately determined. p At each critical age, the compressive strength and elastic modulus of concrete are tested to ensure that both indicators reach 80% of the design value. If mechanical properties do not meet the requirements, linear extrapolation is used to calculate the age required to meet the design requirements, and verification testing is performed. If the requirements are still not met, measures such as adjusting maintenance measures or the concrete mix ratio are implemented to improve the early mechanical properties of the concrete.

[0154] This method avoids problems such as concrete cracking or prestress loss caused by premature release, ensuring the structural safety and durability of the pre-tensioned box girder. Furthermore, the method offers advantages of high operability and precision, making it easy for construction workers to apply in actual projects.

[0155] Step S4: Combining hydration heat and mechanical properties, the timing of prestressing release is determined based on temperature-stress dual control and the prestressing is released:

[0156] Determine the stocking age (T r )

[0157] According to the monitoring results of hydration heat effect, the following ages are determined:

[0158] T h1 : The age corresponding to when the temperature difference between the core and the surface of the box girder does not exceed 10°C;

[0159] T h2: The age corresponding to when the difference between the surface temperature of the box girder and the ambient temperature does not exceed 10°C;

[0160] T h3 : The age corresponding to when the difference between the box girder core temperature and the concrete mold entry temperature does not exceed 10°C;

[0161] According to the requirements of hydration heat effect, the concrete age T corresponding to the release of the pre-tensioned box girder is h Should be T h1 、T h2 and T h3 The maximum value of the three, in h.

[0162] Combined with the hydration thermal effect of box beam and the mechanical properties of concrete, the timing of releasing the pre-tensioned box beam is determined. r , unit is h, T r The value should be greater than 24T p and T h , and based on the temperature-stress dual control requirements, the stress value of the steel strand should not be lower than the 0.98σ0 stress control line when tensioning.

[0163] Release operation:

[0164] At the determined release time T r , the tensioning operation begins, requiring the strand stress to be no less than 0.98σ0. Using a jack-based overall synchronous tensioning process, the oil unloading speeds at both ends of the beam are synchronized, and the tensioning is carried out in stages, evenly, and symmetrically.

[0165] When tensioning, the broken line reinforcement should be tensioned first, and then the straight line reinforcement to ensure uniform stress release in the beam body and avoid local stress concentration causing concrete cracking.

[0166] By combining dual monitoring of the hydration heat effect and concrete mechanical properties, this invention can scientifically and accurately determine the timing for releasing the pre-tensioned box girder. By real-time monitoring of the internal temperature field of the girder and the stress of the steel strands, premature release before the hydration heat effect has fully subsided or the concrete mechanical properties have not met the standards is avoided. This reduces the risk of box girder cracking or loss of excess prestress, significantly improving the load-bearing capacity and durability of the pre-tensioned box girder.

[0167] Example:

[0168] like Figure 1-9 As shown, taking a C50 pre-tensioned box girder as an example, an embodiment of the present invention provides a method for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties, comprising the following steps:

[0169] S1: Install temperature sensor and strain gauge:

[0170] After the template reinforcement operation and bundle threading work is completed, 12 temperature sensors are pre-buried in the corresponding positions inside the beam, and 2 temperature sensors are installed outside the box beam to monitor the hydration heat effect of the concrete and the ambient temperature. A temperature sensor is buried at the core of the bottom plate and web joint position, the core of the top plate and web joint position, the 1 / 2 height core of the web, the 1 / 2 width core of the bottom plate, the 1 / 2 width core of the top plate, and the shallow surface of the two sections in the mid-span and the beam end. Near the mid-span of the pre-tensioned box beam, an internal temperature sensor is installed in the inner mold cavity of the box beam, and an external temperature sensor is installed outside the side mold of the box beam. The sensor arrangement of the beam is as follows: Figure 2 and Figure 3 As shown, the sensor names refer to the mid-span top plate (center-1-1, center-1-2, center-surface), mid-span web (center-2-1), and mid-span bottom plate (center-3-1, center-3-2); and the beam end top plate (end-1-1, end-1-2, end-surface), beam end web (end-2-1), and beam end bottom plate (end-3-1, end-3-2) temperature sensors. Ensure that the temperature sensors are not damaged from the time of installation until the end of monitoring. Temperature monitoring begins with the pouring of the first batch of concrete and lasts for approximately 120 hours. Temperature and relative humidity are recorded throughout the entire process.

