Method for determining tension releasing time of pre-tensioned box girder by combining hydration heat and mechanical properties

Abstract

The invention relates to the field of bridge construction, and discloses a method for determining the tension releasing time of a pre-tensioned box girder by combining hydration heat and mechanical properties, and the method comprises the following steps: arranging a temperature sensor and a strain gauge in a first girder of each batch of prefabricated pre-tensioned box girders to monitor the change of the hydration heat effect and the stress of a steel strand; testing the compressive strength and elastic modulus of concrete at different ages along with a beam curing concrete test block, and preliminarily drawing up a tension releasing age according to the early-age mechanical property; by analyzing the development of the temperature difference between the core part and the superficial part of the pre-tensioned box girder, the temperature difference between the superficial part and the environment and the temperature difference between the core part and the mold entering temperature along with time, the tension releasing time is controlled based on the hydration heat effect; and finally, combining the hydration heat effect of the box girder and the mechanical property of concrete, and comprehensively determining the prestress releasing time based on temperature-stress double control. According to the method, the situation that prestress releasing is conducted when hydration heat is still obvious is avoided, beam body cracking and extra prestress loss caused by too large stress are prevented, and the bearing capacity and durability of the prestress pre-tensioned box beam are guaranteed.

Description

Technical Field

[0001] The 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 according to 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 is not specified, 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, and when the concrete age is used instead of the elastic modulus control, it should be no less than 5d." "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 important process in prestressed concrete construction, prestressing release should not only consider the mechanical properties of concrete materials, but also the temperature field generated by the heat of hydration of concrete at an early age and the corresponding prestress loss. In actual projects, it is common to release the prestress when the heat effect of concrete hydration is still significant. During release, the difference between the temperature at the core of the box girder and the temperature at the surface, and the temperature at the surface and the ambient temperature is too large to form a large temperature stress. Combined with the stress generated by prestressing release, it may cause cracking of concrete due to excessive local tensile stress, weakening the bearing capacity and durability of the beam. In addition, if the difference between the temperature at the core and the temperature at the mold entry is too large during release, it will lead to additional prestress loss, reducing the prestressing strength of the box girder at the beam stage, and it is difficult to guarantee the linear shape and crack resistance of the beam under long-term conditions.

[0004] In view of the above-mentioned problems, it is necessary to introduce a more scientific and reasonable method to determine the timing of releasing the cards. Summary of the invention

[0005] In view of the shortcomings of the prior art, 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. The timing of releasing the pre-tensioned prestressed concrete box girder is determined by combining the hydration heat effect and mechanical properties to avoid the problems of concrete cracking and additional prestress loss.

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

[0007] S1. Arrange temperature sensors and strain gauges at the predetermined positions of the prefabricated pre-tensioned box beams to monitor the hydration heat effect of concrete and the stress changes of the prestressed steel strands;

[0008] S2. Monitor the hydration heat effect and strand stress of the pre-tensioned box girder, and continuously monitor 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 beam according to the mechanical properties of early-age concrete, test the concrete test blocks cured with the beam, 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 comprises:

[0012] Temperature sensors are placed at the core of the bottom plate web joint, the core of the top plate web joint, 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 in the mid-span and beam end;

[0013] Near the mid-span of the pre-tensioned box girder, temperature sensors inside and outside the box girder are installed in the mold cavity inside the box girder 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 zigzag bars, and strain gauges are symmetrically arranged at both ends of the same bundle of steel strands.

[0015] Preferably, the step S2 comprises:

[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, the stress below the initial tensile stress σ 0 2% stress control line, i.e. 0.98σ 0 Line is used to determine the timing of releasing a card.

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

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

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

[0023] S3-2: If the measured values ​​do not all reach 80% of the design values, but are all higher than 70% of the design values, a group of test blocks are 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 designed values, the initial release age period is to be 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 Should not be 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 ;

[0027] If the measured values ​​reach 80% of the design values, the initial release age is set 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 age T corresponding to the time when the design value reaches 80%. p2 , and T p2 Should not be less than 7d;

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

[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 designed values, the initial release age period is to be 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 designed values, the initial release age period is to be 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 maintenance 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, f 1 and f 2 Age T 1 and T 2 The measured mechanical properties at T 1 and T 2 are two known ages, T p1 is the estimated age, and T p1 Should not be less than 5d;

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

[0039]

[0040] Among them, T 3 and T 4 Age 4d and T p1 The age of concrete at that time, f 3 and f 4 are the measured mechanical properties values ​​at the corresponding ages, T p2 is the estimated concrete age, and T p2 Should not be less than 7d.

