Summary:
Today, the design life of the substructure of large concrete projects is 100 years. This puts high demands on the concrete structure designer. At the design stage, it is necessary to carefully consider how to operate and maintain the structure. Because, this year, the lower structure of the main concrete structure in Denmark has installed corrosion monitoring sensors on critical sections, such as the Great Belt Link, Oresund Link, and Copenhagen Metro. Corrosion monitoring makes it possible to check for the intrusion of chloride ions in the early stages. Corrosion protection measures can be taken in the best time period to reduce maintenance costs.
The application of corrosion monitoring is still in the initial stage, but the effect of using this new method is worthy of recognition.
1 Introduction
In the past 15 years, three large concrete structures in Denmark have been improved
. The Great Belt Link, connecting the two islands of Sealand and Funen
. The Oresund link, connecting the island Sealand with Sweden connecting the island Sealand with Sweden
. Copenhagen Metro
The design life of these major concrete projects is 100 years, and the details of construction, maintenance and repair strategies must be considered in the design stage. One of the considerations is to develop and install corrosion sensors in these structures to predict the start of corrosion of reinforced concrete.
Both the Great Belt Link and the Oresund Link Öre Strait Bridge are exposed to sea water, an environment in which chloride ion activity is very active. The deep part of the Copenhagen Metro is in a changeable environment, with the bottom at the natural foundation. The subway structure has several locations that are exposed to groundwater rich in chloride ions. Under these environmental conditions, chloride ions will invade the concrete structure. When the chloride ion content around the reinforcement exceeds the critical value, corrosion will occur, which ultimately leads to a reduction in the service life.
By embedding sensors in the protective layer, the corrosion caused by chloride ion intrusion in the current environment can be monitored. An alternative measure is to use concrete samples for a large number of repeated chloride ion destruction analyses. The durability quality design of concrete will be based on actual environmental conditions. When the corrosion sensor is placed on the concrete protective layer and the critical section, the critical amount of chloride ion intrusion can be monitored. Anti-corrosion measures such as cathodic protection can also be carried out at an appropriate time before the initial rebuilding work, which will reduce the cost of maintenance.
Three major concrete structural engineering projects
Corrosion monitoring system is a tool to estimate the start time of steel bar corrosion. With the support of this information, the designation of maintenance measures can have the best basis in terms of technology and economy.
2. Monitoring principle
Corrosion of steel bars is an electrochemical process. At different positions of steel bars, iron ions are free (anode) and oxides are reduced (cathode), which will produce a potential difference. Due to the potential difference, corrosion current will be generated, and this corrosion current can be measured. When the corrosion process is carried out in a large anode area and cathode area, this corrosion is called macro cell corrosion.
The corrosion sensor is based on the macrocell corrosion principle. Assuming that a black steel is connected to an inert metal as a cathode, when corrosion occurs, the black steel becomes the anode. Generally, titanium metal or titanium covered with metal oxide is used as the cathode, and sometimes stainless black steel (reinforcing steel) can also be used.
In a healthy concrete structure, the electrochemical potentials of the anode and cathode are basically the same. This tiny potential difference makes the current between the anode and the cathode basically zero. When black steel is corroded due to the intrusion of chloride ions, the potential of the anode will decrease, while under normal conditions the potential of the cathode will remain constant. In this way, when the anode and cathode are connected together, the current between them will produce a significant increase in magnitude. The trigger value that can indicate the occurrence of corrosion is related to the quality of the concrete and the surrounding environment such as chloride ion content, humidity, oxygen content, temperature and concrete resistivity. Therefore, the corrosion trigger current value of the entire structure will not be exactly the same. Different concrete components will have corresponding corrosion trigger values. In fact, the corrosion monitoring sensor is composed of several black steel sensors (4 or 6 anodes). These black steel anodes are installed at different depths below the concrete protective layer and are combined with the cathode. When the current of the outermost anode exceeds the trigger value, it means that the active material on the critical surface, especially chloride, has contacted the anode, and corrosion has begun. When the current of the second outermost anode exceeds the trigger value, it means that the active material in the critical plane begins to contact this anode, and the reinforced concrete at the corresponding depth of the protective layer begins to corrode. Correspondingly, the corrosion front will be pushed into the concrete protective layer one after another.
When the quality of concrete is very good, just like the Great Belt Link project, it often takes many years for the sensor closest to the outside to corrode and detect corrosion current. Related to the long-term budget for maintenance costs is to predict the time when the steel bars will corrode as soon as possible. Register the corrosion time of the corrosion sensor and establish a mathematical reasoning model related to the service life and corrosion caused by chloride ion intrusion, and this correlation can be known.
