A discriminant method for the test results of reverse tension of effective prestress under the anchor to eliminate the influence of clamping pieces

Abstract

The present invention provides a discriminant method for the detection result of the effective prestress under the anchor eliminating the influence of the wedge grip. Data collection and processing are carried out through a pre-sensor installed on the steel strand, a jack and connection with a central control tablet, and then detection is carried out, including the reverse tension stage, the load-holding stage and the unloading stage. The present invention realizes dual-control reverse tension by real-time monitoring of the jack tension and the wedge grip displacement; identifies the wedge grip state according to the wedge grip displacement, eliminates the influence of the wedge grip biting action and the dynamic friction influence caused by the misalignment of the wedge grip installation; the method of the present invention can not only ensure the accuracy of the detection result and the safety of the detection process, but also avoid large deviations and losses of the effective prestress under the anchor before and after reverse tension.

Description

Technical Field

[0001] The present invention belongs to the field of prestress detection of bridge steel strands, and particularly relates to a method for discriminating the results of the reverse tension detection of the effective prestress under the anchor by eliminating the influence of the wedge grip. Background Art

[0002] In recent decades, prestressed concrete bridges have gradually become the mainstream bridge type in China's bridge construction, and are favored by industry competent authorities and each construction unit. Prestress is the "lifeline" of prestressed concrete bridges. If the effective prestress under the anchor is too large, it is easy to cause excessive deformation of the concrete main girder. If the effective prestress under the anchor is too small, it is easy to cause the deflection of the concrete main girder and the reduction of the bearing capacity of the bridge. Therefore, it becomes particularly important to guard the "lifeline" of prestressed concrete bridges.

[0003] The detection of the effective prestress under the anchor is the last guarantee for guarding this "lifeline". At present, the detection of the effective prestress under the anchor mainly includes destructive testing and non-destructive testing. Destructive testing will cause varying degrees of damage to the bridge structure and is only used in specific situations, so it is not common. Non-destructive testing includes several methods such as the reverse tension method, the vibration frequency method, and the ultrasonic testing method. Among them, the reverse tension method is adopted by all parties because of its relatively high detection accuracy and convenient on-site detection implementation.

[0004] The detection principle of the reverse tension method is to perform secondary tension on the anchored steel strands by using a jack, and obtain the detection value of the effective prestress under the anchor during the tensioning process. Different detection equipment manufacturers have different methods for discriminating the detection values, resulting in so-called discrimination methods such as the inflection point method, the minimum stress tracking method, and the wedge grip displacement method. The inflection point method takes values according to the inflection point of the steel strand F-S curve, ignoring the influence of stress redistribution caused by the inequality between the external tension of the anchor and the effective prestress under the anchor, making the detection value usually slightly larger than the actual value. And according to a large number of engineering detection examples, it is found that the inflection points of most steel strand F-S curves are not obvious, resulting in the inability to determine the detection value; both the minimum stress tracking method and the wedge grip displacement method consider that the tension corresponding to a certain displacement of the wedge grip minus the anchor mouth friction is the detection value of the effective prestress under the anchor. The detection results of these two methods are closely related to the installation accuracy of the wedge grip and the size of the anchor mouth friction, and it is difficult to ensure the accuracy of the detection results. Summary of the Invention

[0005] Aiming at the defect that various detection value discrimination methods are not accurate enough when using the reverse tension method to detect the effective prestress under the anchor at present, the present invention provides a discrimination method for accurately determining the size of the effective prestress under the steel strand anchor during the reverse tension detection process by eliminating the influence of the wedge grip biting effect and the dynamic friction caused by the misalignment of the wedge grip installation. This method can not only ensure the accuracy of the detection results and the safety of the detection process, but also avoid large deviations and losses of the effective prestress under the anchor before and after reverse tension.

[0006] The present invention is realized through the following technical solutions:

[0007] A method for discriminating the test results of the effective prestress in the reverse tension under the anchor to eliminate the influence of the wedge grips, comprising the following steps:

[0008] (1) Install a pre-sensor and a jack on the steel strand to be tested for reverse tension detection. Collect the displacement data of the wedge grips and the external tension data of the steel strand through the upper computer, and draw a tension-wedge grip displacement curve graph;

[0009] (2) Discriminate the effective prestress under the anchor according to different types of tension-wedge grip displacement curve graphs, specifically including the following four situations:

[0010] a. When the effective prestress redistribution effect of the steel strand is obvious, and the maximum tension of the jack < upper limit value, and the wedge grip displacement < upper limit value, there is an obvious inflection point in the external tension curve of the wedge grips, and both the maximum tension of the jack and the wedge grip displacement do not exceed the limit. At this time, select the external tension value after the effective prestress redistribution under the anchor for holding load. After the external tension value and the wedge grip displacement value are stable for 10 s, enter the unloading stage. Select the tension value corresponding to when the wedge grip displacement just returns to 0 in the unloading stage as the test value of the effective prestress under the anchor.

