How to Measure Reinforcement Corrosion in ConcreteCareBiBi
Suppose reinforced concrete is exposed to a harsh environment for a long time. The acidic substances s and capillaries will pass the passivation layer of reinforcement through the pore channels of concrete, resulting in the corrosion of reinforcement and decreasing the overall structure and stability of reinforced concrete. So, how to find the corrosion of concrete reinforcement?
What are the causes of reinforcement corrosion in concrete?
A reinforcement passivation layer in concrete
There are two reasons why reinforced concrete can be used for a long time and maintain good compression and stability.
First, the concrete freezing layer isolates the internal reinforcement from the external oxygen and acid substances, delaying the corrosion of the reinforcement.
Second, Due to the concrete containing Ca (OH) 2, the pH value is about 12. After the chemical reaction between Ca (OH) 2 and Fe in the reinforcement produces a 200-600mm dense oxide layer. The second protective layer further protects the reinforcement from electrochemical corrosion.
Failure of the reinforcement passivation layer in concrete
Electrochemical corrosion is the root cause of concrete reinforcement corrosion. Due to many parts of the reinforcement will produce a potential difference, it will occur below reactions in the case of water.
Anode reaction: 2fe-4e – → 2fe2 +;
Cathode reaction: O2 + 2H2O + 4E – → y4oh-
Comprehensive reaction: 2Fe + O2 + 2H2O → 2fe2 + + 4OH -; Fe2++2OH-→Fe(OH)2
In practice, the Cl – will not only affect the impermeability of concrete but also directly destroy the passive film of reinforcement in concrete, produce galvanic cells, and accelerate the reaction rate of the galvanic cell.
Detection of reinforcement corrosion in concrete
1) Physical detection
The resistance bar is to detect the remaining area of reinforcement. The corrosion of reinforcement will cause the change of reinforcement surface knot and then produce the change of reinforcement resistance value by reinforcement conductivity measured.
It needs to reserve a resistance probe early in reinforced concrete pouring and keep the corrosion uniform.
it is not suitable for the condition when the reinforcement corrosion is non-even;
It can not detect the corrosion level of reinforcement ;
Damage detection is mainly used for demolishing and evaluating dangerous buildings or buildings with severe reinforcement corrosion. From the visual observation, the concrete wrapped with reinforcement has cracked, and the reinforcement turned out or even broken.
Damage detection is commonly used for reinforced concrete structures to determine the reinforcement corrosion further. The damage detection is to use external force to destroy the detection points, take out the corroded reinforcement, and evaluate the corrosion status of the reinforcement through observation.
It can also evaluate the corrosion of the reinforcement by comparing the section loss rate and weight loss rate of the non-corroded reinforcement.
Directly display corrosion pictures.
The detection range is limited
Damage the stability of components.
Acoustic emission detection
The acoustic emission detection is that the partial expansion of reinforcement corrosion causes local concrete cracking. After the sound wave emitted by the acoustic emission, the device collides with reinforcement in different parts, the different reflected sound wave wavelength, the different reinforcement corrosion, and the different strength of the sound wave.
Due to the external interference of acoustic emission, positioning accuracy has some defects.
Eddy current detection
Eddy current detection is to place the electromagnetic equipment on the concrete component; the excitation current emitted by the electromagnetic device resonates with the infrasonic wave in the reinforcement.
Observe the abnormal electromagnetic field image caused by the corroded reinforcement after magnetic saturation, and determine the loss rate of reinforcement cross-sectional area through data conversion.
The X-ray detection is to photograph the X γ Radiographic photos to observe the corrosion of reinforcement directly. Infrared thermography analyzes the location and degree of reinforcement corrosion by measuring the temperature distribution on the concrete surface.
If too much radiation in a long time, it will harm the surrounding environment and body.
Optical fiber sensor
An optical fiber sensor is a kind of sensor made using the properties of the fiber strain, temperature and interface, the wavelength, energy flow density, frequency, polarization state, and phase of light change when photons propagate along the length direction in fiber.
There are two main methods to detect reinforcement corrosion by optical fiber:
one is to detect the environmental factors around reinforced concrete by optical fiber;
two is to detect the corrosion of reinforcement by the sensitivity of Bragg grating versus strain.
The detection method is based on the magnetic field.
The detection method based on a magnetic field is to use the ferromagnetic properties of the reinforcement, and it will cause the external magnetic field disturbance if the damage level of the reinforcement measures the defects of the reinforcement.