[0171] The strain gauge is installed in Figure 4 As shown in the schematic diagram, strain gauges are arranged on three bundles of prestressed steel strands between the tensioning device and the end mold, of which there are at least two bundles of broken line bars, and strain gauges should be symmetrically arranged at both ends of the same bundle of steel strands.

[0172] S2: Monitoring the hydration heat effect and steel strand stress of pre-tensioned box girders:

[0173] Since the beginning of concrete pouring, the hydration heat effect has been continuously monitored, and the data has been compiled and plotted into a graph to observe the development of the hydration heat temperature of the beam. The curve of the concrete hydration heat effect over time measured at the mid-span and beam end sections is as follows: Figure 5 、 6 As shown in the graph, the hydration heat temperature of each part reaches its highest point about 30 hours after the concrete pouring begins, lasts for about 12 hours, and then slowly decreases. The highest temperature is basically located at the junction of the top plate and the web of the beam end section, where the concrete thickness is the largest in the box girder. After 79 hours of monitoring, the difference between the concrete box girder mid-span and the core of the beam end section and the concrete entering the mold is within 10°C, T h3 Pick and The larger of the two is 79h. After 86h, the temperature difference between the core and the surface of the concrete box beam is within 10℃, T h1 Pick and The larger of the two is 86h. After 91h, the temperature difference between the surface and the environment is within 10℃, Th2 Pick and The larger value of the two, that is, 91h.

[0174] The curve of steel strand force development over time is as follows: Figure 7 As shown in the figure, during the development of the concrete hydration thermal effect, the stress value of the steel strand slowly decreases with the increase of temperature, and basically reaches the lowest value at 35 to 40 hours; as the temperature drops, the stress value of the steel strand slowly recovers and returns to 0.98σ0 at around 80 hours, meeting the requirement of no less than 72 hours as required by the specification, and helping to more accurately determine the timing of prestressing. At this time, the concrete hydration thermal effect weakens and the internal temperature of the concrete gradually decreases.

[0175] S3: Based on the mechanical properties of early-age concrete, the pre-tensioned box girder tensioning age is preliminarily determined:

[0176] Concrete test blocks were prepared simultaneously with the concrete pouring. Five groups of 150×150×150mm cubic test blocks, with three test blocks in each group, were used for the concrete compressive strength test. Five groups of 150×150×300mm prism test blocks, with six test blocks in each group, were used for the concrete elastic modulus test. The test blocks were cured along with the beams.

[0177] Two groups were used to test the mechanical properties of concrete at 3d and 28d ages, and the remaining three groups were used to test the mechanical properties of concrete at other early ages. Figure 8 、 9 As shown in the figure, it can be seen that the compressive strength of concrete at 3d age is 40.9MPa, which is greater than 80% of the design grade value (the design grade value of C50 concrete strength is 50MPa), and the elastic modulus is 28.1GPa, which is greater than 80% of the elastic modulus of concrete at 28d (the elastic modulus of C50 concrete at 28d is 34.5GPa). p According to the mechanical properties of early-age concrete, the initial timing of tensioning is set at 3 days.

[0178] S4: Combine hydration heat and mechanical properties, and determine the timing of prestressing release based on temperature-stress dual control and release the prestress:

[0179] Combined with the monitored hydration thermal effect development curve of pre-tensioned box beams and the initial release timing based on the mechanical properties of concrete, the release timing of this batch of precast pre-tensioned beams is determined based on the temperature-stress dual control. The release age T is initially proposed based on the mechanical properties of early-age concrete. p For 3d, T is determined according to the hydration heat requirement h Take T h1 、T h2 and T h3 The maximum value of the three is 91h.