[0011] b. When the effective prestress redistribution effect of the steel strand is not obvious, and the maximum tension of the jack < upper limit value, and the wedge grip displacement ≥ upper limit value, the inflection point of the external tension curve of the wedge grips is not obvious or does not exist, and the tension of the jack does not exceed the limit. Select the holding load stage when the wedge grip displacement is equal to the upper limit value. When the wedge grip displacement just returns to 0 in the unloading stage, subtract the calculated sliding friction force of the wedge grips from the external tension value at this moment, which is the test value of the effective prestress under the anchor.

[0012] c. When the effective prestress redistribution effect of the steel strand (1) is not obvious, and the maximum tension of the jack ≥ upper limit value, and 0 mm < wedge grip displacement < upper limit value, the inflection point of the external tension curve of the wedge grips is not obvious or does not exist, and the wedge grip displacement does not exceed the limit. Select the holding load stage when the tension of the jack is equal to the upper limit value. When the displacement of the wedge grips just returns to 0 in the unloading stage, subtract the calculated sliding friction force of the wedge grips from the tension value at this moment, which is the test value of the effective prestress under the anchor.

[0013] d. When the maximum tension ≥ upper limit value and the wedge grip displacement = 0 mm, start the unloading stage when the tension of the jack is equal to the upper limit value. At this time, the steel strand is over-tensioned. To ensure safety, the tension of the jack is not increased after reaching the upper limit value, and an accurate test value of the effective prestress under the anchor cannot be obtained.

[0014] As a further improvement of the present invention, the upper limit value of the force value of the jack is the maximum tensile control stress σ of the steel strand con。

[0015] As a further improvement of the present invention, the upper limit value of the displacement of the wedge is 1 mm.

[0016] To ensure the accuracy of the test results and the safety of the test process of the present invention, and to prevent the reverse tension method test from changing the effective prestress value under the anchor of the steel strand or causing the steel strand to be broken, double-control reverse tension is carried out by using the wedge displacement and the jack tension. The present invention sets the upper limit value of the wedge displacement to 1 mm, which can not only ensure the complete elimination of the influence of the wedge biting effect, but also avoid excessive wedge displacement, resulting in a large retraction loss of the steel strand during unloading. The upper limit value of the jack tension is set to the maximum tensile control stress σ of the steel strand according to the provisions of the "Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts" (JTG 3362-2018). con 。

[0017] The specific steps of the reverse tension test of the present invention are as follows:

[0018] S1. Open the data acquisition software of the upper computer, input the basic information of the steel strand, set the upper limit value of the force of the jack and the upper limit value of the displacement of the wedge; then control the jack to load and enter the reverse tension stage. The data acquisition software collects the displacement data of the wedge and the external tension data of the steel strand under the anchor, and draws the tension-wedge displacement curve.

[0019] S2. The system automatically determines whether to enter the holding stage or the unloading stage after the reverse tension stage according to the tension-wedge displacement curve in the reverse tension stage; when the system directly enters the unloading stage from the reverse tension stage, it is determined that the steel strand of this bundle is over-tensioned; when the system enters the holding stage from the reverse tension stage, the holding time of the system is not less than 10 s. After the tension is stable, enter the unloading stage.

[0020] S3. In the unloading stage, the system makes a judgment according to the tension-wedge displacement curve in the reverse tension stage. If there is an obvious inflection point, the external tension of the anchor when the wedge displacement decreases to 0 is the detected value of the effective prestress under the anchor; if there is no obvious inflection point, the system uses the basic information of the steel strand and calculates the sliding friction force of the wedge according to the external tension curve in the unloading stage, and then subtracts the sliding friction force of the wedge from the external tension of the anchor when the wedge displacement decreases to 0. The obtained value is the effective prestress under the anchor.

[0021] S4. Remove the jack and the pre-sensor to complete the detection of the effective prestress under the anchor of the steel strand.

[0022] As a further improvement of the present invention, the basic information of the steel strand includes the elastic modulus E of the steel strand, the cross-sectional area A of the steel strand, the anchorage length x1 of the steel strand, the length x2 from the tool anchor of the external exposed steel strand to the working anchor of the jack, and the included angle θ formed by the inner and outer surfaces of the wedge.

[0023] As a further improvement of the present invention, when the installation accuracy of the wedge grip meets the requirements, at the start of the load-holding stage, the wedge grip relaxes and undergoes displacement. After the prestress redistribution occurs due to the inequality between the external anchor tension and the effective prestress under the anchor until their values are equal, the load-holding process begins; when entering the load-holding stage from the reverse tension stage, there is an obvious inflection point on the tension-wedge grip displacement curve.

[0024] As a further improvement of the present invention, when the installation accuracy of the wedge grip is too low, at the start of the load-holding stage, the wedge grip relaxes and shifts to one side, and a sliding friction force will be generated between the wedge grip and the anchor hole. When the external anchor tension or the wedge grip displacement increases to the upper limit value, the load-holding process begins; when entering the load-holding stage from the reverse tension stage, there is no obvious inflection point on the tension-wedge grip displacement curve.

