Holographic joint monitoring method and device for prestressing and defects in existing structures

Abstract

The present invention discloses a holographic joint monitoring method and device for the prestress and internal defects of an existing structure. By installing a piezoelectric ultrasonic transducer at the end of the prestressed tendon of an internal prestressed concrete structure, an ultrasonic guided wave method is used to detect the internal corrosion and grouting void defects of the structure. The method has the advantages of long detection distance, high detection accuracy, fast detection speed, and no need to repeatedly move the transducer. The present invention takes into account the influence of different prestress levels on the ultrasonic guided wave group velocity, and uses the grouting void coefficient and the corrosion coefficient to achieve simultaneous monitoring of the above two types of damage; finally, the present invention combines holographic simulation means to restore the monitoring signal and damage characteristics and improve the monitoring accuracy. Therefore, the joint monitoring method of the present invention has a simple process, is easy to operate, and has a low cost. It can be used for the detection of prestressed tendons, corrosion, and grouting voids in internal prestressed concrete structures of different specifications. It has strong applicability and is conducive to popularization and application.

Description

Technical Field

[0001] The invention belongs to the technical field of civil engineering monitoring, and in particular relates to a holographic joint monitoring method and device for prestressing of an existing structure and defects in a duct. Background Art

[0002] Internal prestressed concrete structures are widely used in engineering projects. During the construction process, after the prestressed tendons are tensioned, concrete grouting needs to be carried out inside the pipe. If the pouring quality is poor, grouting voids will be formed. During operation, prestressed tendons are prone to prestress loss, resulting in deflection of the beam body and reduced bearing capacity. Environmental factors such as temperature and humidity, as well as loads such as wind and vehicles, will cause cracks in prestressed concrete structures, and further cause corrosion of prestressed tendons, crack expansion, and debonding of prestressed tendons and grouting materials. Prestress loss, corrosion, and grouting voids (including debonding of prestressed tendons and grouting materials) will pose a safety hazard to the structure. It is necessary to monitor the prestress and corrosion of prestressed tendons and grouting voids in internal prestressed concrete structures.

[0003] Common prestress monitoring methods include: (1) vibration frequency method, such as the literature [Jiang Wei. Research on cable force test based on vibration frequency method [D]. Guangxi University of Science and Technology, 2020]. This method is not applicable to prestressed concrete structures because the prestressed tendons are constrained; (2) magnetic flux method, such as the literature [Hao Chao, Pei Minshan, Qiang Shizhong. A new method for cable force test of cable-stayed bridges-magnetic flux method [J]. Highway, 2000, (11): 30-31]. The measurement accuracy of this method is not high; (3) electromagnetic elastic method, such as the literature [Xu Cong. Online monitoring of bridge steel cable stress based on magnetoelastic effect and development of reconfigurable system [D]. Nanchang Hangkong University, 2018]. This method has high measurement accuracy, but the detection accuracy is affected by the installation position.

[0004] Common corrosion detection methods include: (1) magnetic flux leakage detection method such as the literature [Zhou Jianting, Zhang Senhua, Zhang Hong, et al. Analysis of magnetic flux leakage signals of corroded steel bars in concrete [J]. Journal of Jiangsu University (Natural Science Edition), 2019, 40(03): 355-359]. This method requires point-by-point detection and has limited detection accuracy in the presence of concrete protective layer and prestressed channel pipes; (2) eddy current detection method such as the literature [Cheng Jun. Experimental study on detection of internal steel bar corrosion in bridges based on electromagnetic pulse eddy current [D]. Chongqing Jiaotong University, 2015]. This method has high detection accuracy, but is not suitable for existing prestressed concrete structures.

[0005] Common methods for detecting grouting voids include: (1) ground penetrating radar method, such as reference [Shu Zhile, Wang Jie, Wu Rui, et al. Ground penetrating radar model test and forward simulation of prestressed pipe grouting density [J]. Concrete, 2021, (02): 136-140], but this method cannot detect the grouting situation inside the metal bellows; (2) impact echo method, such as reference [Liu Hui, Li Xiangmin, Xu Qingfeng. Experiment on impact echo method in sleeve grouting density detection [J]. Nondestructive Testing, 2017, 39(04): 12-16], this method is difficult to quantify damage; (3) infrared imaging technology, such as reference [Yue Zhuowen, Zhang Decai, Wei Ding, et al. Research on monitoring concrete density in steel pipe columns with infrared imaging technology [J]. Concrete and Cement Products, 2017, (08): 84-86. 2017.08.021], this method has limited detection depth.