2) Electrochemical detection
It is to determine the level or speed of reinforcement corrosion in concrete by measuring the electrochemical property of the reinforced concrete corrosion system, including the half cell potential method, resistivity method, linear polarization method, AC impedance method, etc.
High sensitivity, fast speed, continuous tracking, and in-situ measurement.
Linear polarization method
The linear polarization method is to apply a small disturbance current to the reinforcement corrosion system, detects the corresponding potential change of the corrosion system, and gets the ratio RP of potential change and current change(polarization resistance).
Then, according to the basic formula of the linear polarization method (Stem Gear formula), we can measure the current density of reinforcement corrosion.
The simplest electrochemical method to quantitatively study the corrosion rate of reinforcement in concrete.
Simple and fast
It is suitable for laboratory and field detection.
Require high accuracy of the instrument.
The conductivity of concrete is an electrolytic process that occurs when ions flow in the pore liquid of cement paste. The resistivity of concrete is inversely proportional to the corrosion rate of reinforcement. We can know the corrosion of reinforcement by measuring the resistivity of reinforced concrete. The measurement of concrete resistivity must have two electrodes, one of which can be reinforcement.
The test measures the current by applying an external voltage, and the ratio of the two is the resistance of concrete. There are four-probe methods and two-stage methods according to the specific operation method.
Simple equipment and wide application range,
It is easy to be affected by environmental conditions,
The data measured may be highly discrete.
Half cell potential method
The half cell potential is to measure the potential difference between the electrode composed of reinforcement / concrete and the copper/copper sulfate reference electrode on the concrete surface to evaluate the corrosion of reinforcement.
The potential of the reference electrode is relatively stable, while the potential of the reinforcement / concrete electrode will change due to the corrosion of the reinforcement. Therefore, we can measure the corrosion condition of reinforcement by potential change.
A simple and effective electrochemical test method
Good applicability in laboratory and field.
Only judge the possibility of reinforcement corrosion qualitatively,
It can not measure the corrosion rate of reinforcement quantitatively.
How to measure rebar corrosion with a corrosion tester?
The half cell potential method is commonly used in the detection of reinforcement corrosion. Corrosion of reinforcement in concrete is an electrochemical process that will produce an anode area and a cathode area on the surface of the reinforcement.
An electric current will be generated inside the concrete between these areas with different potentials. The electrical activity of steel bar and concrete can be regarded as a half weak battery pack, and the steel is an electrode, while concrete is an electrolyte.
The half cell potential method uses “Cu + CuSO4 saturated solution” and “reinforcement + concrete” to produce a cell system. Because the potential value of “Cu + CuSO4 saturated solution” is relatively constant, the chemical reaction of reinforcement in concrete is due to corrosion. So we can evaluate the corrosion state of reinforcement by measuring the potential value.
First, prepare Cu + CuSO4 saturated solution. The principle of the half cell potential method requires concrete to become electrolyte, so we must wet the surface of the reinforced concrete structure in advance.
The concrete structure surface is wetted with a liquid composed of 95ml domestic liquid cleaner and 19L drinking water. During testing, keep the concrete wet, but there is no free water on the surface.
One end of the corrosion tester is in contact with the concrete surface, and the other end is connected with the steel bar. During connection, it is required to polish the reinforcement surface to remove rust spots. According to the test principle of the half cell potential method, to ensure that the circuit is closed and the resistance of reinforcement is small enough, we need to use a voltmeter to check the resistance between any two reinforcements in the test area is less than one before the test.
During the testing, we need to determine the measuring line and point according to the reinforcement distribution, and the spacing of the measuring points should be 10-20cm. Read the potential value of each measuring point on each measuring line one by one with the corrosion tester, then record the measuring point value when the potential value remains stable for at least 5min,
1)The measuring area selection should be according to the components’ environmental differences and the appearance inspection results. The measuring area should represent different environmental conditions and different corrosion appearance characteristics. The number of measuring areas should not be less than 3 in each condition.
2)The test grid should be arranged on the measured area. The grid nodes are testing points, and the grid spacing is 200 mm × 200 ㎜、300 ㎜ × 300 mm or 200 mm × L00mm, etc., depending on component size and instrument. The number of measuring points should not be less than 20. The distance between the testing point and the edge of the component should be more than 50mm.
3) The measuring area should be numbered, located, and described its appearance.
There are many detections of reinforcement corrosion in concrete, and they are developing continuously. In the testing process, it is suggested to first master the factors affecting the corrosion of reinforcement in the concrete and environment, then choose the right way to detect the corrosion of reinforcement according to actual need.