Nondestructive evaluation method for grouting defect position and degree of prestressed duct

Abstract

The invention provides a lossless evaluation method for the position and degree of a grouting defect of a prestressed duct, which comprises the following steps: exciting an elastic wave signal by knocking an exposed steel strand of the prestressed duct, receiving the signal by arranging a sensor of a plurality of measuring lines on the surface of concrete, and correcting the actual position of the defect in combination with an elastic wave refraction principle. And the defect degree is dynamically judged by analyzing the delay of the signal propagation time and the normalized change of the head wave amplitude. A dynamic defect threshold value is set based on the vertical position relation of the measuring lines and the hole channel, and high-precision defect positioning and quantification are achieved by integrating cross validation of data of the multiple measuring lines. The method does not need to damage the structure, has the detection depth obviously better than that of a traditional method, is suitable for metal and plastic corrugated pipes, effectively overcomes the problems of insufficient detection depth, large material limitation, low efficiency and the like in the prior art, has the advantages of high anti-interference capability, simplicity and convenience in operation and low cost, and is suitable for popularization and application. The method can be widely applied to grouting quality evaluation of prestressed concrete structures such as bridges and buildings.

Description

Technical Field

[0001] The present invention relates to the technical field of civil engineering structure health monitoring and non-destructive testing, and in particular to a non-destructive evaluation method for the location and degree of grouting defects in prestressed ducts. Background Art

[0002] With the rapid development of transportation infrastructure, such as highways and railways, post-tensioned prestressed concrete beams have been widely used in bridge construction in recent years due to their advantages, such as long spans, light weight, and excellent integrity. Over 95% of newly built bridges in my country are prestressed concrete bridges. The prestressing system is the most critical component of prestressed concrete bridges. The prestressing of the steel strands in the ducts offsets the pressure exerted on the bridge deck by vehicles and pedestrians, and the quality of the grouting determines the lifespan of the bridge.

[0003] To ensure the prestressing effect of the beam and the durability of the structure, and to prevent moisture intrusion and corrosion of the prestressed steel strands, the prestressing ducts must be filled with cement slurry. However, due to process and human factors, defects such as cavities are prone to appearing within the prestressing ducts. These defects, when exposed to air and water, can cause corrosion of the steel strands, thereby reducing the bearing capacity and durability of the concrete structure, and even causing structural failure or collapse. Therefore, rapid detection and defect assessment of the grouting condition of the prestressing ducts is of great significance for practical engineering.

[0004] At present, non-destructive testing methods are mainly used to detect duct grouting defects.

[0005] (1) Infrared thermal imaging detection technology

[0006] Infrared thermography primarily utilizes differences in temperature and emissivity on a material's surface to create visible thermal images, thereby detecting the structural state and defects of the material surface and, in turn, determining the material's properties and internal defects. Research has shown that infrared thermography is highly suitable for detecting shallow voids in concrete structures, with a measurement depth of approximately 10 cm. It can also be used to detect voids within prestressed grouting ducts. A significant advantage of this method is its visual nature, eliminating the need for post-processing data. The presence of defects within the concrete can be directly determined from the image during the inspection process. However, inspecting prestressed ducts requires a relatively long time (approximately 1.5 hours), and heating times are also longer for larger defects. Therefore, this method is not suitable for practical application and can only be used in laboratory model tests. Existing research results indicate that infrared thermography imposes stringent requirements on both time and process, which are often difficult to meet in practice. Furthermore, the limited depth of inspection makes it difficult to implement in practice.

[0007] (2) Geological radar detection technology

[0008] Geological radar (GPR) is currently the most commonly used method for testing the density of grouting in plastic corrugated pipes, both domestically and internationally. However, due to the electromagnetic wave shielding effect of the steel bars and metal bellows, GPR has difficulty detecting grouting defects in metal bellows. Existing research results have shown that while GPR has high accuracy for plastic bellows, it cannot be used for metal bellows. Furthermore, the numerous steel bars arranged vertically and horizontally within the beam body strongly reflect electromagnetic waves, interfering with the identification of anomalies within the bellows.

