JP2016188850A - Concrete cracks detection method and cracks sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately detect cracks of concrete in an early stage, using an optical fiber sensor.SOLUTION: There is provided a method for detecting cracks of concrete, including the steps of: fixing an optical fiber sensor on a surface of a steel material in concrete; measuring distortions of the steel material based on the change of characteristics of a light wave propagating in the optical fiber sensor; and detecting cracks of the concrete based on the change of characteristics with time of the measured distortions.SELECTED DRAWING: Figure 11B

Description

  The present invention relates to a crack detection method and a crack sensor for concrete.

  When cracks occur in structures such as RC (Reinforced-Concrete) and SRC (Steel Reinforced Concrete) structures, the structural performance of the structure is greatly reduced and the cover concrete is peeled off, causing third party damage. I will let you. Therefore, detecting the occurrence of cracks is extremely useful for the maintenance of structures. Concrete cracks are caused by reinforcing steel corrosion, drying shrinkage, temperature stress and external force. In particular, since the volume of corrosion products generated due to corrosion of reinforcing bars increases as compared with iron, the vicinity of the corroded portion expands in volume, which causes corrosion cracks in the concrete and often leads to the fall of the concrete.

  Conventionally, as a method of detecting a crack, a strain gauge is attached to a reinforcing bar or a surface in a concrete structure to detect the strain (Patent Document 1). On the other hand, conventionally, an optical fiber sensor for detecting strain generated in a structure has been proposed. For example, in Patent Document 2, an optical fiber sensor shaped in a spiral shape is attached to a structure to be measured, and a change in light propagation characteristics of the optical fiber sensor is measured by an electro-optical measurement device. Thereby, when a displacement arises in a structure, it is possible to measure the displacement without destroying the structure.

  In Patent Document 3, a tape member that is wound around a spacer member and wound spirally inside the concrete structure, and an optical fiber that is wound along the tape member are used. This makes it possible to detect shear cracks in the concrete member.

Japanese Patent No. 4975420 JP 2000-097647 A Japanese Patent No. 4008623

  In order to measure the strain of concrete, when a strain gauge is attached directly to a steel material, or when a strain gauge is installed in the vicinity of the steel material, the measuring instrument or adhesive covers the steel material. This may affect the generation and may not be able to detect accurate corrosion strain. Furthermore, when a strain gauge is installed on the concrete surface, it may be peeled off or broken by wind and rain, making it impossible to measure, or even if measurement is possible, correct results may not be obtained.

  On the other hand, the method of directly winding an optical fiber around a reinforcing bar in a structure is to detect the damage and deformation of the structure by disposing the optical fiber over the entire structure, and not to detect a crack.

  This invention is made | formed in view of such a situation, and it aims at providing the crack detection method and crack sensor which detect the crack of concrete early and correctly using an optical fiber sensor.

  (1) In order to achieve the above object, the present invention takes the following measures. In other words, the crack detection method of the present invention is a crack detection method for detecting cracks in concrete, the step of fixing an optical fiber sensor on the surface of a steel material in the concrete, and a light wave propagating in the optical fiber sensor. A step of measuring strain of the steel material based on the property change, and detecting cracks in the concrete based on the property of the time-dependent change of the measured strain.

  As described above, since the strain of the steel material is measured using the optical fiber sensor, it is possible to grasp the characteristics of the measured strain over time and to detect cracks in the concrete. In other words, the steel material that has been constrained by concrete is released from the stress due to rebar corrosion and drying shrinkage due to the occurrence of cracks, so that the restraint is weakened, so the behavior of strain changes before and after the occurrence of cracks. By measuring this strain with an optical fiber sensor, it is possible to detect at an early stage the occurrence of cracks that reach the reinforcing bar such as corrosion of the reinforcing bar or drying shrinkage. As a result, the concrete can be prevented from falling, and further deterioration of the structure can be prevented by carrying out repairs as soon as possible. Appropriate maintenance of concrete structures can be performed.

  (2) Further, the crack detection method of the present invention is characterized in that the bending point appearing in the time-dependent change in the measured strain is when a crack of concrete occurs.

