Abstract: In recent years, China has built many large-scale infrastructure projects that will marvel the world. How to ensure the quality of the project and achieve the "100-year project" has become an issue of concern to everyone. Concrete durability is an important factor to ensure the quality of the project. After years of technical development, the durability of concrete in the general environment has been guaranteed, but in some complex and harsh environments, corrosion conditions are prone to occur, which reduces the performance of the concrete. To the designed service life. If corrosion monitoring of the concrete structure can be carried out and the durability redesigned in time for the corrosion situation, it will help to extend the service life of the structure. In Europe, as early as the late 1980s, the corrosion monitoring system was developed, and corrosion monitoring was performed on important or difficult-to-detect concrete structures, and corresponding corrosion repair measures or corrosion protection and other durability redesigns were taken based on the corrosion data. In recent years, corrosion monitoring systems have also been gradually applied in key projects in China, such as Xiamen Xiang’an Subsea Tunnel. This article will mainly introduce the technical principles, development and engineering applications of the current corrosion monitoring system.
Keywords: concrete structure, durability, debarking, corrosion monitoring
1 Overview
Since its advent, concrete has been recognized by the world for its ruggedness and durability. It is a very common material in civil engineering construction. It is estimated that there are nearly 10 billion cubic meters of concrete poured each year worldwide. Since the 1970s, premature failure of concrete structures has gradually emerged, which has caused academics to attach importance to the study of concrete durability. Experts have worked hard to study the corrosion failure of concrete structures and improve the durability of concrete structures. The main causes of concrete failure include steel bar corrosion, concrete carbonization, alkali base material reaction, freeze-thaw damage, etc., among which steel bar corrosion is the most serious. In order to improve the durability and anti-corrosion ability of reinforced concrete structures, in the past, domestic general improvement of concrete formulations, the use of cathodic protection devices, surface protection and other measures to prevent harmful substances such as moisture, air, chloride ions and other harmful media from further intrusion. Many large-scale infrastructure projects, such as Hangzhou Bay Bridge, Xiamen Xiangan Tunnel, Hong Kong-Zhuhai-Macao Bridge, have been built or are under construction in the coastal areas of China, all of which have proposed the slogan of a hundred-year project, but in the harsh engineering environment, these Whether the durability of the concrete structure can pass, and whether the project life can meet the design requirements, is a practical problem before us. In order to avoid and detect the occurrence and development of steel bar corrosion at the early stage, sustainable monitoring of steel bar corrosion technology can be considered in these large-scale projects to accurately measure the early corrosion information in concrete and take effective anti-corrosion measures before the steel bar corrosion. The cost of corrosion protection is lower than that of steel corrosion, and the corrosion protection effect is better.
2. Corrosion principle
The damage to the metal surface and the surrounding medium caused by chemical changes and electrochemical effects is called metal corrosion. There are two major types of steel corrosion, namely chemical corrosion and electrochemical corrosion. There is no flow of electrons in the chemical corrosion process, only a small part of the corrosion phenomenon. The corrosion caused by the electrochemical action between the surface of the steel bar and the medium such as humid air, electrolyte solution, etc. is electrochemical corrosion. There is a flow of electrons in this corrosion process. Most of the corrosion of steel bars is electrochemical corrosion. Several necessary conditions for steel bar corrosion: 1. The presence of a conjugated cathode; 2. The passive film of the steel bar is destroyed; 3. The presence of erosion conditions. As shown in Figure 1:
Figure 1 Corrosive environment of concrete structure
Concrete is a highly alkaline environment (with a PH value of about 13). In this environment, the steel bar forms a passive film, so its corrosion rate is very low. However, when the reinforced concrete is contaminated by Clˉ, such as the marine environment or the bridge structure in the winter when the ice salt is sprinkled, Clˉ gradually diffuses to the surface of the steel bar through the voids on the concrete surface, Clˉ can destroy the surface passivity of the steel bar, and the steel bar changes from passive to active After the steel bar is blunt, if there is still erosion conditions, the anode of the steel bar loses electron rust and the steel bar enters the corrosion stage. The corrosion products of steel bars are mostly oxides such as Fe3O4, whose volume is much larger than the volume of the steel that produces these products, so internal stress is generated and the concrete is cracked. Another reason for the corrosion of reinforced concrete is the infiltration of acidic substances (such as CO2), and the pH value of the pore fluid is reduced. When the pH value drops to 12.5, combined with the effect of Clˉ, the corrosion occurs at a faster rate. The durability of concrete decreases, and the strength degradation can be divided into several stages, see Figure 2:
