This article systematically introduces the principle of the impact echo method and its application in the non-destructive testing of concrete structures, and introduces the latest technology of the impact echo method, the scanning impact echo test system, and lists them at the same time. Some of the latest research results of this technology.
1. Introduction of IE (Impact Echo) shock echo method
(1) Overview of shock echo methods
The impact echo method is a nondestructive testing method based on stress waves to detect the thickness and defects of structures. As early as the 1980s, Dr. Mary Sansalone of Cornell University conducted research on the method. The IE method can not only quickly determine the holes, honeycomb, cracks, peeling and other defects in concrete and masonry structures, but also determine the thickness of structural members and the depth of defects. An important advantage of the IE method is that only one test surface is required for testing.
Shock echo technology is developing very rapidly. There are currently many types of IES scanning shock echo systems, shock echo systems with surface waves, ultra-thin shock echo detection systems, etc., which can be selected according to engineering needs. IES is a major breakthrough in the development of IE technology. It not only can quickly and continuously detect, but also adds inspection items such as prestressed tube grouting, etc. In addition, it can also perform three-dimensional imaging of structural thickness and defects. Surface wave type impact echo system can accurately detect the thickness of concrete without core calibration. The ultra-thin impact echo system can detect a plate-like structure with a minimum thickness of about 5 cm, which broadens the thickness detection range of the impact echo method. At present, the thickness of IE method detection ranges from 5cm to 180cm, which fully meets the detection needs of general engineering.
(2) The principle of the shock echo method
The basic principle of IE test: as shown in Figure 1. A small hammer or impactor is used as an excitation source to impact on the concrete surface to generate a compression wave, and then a receiving sensor placed near the impactor receives the reflected compression wave. After analysis, the thickness of the concrete is calculated, and the defects such as holes, cracks and peeling are detected. For a flat plate or road surface without defects, a reflection wave on the bottom surface will be obtained in the impact echo test, so that when the wave speed of the compression wave is known, the thickness can be calculated
Figure 1: Schematic diagram of the impact echo (where source is the excitation source that generates the compression wave and receiver is the receiver of the reflected wave)
Next, the calculation of the thickness or defect depth of the impact echo method will be described in more detail. As shown in Figure 2, after receiving the reflected wave, the receiver converts the time-domain data into the frequency-domain data by fast Fourier transform, and then determines the frequency peak value F of the echo, and the depth calculation structure thickness or defect D = (b * VP ) / 2f (where b is the shape factor, which is 0.96 for the slab / wall, the value is smaller for the beam and column, determined according to the ratio of thickness and width, VP is the compression wave velocity). Figure 3 shows the time-domain and frequency-domain graphs of a test point, and Figure 4 shows the comparison of the test results at the non-defective and defective locations.
Figure 2: Schematic diagram of shock echo
Figure 3: Time-domain and frequency-domain diagrams of a single-point impact echo test
Figure 4: Comparison of defect-free and defective frequency domain diagrams
(Illustration of Figure 4: Above: There is only one frequency peak at the defect-free location, thickness information can be obtained
Below: There are multiple frequency peaks at the defect, and the nature and depth of the defect can be determined according to the shape of the curve)
(3) The advantages of the impact echo method compared to the ultrasonic method
1. The impact echo method requires only one test surface, while the ultrasonic method requires two test surfaces, which is difficult to achieve in many cases.
2. The shock echo method uses lower frequency sound waves than the ultrasound (IE frequency range is usually 2 ~ 20 kHz), which makes the shock echo method avoid the high signal attenuation and excessive Clutter interference.
3. The impact echo method does not require a coupling agent, and can be operated with one hand. After calibration, each measurement point directly obtains the structure thickness or defect position and depth information. While the ultrasonic method requires a coupling agent, two probes increase the difficulty of operation. At the same time, a large amount of data comparison is required to determine the location of the defect, but the depth of the defect cannot be determined.
4. The impact echo method can measure the structure up to 180cm, and it is very difficult to test the same thickness with the ultrasonic method, especially when the two test surfaces are not easy to contact.
From the above, it can be seen that the IE impact echo method has unparalleled advantages of the ultrasonic method. This method will become an important method for the non-destructive testing of concrete structures with the continuous improvement and development of technology.