  With this configuration, it is possible to grasp the timing of occurrence of cracks in concrete. That is, in a concrete structure, the behavior of strain changes abruptly before and after the occurrence of a crack, so the inflection point of the strain curve that appears here can be recognized as the occurrence of a crack in the concrete. In addition, if a bending point is extracted by numerical analysis, the occurrence of a crack can be automatically detected.

  (3) Moreover, the crack detection method of the present invention is characterized in that the crack of the concrete is caused by corrosion expansion of a steel material in the concrete.

  With this configuration, the optical fiber sensor detects the increase in strain due to the corrosion expansion of the steel material before the occurrence of cracking, so it is possible to easily detect fluctuations in strain behavior due to cracking and to accurately detect the occurrence of cracking. it can.

  (4) Further, the crack detection method of the present invention is characterized in that a plurality of the optical fiber sensors are fixed in a certain range of the measurement object.

  With this configuration, when cracks occur, the strain behavior of multiple optical fibers around the cracks changes, so it is possible to discriminate between noise and failures that have occurred in some optical fibers, fine cracks, and through cracks. Is possible.

  (5) Moreover, the crack detection method of this invention installs the dummy sensor which fixed the optical fiber sensor in the concrete in the location processed so that the said steel materials may not corrode, and it is a factor other than corrosion by the said dummy sensor. The generated strain is detected, and the strain detected by the dummy sensor is used to correct the strain of the steel material.

  With this configuration, it is possible to remove the influence of strain caused by factors other than corrosion, such as temperature strain, using the strain detected by the dummy sensor, and more accurately detect the occurrence of cracks. .

  (6) Moreover, the crack sensor of this invention is a crack sensor applied to the crack detection method in any one of said (1) to (5), Comprising: An optical fiber sensor and the said optical fiber sensor are hold | maintained A characteristic of the light wave propagating through the optical fiber sensor due to the occurrence of the crack.

  With this configuration, since the strain of the steel material is measured using an optical fiber sensor, it is possible to grasp the characteristics of the measured strain over time and to detect cracks in the concrete.

  According to the present invention, corrosion of a steel material can be detected, the progress of corrosion can be detected, and the occurrence of cracks can be detected, which is useful for maintenance and management of concrete.

It is a figure which shows Embodiment 1 which concerns on this invention. It is a figure which shows Embodiment 2 which concerns on this invention. It is a figure which shows Embodiment 3 which concerns on this invention. It is a figure which shows Embodiment 3 which concerns on this invention. It is a figure which shows Embodiment 4 which concerns on this invention. It is a figure which shows Embodiment 5 which concerns on this invention. It is a figure which shows Embodiment 6 which concerns on this invention. It is a figure which shows the test body used in order to verify whether the concept of the corrosion detection method which concerns on this embodiment detects a distortion | strain by an optical fiber sensor by metal corrosion. It is a graph which shows the relationship between the elapsed time and distortion in a corrosive environment in the verification example made into 1 volume. It is a graph which shows the relationship between the elapsed time and distortion in a corrosive environment in the verification example made into 3 windings. It is a figure which shows the outline | summary of an electric corrosion test. It is a figure which shows the outline | summary of the polishing bar steel 160 around which the optical fiber sensor 161 was wound. It is a figure which shows the elapsed time of an electric corrosion test, and the distortion of each measuring point. It is a figure which shows the elapsed time of an electric corrosion test, and the distortion of each measuring point. It is a figure which shows the elapsed time of an electric corrosion test, and the distortion of each measuring point. It is a figure which shows the elapsed time of an electric corrosion test, and the distortion of each measuring point. It is a figure which shows the elapsed time of an electric corrosion test, and the distortion of each measuring point.