[0025] As a further improvement of the present invention, after the pressure value of the pre-mounted sensor reaches 30 kN, the displacement data of the wedge grip starts to be recorded. The pressure sensor in the pre-mounted sensor starts to record data at the start of the reverse tension stage. However, since there is an external leakage section during the installation of the wedge grip, in order to avoid the influence of the external leakage section on data acquisition, it is selected to start recording the data of the displacement sensor after the pressure sensor value reaches 30 kN.

[0026] The present invention detects the effective prestress under the strand anchor by the reverse tension method. The jack is used to perform secondary tension on the already anchored strand, and the effective prestress detection value under the anchor is obtained according to the F-S curve of the strand during the tension process. The reverse tension method of the present invention is divided into three stages: the reverse tension stage, the load-holding stage, and the unloading stage. At the same time, the difference between the external anchor tension value and the effective prestress value under the anchor when the wedge grip is about to undergo displacement is defined as the wedge grip biting force. Obviously, to determine the effective prestress under the anchor through the external anchor tension, it is first necessary to determine the magnitude of the wedge grip biting force or eliminate the influence of the wedge grip biting force. Since the hardness of the wedge grip is much greater than that of the strand, and during the relaxation process of the strand, the thread teeth of the wedge grip cut into the strand by about 0.5 mm, resulting in a relatively large friction coefficient between the wedge grip and the strand. During the reverse tension process, there is no relative slip between the strand and the wedge grip. Therefore, the wedge grip biting force is mainly generated by the horizontal friction force between the wedge grip and the anchor hole. Through the force analysis of the wedge grip (such as Figure 2 ) the following relational expression is obtained:

[0027]

[0028] In the formula: F mj is the horizontal friction force between the anchor hole and the wedge grip; L is the length of the wedge grip; p mj is the pressure of the anchor hole on the wedge grip; f mj is the friction force between the anchor hole and the wedge grip; θ is the angle formed by the inner and outer surfaces of the wedge grip; r(x) is the radius of the wedge grip at x.

[0029] As can be seen from Equation 1, the clamping force of the wedge is closely related to the type of wedge and the installation accuracy of the wedge. In practical engineering applications, there are a wide variety of wedge types, and it is not easy to control the installation accuracy, resulting in a large difference in the clamping force of each steel strand. It is difficult to determine the clamping force of the wedge according to theoretical calculations. Therefore, how to eliminate the influence of the clamping force of the wedge during the reverse tension test is a key factor in accurately determining the effective prestress under the anchor. According to the present invention, when the displacement of the wedge returns to 0 during the unloading stage, the clamping force of the wedge is exactly 0, and at this time, the external tension of the anchor is the effective prestress under the anchor, so as to eliminate the influence of the clamping force of the wedge.

[0030] After most wedges are anchored, their top surfaces are not flush, and the misalignment between them is between 0.5 mm and 2.0 mm, resulting in a large difference in the pressure of the steel strand on the wedge. When the reverse tension method is used for detection, at the moment when the wedge relaxes, the clamping force of the wedge disappears, but due to the unequal pressure of the steel strand on the wedge, the whole of the steel strand and the wedge shift to one side, and one side of the wedge fits with the anchor hole again, generating sliding friction. Only when the wedge completely exits the anchor hole will the sliding friction completely disappear. Since the clamping force of the wedge is transformed into the sliding friction of the wedge in a very short time, there is no inflection point or the inflection point is not obvious in the external tension curve of the anchor, and the curve still develops according to the original trend.

[0031] Regarding the steel strand as a linear elastic body, through the force analysis of the slipping wedge and the steel strand (such as Figure 4 ), and according to the mechanical properties of the material, the following relational expression is obtained:

[0032] F0 = F1 - F2cosθ (Equation 2)

[0033]

[0034]

[0035] In the formula: ΔL is the elongation of the steel strand. After the wedge slips, the displacement of the wedge is equivalent to the elongation of the steel strand; F0 is the effective prestress under the anchor; F1 is the external tension of the anchor; F2 is the sliding friction of the anchor hole on the wedge; θ is the angle formed by the inner and outer surfaces of the wedge; x1 is the anchoring length of the steel strand; x2 is the length of the steel strand from the tool anchor outside the exposed section to the working anchor of the jack; E is the elastic modulus of the steel strand; A is the cross-sectional area of the steel strand.

[0036] As can be seen from Equations 2 - 4, E, A, x1, x2, and θ can all be regarded as constants. Then, according to the displacement of the wedge and the magnitude of the external tension of the anchor at a certain moment, the magnitude of the sliding friction F2 of the wedge at this moment can be calculated.

[0037] Therefore, according to the installation accuracy of the wedge, the present invention adjusts the specific test methods at each stage.