[0006] The size of structural prestress, corrosion, and grouting void damage have a huge impact on the structural bearing capacity and are the subject of great concern in the engineering community; however, from the above analysis of the prior art, it can be seen that those skilled in the art still need to conduct in-depth research to obtain a simpler, more practical, wide-range, and high-precision joint monitoring method for prestressed tendons and corrosion, and grouting voids in prestressed concrete structures. Summary of the invention

[0007] In view of the above, the present invention provides a holographic joint monitoring method and device for prestressing and defects in existing structures, which uses an electromagnetic elastic absolute stress sensor arranged at the anchor head to monitor the prestress level of the prestressed tendons, and uses a piezoelectric ultrasonic transducer arranged at both ends of the prestressed tendons to detect corrosion and grouting voids in the prestressed structure. It has the advantages of easy operation, high monitoring accuracy, and strong applicability.

[0008] A holographic joint monitoring method for prestressing of an existing structure and defects in a hole comprises the following steps:

[0009] (1) Using an electromagnetic elastic absolute stress sensor to collect the magnetic characteristic signal at the anchor head of the prestressed tendon standard sample under different prestress sizes, the relationship curve between the prestress size and the magnetic characteristic signal of the anchor head is obtained by fitting;

[0010] (2) An electromagnetic elastic absolute stress sensor is mounted on the periphery of the anchor head at the end of the prestressed tendon to be tested, and the magnetic characteristic signal at the anchor head is collected, and then the prestress level of the prestressed tendon to be tested is calculated according to the above relationship curve;

[0011] (3) Calculate the propagation group velocity of the ultrasonic guided wave in the prestressed tendon to be tested based on the size and prestress level of the prestressed tendon to be tested;

[0012] (4) Piezoelectric ultrasonic transducers are installed at both ends of the prestressed tendons to be tested. The piezoelectric ultrasonic transducer at one end excites ultrasonic guided waves through the piezoelectric principle. According to the propagation group velocity of the ultrasonic guided waves in the prestressed tendons to be tested, the characteristics of the transmitted wave signal are analyzed to achieve quantitative monitoring of the corrosion and grouting voids of the prestressed tendons to be tested.

[0013] (5) Combined with holographic simulation technology, the monitoring information is restored to vividly display the damage characteristics.

[0014] When the prestressed steel bar to be tested has corrosion damage, part of the ultrasonic guided wave energy is reflected at the damage site, thereby reducing the ultrasonic guided wave energy transmitted from the damage site. The more severe the damage, the smaller the transmitted ultrasonic guided wave energy. When the steel bar to be tested has grouting cavity damage, the energy scattered to the anchor body is reduced, thereby increasing the transmitted ultrasonic guided wave energy. The longer the grouting cavity area, the greater the transmitted ultrasonic guided wave energy. Therefore, in step (4), the peak value of the transmitted wave signal is obtained by using the piezoelectric ultrasonic transducer at the other end, and compared with the peak value of the transmitted wave signal in the non-destructive state, so as to quantitatively monitor the corrosion of the prestressed steel bar to be tested and the grouting cavity damage.

[0015] In the area where grouting voids are generated, the prestressed tendons to be tested are in a non-gripped state, which increases the group velocity of ultrasonic guided wave propagation. Therefore, in step (4), the arrival time of the transmitted wave signal is obtained by the piezoelectric ultrasonic transducer at the other end. t 1. Combined with the propagation group velocity of ultrasonic guided waves in the prestressed tendons to be tested under the wrapped and free states, the length of the grouting cavity of the prestressed tendons to be tested can be monitored. The specific calculation expression is as follows:

[0016] t 1= L a1 / v a + L f1 / v f , L=L a1 + L f1 , L f1 =L f0 + L fm

[0017] in: L fm is the length of the grouting cavity, L f0 is the length of the free section of the prestressed tendon to be tested in the non-destructive condition, L f1 is the length of the free section of the prestressed tendon to be tested under cavity damage,L a1 is the length of the prestressed tendon gripping section to be tested under cavity damage, L is the total length of the prestressed tendons to be tested, v a is the propagation group velocity of ultrasonic guided waves in the prestressed tendons to be tested in the gripping state, v f is the propagation group velocity of ultrasonic guided waves in the prestressed tendons to be tested in the free state.