Figure 2 Schematic diagram of strength degradation of concrete structure
Take the time T when the anode is deactivated to start losing electron rust as the boundary point. Therefore, if we can use a means to accurately test the time when the anode begins to lose electronic rust, it is particularly important for us to design and construct corrosion repairs. At present, China mainly relies on the parameters obtained by rapid laboratory tests and the in-situ component damage or non-destructive test structure to indirectly infer this time. However, for various reasons, the accuracy of this time inference is difficult to guarantee, and there is a disadvantage that it cannot be fed back dynamically. If a sensor that can monitor the entire deactivation process is embedded inside the concrete structure, and the progress of depassion and some key parameter feedbacks can be obtained dynamically and for a long time, then the time when corrosion begins can be accurately predicted. We know that the blunt front line of freshly poured concrete is located on the surface of the concrete. With time, the blunt front line will advance through the protective layer towards the reinforcement. Then, within the scope of the protective layer of the concrete structure, bury multiple desensitization sensors at different depths, and each sensor is distributed on the concrete surface to reach the protective layer of the steel bar. Time, establish a mathematical model of the development process of the front face, so that the time for the debarking of the steel bar can be calculated, and this time value T can be continuously corrected dynamically. If T is less than the design period, you can redesign the structure for durability, start the corrosion protection plan in time, and continue to monitor the front surface to confirm the effectiveness of the corrosion protection measures. If after taking measures, T is still less than the design period, then there is still a chance to take corresponding remedial measures before the project enters the corrosion stage.
3. Research and application of corrosion detection system
3.1 Embedded corrosion monitoring system
In the late 1980s, Europe began to develop corrosion monitoring systems, including the trapezoidal anode concrete structure embedded corrosion monitoring and sensing system (Anoden-Leiter-Sysetem, Figure 3) of Germany S + R SensorTech Company and FORCE Technology of Denmark. Circular multi-probe anode concrete structure corrosion monitoring system (Nagel-System, Figure 4), these two systems have been well applied in many large concrete structure projects in Europe and Africa. The common principle of both is that the sensor is installed inside the structure, and the corrosion time of the steel bar is pre-warned in advance according to the deactivation corrosion of the anode at different heights.
Figure 3 Anoden-Leiter-Sysetem system
Figure 4 Nagel-System corrosion monitoring system
The Nagel-System corrosion monitoring system of the Danish FORCE Technology company is taken as an example to introduce this kind of system in detail.
The Nagel-System corrosion monitoring system consists of a data acquisition instrument, CorroWatch corrosion sensor, and ERE20 reference electrode. CorroWatch Corrosion Sensor is a multi-probe sensor (Figure 5). It consists of four anodes (black steel material) and one cathode (titanium mesh) at different heights and interconnecting lead wires. The sensor base is also built in. A temperature sensor can measure electrical parameters and temperature data at various stages of short corrosion of steel bars.
Figure 5 CorroWatch multi-probe corrosion sensor
The sensor base is fixed above the main reinforcement mesh (must be insulated from the reinforcement section with insulating material), and is located in the concrete protective layer. depth. The common cathode is made of platinum-coated titanium mesh and has a very high positive potential. When the depassive front line is advanced to the anode, when the anode is deactivated, the macro current of the circuit between the anode and the cathode will change. Two metals with different potentials can form a galvanic cell through the wire. The greater the potential difference, the corrosion The greater the current.
The corrosion potential of steel bars in different electrochemical states is different. When the steel bar is passivated, its corrosion potential increases, and after depassivation, its corrosion potential decreases. The corrosion status of the steel bar can be judged according to the corrosion potential. But the corrosion potential measured on the concrete surface is not accurate enough. Therefore, an embedded reference electrode can be used, but the reference electrode should have long-term stability and accuracy. Currently, the most commonly used reference electrode in the world is the ERE20 reference electrode from FORCE. The main materials of ERE20 are manganese dioxide MnO2 and alkaline chloride-free gel materials, which are used with CorroWatch to form a half-cell structure to monitor the corrosion state of steel bars.