[0038] (1) When the installation accuracy of the wedge grips meets the requirements, the curves of the tension outside the anchor, the effective prestress under the anchor, and the displacement of the wedge grips during the whole detection process are as shown in Figure 1 (a), and each stage is as follows:

[0039] During the reverse tension stage, the wedge grips do not displace. In the early stage, the tension outside the anchor gradually increases from 0. When the tension outside the anchor is equal to the effective prestress under the anchor, due to the existence of the bite force of the wedge grips, the wedge grips will not displace, and the curve of the tension outside the anchor continues to increase in the original trend. Therefore, it is impossible to determine the specific time when the tension outside the anchor is equal to the effective prestress under the anchor based on the curve of the tension outside the anchor. In the later stage, the tension outside the anchor gradually exceeds the effective prestress under the anchor and reaches a peak at a certain moment. At this time, the wedge grips are about to displace.

[0040] During the load-holding stage, the wedge grips displace outward and then remain stable. At the beginning of this stage, the wedge grips relax and displace, and the bite force of the wedge grips disappears. The inequality between the tension outside the anchor of the steel strand and the effective prestress under the anchor leads to stress redistribution. After stress redistribution, the two values are almost equal and greater than the effective prestress under the anchor before reverse tension. Stress redistribution is mainly due to the influence of the bite action of the wedge grips. However, due to the different bite actions of the wedge grips for different steel strands, it is difficult to accurately calculate the influence of stress redistribution. That is, it is difficult to obtain an accurate value of the effective prestress under the anchor in this stage. After stress redistribution is completed, the load can be held at a specific load according to the situation, and finally the displacement of the wedge grips and the tension value of the jack are kept in a stable state, and the duration of the stable state is not less than 10 s. The purpose of the load-holding stage is to let the stress of the steel strand be fully redistributed and eliminate the interference of other factors, which is convenient for the implementation of the next stage.

[0041] During the unloading stage, the hydraulic pump returns oil, the jack unloads, the wedge grips retract to their original positions, the tension outside the anchor gradually decreases to 0, and the effective prestress under the anchor returns to its initial value. When the displacement value of the wedge grips decreases to 0, that is, the wedge grips return to their original positions, the bite force of the wedge grips is just 0. At this time, there is no influence of the bite action of the wedge grips, and the tension outside the anchor of the steel strand is equal to the effective prestress under the anchor. The tension value corresponding to this point is the detected value of the effective prestress under the anchor.

[0042] (2) When the installation accuracy of the wedge grips is too low, the curves of the tension outside the anchor, the effective prestress under the anchor, and the displacement of the wedge grips during the whole detection process are as shown in Figure 1 (b), and each stage is as follows:

[0043] During the reverse tensioning stage, the wedge grips do not displace. In the early stage, the external anchor tension gradually increases from 0. When the external anchor tension equals the effective prestress under the anchor, due to the existence of the wedge grip biting force, the wedge grips do not displace, and the external anchor tension curve continues to increase in the original trend. Therefore, it is impossible to determine the specific time when the external anchor tension equals the effective prestress under the anchor based on the external anchor tension curve. In the later stage, the external anchor tension gradually exceeds the effective prestress under the anchor. At the moment when the external anchor tension equals the sum of the effective prestress under the anchor and the wedge grip biting force, the wedge grips are about to displace.

[0044] During the load holding stage, the wedge grips displace outward and then remain stable. At the beginning of this stage, the wedge grips displace due to relaxation, and the wedge grip biting force disappears. However, due to the too low installation accuracy of the wedge grips, the steel strand and the wedge grips as a whole shift to one side, generating sliding friction. At this time, the effective prestress value under the anchor remains at the initial level or slightly increases. Since the wedge grip biting force is transformed into the wedge grip sliding friction in a very short time, the prestress redistribution is not obvious, resulting in no inflection point or an unclear inflection point on the external anchor tension curve. As the oil pressure of the jack increases, the external anchor tension curve continues to develop in the original trend. When the displacement of the wedge grips or the external anchor tension increases to the upper limit value, the load can be held, and finally, the displacement of the wedge grips and the jack tension value remain stable, and the stable state lasts for no less than 10 s.

[0045] During the unloading stage, the hydraulic pump returns the oil, the jack unloads, the wedge grips retract to their original positions, the external anchor tension gradually decreases to 0, and the effective prestress under the anchor returns to the initial size. The displacement value of the wedge grips decreases to 0, and the wedge grip sliding friction is transformed into the wedge grip biting force. Subtracting the wedge grip sliding friction calculated according to the formula from the external anchor tension when the displacement of the wedge grips is 0 is the detected value of the effective prestress under the anchor.

[0046] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0047] 1. Based on the basic principle of the reverse tensioning method for detection, the present invention proposes a detection and discrimination method that eliminates the influence of the wedge grip biting effect and the dynamic friction of the wedge grips, and accurately obtains the true effective prestress under the anchor.

[0048] 2. For the detection of the effective prestress under the anchor by the reverse tensioning method, the present invention first proposes three stages: reverse tensioning, load holding, and unloading. By maintaining the stability of the reverse tensioning system during the load holding stage, it is convenient for the accuracy and effectiveness of data collection during the unloading stage.