[0180] The concrete age T corresponding to the time of placement r Take T p and T h The maximum value of the two, which must be greater than the 80h corresponding to the stress control line of the steel strand stress value of not less than 0.98σ0, determines the release timing T of the prefabricated pre-tensioned box beams of this batch. r The concrete compressive strength and elastic modulus reached 80% of the 28-day design value for C50 concrete at this time. The hydration heat effect of the concrete was also reduced. The temperature differences between the core and the surface, between the surface and the ambient temperature, and between the core and the concrete mold entry temperature were all within 10°C. The stress in the steel strands had recovered to a level above the 0.98σ0 stress line. While meeting regulatory requirements, this approach enabled more precise determination of the timing for tensioning, ensuring beam quality in a more scientific manner.

[0181] When tensioning, the oil unloading speed at both ends of the beam should be kept synchronized, and the overall tensioning process using jacks should be carried out in stages, evenly, symmetrically, and staggered. The broken line reinforcement should be tensioned first, and then the straight line reinforcement.

[0182] The device for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties described below and the method for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties described above can be used in correspondence with each other.

[0183] Please see the attached Figure 10 The present invention also provides a device for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties, comprising:

[0184] Temperature monitoring module 100, used to monitor the hydration heat effect of concrete and ambient temperature;

[0185] The stress monitoring module 200 is used to monitor the stress changes of the steel strand;

[0186] The data acquisition and processing module 300 is used to collect and process temperature and stress data in real time and draw the temperature and stress development curve over time;

[0187] The release timing calculation module 400 is used to comprehensively determine the prestress release timing based on the mechanical properties of concrete and the hydration heat effect, based on the temperature-stress dual control principle;

[0188] The control module 500 is used to control the sheet placing device to perform the sheet placing operation according to the calculated sheet placing timing and sequence.

[0189] The device of this embodiment can be used to execute the above method embodiment, and its principles and technical effects are similar, so they will not be repeated here.

[0190] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining the timing of releasing a pre-tensioned box girder by combining hydration heat and mechanical properties, characterized in that: The following steps are involved: S1. Temperature sensors and strain gauges are placed at predetermined locations on the prefabricated pre-tensioned box beam to monitor the hydration heat effect of concrete and stress changes in the prestressed steel strands. S2. Monitor the hydration heat effect and strand stress of the pre-tensioned box girder, continuously monitoring from the pouring of the girder concrete to obtain the temperature and stress development curve over time; S3. Preliminarily determine the age of the pre-tensioned box girder based on the mechanical properties of early-stage concrete. Test the concrete specimens cured with the beams to obtain the compressive strength and elastic modulus at different ages, and preliminarily determine the timing of tensioning. S4. Combine hydration heat and mechanical properties, determine the timing of prestressing release based on temperature-stress dual control, and release according to design requirements; In step S4, the release timing T r The following conditions are met: Time to release T r The value should be greater than 24 times the tensioning age T calculated based on the mechanical properties of concrete. p and the expansion age T based on the control of concrete hydration thermal effect h The larger of The stress value of the steel strand during tensioning should not be less than 98% of the initial tensile stress.

2. The method for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties according to claim 1, characterized in that: The step S1 comprises: Temperature sensors are placed at the core of the bottom plate and web junction, the core of the top plate and web junction, the core of 1 / 2 height of the web, the core of 1 / 2 width of the bottom plate, the core of 1 / 2 width of the top plate, and the shallow surface of the two sections at the mid-span and beam ends; Near the mid-span of the pre-tensioned box girder, install temperature sensors inside the box girder mold cavity and outside the box girder respectively; Strain gauges are arranged on at least three bundles of prestressed steel strands, at least two of which are broken line bars, and strain gauges are symmetrically arranged at both ends of the same bundle of steel strands.

3. The method for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties according to claim 1, characterized in that: The step S2 comprises: The frequency of collecting data on hydration heat effect and steel strand strain shall not be less than once every 5 minutes; Monitor and record the temperature data of each temperature measuring point, and calculate the temperature difference between the core and the surface, the surface and the ambient temperature, and the core and the mold temperature over time; Monitor and record the stress data of each strain gauge and draw the stress development curve of the steel strand over time.