[0018] Furthermore, in the step (4), in view of the situation where both grouting voids and corrosion are damaged at the same time, a 3D model of the prestressed tendon to be tested in this state is firstly constructed, in which an ultrasonic guided wave is excited at one end and a transmitted wave signal is obtained at the other end, and the ratio of the amplitude of the transmitted wave signal of the prestressed tendon to be tested at different grouting void lengths to the amplitude of the transmitted wave signal in the intact state is calculated; and then in the actual calculation, the piezoelectric ultrasonic transducer at the other end is used to measure the transmitted wave signal and the degree of damage is calculated using the following expression:

[0019]

[0020] in: V 0 is the amplitude of the transmitted wave signal measured by the piezoelectric ultrasonic transducer at the other end in a lossless condition, V 1 is the amplitude of the transmitted wave signal measured by the piezoelectric ultrasonic transducer at the other end in the case of damage, which reflects the degree of damage. is the length of the grouting cavity of the prestressed tendon to be tested when only grouting cavity damage exists L fm The ratio of the amplitude of the transmitted wave signal under lossless conditions to the amplitude of the transmitted wave signal under lossless conditions, α 2 is the corrosion coefficient (used to characterize the degree of corrosion of embedded steel bars).

[0021] Furthermore, in step (5), holographic simulation technology is used to establish a 3D model of the prestressed tendon to be tested in the finite element software, and the position, shape and material parameters of the damage are modified to restore the monitoring information in the actual situation and to intuitively and vividly display the damage characteristics.

[0022] A holographic joint monitoring device for prestress and defects in existing structures comprises a piezoelectric ultrasonic transducer, an electromagnetic elastic absolute stress sensor and a host computer, wherein the piezoelectric ultrasonic transducer is installed and fixed at the front and rear ends of the test beam where the prestressed tendons to be tested extend out, and is used to stimulate the generation of ultrasonic guided waves and receive transmission wave signals at the same time; the electromagnetic elastic absolute stress sensor is sleeved on the periphery of the anchor head at the end of the prestressed tendons to be tested, and collects magnetic characteristic signals at the anchor head of the prestressed tendons to be tested under the action of a DC pulse;

[0023] The host computer is used to analyze the characteristics of the transmission wave signal received by the piezoelectric ultrasonic transducer, so as to quantitatively monitor the corrosion and grouting voids of the prestressed tendons to be tested; at the same time, the prestress level of the prestressed tendons to be tested is calculated based on the magnetic characteristic signal collected by the electromagnetic elastic absolute stress sensor and the relationship between the prestress size and the magnetic characteristic signal of the anchor head.

[0024] Furthermore, the electromagnetic elastic absolute stress sensor is cylindrical, and has an excitation coil for excitation and a magnetoelectric conversion element for receiving magnetic characteristic signals built in it, and the magnetoelectric conversion element is located between the excitation coil and the anchor head; the electromagnetic elastic absolute stress sensor is prefabricated, and is designed and manufactured in advance according to different anchor head specifications, and does not require on-site assembly and winding.

[0025] Through in-depth research, the present invention proposes to use an electromagnetic elastic absolute stress sensor at the anchor head to monitor the prestress of the prestressed tendons, and to use a piezoelectric transducer arranged at both ends of the prestressed tendons to detect the rust and grouting cavity defects inside the prestressed tendons. The present invention has the characteristics of convenient operation, low cost, strong applicability, fast detection speed and high detection accuracy. First of all, no one in the field has applied the electromagnetic elastic absolute stress sensor to the absolute stress monitoring of the prestressed tendons of the existing internal prestressed concrete structure, and the present invention attempts to use it for the absolute stress monitoring of the prestressed tendons of the internal prestressed concrete structure, and proposes to reflect the size of the prestress of the prestressed tendons through the change of the magnetic characteristic quantity at the anchor head. The reason for this design is that the anchor head is a magnetizable component, and the stress of the prestressed tendons is transmitted to the anchor head through the clip at the anchor hole, so that the anchor head is in a compressed state. Different pressures will affect the magnetic characteristics of the anchor head, so the size of the prestress of the corresponding structure prestressed tendons can be monitored according to the size of the magnetic characteristic quantity at the anchor head.

[0026] In addition, no one in the field has applied ultrasonic guided waves to the detection of corrosion and grouting voids of prestressed tendons in existing internal prestressed concrete structures. The present invention uses this technology and proposes to arrange a piezoelectric ultrasonic transducer at the end of the prestressed tendon. The reason for this design is: for existing internal prestressed concrete structures, the ultrasonic guided wave transducer is limited in arrangement due to the short length of the prestressed tendons extending out of the structure. At the same time, when the prestressed tendons are corroded, the ultrasonic guided waves will be reflected at the corrosion damage, thereby reducing the energy of the transmitted wave; when the prestressed tendons are grouting void damaged, the energy scattered to the surrounding anchor bodies is reduced, increasing the energy of the transmitted wave, and at the same time the group velocity of the ultrasonic guided waves will increase. Therefore, the corrosion of the prestressed tendons and the grouting void damage of the existing internal prestressed concrete structure can be detected based on the flight time of the transmitted wave packet of the ultrasonic guided wave and the change in the wave packet peak.