[0049] 3. The present invention uses the jack tension and the displacement of the wedge grips for double-control reverse tensioning, which not only reduces the risk of the steel strand being broken during reverse tensioning but also avoids the deviation of the prestress under the anchor before and after reverse tensioning. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1The curves of the external anchor tension, the effective prestress under the anchor, and the displacement of the wedge during the whole process of the reverse tension method detection under different installation precisions of the wedge.

[0051] Figure 2 It is the force balance diagram of the wedge. In the figure, L is the length of the wedge, p mj is the pressure of the anchor hole on the wedge, f mj is the friction force between the anchor hole and the wedge, p gj is the pressure of the steel strand on the wedge, f gj is the friction force between the steel strand and the wedge, θ is the angle formed by the inner and outer surfaces of the wedge, and r(x) is the radius of the wedge at x.

[0052] Figure 3 It is the installation schematic diagram of the wedge.

[0053] Figure 4 It is the force schematic diagram of the steel strand and the wedge when the wedge slips. In the figure, F0 is the effective prestress under the anchor, F1 is the external anchor tension, F2 is the sliding friction force of the anchor hole on the wedge, and θ is the angle formed by the inner and outer surfaces of the wedge.

[0054] Figure 5 It is four typical test curves of the reverse tension method detection.

[0055] Figure 6 It is the schematic diagram of the reverse tension method detection.

[0056] Figure 7 It is the sectional view of the pre - sensor.

[0057] Reference numerals: 1 - steel strand, 2 - pre - sensor, 3 - jack, 4 - 485 data cable, 5 - integrated hydraulic pump, 6 - oil pressure pipe, 7 - wedge, 8 - upper computer, 9 - displacement sensor, 10 - pressure sensor. Detailed implementation mode

[0058] The present invention will be further described below with reference to the accompanying drawings.

[0059] Embodiment

[0060] The MAG - 6500W type microcomputer - controlled electro - hydraulic servo static load anchorage testing machine is used for the tension testing. The upper computer 8 is a central control tablet. The tension testing is divided into 10 groups, and the single - strand tension anchorage of 10 prestressed steel strands of the same batch under different loads is carried out respectively, and then the effective prestress under the anchor is detected by using the discrimination method of the present invention.

[0061] The standard tensile strength f of the steel strand in this detection test pk = 1860 MPa, the maximum tension control stress σ con = 0.8f pk , the elastic modulus E = 1.95×10 5MPa, with a diameter d = 15.2 mm and a nominal area A = 140 mm 2 , an anchorage length x1 = 4.00 m, the length x2 of the external strand from the tool anchor to the working anchor of the jack is 0.40 m, the included angle θ between the inner and outer surfaces of the wedge is 3.0°, the anchor type is OVM15-1, and the anchors and wedges are all produced in the same batch.

[0062] The steps for the reverse tension detection of all steel strands in this test are as follows:

[0063] S1. Install the front sensor 2 and the jack 3 on the steel strand 1 in sequence. The front sensor 2 is connected to the data acquisition module in the integrated hydraulic pump 5 through the 485 data cable 4, and the jack 3 is connected to the oil pump in the integrated hydraulic pump 5 through the oil pressure pipe 6, as Figure 6 shown.

[0064] The front sensor 2 includes a displacement sensor 9 and a pressure sensor 10, as Figure 7 shown. The displacement sensor 9 uses a high-precision ring displacement gauge and is located at the top of the front sensor 2 for measuring the displacement of the wedge 7.

[0065] The pressure sensor 10 uses a high-precision pressure ring and is located at the end of the front sensor 2 for measuring the tension of the jack 3.

[0066] S2. Turn on the power of the integrated hydraulic pump 5 and connect it to the central control tablet 8 through wifi.

[0067] S3. Open the data acquisition software on the central control tablet 8 and input the standard tensile strength f pk , elastic modulus E, diameter d, nominal area A, anchorage length x1, the length x2 of the external strand of the steel strand 1 from the tool anchor to the working anchor of the jack 3, etc. information, and then set the upper limit of the force value of the jack 3 to 0.8f kp , and the upper limit of the displacement of the wedge 7 to 1 mm.

[0068] S4. Start the detection. The central control tablet 8 controls the jack 3 to load and enters the reverse tension stage. The data acquisition software records the values of the displacement sensor 9 and the pressure sensor 10, and draws different tensile force - wedge displacement curves according to the installation accuracy of the wedge 7.

[0069] The value of the pressure sensor 10 is recorded at the beginning of the reverse tension stage. After the value of the pressure sensor 10 reaches 30 kN, the value of the displacement sensor 9 is recorded to eliminate the influence of the external section of the wedge 7.

[0070] S5. Remove the jack 3 and the front sensor 2 to complete the detection of the effective prestress under the anchor of this bundle of steel strand 1.