  The method for detecting cracks in concrete according to an embodiment of the present invention is a method in which an optical fiber sensor is attached to a steel material such as a reinforcing bar in concrete, the strain of the concrete steel material is measured, and the behavior change of the strain at the time of crack occurrence is captured. It is. In other words, the steel material that has been constrained by concrete is released from the stress due to the occurrence of cracks, so that the restraint is weakened, and the behavior of strain changes before and after the occurrence of cracks. Therefore, the crack detection method for concrete according to the embodiment of the present invention can detect the occurrence of cracks reaching the reinforcing bars due to corrosion, drying shrinkage, temperature stress, external force, etc. at an early stage.

  The optical fiber sensor in the concrete crack detection method according to the embodiment of the present invention can also detect the strain environment of the steel material itself. For example, when steel material corrodes, the distortion by the volume expansion of a corrosion product arises. By detecting this strain with a measuring instrument such as a data logger via an optical fiber sensor, whether the concrete interior is in a corrosive environment, whether a corrosive factor has entered, and the progress of the corrosion status of the steel Can be captured. After that, steel corrosion progresses, and changes in strain behavior due to cracking are captured.

[Embodiment 1]
FIG. 1 is a diagram showing Embodiment 1 according to the present invention. In the first embodiment, the optical fiber sensor 62 is directly wound around the round steel bar 60. As a result, the optical fiber sensor 62 can be provided directly on the steel material without processing the steel material. As a result, early detection of concrete cracks becomes possible. In addition, it is possible to detect the progress of strain due to corrosion of reinforcing bars.

  As the optical fiber sensor, an FBG sensor or the like can be used. When the optical fiber sensor 62 is wound around the round steel 60, the optical fiber sensor 62 may be arranged in a straight line or bend in a wave shape, but preferably spiral so as to circulate, Or wrap it in a loop. When capturing the progress of strain due to corrosion, the more the number of turns, the more the corroded portion and the optical fiber overlap, so that detection is performed early. However, if the number of turns is too large, the arrival of the corrosion factor to the round steel 60 will be hindered.

  When the optical fiber sensor 62 is wound around the round steel 60, it is wound so as to be in close contact with each other, and preferably a tensile force is applied, and both ends are fixed with an adhesive. Thereby, both the expansion side and the contraction side strain can be measured. In order to detect cracks with high accuracy, the length of the detection section of the optical fiber sensor with both ends fixed is preferably 5 mm to 500 mm, although it depends on the conditions of the structure and the steel material. When the length of the optical fiber sensor is 500 mm or more, it is difficult to handle the steel fiber when it is installed, and the possibility of damage during placement becomes large. Moreover, since the measurement range is narrow when the length of the optical fiber sensor is 5 mm or less, the detection accuracy is poor. After that, concrete is placed and the strain is detected by measuring the strain with a data logger. In addition, although it was set as the round bar 60 here, the said arrangement | positioning can also be performed to a deformed bar.

[Embodiment 2]
FIG. 2 is a diagram showing Embodiment 2 according to the present invention. In the second embodiment, a deformed reinforcing bar 65 is used. In FIG. 2, the deformed reinforcing bar 65 is provided with ribs 66 at positions above and below the paper surface. The deformed reinforcing bar 65 is provided with a plurality of nodes 67 protruding from the surface and provided at a predetermined interval. In the second embodiment, the optical fiber sensor 62 is fixed to the top portion of the node 67. When the optical fiber sensor 62 is fixed, it is fixed at two adhesion points 69 as shown in FIG. The adhesion point may be provided after the surface of the reinforcing bar is cut and polished so as to adhere firmly. Moreover, the location where the optical fiber sensor 62 is fixed is installed toward the surface of the concrete. In this manner, since the optical fiber sensor 62 is fixed to the top portion of the node 67, it can be brought close to the concrete surface on which cracking occurs. Thereby, it becomes possible to detect a crack at a relatively early stage.