[0009] (3) Impact echo detection technology

[0010] The impact echo method is a method primarily suitable for detecting internal defects in planar plate structures. It can be used to detect the thickness of concrete slabs, delamination defects, and interface bonding quality. Studies have found that the impact echo method can effectively detect grouting defects in metal bellows, but cannot accurately detect plastic bellows. Existing research results indicate that the impact echo method can provide a preliminary assessment of the grouting quality of metal bellows, but quantitative defect calculation is difficult. For plastic bellows, this method also has difficulty assessing the internal conditions of the pipe. Furthermore, the use of the impact echo method for testing also presents issues such as low efficiency, low accuracy, and difficulty in detecting double-layer ducts. Furthermore, in actual testing, it has been found that the frequency spectrum obtained by the impact echo method is sometimes complex, and simply relying on spectral characteristics cannot accurately determine the location, size, and other information of duct grouting defects.

[0011] (4) Ultrasonic detection technology

[0012] In recent years, ultrasonic methods have been widely studied and applied in the field of nondestructive testing of concrete. Ultrasonic waves have good directivity; the higher the frequency, the better the directivity. Ultrasonic waves have high propagation energy and strong penetration into various materials. The characteristics of ultrasonic waves, such as amplitude, frequency, and phase changes, provide a wealth of information for ultrasonic detection. Existing research and engineering practice have found that ultrasonic methods can detect grouting defects within ducts and have further discovered the impact of grouting defects on the propagation and attenuation characteristics of ultrasonic waves. Related research results show that concrete aggregates have strong scattering and reflection effects on ultrasonic waves, resulting in rapid attenuation of ultrasonic energy within concrete structures. To reduce the attenuation of ultrasonic energy, low-frequency ultrasonic waves are required, but such low frequencies also result in reduced resolution. In addition, due to the attenuation of ultrasonic amplitude and the strong reflection of steel strands within the duct, the imaging effect of defects is sometimes unsatisfactory.

[0013] These issues severely limit the engineering applicability of prestressed duct grouting defect detection. Therefore, there is an urgent need for an efficient, non-destructive, interference-resistant detection method applicable to corrugated pipes of different materials to improve detection accuracy and meet the needs of complex engineering scenarios. Summary of the Invention

[0014] In view of the above-mentioned technical deficiencies, the purpose of the present invention is to provide a non-destructive evaluation method for the location and degree of grouting defects in prestressed ducts, in which the excitation position does not change with the position of the sensor, thereby improving the test efficiency.

[0015] To achieve the above object, the present invention adopts the following technical solutions:

[0016] A nondestructive evaluation method for the location and extent of grouting defects in a prestressed duct comprises the following steps:

[0017] S1. Select the test piece to be inspected. Obtain the location, diameter, and number of inserted steel strands of the prestressed channel according to the design drawings. Mark the channel location on the test surface. Use the channel centerline as the primary measurement line. Arrange multiple auxiliary measurement lines at preset distances above and below the primary measurement line. Mark the measurement points on each measurement line at the set intervals.

[0018] S2. Calculate the grouting defect level corresponding to each measuring line: Determine the grouting defect level threshold corresponding to each measuring line based on the relative positional relationship between the measuring line and the duct, combined with the duct diameter and the measuring line spacing;

[0019] S3. Calculate the equivalent steel strand diameter based on the number of inserted steel strands and calculate the cement slurry layer thickness b based on the hole diameter; and calculate the concrete layer thickness d based on the distance between the hole and each measuring line;

[0020] S4. Measure the elastic wave velocity v of the concrete specimens in the same batch 硂 Based on Snell's law (1), the refraction angle β when the elastic wave enters the concrete from the steel strand at an incident angle α = 90° is calculated as follows:

[0021]

[0022] Where, v 钢 is the elastic wave velocity of the steel strand, which is 5800 m / s;