[0071] Among them, for the steel strands with the installation accuracy of the wedge grips meeting the requirements, the specific three stages of the reverse tension test are as follows:

[0072] Reverse tension stage: In this stage, the wedge grip 7 does not displace. In the early stage, the tension outside the anchor starts to gradually increase from 0. When the tension outside the anchor is equal to the effective prestress under the anchor, i.e., at point A, due to the biting force of the wedge grip 7, the wedge grip 7 will not displace, and the tension curve outside the anchor still continues to increase in the original trend. Therefore, the specific time when point A occurs cannot be determined based on the tension curve outside the anchor. In the later stage, the tension outside the anchor gradually exceeds the effective prestress under the anchor and reaches the peak at point B. At this time, the wedge grip 7 is about to displace.

[0073] Load holding stage: In this stage, the wedge grip 7 displaces outward to a certain extent and then remains stable. At the beginning of the stage, the wedge grip 7 relaxes and displaces, and the biting force of the wedge grip 7 disappears. The inequality between the tension outside the anchor of the steel strand 1 and the effective prestress under the anchor leads to stress redistribution. After stress redistribution, the two values are almost equal and greater than the effective prestress value under the anchor before reverse tension. Stress redistribution is mainly due to the influence of the biting action of the wedge grip 7. However, due to the different biting actions of the wedge grips 7 of different steel strands 1, it is difficult to accurately calculate the influence of stress redistribution. That is, it is difficult to obtain the accurate effective prestress value under the anchor in this stage. After stress redistribution is completed, the load can be held at a specific load according to the situation, and finally the displacement of the wedge grip 7 and the tension value of the jack 3 are kept in a stable state, and the duration of the stable state is not less than 10 s. The purpose of the load holding stage is to let the stress of the steel strand 1 be fully redistributed and eliminate the interference of other factors to facilitate the implementation of the next stage.

[0074] Unloading stage: The integrated hydraulic pump 5 returns oil, the jack 3 unloads, the wedge grip 7 retracts to its original position, the tension outside the anchor gradually decreases to 0, and the effective prestress under the anchor returns to its initial size. When reaching point K, the displacement value of the wedge grip 7 decreases to 0, that is, the wedge grip 7 returns to its original position, and the biting force of the wedge grip 7 is just 0. At this time, without the influence of the biting action of the wedge grip 7, the tension outside the anchor of the steel strand 1 is equal to the effective prestress under the anchor, and the tension value corresponding to this point is the detected value of the effective prestress under the anchor.

[0075] Among them, for the steel strands with too low installation accuracy of the wedge grips, the specific three stages of the reverse tension test are as follows:

[0076] Reverse tension stage: In this stage, the wedge grip 7 does not displace. In the early stage, the tension outside the anchor starts to gradually increase from 0. When the tension outside the anchor is equal to the effective prestress under the anchor, i.e., at point A, due to the biting force of the wedge grip 7, the wedge grip 7 will not displace, and the tension curve outside the anchor still continues to increase in the original trend. Therefore, the specific time when point A occurs cannot be determined based on the tension curve outside the anchor. In the later stage, the tension outside the anchor gradually exceeds the effective prestress under the anchor, and at point B, the tension outside the anchor is equal to the sum of the effective prestress under the anchor and the biting force of the wedge grip 7. At this time, the wedge grip 7 is about to displace.

[0077] During the load-holding stage, after the wedge 7 undergoes a certain displacement outward, it remains in a stable state. At the beginning of this stage, the wedge 7 relaxes and displaces, and the biting force of the wedge 7 disappears. However, due to the too low installation accuracy of the wedge 7, the overall steel strand 1 and the wedge 7 shift to one side, generating sliding friction. At this time, the effective prestress value under the anchor remains at the initial level or increases slightly. Since the biting force of the wedge 7 is transformed into the sliding friction of the wedge 7 in a very short time, there is no inflection point or the inflection point is not obvious in the external anchor tension curve. As the oil pressure of the jack increases, the external anchor tension curve continues to develop according to the original trend. When the displacement of the wedge 7 or the external anchor tension increases to the upper limit value, the load can be held, and finally the displacement of the wedge 7 and the tension value of the jack 3 are kept in a stable state, and the duration of the stable state is not less than 10 s.

[0078] During the unloading stage, the integrated hydraulic pump 5 returns oil, the jack 3 unloads, the wedge 7 retracts to its original position, and the external anchor tension gradually decreases to 0, and the effective prestress under the anchor returns to the initial size. At the moment of point K, the displacement value of the wedge 7 decreases to 0, and the sliding friction of the wedge 7 is transformed into the biting force of the wedge 7. Subtracting the sliding friction of the wedge 7 calculated according to the formula from the external anchor tension corresponding to point K when the displacement of the wedge 7 is 0 is the detected value of the effective prestress under the anchor.