[Embodiment 3]
3A and 3B are diagrams showing Embodiment 3 according to the present invention. FIG. 3A is a diagram illustrating the deformed reinforcing bar 65 from the direction in which the rib 66 is positioned above and below the paper surface, and FIG. 3B is a diagram illustrating the deformed reinforcing bar 65 from any one direction of the rib 66. In the third embodiment, the optical fiber sensor 62 is continuously arranged with respect to the plurality of nodes 67. As shown in FIG. 3A, in each node 67, the optical fiber sensor 62 is bonded at two bonding points 69, respectively. As described above, by arranging the optical fiber sensors 62 at the plurality of nodes 67, a portion where rust is generated and a portion where no rust is generated at the initial stage when the corrosive environment is generated. Then, it becomes possible to discriminate between distortion caused by corrosion and distortion not caused, that is, distortion caused by temperature or external force, thereby eliminating the influence of distortion caused by temperature or external force. As a result, it is possible to detect only the strain due to the start of corrosion with high accuracy.

[Embodiment 4]
FIG. 4 is a diagram showing Embodiment 4 according to the present invention. In the fourth embodiment, the optical fiber sensor 62 is disposed along the rib 66 facing the surface of the concrete. This enables early detection of concrete cracks. In addition, it is possible to detect corrosion expansion caused by a corrosion factor that has entered from the concrete surface at an early stage. In addition, it is possible to increase the adhesive force by removing the black skin and rust of the adhesion point 69, and to extend the measurement period by applying a rust prevention treatment to the adhesion point 69.

[Embodiment 5]
FIG. 5 is a diagram showing Embodiment 5 according to the present invention. In the fifth embodiment, the coating layer 72 is provided between the deformed reinforcing bar 65 and the optical fiber sensor 62. The coating layer 72 is composed of a hardened cement body such as mortar. The mortar may be made with a blend of water cement ratio similar to that of concrete, or may be made with a more porous blend. This is because, when the coating layer 72 is provided between the deformed reinforcing bar 65 and the optical fiber sensor 62, if the hardened cement body is soft, strain transmission to the deformed reinforcing bar 65 due to cracking or corrosion of the optical fiber sensor 62 is delayed. It is.

  Also, the water cement ratio should be equal to or higher than that of concrete in reinforced concrete structures so that it is in the same environment as the surroundings as much as possible and does not hinder the passage of corrosion factors from the concrete surface to the reinforcing bars. It is preferable that The covering layer 72 preferably has a thickness of 3 to 15 mm so that the irregularities on the surface of the deformed reinforcing bar 65 are eliminated and are not damaged. However, the present invention is not limited to these.

  As described above, since the coating layer 72 is provided and the unevenness can be eliminated regardless of the location of the deformed reinforcing bar 65, the optical fiber sensor 62 can be disposed at any location. Moreover, since the whole site | part including a mortar expand | swells when the deformed reinforcing bar 65 corrodes, it becomes easy to detect the displacement by the corrosion start including the corroded place.

[Embodiment 6]
FIG. 6 is a diagram showing Embodiment 6 according to the present invention. In FIG. 6, the left side of the paper surface has a configuration in which the deformed reinforcing bar 65 is provided with the covering portion 74, but is characterized in that the optical fiber sensor 62 is wound in a state where the mortar is not cured. In this case, the optical fiber sensor 62 is sunk into the mortar. Thus, the mortar can also function as an adhesive, and the adhesive point 69 may not be provided or temporarily fixed.

  On the other hand, in FIG. 6, the right side with respect to the paper surface is a mode in which a covering portion 74 is formed with a hardened cement body at a portion where the optical fiber sensor 62 is fixed. As to the method of fixing the optical fiber sensor 62, any one of the first to fifth embodiments may be used. It is possible to make the mortar function as an adhesive, and the adhesive point 69 may not be provided or may be temporarily fixed.

  As the cement hardened body of the present embodiment, a cement paste or mortar that is alkaline and becomes porous after solidification can be used. The water-cement ratio of the hardened cement should be equal to or higher than that of concrete in the reinforced concrete structure so that it is in the same environment as the surroundings as much as possible and does not hinder the passage of corrosion factors from the concrete surface to the rebar. A cement ratio is preferable. The cover 74 preferably has a thickness of 3 to 15 mm so as not to break.