[0023] S5. Arrange acceleration receiving sensors at each measuring point, use a striking tool with an acceleration trigger sensor to strike the extended steel strand to trigger an elastic wave signal, and receive the signal;

[0024] S6. Perform spectrum analysis on the elastic wave signals collected by the trigger sensor and the receiving sensor, filter out low-frequency drift and high-frequency noise, and extract the first arrival time and the first wave amplitude;

[0025] S7. Calculate the signal propagation time: Subtract the first arrival time of the trigger signal from the first arrival time of the received signal, and calculate the hole position x corresponding to the defect by combining the refraction angle β and the concrete layer thickness d using formula (2) c , the formula is:

[0026] x c =x-dtanβ#(2)

[0027] Where x is the position of the sensor on the measuring line;

[0028] S8. Calculate the normalized first wave amplitude: divide the first wave amplitude of the received signal by the first wave amplitude of the trigger signal;

[0029] S9.x c The propagation time variation curve is plotted with x as the horizontal coordinate and the signal propagation time as the vertical coordinate; the amplitude variation curve is plotted with x as the horizontal coordinate and the normalized first wave amplitude as the vertical coordinate. By analyzing the propagation time delay and the amplitude drop area in the curve, combined with the grouting defect degree threshold in step S2, the location and degree of the defect are determined, and the overall defect range is evaluated by integrating the results of multiple measurement lines.

[0030] Preferably, in step S1, the number of auxiliary measuring lines is no less than 4, and the distance between measuring points is 5 cm.

[0031] Preferably, the knocking tool in step S5 is a small ball with an acceleration trigger sensor, and the number of knocks matches the number of measuring points and the arrangement of the sensors.

[0032] Preferably, the criteria for determining the degree of defect in step S9 are:

[0033] When the amplitude decrease of the normalized first wave exceeds the defect degree threshold of the corresponding measurement line, it is determined to be a defect; the actual degree of the defect is quantified based on the difference between the amplitude decrease ratio and the threshold range.

[0034] Preferably, the method is applicable to the detection of prestressed duct grouting defects in metal bellows or plastic bellows, and the excitation position is fixed.

[0035] Preferably, the defect position x in step S7 c The calculation of needs to be cross-validated with data from at least two survey lines to correct the positioning error.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] The present invention excites elastic waves by striking the steel strands and receives signals on the concrete surface. There is no need to drill, sample or perform other destructive operations on the test piece, and the structure of the prestressed duct and the surrounding concrete is completely preserved, avoiding secondary damage caused by the detection means. Fixed excitation position: The position of striking the steel strands does not change with the movement of the sensor, reducing the equipment adjustment time; Fast signal processing: Rapidly extract the first arrival time and the first wave amplitude through spectrum analysis and bandpass filtering, shortening the data processing cycle; Multi-line synchronous analysis: Rapidly locate defects by integrating data from multiple lines to avoid misjudgment of a single line. Reduce material influence: Elastic waves are refracted to concrete after propagating through the steel strands, effectively avoiding interference of metal bellows on electromagnetic waves (such as geological radar) or plastic bellows on impact echoes; Penetration depth optimization: Based on the refraction angle calculation modified by Snell's law, combined with low-frequency elastic waves, taking into account both penetration depth and resolution, grouting defects with a depth of more than 20 cm can be detected. Geometric correction positioning: Corrects the geometric deviation between the sensor position and the actual defect position in the duct to improve positioning accuracy; Dynamic threshold judgment: Dynamically sets the defect degree threshold based on the vertical height of the survey line and the duct diameter to avoid misjudgment caused by a single standard; Multi-parameter comprehensive evaluation: Simultaneously analyzes the propagation time delay and the normalized first wave amplitude drop to quantify the defect degree and improve the reliability of the results. Material compatibility: Applicable to grouting defect detection of metal bellows and plastic bellows, overcoming the limitations of existing technologies (such as geological radar's insensitivity to metal pipe failure and impact echoes to plastic pipes); Strong engineering adaptability: Applicable to prestressed duct detection in different scenarios such as bridges and buildings, supporting double-layer ducts or complex steel bar layout environments. Simple equipment: Only requires an accelerometer, a tapping tool, and a signal analysis system, without the need for expensive thermal imagers or geological radar equipment; Low operating threshold: Standardized processes (marking survey lines, tapping, signal analysis) reduce dependence on professionals. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Schematic diagram of the test piece structure in the present invention;