[0079] The discrimination methods for the detected values of the effective prestress under the anchor of 10 groups of steel strands in this test are as follows:

[0080] (1) When the effective prestress redistribution effect of the steel strand is obvious and the maximum tension of the jack < 0.8f pk , and the wedge displacement < 1 mm, there is an obvious inflection point in the external anchor tension curve, and both the maximum tension of the jack and the wedge displacement do not exceed the limit. At this time, select the external anchor tension value after the effective prestress redistribution under the anchor for load holding. After the external anchor tension value and the wedge displacement value are stable for 10 s, enter the unloading stage. Select the tension value corresponding to the moment when the wedge displacement just returns to 0 in the unloading stage as the detected value of the effective prestress under the anchor; the test curve is as Figure 5 (a) shown.

[0081] (2) When the effective prestress redistribution effect of the steel strand is not obvious and the maximum tension of the jack < 0.8f pk , and the wedge displacement ≥ 1 mm, the inflection point in the external anchor tension curve is not obvious or does not exist, and the tension of the jack does not exceed the limit. Select the moment when the wedge displacement is equal to 1 mm to start the load-holding stage. When the wedge displacement just returns to 0 in the unloading stage, subtract the calculated sliding friction of the wedge from the external anchor tension value at this moment, which is the detected value of the effective prestress under the anchor; the test curve is as Figure 5 (b) shown.

[0082] (3) The effective prestress redistribution effect of the steel strand is not obvious, and the maximum jacking force ≥ 0.8f pk , when 0 mm < the displacement of the wedge grip < 1 mm, the inflection point of the tension curve outside the anchor is not obvious or does not exist, and the displacement of the wedge grip is not exceeded. Select the jacking force equal to 0.8f pk to start entering the holding load stage. When the displacement of the wedge grip just returns to 0 during the unloading stage, subtract the calculated sliding friction force of the wedge grip from the jacking force value at this moment, which is the measured value of the effective prestress under the anchor; The test curve is as Figure 5 (c) shown.

[0083] (4) The maximum jacking force of the steel strand ≥ 0.8f pk , the displacement of the wedge grip = 0 mm. When the jacking force of the jack is equal to 0.8f pk to start entering the unloading stage. At this time, the steel strand is over-tensioned. To ensure safety, the jacking force of the jack will not be increased after reaching the upper limit value, and the accurate measured value of the effective prestress under the anchor cannot be obtained; The test curve is as Figure 5 (d) shown.

[0084] The test data of each steel strand in this test are shown in Table 1.

[0085] Table 1:

[0086] Serial number Tensile force (kN) Measured value (kN) Sliding friction force (kN) Difference (kN) Error (%) 1 155 154.42 / -0.58 -0.37 2 160 163.71 2.86 3.71 2.32 3 165 166.20 / 1.20 0.73 4 170 173.41 / 3.41 2.01 5 175 174.27 5.18 -0.73 -0.42 6 180 182.46 5.04 2.46 1.37 7 185 189.15 / 4.15 2.24 8 190 187.22 / -2.78 -1.46 9 195 Over-tension / / / 10 200 Over-tension / / /

[0087] From the data in Table 1, it can be seen that by using the method of the embodiment to detect the effective prestress under the anchor, the maximum data error is only 2.32%. It can be seen that the detection method of the present invention has a small deviation and high accuracy, and is applicable to the detection of the effective prestress under the anchor.

[0088] The above embodiments are only exemplary embodiments of the present invention and are not used to limit the present invention. The protection scope of the present invention is defined by the claims. Those skilled in the art can make various modifications or equivalent replacements within the essence and protection scope of the present invention, and such modifications or equivalent replacements should also be regarded as falling within the protection scope of the present invention.

Claims

1. A discriminant method for the detection results of the effective prestress reverse tension under the anchor to eliminate the influence of the wedge grip, characterized in that, It includes the following steps: (1) Install a pre - sensor and a jack on the steel strand to be tested for back - tension detection. Collect the displacement data of the wedge grip and the external - anchor tensile force data of the steel strand through the upper computer, and draw a tensile - force - wedge - grip displacement curve; (2) Discriminate the effective prestress under the anchor according to different types of tensile - force - wedge - grip displacement curves, which specifically includes the following four situations: a. When the effective prestress redistribution effect of the steel strand is obvious, and the maximum tensile force of the jack < upper limit value, and the wedge - grip displacement < upper limit value, there is an obvious inflection point in the external - anchor tensile - force curve, and both the maximum tensile force of the jack and the wedge - grip displacement are not over - limit. At this time, select the external - anchor tensile - force value after the effective prestress redistribution under the anchor for holding load. After the external - anchor tensile - force value and the wedge - grip displacement value are stable for 10 s, enter the unloading stage. Select the tensile - force value corresponding to when the wedge - grip displacement just returns to 0 in the unloading stage as the detected value of the effective prestress under the anchor; b. When the effective prestress redistribution effect of the steel strand is not obvious, and the maximum tensile force of the jack < upper limit value, and the wedge - grip displacement ≥ upper limit value, the inflection point of the external - anchor tensile - force curve is not obvious or does not exist, and the tensile force of the jack is not over - limit. Select the stage when the wedge - grip displacement is equal to the upper limit value to start the holding - load stage. When the wedge - grip displacement just returns to 0 in the unloading stage, subtract the calculated sliding friction force of the wedge grip from the external - anchor tensile - force value at this moment, which is the detected value of the effective prestress under the anchor; c. When the effective prestress redistribution effect of the steel strand (1) is not obvious, and the maximum tensile force of the jack ≥ upper limit value, and 0 mm < wedge - grip displacement < upper limit value, the inflection point of the external - anchor tensile - force curve is not obvious or does not exist, and the wedge - grip displacement is not over - limit. Select the stage when the tensile force of the jack is equal to the upper limit value to start the holding - load stage. When the displacement of the wedge grip just returns to 0 in the unloading stage, subtract the calculated sliding friction force of the wedge grip from the tensile - force value at this moment, which is the detected value of the effective prestress under the anchor; d. When the maximum tensile force ≥ upper limit value and the wedge - grip displacement = 0 mm, when the tensile force of the jack is equal to the upper limit value, start the unloading stage. At this time, the steel strand is over - tensioned. To ensure safety, the jack will not be tensioned anymore after the tensile force reaches the upper limit value, and an accurate detected value of the effective prestress under the anchor cannot be obtained.