[0027] Therefore, the present invention has the following beneficial technical effects:

[0028] 1. The present invention installs a piezoelectric ultrasonic transducer at the end of the prestressed tendon of the internal prestressed concrete structure and uses an ultrasonic guided wave method to detect internal corrosion and grouting cavity defects of the structure. The invention has the advantages of long detection distance, high detection accuracy, fast detection speed, and no need to repeatedly move the transducer.

[0029] 2. The present invention tests the in-vivo prestressed concrete structure standard sample and collects the magnetic characteristic signal at the anchor head to obtain a relationship curve between the prestress of the prestressed tendons in the standard sample and the magnetic characteristic signal of the anchor head as a reference curve for detection. The calibration accuracy is high and the test operation is simple.

[0030] 3. The present invention prefabricates specific electromagnetic elastic absolute stress sensors for anchor heads of different specifications, eliminating the need for on-site assembly and winding.

[0031] 4. The present invention improves the monitoring accuracy of grouting voids by considering the ultrasonic guided wave velocity under different prestress levels, and uses holographic simulation technology to restore and display damage characteristics more intuitively and vividly.

[0032] 5. The monitoring method of the present invention has a simple process, is easy to operate, and has a low cost. It can be used for the joint monitoring of prestressing and corrosion of prestressed tendons and grouting voids in prestressed concrete structures of different specifications. It has strong applicability and is conducive to popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the arrangement of the holographic joint monitoring device for prestressing of existing structures and defects in ducts in the present invention in the prestressed tendons to be tested, wherein (a) is a vertical view, (b) is a side view, (c) is a cross-sectional view, and (d) is an end top view.

[0034] Figure 2 It is a schematic structural diagram of the electromagnetic elastic absolute stress sensor of the present invention.

[0035] Figure 3 It is a schematic flow chart of the holographic joint monitoring method of the existing structure prestress and defects in the pores of the present invention.

[0036] Figure 4 It is a schematic diagram of a calibration curve between the magnetic characteristic signal (magnetic permeability) at the anchor head and the prestress size in an embodiment of the present invention.

[0037] Figure 5 is the grouting void coefficient in the embodiment of the present invention Schematic diagram of the calibration curve of grouting cavity length.

[0038] Figure 6 Schematic diagram of transmission wave signals of lossy and non-destructive test pieces in an embodiment of the present invention.

[0039] Figure 7It is a schematic diagram of a 3D finite element model established in an embodiment of the present invention.

[0040] Figure 1 and Figure 2 In the figure: 1—monitoring device, 2—test beam, 3 and 3'—piezoelectric ultrasonic transducer, 4—prestressed tendon to be tested, 5—anchor head, 6—anchor pad, 7 and 7'—electromagnetic elastic absolute stress sensor, 8—grouting cavity damage, 9—corrosion damage, 10—magnetoelectric conversion element, 11—excitation coil. DETAILED DESCRIPTION

[0041] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0042] like Figure 1 As shown, the holographic joint monitoring device 1 for existing structural prestress and defects in the channel of the present invention is installed at the front and rear ends of the prestressed tendon 4 to be tested, and includes piezoelectric ultrasonic transducers 3 and 3' and electromagnetic elastic absolute stress sensors 7 and 7', wherein the piezoelectric ultrasonic transducers 3 and 3' are respectively installed and fixed at the front and rear ends of the prestressed tendon 4 to be tested extending out of the test beam 2, and are used to stimulate the generation of high-energy ultrasonic guided waves and receive transmission wave signals at the same time; the electromagnetic elastic absolute stress sensors 7 and 7' are sleeved on the periphery of the anchor heads 5 at both ends of the prestressed tendon 4 to be tested, and are used to collect magnetic characteristic signals at the anchor heads 5 of the prestressed tendon 4 to be tested; an anchor pad 6 is provided between the anchor head 5 and the end of the test beam 2.