[0041] Preferably, step S4 comprises:

[0042] Determine the concrete age T h1 , the corresponding age when the temperature difference between the core and the shallow 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 environment 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 h3The maximum value in .

[0046] Preferably, in step S4, the timing of placing a 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 proposed based on the mechanical properties of concrete. p and the setting 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 release step, the release is carried out in the order and steps required by the design, first the folded line reinforcement is released, and then the straight line reinforcement is released. 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 beam 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 the stress changes of steel strands;

[0053] Data acquisition and processing module, used to collect and process temperature and stress data in real time, and draw the curve of temperature and stress development 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 placing device to perform the sheet placing operation according to the calculated sheet placing timing and sequence.

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

[0057] It has the following beneficial effects:

[0058] 1. The present invention can scientifically determine the best time for prestressing release by combining dual monitoring of hydration heat effect and concrete mechanical properties, avoiding the errors caused by relying solely on age or empirical judgment, 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 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 prestress of the structure.

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

[0062] 5. The present invention avoids the problem of concrete cracking or prestress loss caused by improper tensioning by accurately 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 suitable for the construction of prestressed concrete box girders under 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 It is a schematic diagram of the method flow of the present invention;

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

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

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

[0069] Figure 5 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 6It is a curve diagram showing the development of the hydration heat effect of concrete at the end section of a box girder according to an embodiment of the present invention over time;

[0071] Figure 7 A curve diagram showing the development of the stress value of the steel strand 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] Fig. 9 The concrete elastic modulus development curve with age according to the embodiment of the present invention;

[0074] Fig.10 It is a 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 be combined with the drawings in the specification of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0077] Please refer to the attached Figure 1 The 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, aiming to optimize the method for determining the timing of releasing the pre-tensioned prestressed concrete box girder in bridge construction, avoid releasing the concrete when the hydration heat effect is still intense, solve the problems of cracking of the box girder structure and loss of additional prestress, thereby improving the bearing capacity and durability of the box girder. By real-time monitoring of the hydration heat effect and mechanical property changes of the concrete, and dual control of temperature and stress, the accurate timing of releasing the girder is finally 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] In the mid-span and beam end sections, temperature sensors are placed at the following key locations:

[0082] The core at the junction of the bottom plate and the web, the core at the junction of the top plate and the web, the core at 1 / 2 the height of the web, the core at 1 / 2 the width of the bottom plate, the core at 1 / 2 the 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 special attention paid to the temperature difference between the core and the shallow surface, the difference between the shallow surface and the ambient temperature, and the difference between the core temperature and the temperature of the concrete entering the mold, to ensure a comprehensive grasp of the temperature changes inside the beam.

[0084] In addition, temperature sensors are installed in the inner cavity of the box girder near the mid-span to monitor the internal temperature of the girder, and temperature sensors are installed outside the box girder to monitor the external ambient temperature. The data from these sensors are used to evaluate the temperature difference between the inside and outside of the girder to analyze the development of the hydration heat effect.

[0085] Strain gauge arrangement:

[0086] Strain gauges are arranged on three bundles of prestressed steel strands, of which at least two are zigzag bars, and strain gauges are symmetrically arranged at both ends of the same bundle of steel strands. The strain gauges are used to measure the changes in tensile stress of the steel strands, especially the changes in stress of the steel strands under the action of hydration heat effect. The arrangement of strain gauges ensures that the stress state of the steel strands is accurately monitored.

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

[0088] Since the beginning of concrete pouring, the temperature sensor and strain gauge data are collected through the automated monitoring system. Temperature data and stress data are collected every 5 minutes to ensure that the temperature field changes and stress state of the beam can be grasped in real time. The monitoring work should last for at least 120 hours to cover the critical stage of hydration heat effect.

[0089] Monitoring of hydration heat effect:

[0090] About 30 hours after the concrete is poured, 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 larger cross-section thickness at the end of the beam.

[0091] After that, the temperature will slowly drop, and 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 mold entry 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 release.

[0092] Monitoring of steel strand stress:

[0093] The stress of the steel strand changes with the change of concrete temperature. The hydration heat reaction causes the stress of the steel strand to gradually decrease. When the temperature reaches the peak, the stress of the steel strand usually drops to the lowest point. As the temperature gradually decreases, the stress of the steel strand recovers. Usually, in about 80 hours, the stress value of the steel strand rises to more than 98% of the original tensile stress. According to the monitored stress change curve, it is determined that the stress of the steel strand is not less than 0.98σ 0 moment.