3 Monitoring
3.1 The Great Belt Link
Great Belt Link was built in 1989 and put into operation in 1998. The sea passage consists of two bridges and a tunnel. They are called West Bridge, East Bridge and East Tunnel. The West Bridge consists of two parallel continuous box beams approximately 6.6 kilometers long, one is a highway bridge and the other is a railway bridge. The caissons, bridge shafts and beams are all prefabricated in concrete, while the connectors are all cast-in-place concrete. The East Bridge is a suspension bridge, including the approach bridge is about 6.8 kilometers long. The caisson, bridge shaft, anchor pier and tower are all reinforced concrete structures, partially prefabricated and partially cast-in-place. The East Tunnel consists of two parallel tunnel pipes approximately 7.5 kilometers long. The lining of the tunnel pipes are prefabricated with concrete. With two ends, the total length of the tunnel is close to 8 kilometers, and the deepest immersion in the sea water exceeds 75 meters.
Figure 3 The great belt Link
The durability requirement of Great belt link is 100 years. This includes comprehensive consideration of structural layout, material composition, execution quality, construction and maintenance strategies. It is difficult to achieve a service life of 100 years without considering maintenance and repair as the main part in structural design and operation. Corrosion detection will serve as a very important tool to confirm the signs of quality degradation and take preventive measures before quality degradation.
The Great Belt Link's corrosion monitoring uses a total of 446 sets of corrosion sensors, of which 180 are installed on the axle and beam of the West Bridge, 42 are installed on the tower, axle and anchor of the East Bridge, and 225 are installed on the The interface of the east tunnel and the inner and outer ring lining of the protective layer.
The corrosion sensor used by The Great Belt Link is the anode ladder of German S + R sensortech company. The anode ladder is composed of 6 sections of black steel anode and temperature sensor (PT-1000), and the cathode is a section of titanium rod. At the same time equipped with ERE10 reference electrode of Denmark FORCE Technology company. The sensor is installed on critical weak sections such as structural connections and components that are difficult to reach or difficult to carry out inspection work. The sensor was installed in the East Tunnel from 1989-1991.
Figure 4 Trapezoidal corrosion sensor
3.2 Øresund Bridge
The Øresund Bridge was built in 1995 and completed in 2000. The bridge includes a cable-stayed bridge and a tunnel connecting Copenhagen and Malmo, Sweden ’s third largest city. The cable-stayed bridge is 7.8km long and uses reinforced concrete as the foundation. All caissons, bridge piers and pier shafts are prefabricated, and the towers are cast-in-place. The total length of the concrete tunnel is close to 4 kilometers, and the length of the seabed is about 3.5KM.
Figure 5 The Oresund Bridge
A total of 249 sets of corrosion sensors are used in the concrete tunnel structure, of which 60 sets are used in the pier shafts and pylons of the cable-stayed bridge and the railway trough. Another 189 sets of sensors are used in the tunnel. The corrosion sensor used on the bridge is the anode ladder of the German S + R sensortech company. It has also been used on the Great Belt Link in the past. The reference electrode is ERE20 from Denmark FORCE Technology. The sensors in the bridge poles and towers are installed in the mass concrete parts and structural joints. These are the places that are exposed to the chloride ion erosion environment and are easy to corrode. The sensors in the railway trough are set at locations prone to carbonization corrosion and cracks that may occur during the construction phase. The corrosion sensor used in the tunnel is Corrowatch of FORCE Technology, Denmark. This sensor has 4 ordinary black steel anodes and one cathode using a high-purity titanium alloy mesh. The corresponding reference electrode is also Force ERE20. These sensors are placed on critical weak sections such as structural joints and components that are difficult to reach or difficult to carry out inspection work.
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Figure 6. Corrowatch corrosion sensor
3.3 Copenhagen Metro
The Copenhagen Metro was built in 1996-2003. The subway is located in the center of Copenhagen with a tunnel structure, and the suburbs with dikes or light rail. The underground part of the Copenhagen Metro is approximately 8 kilometers long. The tunnel is excavated by a shield. The lining is prefabricated reinforced concrete slabs. The interface and the upper cover are also reinforced concrete structures, similar to the NATM tunnel. Most subway stations are also supported underground with cross piles along with the excavation, which is also a reinforced concrete structure. The foundation and superstructure of the above-ground elevated railway are also reinforced concrete structures, and these generally face only low-risk corrosion of steel bars, unless the main roads are exposed to chloride ion corrosion due to the spray of deicing salt in winter.