[0043] like Figure 2 As shown, the electromagnetic elastic absolute stress sensors 7 and 7' are cylindrical in shape, and are equipped with an excitation coil 11 for excitation and a magnetoelectric conversion element 10 for receiving magnetic characteristic signals. The magnetoelectric conversion element 10 is located between the excitation coil 11 and the anchor head 5. The electromagnetic elastic absolute stress sensors 7 and 7' are prefabricated and can be designed and manufactured in advance according to the specifications of different anchor heads 5, without the need for on-site assembly and winding.

[0044] Before measurement, electromagnetic elastic absolute stress sensors 7 and 7' are used to collect magnetic characteristic signals at the anchor heads of prestressed tendon standard samples at different prestress magnitudes, and the relationship curve between the prestress magnitude and the magnetic characteristic signals of the anchor heads is obtained by fitting; during measurement, electromagnetic elastic absolute stress sensors 7 and 7' are mounted on the periphery of the anchor head 5 at the end of the prestressed tendon 4 to be measured, and the magnetic characteristic signals at the anchor head 5 are collected and sent to the host computer, and the prestress level of the prestressed tendon 4 to be measured is calculated based on the magnetic characteristic signals collected by the electromagnetic elastic absolute stress sensors 7 and 7' and the relationship curve between the prestress magnitude and the magnetic characteristic signals of the anchor heads. At the same time, finite element software is used to perform 3D modeling of the component to be measured, and the transmission wave signal of the piezoelectric transducer at the other end of the model is obtained, and the ratio of the transmission wave signal amplitude when the component to be measured is damaged by grouting cavities of different lengths to the transmission wave signal amplitude in the non-destructive state is calculated. ( x 1, x 2,…, x n ≥0 is the grouting cavity length).

[0045] According to the size of the component to be tested, the propagation group velocity of the ultrasonic guided wave in the free state and the gripped state of the steel bar to be tested is calculated at different prestress levels, which serves as the basis for determining the ultrasonic guided wave group velocity after obtaining the prestress value of the steel bar.

[0046] During the measurement process, when the prestressed tendon 4 to be tested has corrosion damage, part of the ultrasonic guided wave energy is reflected at the damage, thereby reducing the ultrasonic guided wave energy transmitted from the damage. The more serious the corrosion damage, the smaller the transmitted ultrasonic guided wave energy. When the prestressed tendon 4 to be tested has grouting cavity damage, the energy scattered to the test beam 2 is reduced, thereby increasing the transmitted ultrasonic guided wave energy. The longer the grouting cavity damage area 8, the greater the transmitted ultrasonic guided wave energy. The peak value of the transmission wave signal is obtained by using the piezoelectric ultrasonic transducer 3' at the other end, and compared with the peak value of the transmission wave signal in the lossless state, so as to quantitatively monitor the corrosion and grouting cavity damage of the prestressed tendon 4 to be tested.

[0047] At the grouting cavity damage 8, the prestressed tendon 4 to be tested is in a non-gripped state, which increases the group velocity of ultrasonic guided wave propagation. The arrival time of the transmission wave signal is obtained through the piezoelectric ultrasonic transducer 3' at the other end. t 1. Combined with the group velocity of ultrasonic guided wave propagation in the prestressed tendon 4 under the gripping and free states, the grouting cavity length of the prestressed tendon 4 under the gripping and free states is monitored. The specific calculation expression is as follows:

[0048] t 1= L a1 / v a + L f1 / v f , L=L a1 + L f1 , L f1 =L f0 + L fm

[0049] in: L fm is the length of the grouting cavity, L f0 is the length of the free section of the prestressed tendon 4 to be tested in the non-destructive condition; Lf1 is the length of the free section of the prestressed tendon 4 to be tested under the condition of cavity damage; L a1 is the length of the 4-wrapped section of the prestressed tendon to be tested under grouting cavity damage, L is the total length of the prestressed tendon 4 to be tested, v a When the prestressing force is a certain level, the group velocity of ultrasonic guided wave propagation in the prestressed tendon 4 to be tested in the gripping state; v f When the prestressing force is at a specific level, the ultrasonic guided wave propagation group velocity in the prestressed tendon 4 to be tested in the free state.

[0050] In the case of grouting voids and corrosion coexisting, on the basis of calculating the length of the grouting void, the transmission wave signal of the piezoelectric ultrasonic transducer 3' at the other end is obtained, and the damage degree is calculated using the following expression:

[0051]

[0052] in: V 0 is the amplitude of the transmitted wave signal in the lossless case, V 1 is the measured amplitude of the transmitted wave signal, is the length of the grouting void when only grouting void damage exists L fm The ratio of the amplitude of the transmitted wave signal under lossless conditions to the amplitude of the transmitted wave signal under lossless conditions, α 2 is the corrosion coefficient, which is used to characterize the corrosion degree of the prestressed tendon 4 to be tested.