[0039] Figure 2 Schematic diagram for measuring point marking;

[0040] Figure 3 This is a schematic diagram of survey line distribution;

[0041] Figure 4 The schematic diagram of the arrangement of steel strands;

[0042] Figure 5 (a) is the signal propagation time variation curve of measurement line 1 and measurement line 2. Figure 5 (b) is the normalized first wave amplitude change curve of survey line 1 and survey line 2;

[0043] Figure 6 (a) is the signal propagation time variation curve of measurement line 3 and measurement line 4. Figure 6 (b) is the normalized first wave amplitude change curve of measurement lines 3 and 4.

[0044] in:

[0045] 1. Cement slurry; 2. Foam paper; 3. Concrete; 4. Steel strand; 5. Measuring point; 6. Measuring line. DETAILED DESCRIPTION

[0046] The present invention will be further described below with reference to the accompanying drawings.

[0047] like Figures 1 to 6 As shown, a non-destructive evaluation method for the location and extent of grouting defects in prestressed ducts includes the following steps:

[0048] 1. Select the test piece to be tested. Obtain the prestressed hole location, hole diameter, and number of inserted steel strands 4 according to the design drawing. Mark the hole location on the test surface. Use the hole centerline as the measuring line and arrange multiple measuring lines at a certain distance above and below this measuring line. Mark measuring points 5 at appropriate intervals:

[0049] like Figure 1 As shown, three concrete slab specimens were cast, with a hole diameter of 8 cm, and three 7×1 steel strands 4 with a nominal diameter of 15.2 mm were inserted. Foam papers 2 of different heights were tied at designated positions of the steel strands 4 as grouting defects of different degrees. The length of defect V1 is 10 cm, and that of defect V2 is 15 cm. The degree of defect is quantified by the ratio of defect height to hole diameter. The height of defect V1 is 2 cm, which is a 25% defect; the height of defect V2 is 4 cm, which is a 50% defect. The center line of the hole is used as the measuring line 6, and four more measuring lines are arranged 5 cm and 10 cm above and below the center line, that is, a total of 5 measuring lines are arranged, and the measuring points 5 are spaced 5 cm apart, as shown in Figure 2 shown.

[0050] 2. Calculate the degree of grouting defects corresponding to each measuring line:

[0051] Calculate the degree of grouting defects corresponding to the five measuring lines 6: The horizontal line height of the intersection of the elastic wave propagation path of measuring line 1 and the duct is 6.5 cm (e.g. Figure 3 As shown in the figure, the hole diameter is 8 cm, and the grouting defect degree corresponding to measuring line 1 is 81%. Based on this, the grouting defect degrees corresponding to measuring lines 2 to 5 are 69%, 50%, 31%, and 19%, respectively.

[0052] 3. Calculate the equivalent steel strand diameter 4 based on the number of inserted steel strands 4, and calculate the thickness b of the cement slurry layer 1 based on this and the hole diameter; calculate the thickness d of the concrete layer 3 based on the distance between the hole and each measuring line;

[0053] Insert three 7×1 steel strands 4 with a nominal diameter of 15.2 mm, and calculate the equivalent diameter of the steel strand 4 to be 2.8 cm. Figure 4 As shown. The thickness of the cement paste layer b = (8-2.8) / 2 = 2.6 cm, and the thickness of the concrete layer 3 of the measuring line 1 and the measuring line 5 is d1 = 11.9 cm ( Figure 3 ), the thickness d2 of the three layers of concrete in measuring lines 2 and 4 is 9.4 cm, and the thickness d3 of the three layers of concrete in measuring line 3 is 8.5 cm.