  Thereby, the cement hardened body constituting the covering portion 74 has a function of fixing the position of the optical fiber sensor 62 after manufacture, preventing corrosion of reinforcing bars until the time of placing, and protecting the optical fiber sensor 62 at the time of placing. Fulfill.

  In addition, although the coating layer 72 and the coating | coated part 74 of a cement hardening body have made the deformed reinforcing bar 65 into the example, this invention is not limited to this, Between the round steel 60 and the optical fiber sensor 62, the coating layer 72 or A covering portion 74 may be provided.

  In all the above embodiments, a dummy sensor can also be used. That is, an optical fiber sensor is manufactured in the same manner as in the above embodiment, but a dummy sensor having a rust-proofing treatment on the entire surface. Then, a strain caused by factors other than corrosion may be detected using a dummy sensor, and the strain detected by the optical fiber sensor may be corrected. Thereby, for example, it becomes possible to remove the influence of temperature distortion and the like. As for the dummy sensor, there is a method in which the reinforcing bar or the covering portion where the optical fiber sensor is arranged is coated with an epoxy resin or the like to prevent neutralization or deterioration factors from entering and to prevent corrosion of the internal carbon steel. Further, when a plurality of detection units are provided as in the third embodiment, a part thereof may be subjected to rust prevention treatment.

[Detection method]
The optical fiber sensor is installed in an actual structure or product before placing concrete in the above-described form. Or, it is attached to a simulated member having the same concrete composition and cover thickness as the actual structure targeted for the optical fiber sensor, and cracks are generated by installing the simulated member in the same environment as the actual structure targeted. The situation and the progress of strain due to corrosion may be referred to.

  The optical fiber can be arbitrarily installed at a location where the structure is desired to be measured. For example, the optical fiber is installed at a location where corrosion is expected to occur or a location where stress cracking is likely to occur due to drying or temperature change. Moreover, since it is assumed that noise, failure, or the like due to fine cracks or the like occurs in some of the optical fibers, it is preferable to provide a plurality of optical fibers at a location to be measured. As will be described later, when cracks occur, the strain behavior of a plurality of optical fibers around the cracks changes, so that it is possible to discriminate noise and the like. The range in the case where a plurality of optical fibers are provided at a location to be measured is preferably within 50 cm, although it depends on the structure. In this embodiment, if an optical fiber is wired, it can be applied to a place where a person cannot enter or a place where it cannot be visually observed. Moreover, you may enable it to monitor data in a remote place.

  If a crack occurs during measurement, the stress on the steel or corrosion product that was previously constrained by the concrete is released, and the restraint is weakened. Arise. The inflection point can be easily detected visually by expressing the amount of strain with respect to the elapsed time in the figure (strain curve). The shorter the strain measurement interval, the more accurately it can be detected, and it is preferably shorter than 30 minutes.

  Alternatively, a system that performs numerical processing, automatically detects the inflection point of the strain curve due to the occurrence of a crack, and can be notified. What is necessary is just to obtain a regression equation for the strain curve and detect a sudden change in the gradient. Alternatively, the strain curve is regressed as a polynomial curve, the tangent slope at the measurement point is obtained, and the time when the absolute value fluctuates can be recognized as the occurrence of a crack.

When the occurrence of a crack is detected, it is possible to immediately take measures to prevent the concrete from falling or to repair and reinforce the concrete. In particular, the expansion strain due to corrosion can be predicted by the optical fiber sensor, so that the occurrence of cracks can be predicted.

[Verification example]
FIG. 7 is a view showing a test body used for verifying whether the optical fiber sensor detects the strain due to the corrosion of the metal, based on the concept of the corrosion detection method according to the present embodiment. As shown in FIG. 7, the test body 40 is configured by winding an optical fiber sensor 51 around a steel bar 50 and covering with a hardened cement body 52 such as mortar. With respect to the test body 40, the vertical fog is 20 mm, and the horizontal fog is 10 mm. In FIG. 7, an example in which the optical fiber sensor 51 is wound once with respect to the steel bar 50 is shown. Next, a specific verification example of the test body 40 is shown.