[0053] In addition, the present invention adopts holographic simulation to establish a 3D model of the monitored object in the finite element software, and modifies the position, shape, material parameters, etc. of the damage to restore the actual monitoring signal and display the damage characteristics more intuitively and vividly.

[0054] like Figure 3 As shown, the method for holographically jointly monitoring the prestress and defects in the duct of the prestressed tendons using the above device is as follows:

[0055] Firstly, the model of the component to be tested (prestressed tendons and anchor heads to be tested) is determined, and the electromagnetic elastic absolute stress sensor is designed and manufactured in advance according to the specifications of the prestressed tendons and anchor heads to be tested, without the need for on-site assembly and winding.

[0056] An electromagnetic elastic absolute stress sensor is installed on the anchor head of the prestressed tendon standard sample. The magnetic characteristic signal at the anchor head is collected under different prestressed tendon stress sizes. The prestress is applied to the prestressed tendon standard sample step by step using a hydraulic jack, and the magnetic characteristic signal at the anchor head under each level of prestress load is recorded. Then, the relationship curve between the prestress size and the magnetic characteristic signal of the anchor head is obtained by fitting according to the prestress size and its corresponding magnetic characteristic signal at the anchor head.

[0057] Since the prestressed tendon standard sample and anchor head are of the same specifications as the prestressed tendon and anchor head to be tested, an electromagnetic elastic absolute stress sensor is used to cover the periphery of the anchor head of the prestressed tendon to be tested in the existing internal prestressed concrete structure during the actual measurement. The built-in excitation coil completes the excitation function, and the magneto-electric conversion element completes the function of receiving the magnetic characteristic signal. By collecting and acquiring the magnetic characteristic signal at the anchor head, the prestress level of the prestressed tendon to be tested can be obtained in combination with the above relationship curve.

[0058] At the same time, in this embodiment, the piezoelectric ultrasonic transducer is vertically arranged at the two ends of the prestressed tendon to be tested, and the piezoelectric ultrasonic transducer at one end is used to excite the ultrasonic guided wave signal, and the piezoelectric ultrasonic transducer at the other end is used to obtain the transmitted wave signal. Specifically:

[0059] First, according to the size of the component to be tested, the group velocity of the ultrasonic guided wave propagating in the free state and the gripped state of the steel bar to be tested at different prestressing levels is calculated in advance; then, according to the measured prestressed steel bar stress, the group velocity of the ultrasonic guided wave propagating in the free state is obtained. v f and the group velocity of propagation in the grip state v a .

[0060] At the same time, the finite element software is used to build a 3D model of the component to be tested, and the transmission wave signal of the piezoelectric transducer at the other end of the model is obtained. The ratio of the transmission wave signal amplitude when the component to be tested has grouting cavity damage of different lengths to the transmission wave signal amplitude in the intact state is calculated. ( x 1, x 2,…, x n ≥0 is the grouting cavity length).

[0061] The length of the prestressed tendon to be tested is L At one end, the piezoelectric ultrasonic transducer excites the ultrasonic guided wave signal, and at the other end, the piezoelectric ultrasonic transducer obtains the transmitted wave signal and obtains the wave packet flight time. t 1. Assume that when the component leaves the factory (in a non-destructive state), the length of the free section of the prestressed tendon is L f0 , the length of the prestressed tendon grip section is L a0When there is no grouting cavity damage, the time for the other end of the prestressed tendon to obtain the direct ultrasonic guided wave signal is t 0, then L=L a0 + L f0 , t 0= L a0 / v a + L f0 / v f ;like t 1< t 0, it indicates that there is grouting cavity damage. Assuming that in this state, the length of the free section of the prestressed tendon is L f1 , the length of the prestressed tendon grip section is L a1 , the length of the grouting cavity area is L fm ,according to t 1= L a1 / v a + L f1 / v f , L=L a1 + L f1 , L f1 =L f0 + L fm , the above unknown quantities can be solved to determine the length of the grouting void area L fm If it is determined that there is no grouting cavity damage, then L fm =0.

[0062] Based on the calculated length of the grouting cavity, the amplitude of the transmitted wave signal of the piezoelectric ultrasonic transducer at the other end is obtained, and the damage degree is calculated using the following expression:

[0063]

[0064] in: V 0 is the amplitude of the direct packet signal wave in the lossless case, V 1 is the measured direct packet signal amplitude, is the specific grouting void length when only grouting void damage exists L fmThe ratio of the amplitude of the transmitted wave signal packet under lossless conditions to the amplitude of the transmitted wave signal under lossless conditions, α 2 is the corrosion coefficient, which is used to characterize the degree of corrosion of prestressed tendons.