[0054] 4. Measure the elastic wave velocity v of concrete 3 of concrete 3 test blocks from the same batch 砼 ; and calculate the refraction angle β of the elastic wave in the concrete 3 when the incident angle α=90° from the steel strand 4 according to Snell's law;

[0055] The elastic wave velocity v of 3 specimens of the same batch of concrete was measured 砼 =3998m / s, and the refraction angle β calculated according to formula (1) is =44°.

[0056] 5. Arrange an acceleration receiving sensor at the measuring point, use a knocking ball with an acceleration trigger sensor to knock on the extended steel strand 4, and move the acceleration sensor to the next batch of measuring points after receiving the signal;

[0057] 6. Perform spectrum analysis on the elastic wave signals received by the trigger sensor and the receiving sensor, filter out the low-frequency drift and high-frequency noise, and extract the first arrival time and first wave amplitude of the filtered trigger signal and the receiving signal;

[0058] Perform spectrum analysis on the trigger sensor and receiving sensor signals, use a bandpass filter to filter out frequency components outside 3000Hz to 10000Hz, and extract the first arrival time and first wave amplitude of the filtered trigger signal and receiving signal.

[0059] 7. Subtract the first arrival time of the trigger signal of the same tap from the first arrival time of the received signal of the same tap to obtain the signal propagation time; and calculate the channel position x corresponding to the signal propagation time of each measuring line as follows: c :

[0060] x c =x-dtanβ#(2)

[0061] Where x is the position of the sensor on the measuring line.

[0062] 8. Divide the first wave amplitude of the received signal of the same tap by the first wave amplitude of the trigger signal of the same tap to obtain the normalized first wave amplitude;

[0063] 9. x cWith x as the abscissa and the signal propagation time of each measuring line as the ordinate, the signal propagation time variation curves of all measuring lines are plotted; with x as the abscissa and the normalized first wave amplitude of the signal of each measuring line as the ordinate, the normalized first wave amplitude variation curves of the signals of all measuring lines are plotted. The portion where the signal propagation time variation curve of a single measuring line is delayed and the portion where the normalized first wave amplitude variation curve drops sharply is the portion where the grouting defect occurs. The degree of the grouting defect is determined by comprehensively considering the grouting defect conditions of multiple measuring lines:

[0064] The signal propagation time and normalized first wave amplitude change curves of survey line 1 and survey line 2 are shown in the figure. Figure 5 The signal propagation time and normalized first wave amplitude change curves of measurement lines 3 and 4 are shown in Figure 6 shown.

[0065] For the V1 defect, the signal propagation time of the survey line 1 is delayed at 25cm~35cm, and the normalized amplitude is greatly reduced at 25cm~35cm. However, the signal propagation time and normalized first wave amplitude of the survey lines 2, 3, and 4 do not fluctuate significantly at 25cm~35cm. Therefore, it can be judged that there is a defect here. The defect is about 10cm long, and the defect degree exceeds the grouting defect degree corresponding to the survey line 1 (19%), but is less than the grouting defect degree corresponding to the survey line 2 (31%). That is, the defect degree of the V1 defect is judged to be between 19% and 31%, which is consistent with the actual situation. For the V2 defect, the signal propagation time of the survey lines 1, 2, and 3 is delayed at 65cm~80cm, and the normalized amplitude is greatly reduced at 65cm~80cm. However, the signal propagation time and the normalized first wave amplitude of the survey line 4 do not fluctuate significantly at 65cm~80cm. Therefore, it can be judged that there is a defect here. The defect is about 15cm long, and the defect degree exceeds the grouting defect degree corresponding to the survey line 3 (50%), but is less than the grouting defect degree corresponding to the survey line 4 (68%). That is, the defect degree of the V2 defect is judged to be between 50% and 68%, which is consistent with the actual situation.