[0094] Step S3: Preliminarily determine the age of pre-tensioned box beam according to the mechanical properties of early-age concrete:

[0095] While pouring concrete, make at least 5 groups 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 beams to ensure that the mechanical properties of the test blocks are consistent with those of the actual beams.

[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 4d age.

[0107] Judging the test results:

[0108] If the measured compressive strength and elastic modulus both reach 80% of the design value, the initial release age T is determined. 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 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] f 1 and f 2 :Age T 1 (3d) and T 2 (4d) Measured mechanical property values;

[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 The 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 release age T is determined. 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] T 3 and T 4 Age 4d and T p1 The age of concrete at the time of

[0130] f 3 and f 4 are the measured mechanical properties 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 28d 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 release age T is determined. 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

[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 release age T is determined. 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 maintenance measures

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

[0150] Improve maintenance measures: such as adjusting the maintenance temperature, humidity and other conditions to promote the 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. When the mechanical properties do not meet the standards, the linear extrapolation method is used to calculate the age required to meet the design requirements and conduct verification tests. If the standards are still not met, the maintenance measures or concrete mix ratio are adjusted to improve the early mechanical properties of 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. At the same time, this method has the advantages of strong operability and high precision, which is convenient for construction personnel to apply in actual projects.

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

[0156] Determine the stocking age (T r )

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

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

[0159] T h2 : The age corresponding to when the difference between the superficial 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 pre-tensioned box girder when it is released 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 the box girder and the mechanical properties of concrete, the timing of releasing the pre-tensioned box girder T 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 less than 0.98σ when tensioning 0 Stress control line.

[0163] Release operation:

[0164] At the determined release time T r , start the tensioning operation, requiring the stress of the steel strand to be no less than 0.98σ 0 The jack-based overall synchronous tensioning process is adopted, and the oil circuit unloading speeds at both ends of the beam body are kept synchronous, and the tensioning is carried out in stages, evenly and symmetrically.

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

[0166] The present invention can scientifically and accurately determine the timing of releasing the pre-tensioned box girder by combining dual monitoring of the hydration heat effect and the mechanical properties of concrete. By real-time monitoring of the temperature field inside the beam and the stress of the steel strands, premature release when the hydration heat effect has not completely subsided or the mechanical properties of the concrete have not reached the standard is avoided, the risk of cracking of the box girder or loss of additional prestress is reduced, and the bearing capacity and durability of the pre-tensioned box girder are greatly improved.

[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 of 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 web junction, the core of the top plate web junction, 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 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 on the outside of the box beam side mold. The sensors of the beam are arranged as follows. Figure 2 and Figure 3 As shown, the sensor names refer to the mid-span top plate (middle-1-1, middle-1-2, middle-surface), mid-span web plate (middle-2-1), mid-span bottom plate (middle-3-1, middle-3-2); beam end top plate (end-1-1, end-1-2, end-surface), beam end web plate (end-2-1) and beam end bottom plate (end-3-1, end-3-2) temperature sensors. It is necessary to ensure that the temperature sensor is not damaged after the installation work is completed until the end of monitoring. Temperature monitoring starts from the pouring of the first batch of concrete and lasts for about 120 hours. Record the temperature and relative humidity throughout the process.

[0171] The strain gauges are installed in Figure 4 On the prestressed steel strands between the tensioning device and the end die shown in the schematic diagram, strain gauges are arranged on three bundles of prestressed steel strands, 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 beams:

[0173] Since the beginning of concrete pouring, the hydration heat effect has been continuously monitored, and the data has been collated and plotted into a graph to observe the development of the hydration heat temperature of the beam body. The curve of the concrete hydration heat effect over time measured at the mid-span and beam end section is as follows: Figure 5 , 6 As shown in the figure, the hydration heat temperature of each part reaches the highest 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 monitoring for 79 hours, the difference between the concrete box girder mid-span and the core of the beam end section and the concrete mold entry temperature is within 10°C, T h3 Pick and The larger value of the two is 79h. After 86h, the temperature difference between the core and the surface of the concrete box girder 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℃, T h2 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 minimum at 35 to 40 hours; as the temperature drops, the stress value of the steel strand slowly rises again, and returns to 0.98σ at around 80 hours. 0 , which meets the requirement of no less than 72h in the specification and helps to more accurately determine the timing of prestressing release. At this time, the heat effect of concrete hydration is weakened and the internal temperature of the concrete gradually decreases.