2. The method for judging the detection result of the effective prestress reverse tension under the anchor to eliminate the influence of the clamping piece according to claim 1, characterized in that: The upper limit value of the force of the described jack is the maximum tensile control stress of the steel strand σ con .

3. The method for judging the detection result of the effective prestress reverse tension under the anchor to eliminate the influence of the clamping piece according to claim 1, characterized in that: The upper limit value of the displacement of the wedge grip is 1 mm.

4. The method for judging the detection result of the effective prestress back-tension under the anchor to eliminate the influence of the clamping piece according to any one of claims 1-3, characterized in that, The specific steps of the back - tension detection are as follows: S1. Open the data - acquisition software of the upper computer, input the basic information of the steel strand, set the upper limit value of the force of the jack and the upper limit value of the displacement of the wedge grip; then control the jack to load and enter the back - tension stage. The data - acquisition software collects the displacement data of the wedge grip and the external - anchor tensile - force data of the steel strand, and draws a tensile - force - wedge - grip displacement curve; S2. The system automatically determines whether to enter the holding - load stage or the unloading stage after the back - tension stage according to the tensile - force - wedge - grip displacement curve in the back - tension stage; When the system directly enters the unloading stage from the back - tension stage, it is determined that the steel strand of this bundle is over - tensioned; when the system enters the holding - load stage from the back - tension stage, the holding - load time of the system is not less than 10 s. After the tensile force is stable, enter the unloading stage; S3. Unloading stage: The system determines according to the tensile force-wedge displacement curve graph in the reverse tension stage. If there is an obvious inflection point, the external anchor tensile force when the wedge displacement decreases to 0 is the detected value of the effective prestress under the anchor; if there is no obvious inflection point, the system uses the basic information of the steel strand, calculates the sliding friction force of the wedge according to the external anchor tensile force curve in the unloading stage, and then the value obtained by subtracting the sliding friction force of the wedge from the external anchor tensile force when the wedge displacement decreases to 0 is the effective prestress under the anchor. S4. Remove the jack and the front sensor to complete the detection of the effective prestress under the anchor of the steel strand.

5. The method for judging the test result of the effective prestress reverse tension under the anchor to eliminate the influence of the clamping piece according to claim 4, characterized in that: After the pressure value of the front sensor reaches 30 kN, start recording the displacement data of the wedge.

6. The method for judging the test result of the effective prestress reverse tension under the anchor to eliminate the influence of the clip as claimed in claim 4, characterized in that: When the installation accuracy of the wedge meets the requirements, at the beginning of the load-holding stage, the wedge relaxes and displaces. Since the external anchor tensile force is not equal to the effective prestress under the anchor, prestress redistribution occurs until the two values are equal, and then load-holding is carried out; when entering the load-holding stage from the reverse tension stage, there is an obvious inflection point in the tensile force-wedge displacement curve graph.

7. The method for judging the test result of the effective prestress reverse tension under the anchor to eliminate the influence of the clamping piece according to claim 4, characterized in that: When the installation accuracy of the wedge is too low, at the beginning of the load-holding stage, the wedge relaxes and deflects to one side, and a sliding friction force will be generated between the wedge and the anchor hole. When the external anchor tensile force or the wedge displacement increases to the upper limit value, load-holding is carried out; when entering the load-holding stage from the reverse tension stage, there is no obvious inflection point in the tensile force-wedge displacement curve graph.

8. The method for judging the detection result of the effective prestress reverse tension under the anchor to eliminate the influence of the clamping piece according to claim 4, characterized in that: The basic information of the steel strand includes the included angle formed by the inner and outer surfaces of the wedge θ , the anchorage length of the steel strand x 1 , the length from the tool anchor of the external exposed steel strand to the working anchor of the jack x 2 , the elastic modulus of the steel strand E , the cross-sectional area of the steel strand A .

Images