Figure 7. Copenhagen Metro Tunnel
In 1997-2004, during the construction of the subway, a total of 147 sets of corrosion sensors were installed. The sensors are installed in several structures of the following shield tunnels, such as the interface, the upper cover, the slope, the cross piles, the cross wall, the elevated pier column and the bridge pier. The corrosion sensor of the Copenhagen Metro uses the DURMONTM durability monitoring system, which is a product of Corrosion Control Services Ltd. This corrosion sensor consists of four black steel anodes, a platinum-plated titanium gold cathode, a PT1000 temperature sensor and an ERE20 reference electrode. This sensor is installed inside the concrete structure, the anode is at different depths of the protective layer. As a complete corrosion monitoring system, a cable socket will be installed as part of the concrete together with the anode and cathode.
Figure 8. Copenhagen Metro, buried sensor Durmon
4. Testing and evaluation
The most typical instrument for measuring corrosion sensors is a multimeter, which can be used for electrochemical potential measurement, a zero-impedance current measurement, and an AC impedance resistivity measurement. A typical measurement includes the following:
1. Electrochemical potential test of sensor electrode (anode and cathode) and potential test of steel bar and reference electrode and steel bar and cathode;
2. Unfixed macro cell current test, measured 5-10 seconds after each anode and cathode are coupled. Once corrosion current is generated, this measurement will be used as a supplement to other fixed current tests within a few minutes to an hour;
3. Resistivity test of different sensors, resistivity test between reference electrode and rebar;
4. Concrete temperature test.
In the risk of corrosion, the first assessment is based on the current between the anode and the cathode, followed by other measurements. The potential difference between the anode and the cathode will have some small differences due to factors such as oxygen content and humidity, resulting in a small current, but the sensor at this time is still passive. Therefore, it is necessary to work out a critical value (trigger value) of the current value to indicate whether corrosion has occurred. Different types of concrete originals have their own corrosion trigger values, which vary with the actual environment, which means that a structure may have different trigger values.
The trigger value will be determined based on the actual measurement results of corrosion sensors constructed with different concrete. Before this basis is worked out, the occurrence of corrosion will be based on the outermost anode.
5. Experience
The large-scale offshore concrete structure project in Denmark is still in the initial service stage, and the corrosion sensor has not yet begun to corrode.
The corrosion monitoring probe is operating as expected under normal conditions. However, at some specific locations, we know that the potential of platinum-plated titanium gold (cathode) is lower than the anode potential of the corrosion sensor. In this case, the short-circuit current of the macro cell structure often obtains the opposite information. These conditions all occur in very humid or dense concrete structures with little or no oxygen. Sometimes the potential of the rebar at the same location will be higher than the potential of the corrosion sensor. The solution to this problem is to use rebar as the cathode. In addition, the adjacent platinum-plated titanium in an oxygen-rich environment can also be considered as a cathode, but at the same time, the resistivity of the concrete must be considered.
Under the premise of ensuring low impedance, the distance between the anode and the cathode in the concrete is often a few meters, but this does not happen in a very dense concrete structure. One of the problems mentioned above is very much. A typical case is a submarine concrete tunnel in the Great Belt Link where oxygen penetrates through the tunnel lining. The cathode potential of the macrocell installed on the outermost steel bar is lower than that of the corrosion sensor, but the macrocell installed on the innermost steel bar is normal.
6 Conclusion
Corrosion monitoring is an important supplementary method for traditional monitoring of concrete structures. It is suitable for concrete structures that require regular maintenance (predictive maintenance) measures and are difficult or impossible to detect and evaluate. Routine quality inspection provides a view on the obvious characteristics of concrete quality degradation. However, corrosion monitoring provides information on the location of corrosion development. Therefore, it is necessary to develop early warning methods before the concrete quality is obviously degraded
The most obvious advantage of the corrosion monitoring system is that it can test the characteristics that are very relevant to the risk of corrosion (such as chloride ion content, humidity, oxygen or other), that is, the time when corrosion begins at different depths below the exposed contact surface. Based on this consideration, corrosion monitoring is the most direct method of assessing corrosion among all methods. Through comprehensive corrosion monitoring and other nondestructive testing methods, an optimal method can be obtained to inspect or adjust the previously established operation and maintenance measures. Therefore, use all the collected information to test and upgrade the service life model, and use a unified and reasonable view method to ensure that the service life of the large-scale offshore concrete structures mentioned in this paper and the structures with high durability requirements in the future is not low. In 100 years.
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