Claims

1. A non-destructive evaluation method for the location and extent of grouting defects in prestressed ducts, characterized in that: The following steps are involved: S1. Select the test piece to be inspected. Obtain the location, diameter, and number of inserted steel strands of the prestressed channel according to the design drawings. Mark the channel location on the test surface. Use the channel centerline as the primary measurement line. Arrange multiple auxiliary measurement lines at preset distances above and below the primary measurement line. Mark the measurement points on each measurement line at the set intervals. S2. Calculate the grouting defect level corresponding to each measuring line: Determine the grouting defect level threshold corresponding to each measuring line based on the relative positional relationship between the measuring line and the duct, combined with the duct diameter and the measuring line spacing; S3. Calculate the equivalent steel strand diameter based on the number of inserted steel strands and calculate the cement slurry layer thickness b based on the hole diameter; and calculate the concrete layer thickness d based on the distance between the hole and each measuring line; S4. Measure the elastic wave velocity v of the concrete specimens of the same batch 砼 Based on Snell's law (1), the refraction angle β when the elastic wave enters the concrete from the steel strand at an incident angle α = 90° is calculated as follows: Where, v 钢 is the elastic wave velocity of the steel strand, which is 5800 m / s; S5. Arrange acceleration receiving sensors at each measuring point, use a striking tool with an acceleration trigger sensor to strike the extended steel strand to trigger an elastic wave signal, and receive the signal; S6. Perform spectrum analysis on the elastic wave signals collected by the trigger sensor and the receiving sensor, filter out low-frequency drift and high-frequency noise, and extract the first arrival time and the first wave amplitude; S7. Calculate the signal propagation time: Subtract the first arrival time of the trigger signal from the first arrival time of the received signal, and calculate the hole position x corresponding to the defect by combining the refraction angle β and the concrete layer thickness d using formula (2) c , the formula is: x c =x-dtanβ#(2) Where x is the position of the sensor on the measuring line; S8. Calculate the normalized first wave amplitude: divide the first wave amplitude of the received signal by the first wave amplitude of the trigger signal; S9.x c The propagation time variation curve is plotted with x as the horizontal coordinate and the signal propagation time as the vertical coordinate; the amplitude variation curve is plotted with x as the horizontal coordinate and the normalized first wave amplitude as the vertical coordinate. By analyzing the propagation time delay and the amplitude drop area in the curve, combined with the grouting defect degree threshold in step S2, the location and degree of the defect are determined, and the overall defect range is evaluated by integrating the results of multiple measurement lines.

2. A non-destructive evaluation method for the location and extent of grouting defects in prestressed ducts according to claim 1, characterized in that: In step S1, the number of auxiliary measuring lines is no less than 4, and the distance between measuring points is 5 cm.

3. The non-destructive evaluation method for the location and extent of grouting defects in prestressed ducts according to claim 1, characterized in that: The knocking tool in step S5 is a small ball with an acceleration trigger sensor, and the number of knocks matches the number of measuring points and the sensor arrangement.

4. A non-destructive evaluation method for the location and extent of grouting defects in prestressed ducts according to claim 1, characterized in that: The criteria for determining the degree of defect in step S9 are: When the amplitude decrease of the normalized first wave exceeds the defect degree threshold of the corresponding measurement line, it is determined to be a defect; the actual degree of the defect is quantified based on the difference between the amplitude decrease ratio and the threshold range.

5. The non-destructive evaluation method for the location and extent of grouting defects in prestressed ducts according to claim 1, characterized in that: The method is applicable to the detection of prestressed duct grouting defects in metal bellows or plastic bellows, and the excitation position is fixed.

6. A non-destructive evaluation method for the location and extent of grouting defects in prestressed ducts according to claim 1, characterized in that: Defect position x in step S7 c The calculation of needs to be cross-validated with data from at least two survey lines to correct the positioning error.

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