1, rebound method and its application
The rebound method is to estimate the quality and compressive strength of concrete by measuring the hardness of concrete surface with a rebound instrument. The rebound instrument consists of an impact hammer and an impact dowel bar. The impact hammer is driven by the spring, and the driving force acts on the concrete through the dowel bar. The impact hammer bounces back to a certain distance, and the measured distance is the rebound distance. Then the strength-related index is the ratio of the rebound distance to the initial length of the spring. The strength of concrete can be estimated by this index to test the quality and compressive strength of concrete. Compared with other nondestructive testing technologies, rebound method has the advantages of simple operation, easy mastery, convenient instrument carrying, high efficiency, strong flexibility and low cost. However, the rebound method also has some defects, that is, it is not accurate enough and can only be used to detect the quality of concrete with thin surface. If there is obvious quality difference between the surface and the interior of concrete or there are some defects in concrete, the rebound method can not meet the engineering requirements. In addition, the rebound method can not be applied to the concrete in the anchorage area of prestressed steel bars or in the dense area of steel bars.
2. Ultrasonic method and its application
In the nondestructive testing technology of concrete, the position of acoustic wave method can not be ignored. Based on wave propagation theory, acoustic wave method mainly includes ultrasonic detection technology, acoustic divergence detection technology and impact echo technology. And ultrasonic testing technology is a common technology in engineering construction. The following focuses on ultrasonic testing technology.
The detection principle of ultrasonic method is that the ultrasonic instrument generates high-voltage electrical pulse, and the high-voltage electrical pulse is transmitted to the piezoelectric crystal in the mixed-wave transducer by exciting the acoustic pulse, thus obtaining high-frequency acoustic pulse. The acoustic signal from concrete propagates in concrete medium and is finally received by the receiving transducer. Measure the time and distance of ultrasonic wave propagation in concrete, and calculate the propagation speed of ultrasonic wave in concrete. The data processing and correlation analysis software of the instrument is used to comprehensively analyze various acoustic parameters of the received signal to evaluate the strength and defects of concrete members.
Ultrasonic detection technology has good directivity and large transmission energy; Can be well adapted to various concrete materials; Strong penetration and good detection sensitivity; Low cost, non-toxic and harmless, etc. , has aroused people's attention and wide application. Among all kinds of nondestructive testing technologies, ultrasonic method is the most widely used and fastest developing testing technology, and its position can not be ignored. At the same time, ultrasonic method also has some shortcomings. For example, because people don't know enough about the theoretical basis of ultrasonic wave at present, most concrete structures are complex, especially because many propagation laws of ultrasonic wave will change with various factors, so it is easy to have big or small errors when using ultrasonic wave to detect concrete, which will affect the interpretation of the phenomenon and the accuracy of the detection results.
3. Ultrasonic rebound synthesis method and its application
As the name implies, ultrasonic rebound synthesis method is to measure the sound value and rebound value of concrete simultaneously by using ultrasonic instrument and rebound instrument. It is worth noting that the instrument should be measured on the same side of the concrete structure. The rebound value is used to measure the quality and strength of the thin layer on the surface of concrete, the acoustic value is used to reflect the measured mechanical properties of concrete structure, and then the obtained mechanical properties are used to reflect the internal strength and quality of concrete. In the application process, the advantage of ultrasonic rebound synthesis method is that it can not only reflect the strength of thin concrete surface, but also reflect the strength of concrete internal structure. In addition, some factors related to the physical quantity and strength of concrete can be offset, thus improving the accuracy of concrete strength measurement. Its disadvantage is that it still cannot achieve very accurate detection results, so the technology related to the accuracy of detection results needs to be improved.
4. Vertical reflection method and its application
The offset distance measured by vertical reflection method is very small, that is, the sending and receiving distance is very small, thus improving the accuracy of the results. The detection principle is that the transmitting probe emits a pulse wave, and the pulse wave will encounter certain wave impedance when it propagates in the concrete medium. If the wave impedance is too obvious, it will produce a reflected wave, which will be received by the receiving sensor after returning to the concrete surface. By recording the amplitude, phase and frequency of the reflected wave signal and analyzing it, the defects in concrete can be judged.
5. Geological radar method and its application
With the continuous improvement of GPR method, it has been successfully applied to many fields such as highway pavement survey, bridge tunnel survey, concrete detection and so on.
The working principle of GPR method is to transmit high-frequency electromagnetic waves from the ground to the underground in the form of broadband short pulses by using transmitting antennas, and then reflect them when encountering obviously different wave impedances, thus returning to the ground, where they are received by receiving antennas. Make the transmitting antenna and the receiving antenna move synchronously along the survey line, and pay attention to keeping the distance between the transmitting antenna and the receiving antenna fixed during the movement, so as to obtain the radar image of the defect distribution of concrete under the survey line. When there are obvious differences in concrete, such as honeycomb or overhead, the reflected wave will be enhanced; Then when the concrete is relatively complete and dense, and its properties are relatively uniform, the reflected wave will be very weak. The strength and internal defects of concrete can be obtained by analyzing the recorded waveform. Its advantages are visibility and flexibility, and it can be tested repeatedly in the same part. The disadvantage is that the reinforcement in concrete makes it difficult to identify the defects in concrete with radar signals.
6. Core drilling method and its application
Core drilling method adopts special drilling rig and artificial diamond hollow thin-walled bit. The detection principle is to directly drill core samples on concrete structural members by using special drilling rig and artificial diamond hollow thin-walled drill bit, and the strength and internal defects of concrete can be inferred from the core samples obtained by detection. This method has the advantages of high accuracy and reliable test results, but at the same time, it also has some disadvantages, such as damage to concrete, high labor intensity, expensive price and easy to be limited by various related factors.
Non-destructive testing technology of concrete is an important monitoring method for various civil engineering. In order to prevent the occurrence of quality and safety problems, we must do a good job of monitoring, testing and evaluation in advance, and judge and check the defects of concrete structures in time to nip in the bud. With the continuous improvement of people's quality requirements, there are more and more nondestructive testing methods for concrete, which are more and more advanced and more efficient. At the same time, the rapid development of modern computer technology provides good technical support for nondestructive testing technology, provides a certain foundation for the reliability of its testing results and the improvement of testing level, and promotes the processing speed of a large amount of information.
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