(4) Application examples of shock echo method
The IE test can be used to evaluate the internal conditions of structures such as slabs, beams, columns, walls, road surfaces, airstrips, tunnels, and dams. The IE method can detect holes, honeycombs, cracks, peeling, and other defects in concrete, wood, stone, and masonry. If the thickness of the concrete member is known, the IE method can also predict the strength of early concrete.
The following are several on-site examples of the impact echo detection structure, in which the surface wave and the impact echo are used in combination in Figure 8, in a single-sided case, the wave velocity of the concrete can be accurately obtained, and then a more accurate thickness value can be obtained:
Figure 5: Impact echo method to detect bridges Figure 6: Impact echo method to detect tunnels
Figure 7: Impact echo method to detect highway Figure 8: Surface wave and impact echo method to detect bridge deck
The following picture shows the impact echo test results of an 11-inch thick concrete slab (compact) and a 17-inch thick concrete slab (with cracks). Test results show that there is a crack at a depth of 6.3 inches from the surface of the concrete slab
Figure 9: Comparison of IE results for dense concrete slabs and cracked concrete slabs
2. Introduction of IES (Impact Echo Scanner) scanning impact echo method
(1) Overview of scanning shock echo method
The IE method has always been recognized as a powerful tool for non-destructive testing of concrete components. This test method not only meets the relevant requirements of the American ASTM Standard C1383-98 thickness determination standard, but also meets the relevant requirements of the American ACI 228.2R-98 standard for determining defects such as holes, honeycomb, cracks, and delamination.
So far, the biggest limitation of the IE method is the low test rate. In general, the average IE system can measure 30 to 60 points per hour, which limits the method to only be used for testing small, very critical parts. The successful development of the scanning impact echo system completely solved the problem. The IES method can measure 2000 to 3000 points per hour, which greatly improves the detection efficiency. In addition, because the method can be quickly tested at intervals of a few centimeters along a straight line, it has been proven many times in practice that it can also be used to detect grouting in post-tensioned prestressed pipes. After three-dimensional processing of the IES data using the software, the thickness of the structure, the position of the defect and the grouting in the post-tensioned prestressed tube can also be displayed very intuitively.
(2) Principle of scanning shock echo method
The IES method is based on IE, the fixed single sensor into a rolling sensor, thereby greatly speeding up the test speed. As shown in Figure 11, the IE method has only one receiving sensor, and the solenoid is impacted once per test point. The IES method uses a scanning rolling sensor and a solenoid impactor for continuous impact, which greatly improves the detection efficiency.
Figure 10: Scanning shock echo principle diagram Figure 11: The upper left corner is the IES system, which uses a rolling receiving sensor; the lower right corner is the traditional IE system, with only one receiving sensor
(3) IES test examples:
1. An example of testing on a large board with the IES method:
Test board information:
l Board size = 10 × 4 meters
l IE Scanner scan every 5 cm.
l There are honeycomb defects in the board
l Board thickness changes
l Prestressed pipes exist in the board (there are dense grouting area, non-compact area and empty pipe area)
Figure 12: IES method testing large board site
Three-dimensional imaging test results using IES:
Figure 13: Three-dimensional imaging of thickness changes Figure 14: Three-dimensional imaging of honeycomb defects in the plate
Figure 15: Defect imaging of ungrouted areas in prestressed tubes Figure 16: 3D imaging of hole defects in plates
Figure 17: Graphical explanation of IES testing of prestressed pipe grouting
1. Summary
The IE impact echo method has proved to be a powerful tool for non-destructive testing of concrete structure thickness and defects, and has very mature experience in many countries. The improvement of its technology will make it an increasingly important nondestructive testing method. Compared with the IE method, the IES method is a huge improvement, which not only greatly improves the detection speed of the traditional impact echo, but also solves the big problem of detecting the grouting condition of the prestressed pipe. In addition, software imaging based on IES data can clearly and intuitively display the thickness variation, defect type and location of the structure. Practice has proved that the IES method is a very reliable and effective detection method, and is a breakthrough in the detection technology of concrete structures, which will definitely play a very positive role in the non-destructive testing of domestic concrete structures.
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