[0065] In addition, this embodiment uses holographic simulation to establish a 3D model of the monitored object in the finite element software, and modifies the location, shape, material parameters, etc. of the damage to restore the actual monitoring signal, display the damage characteristics more intuitively and vividly, and make the monitoring results more accurate.

[0066] It should be noted that in actual measurement, the specifications of the piezoelectric transducer and electromagnetic elastic absolute stress sensor, the installation method of the sensor, the excitation voltage, and the amplification factor of the collected signal must be the same as those used for the standard.

[0067] Below we choose a steel strand with an outer diameter of 15.2mm, a length of 3m, and a density of 7850kg / m 3 , elastic modulus 209Gpa, Poisson's ratio 0.29; concrete beam material is C40, cross-sectional dimensions are 0.1m*0.1m, length 2.8m, a hole is reserved in the center of the cross-sectional area of ​​the beam, and the cross-sectional area of ​​the hole is slightly larger than the steel strand to facilitate the arrangement of the steel strand. In this embodiment, we first calibrate the magnetic characteristic signal and prestress size at the anchor head of the structure, and the corresponding calibration curve is as follows Figure 4 As shown; at the same time, a corresponding 3D model is established in the finite element model, and the excitation signal is a 3-cycle Hanning window modulated sine wave with a center frequency of 15kHz to calibrate the grouting void coefficient The relationship between the length of the grouting cavity and the corresponding calibration curve is as follows: Figure 5 shown.

[0068] Then the group velocity of ultrasonic guided waves propagating in the steel bars to be tested is calculated under different prestress levels.

[0069] Next, two specimens with the same parameters as in the above steps were made, one of which was a non-destructive specimen, and the other was pre-embedded with 0.7m of grouting cavity damage and 0.1m of uniform corrosion damage (electrochemical corrosion was used, and the steel bars were corroded for one week before the specimens were made). Both specimens were prestressed with 1000MPa, and piezoelectric sensors were arranged on both sides of the protruding steel bars. The excitation signal was a 3-cycle Hanning window modulated sine wave with a center frequency of 15kHz. The transmission wave signals of the lossy and non-destructive specimens were obtained as shown in the figure. Figure 6Under the above conditions, the group velocity of the ultrasonic guided wave L(0,1) in the free state is 5093m / s, and the group velocity in the anchored state is 3024m / s. The arrival times of the lossless signal and the lossy signal L(0,1) modal transmission waves are 0.965ms and 0.867ms, respectively, with a difference of 0.098ms. The length of the grouting cavity is calculated to be 0.73m based on the wave packet arrival time, which is close to the preset 0.7m. The signal amplitudes of the two are V 0 and V 1 are 1.315 and 2.028 respectively, according to Figure 5 The calibration curve of grouting void coefficient is obtained , according to the formula Calculate α 2=0.700<1, it is judged that the specimen has rust damage.

[0070] After obtaining the damage coefficient, we Figure 7 The damage position, shape, material parameters, etc. are modified in the finite element model shown to restore the actual monitoring signal, present the damage characteristics more intuitively and vividly, and make the monitoring results more accurate.