4. The method for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties according to claim 1, characterized in that: In step S3, the test of the mechanical properties of concrete and the setting age T based on the mechanical properties of concrete are p The determination includes the following steps: S3-1: Randomly select a group of test blocks and test the compressive strength and elastic modulus of concrete at 3 days of age; If the measured values ​​all reach 80% of the design values, the initial setting age period is no less than 3 days; S3-2: If the measured values ​​do not all reach 80% of the design values, but are all above 70% of the design values, a group of test blocks shall be randomly selected to test the compressive strength and elastic modulus of the concrete at 4 days of age; If the measured values ​​all reach 80% of the design values, the initial setting age period is no less than 4 days; If there are still items that do not meet the standards, the linear extrapolation method is used to calculate the age T corresponding to the time when the design value is 80%. p1 , and T p1 Not less than 5d; S3-2-1: Randomly select a group of test blocks and test the concrete age T p1 Compressive strength and elastic modulus at 3000 nm; If the measured values ​​all reach 80% of the design values, the initial release age period is to be no less than T p1 ; If the measured values ​​still do not reach 80% of the design values, the linear extrapolation method is used to calculate the corresponding age T when the unreached items reach 80% of the design values. p2 , and v p2 Not less than 7 days; S3-2-2: Preliminary plan is to set the stocking age to be no less than T p2 , and test the concrete age T p2 Compressive strength and elastic modulus at 3000 nm; S3-3: If the measured mechanical properties of concrete at 3 days are not all higher than 70% of the design values, a group of test blocks shall be randomly selected to test the compressive strength and elastic modulus of concrete at 5 days; If the measured values ​​all reach 80% of the design values, the initial setting age period is no less than 5 days; If there are still items that do not meet the standards, test the mechanical properties of the concrete at 7 days of age; If the measured values ​​all reach 80% of the design values, the initial setting age period is no less than 7 days; If the measured values ​​still do not reach 80% of the design values, it is necessary to adjust the concrete mix ratio or curing measures to improve the early mechanical properties; The linear extrapolation method is used to calculate the concrete age T p1 The formula is: Among them, f d is the design value of compressive strength or elastic modulus, f1 and f2 are the measured mechanical property values ​​at ages T1 and T2 respectively, T1 and T2 are two known ages, T p1 is the estimated age, and T p1 Not less than 5d; The linear extrapolation method is used to calculate the concrete age T p2 The formula is: Among them, T3 and T4 are age 4d and T p1 The concrete age at that time, f3 and f4 are the measured mechanical properties values ​​at the corresponding ages, T p2 is the calculated concrete age, and T p2 Should not be less than 7d.

5. The method for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties according to claim 1, characterized in that: The step S4 comprises: Determine the concrete age T h1 , the corresponding age when the temperature difference between the core and the surface of the box girder does not exceed 10℃; Determine the concrete age T h2 , the corresponding age when the temperature difference between the shallow surface of the box girder and the ambient temperature does not exceed 10℃; Determine the concrete age T h3 , the age corresponding to when the difference between the box girder core temperature and the concrete mold entry temperature does not exceed 10℃; Concrete age T controlled by concrete hydration heat effect during tensioning h Take T h1 、T h2 and T h3 The maximum value in .

6. The method for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties according to claim 1, characterized in that: In the said tensioning step, the tensioning is carried out in the order and steps required by the design, first the broken line reinforcement is tensioned, then the straight line reinforcement is tensioned, and the overall synchronous tensioning process of the jack is adopted to ensure that the oil circuit unloading speed at both ends of the beam is synchronized, and the tensioning is carried out in stages, evenly, symmetrically and staggeredly.

7. A device for determining the timing of releasing a pre-tensioned box girder by combining hydration heat and mechanical properties, for implementing the method according to any one of claims 1 to 6, characterized in that: include: Temperature monitoring module, used to monitor the hydration heat effect of concrete and ambient temperature; Stress monitoring module, used to monitor stress changes in steel strands; Data acquisition and processing module, used to collect and process temperature and stress data in real time, and draw temperature and stress development curves over time; The release timing calculation module is used to comprehensively determine the prestress release timing based on the mechanical properties of concrete and the hydration heat effect, based on the temperature-stress dual control principle; The control module is used to control the sheet-releasing device to perform the sheet-releasing operation according to the calculated sheet-releasing timing and sequence.

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