The advantage of the Nagel-System system is the industrial design of the CorroWatch sensor. Its base is a flat ring structure, which can be easily fixed on the main reinforcement mesh without considering the angle. The height of the anode is also very easy to determine, and the height can be adjusted during installation. According to the time point of deactivation corrosion of different anodes, the time when the main reinforcement mesh begins to corrode can be predicted in advance. In the macro current test, the cathode and anode spacing is required to be small, otherwise the macro current value obtained by the test will be small due to the influence of concrete resistance, and it is not easy to judge the corrosion of the steel bar. Compared with other corrosion monitoring sensors, the CorroWatch sensor has the smallest anode and cathode spacing, and the macro current data is relatively easy to judge. Moreover, due to the small occupied space, it is pre-buried in concrete and has little effect on the protective layer and bearing capacity.
The monitoring system with the same principle includes the ECI corrosion monitoring system (Figure 6) developed by Virginia Technologies in the United States and the SENSCORE corrosion monitoring system of ROCKTEST. However, these two systems have not been available for a long time, and there is no real large-scale application in engineering.
Figure 6 US ECI corrosion monitoring system
3.2 Rear-mounted corrosion monitoring system
For the completed infrastructure construction projects, in order to track the durability of the concrete structure, a corresponding post-installed corrosion monitoring system has also been developed. The Speizring-Anoden-System (Figure 7) of the German H + R Sensor Company, which consists of an anode ring and a cathode rod, is installed in place by drilling holes in the structure. CorroRisk (Figure 8) of FORCE Technology, Denmark, is composed of 4-8 anodes and a combined electrode (composite of titanium mesh and ER20). It can be used for both new concrete structures and existing concrete structures. The hole is installed in place.
Figure 7 Speizring-Anoden-System system Figure 8 CorroRisk system
3.3 Application Case
The corrosion monitoring system in Europe has developed very maturely. Since the 1990s, the corrosion monitoring system has been successively put into engineering applications in various countries around the world. Important infrastructure such as offshore platforms and decks. Today, nearly 1,000 sets of trapezoidal anode corrosion monitoring systems and five or six hundred sets of annular multi-probe anode corrosion monitoring systems are used worldwide. The most influential are Denmark ’s Buildings of the Great-Belt-Link (446 sets), Denmark-Sweden Oresund-Link (249 sets), Egypt ’s Monitoring of the walls of the A Sukhna Por (71 sets), Japan Tunnel Project in Tokyo (15 sets), Xiamen Xiangan Subsea Tunnel in China (34 sets). In all of these projects, although two different corrosion monitoring systems are used, the important part of the reference electrode in the corrosion monitoring system is the Danish FORCE ERE20 reference electrode. Now take the Danish-Swedish Oresund-Link as an example to introduce the application of the corrosion monitoring system. The total length of The Oresund Link is 15410 meters, of which the bridge is 7800 meters long and the tunnel is 3510 meters long. The corrosion monitoring system of the bridge used 60 sets of trapezoidal anode corrosion sensors of S + R Sensor Company and dozens of sets of ERE20 reference electrodes of Denmark FORCE company; the corrosion monitoring system of the tunnel used 189 sets of ring multi-probe corrosion sensors of Denmark FOREC company (CorroWatch) and 243 sets of ERE20 reference electrodes. There are 9 passage blocks in the tunnel, of which 7 positions (C1-C7): 3 sets of CorroWatch and 1 ERE20 are set. The other 10 positions (R1-R10) are set of 2 ERE20 respectively. ![]()
3.4 Data collection and processing
There are two ways to collect data from the corrosion monitoring system. The first is real-time online monitoring and acquisition. For example, the Corrologger data acquisition instrument of Denmark's FORCE Technology company includes a data collector, GSM / GPRS module, battery and solar panel. It can be directly connected to CorroWatch, CorroRisk, ERE20 and temperature, Humidity sensor for remote online monitoring. The second type is random collection. A dedicated data collection instrument or a multimeter can be used. The collection interval can be two or three times a year or longer. In addition, information such as temperature and humidity of concrete and concrete impedance are also very important. Combining these data, a mathematical model is established through laboratory tests to estimate the time for the steel to pass through.
4. Conclusion
Practice has proved that for large-scale infrastructure projects, the establishment of a complete set of concrete corrosion monitoring system can obtain the key data of concrete durability decline, strength degradation, durability redesign, and anticorrosion measures in advance. For structures that are difficult to reach, such as underwater foundations, bridge foundations across the sea, submarine tunnels, etc., corrosion monitoring is irreplaceable by other detection methods. In order to improve the quality of the project in our country, it is very meaningful and necessary to introduce a corrosion monitoring system to build a 100-year project. We also hope that our scholars can develop independently innovative corrosion monitoring systems.
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