[0071] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art to the present invention based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A holographic joint monitoring method for prestressing of an existing structure and defects in a duct, comprising the following steps: (1) Using an electromagnetic elastic absolute stress sensor to collect the magnetic characteristic signal at the anchor head of the prestressed tendon standard sample under different prestress sizes, the relationship curve between the prestress size and the magnetic characteristic signal of the anchor head is obtained by fitting; (2) An electromagnetic elastic absolute stress sensor is mounted on the periphery of the anchor head at the end of the prestressed tendon to be tested, and the magnetic characteristic signal at the anchor head is collected, and then the prestress level of the prestressed tendon to be tested is calculated according to the above relationship curve; (3) Calculate the propagation group velocity of the ultrasonic guided wave in the prestressed tendon to be tested based on the size and prestress level of the prestressed tendon to be tested; (4) Piezoelectric ultrasonic transducers are installed at both ends of the prestressed tendons to be tested. The piezoelectric ultrasonic transducer at one end excites ultrasonic guided waves through the piezoelectric principle. According to the propagation group velocity of the ultrasonic guided waves in the prestressed tendons to be tested, the characteristics of the transmitted wave signal are analyzed to achieve quantitative monitoring of the corrosion and grouting voids of the prestressed tendons to be tested. Specifically: The peak value of the transmission wave signal is obtained by using the piezoelectric ultrasonic transducer at the other end, and compared with the peak value of the transmission wave signal in the lossless state, so as to quantitatively monitor the corrosion of the prestressed tendons and the grouting cavity damage to be tested; the arrival time of the transmission wave signal is obtained by using the piezoelectric ultrasonic transducer at the other end t 1. Combined with the propagation group velocity of ultrasonic guided waves in the prestressed tendons to be tested under the wrapped and free states, the length of the grouting cavity of the prestressed tendons to be tested can be monitored. The specific calculation expression is as follows: t 1= L a1 / v a + L f1 / v f ， L=L a1 + L f1 ， L f1 =L f0 + L fm Where: i>L fm is the length of the grouting cavity, L f0 is the length of the free section of the prestressed tendon to be tested in the non-destructive condition, L f1 is the length of the free section of the prestressed tendon to be tested under cavity damage, L a1 is the length of the prestressed tendon gripping section to be tested under cavity damage, L is the total length of the prestressed tendons to be tested, v a is the propagation group velocity of ultrasonic guided waves in the prestressed tendons to be tested in the gripping state, v f is the propagation group velocity of ultrasonic guided waves in the prestressed tendons to be tested in the free state; In the case of simultaneous damage of grouting voids and corrosion, a 3D model of the prestressed tendons to be tested in this state is built in advance. In the model, ultrasonic guided waves are excited at one end, and the transmitted wave signal is obtained at the other end. The ratio of the amplitude of the transmitted wave signal of the prestressed tendons to the amplitude of the transmitted wave signal in the intact state at different grouting void lengths is calculated. Then, in the actual calculation, the piezoelectric ultrasonic transducer at the other end is used to measure the transmitted wave signal and the degree of damage is calculated using the following expression: ; in: V 0 is the amplitude of the transmitted wave signal measured by the piezoelectric ultrasonic transducer at the other end in a lossless condition, V 1 is the amplitude of the transmitted wave signal measured by the piezoelectric ultrasonic transducer at the other end in the case of damage, which reflects the degree of damage. is the length of the grouting cavity of the prestressed tendon to be tested when only grouting cavity damage exists L fm The ratio of the amplitude of the transmitted wave signal under lossless conditions to the amplitude of the transmitted wave signal under lossless conditions, α 2 is the corrosion coefficient; (5) Combined with holographic simulation technology, the monitoring information is restored to vividly display the damage characteristics.

2. According to claim 1, a holographic joint monitoring method for prestressing of an existing structure and defects in a hole is characterized by: In the step (5), holographic simulation technology is used to establish a 3D model of the prestressed tendon to be tested in the finite element software, and the position, shape and material parameters of the damage are modified to restore the monitoring information in the actual situation and to intuitively and vividly display the damage characteristics.

3. A holographic joint monitoring device for existing structural prestress and defects in the duct, used to implement a holographic joint monitoring method for existing structural prestress and defects in the duct as claimed in claim 1 or 2, characterized in that: The device comprises a piezoelectric ultrasonic transducer, an electromagnetic elastic absolute stress sensor and a host computer. The piezoelectric ultrasonic transducer is installed and fixed at the front and rear ends of the test beam where the prestressed tendon to be tested extends, and is used to stimulate the generation of ultrasonic guided waves and receive transmission wave signals at the same time; the electromagnetic elastic absolute stress sensor is sleeved on the periphery of the anchor head at the end of the prestressed tendon to be tested, and collects magnetic characteristic signals at the anchor head of the prestressed tendon to be tested under the action of a DC pulse; The host computer is used to analyze the characteristics of the transmission wave signal received by the piezoelectric ultrasonic transducer, so as to quantitatively monitor the corrosion and grouting voids of the prestressed tendons to be tested; at the same time, the prestress level of the prestressed tendons to be tested is calculated based on the magnetic characteristic signal collected by the electromagnetic elastic absolute stress sensor and the relationship between the prestress size and the magnetic characteristic signal of the anchor head.

4. The holographic joint monitoring device for prestressing of existing structures and defects in ducts according to claim 3 is characterized by: The electromagnetic elastic absolute stress sensor is cylindrical, and has an excitation coil for excitation and a magnetoelectric conversion element for receiving magnetic characteristic signals built in it, and the magnetoelectric conversion element is located between the excitation coil and the anchor head; the electromagnetic elastic absolute stress sensor is prefabricated, and is designed and manufactured in advance according to different anchor head specifications, and does not require on-site assembly and winding.

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