[0175] S3: Based on the mechanical properties of early-age concrete, the preliminarily proposed age of pre-tensioned box beams:

[0176] Concrete test blocks were made while pouring concrete. The concrete compressive strength test used 150×150×150mm cubic test blocks, with 5 groups of 3 test blocks in each group. The concrete elastic modulus test used 150×150×300mm prism test blocks, with 5 groups of 6 test blocks in each group. 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 , 9It 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 strength design grade value of C50 concrete 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:

[0179] Combined with the monitored development curve of hydration heat effect of prestressed box beams and the initial release time based on the mechanical properties of concrete, the release time of this batch of precast prestressed beams is determined based on the temperature-stress dual control. p For 3 days, 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, and must be greater than the stress value of the steel strand by no less than 0.98σ 0 The stress control line corresponds to 80h, which determines the release time T of the prefabricated pre-tensioned box beams in this batch. r At this time, the concrete compressive strength and elastic modulus have reached 80% of the 28d design value of C50 concrete, and the concrete hydration heat effect has weakened. The temperature difference between the core and the surface of the box beam, the surface and the ambient temperature, and the core and concrete mold entry temperature are all within 10°C. The stress value of the steel strand has recovered to above 0.98σ 0 The level of stress line. While meeting the requirements of the specifications, it is possible to more accurately determine the timing of tensioning and more scientifically ensure the quality of beam manufacturing.

[0181] When tensioning, the unloading speed of the oil circuits 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, with the broken line reinforcement being tensioned first, and then the straight line reinforcement.

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

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

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

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

[0186] The data acquisition and processing module 300 is used to acquire 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 according to 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, which will not be repeated here.

[0190] Although 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 the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties, characterized in that: The following steps are involved: S1. Arrange temperature sensors and strain gauges at the predetermined positions of the prefabricated pre-tensioned box beams to monitor the hydration heat effect of concrete and the stress changes of the prestressed steel strands; S2. Monitor the hydration heat effect and strand stress of the pre-tensioned box girder, and continuously monitor 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 beam according to the mechanical properties of early-age concrete, test the concrete test blocks cured with the beam, 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.

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 web joint, the core of the top plate web joint, 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 in the mid-span and beam end; Near the mid-span of the pre-tensioned box girder, temperature sensors inside and outside the box girder are installed in the mold cavity inside the box girder 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 zigzag 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 3, characterized in that: 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.

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: In step S3, the mechanical properties of concrete are tested and the tensioning age T is determined based on the mechanical properties of concrete. p The determination includes the following steps: S3-1: Randomly select a group of test blocks to test the compressive strength and elastic modulus of concrete at 3d age; If the measured values ​​all reach 80% of the designed values, the initial release age period is to be no less than 3 days; S3-2: If the measured values ​​do not all reach 80% of the design values, but are all higher than 70% of the design values, a group of test blocks are 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 designed values, the initial release age period is to be 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 Should not be 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 ; If the measured values ​​reach 80% of the design values, the initial release age is set 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 age T corresponding to the time when the design value reaches 80%. p2 , and T p2 Should not be less than 7d; S3-2-2: Preliminary plan is to set the stock age to be no less than T p2 , and test the concrete age T p2 Compressive strength and elastic modulus at ; 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 designed values, the initial release age period is to be 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 designed values, the initial release age period is to be 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 maintenance measures to improve the early mechanical properties.

6. The method for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties according to claim 5, characterized in that: 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 Should not be 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 estimated concrete age, and T p2 Should not be less than 7d.

7. 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 shallow 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 environment 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 .

8. 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 S4, the timing of placing a sheet is T r The following conditions are met: Time to release T r The value should be greater than 24 times the tensioning age T proposed based on the mechanical properties of concrete. p and the setting 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.

9. 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 release step, the release is carried out in the order and steps required by the design, first the folded line reinforcement is released, and then the straight line reinforcement is released. 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.

10. A device for determining the timing of releasing the pre-tensioned box girder by combining hydration heat and mechanical properties, used for implementing the method according to any one of claims 1 to 9, 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 the stress changes of steel strands; Data acquisition and processing module, used to collect and process temperature and stress data in real time, and draw the curve of temperature and stress development 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 placing device to perform the sheet placing operation according to the calculated sheet placing timing and sequence.

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