The primary problem of durability failure of reinforced concrete structures is steel corrosion. During the inspection, the concrete structure has been seriously damaged. Therefore, it is very necessary to find and accurately diagnose the corrosion of steel bars in time, which is very important for the durability evaluation of concrete structures. The corrosion of steel bars in concrete is a very complicated process, but people have never given up the research on the detection and monitoring methods of steel bar corrosion in concrete. For the development of reinforced concrete structure detection, nondestructive testing is the main research direction, and acoustic emission technology, as a new nondestructive testing technology and electrochemical method, has become the focus of nondestructive testing research in the future.
Due to many factors, the electrochemical detection speed is fast, but the accuracy is greatly affected, which can only give the possibility of steel corrosion. The acoustic emission technology can well judge the corrosion degree of steel bars. However, due to the complexity of concrete itself, the application and research of acoustic emission technology in monitoring the corrosion of steel bars in concrete are still insufficient. When detecting the corrosion of reinforced concrete, it is suggested to combine the two methods to analyze and judge the corrosion of reinforced concrete.
1, electrochemical method
Semi-battery method Semi-battery method can detect the corrosion degree of steel bars, which is widely used in concrete structures at present, and there are mature detection standard methods. In this detection method, there are two electrodes, and the corrosion state of steel bars is evaluated by the potential difference measured by the two electrodes. The half-potential tester mainly consists of a constant potential reference electrode, a reinforced connecting wire and a high input resistance voltmeter. The test principle is shown in figure 1. In this method, the potential relative to the standard half cell is obtained by detecting the position of the surface of the element. By analyzing the impedance spectrum and according to the assumed equivalent circuit diagram, the ohmic resistance, interfacial capacitance and polarization resistance of concrete can be determined. EIS can reflect the information of reinforced concrete corrosion system more comprehensively, but it is rarely used in engineering testing because of long testing time, difficult analysis and expensive testing instruments. Concrete resistivity method When ions flow in the pore fluid of cement paste, electrolysis will occur. This electrolytic process refers to the conductivity of concrete. This method must have two electrodes to measure, and the steel bar can be used as one electrode. Through the experiment of applying applied voltage and measuring current, the ratio of the two is the resistance value of concrete. According to the different operation methods, it can also be divided into four-probe method, disk method and two-stage method. Although the resistivity detection method is simple and has a wide application range, it is easily influenced by environmental conditions and the measured data is discrete.
2. Acoustic emission technology
Compared with traditional nondestructive testing methods, acoustic emission technology, as a new nondestructive testing method, was introduced into the field of civil engineering in the 1990s. Acoustic emission is a very common physical phenomenon. In fact, for some reason, the internal damage of the material releases energy and produces elastic waves, which will propagate to the surface of the material and cause the surface displacement of the material. Acoustic emission probe can detect these mechanical vibrations and convert them into electrical signals, which are amplified by pre-amplification gain and finally collected and recorded by signal acquisition system. In concrete structures, the degree, position and form of steel bar corrosion are dynamic. Moreover, the corrosion of steel bars occurs in concrete, and the signal sources are numerous and complicated. It is difficult to determine the degree of corrosion before the protective layer cracks. Acoustic emission technology can dynamically monitor materials in real time and is very sensitive to the transient energy released by the generation and development of internal defects in materials. The location method and moment tensor theory can be used to determine the damage location and crack type of concrete quickly and accurately.
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