  JIS G 3108 SGD3M was used as the polished steel bar. The method of winding the optical fiber around the polished steel bar is under certain tension, for example, after confirming that some tensile strain has occurred during winding, and then winding the wire with CN (manufactured by Tokyo Sokki). Fixed. An optical fiber sensor (FBG sensor) having the specifications shown in Table 2 was used. In addition, it is preferable to detect corrosion based on the difference in strain behavior of the dummy sensor, and it is expected that the influence of volume change and moisture content of the coated mortar will appear in the strain. The body was made.

  Next, the covering mortar will be described. The materials used for the mortar are as shown in the following table.

  Next, the composition of the mortar is as shown in the following table.

In the above table, “B” is a mixture of “C” and “L”.

[Mortar mixing method]
The mortar used for the test body was kneaded using a “Maruto Seisakusho mortar mixer (2 L kneading)”. The mixing procedure is as follows. The mortar was kneaded in a constant temperature and humidity chamber at 20 ± 2 ° C. and a humidity of 50% or more.

  Put mortar into a PVC mold (inner diameter φ40 mm × height 50 mm, or inner diameter φ40 mm × height 65 mm), put a steel bar wrapped with optical fiber in the center, and then in the same constant temperature and humidity chamber After curing for 3 hours, it was cured at 20 ° C. and a humidity of 95% or more for 7 days and demolded.

  Specimen 40 under corrosive environment (temperature 40 ° C., immersed in NaCl: 10% aqueous solution for 1 day, humidity 60% dried for 3 days, again immersed in NaCl: 10% aqueous solution for 1 day, and thereafter dried at 60% humidity) The change in wavelength was measured using a measuring instrument (manufactured by Watanabe Seisakusho Co., Ltd.) In the immersion of the NaCl: 10% aqueous solution, a portion of 30 mm from the lower end of the test specimen (45 mm from the lower end of the test specimen in the case of three windings) was so immersed that the NaCl aqueous solution did not enter from the optical fiber lead-out portion. . The dummy specimen used here was the same specimen as the corrosion sensor without using a steel bar that does not corrode experimentally, and was immersed in pure water that does not corrode instead of using the NaCl aqueous solution.

  Using the following equation, the wavelength was converted to strain, and the change in strain due to corrosion was confirmed.

Here, ε: strain (μ), λ: wavelength at the time of measurement (nm), and λ ^* : initial wavelength (nm).

  FIG. 8A is a graph showing the relationship between elapsed time and strain in a corrosive environment. In the verification example with one winding shown in FIG. 8A, since the sample was immersed in salt water or water on the 1st and 4th, the strain temporarily changed due to temperature change or mortar water absorption, but the dummy specimen also changed in the same way. This is not due to corrosion. On the 16th, the strain amount of the test specimen and the dummy test specimen deviated. Then, when the covering mortar of the test body was removed, it was confirmed that the steel bar was corroded.

  The verification example with 3 windings shown in FIG. 8B is an example in which measurement is started after the temperature becomes constant and measurement is continued after corrosion. Corrosion cracks occurred on the 45th measurement day, and expansion stopped. It can be seen that the increase in strain curve stopped due to the occurrence of cracks, and the inflection point appeared. In addition, it was proved that expansion due to corrosion can be continuously detected until cracking occurs.

  From this verification example, it was proved that the optical fiber sensor that detects the strain wound around the iron member detects the strain caused by corrosion of the iron member and the occurrence of cracks.

[Example]
FIG. 9 is a diagram showing an outline of the electric corrosion test. A steel bar 160 having a diameter of 30 mm and a length of 350 mm is used. The optical fiber sensor 161 is wound, and the cable 162 is connected and embedded in the concrete 164. At that time, the horizontal fog is set to 135 mm equally on the left and right, and the depth fog is set to 50 mm from the upper end and 220 mm from the lower end. This is used as a specimen 166, and the specimen 166 is submerged in a container 169 having an internal method of 310 mm, and a copper plate electrode 168 as a cathode material is provided at a position 10 mm away from the specimen in water. The copper plate electrode 168 has a width of 100 mm, a length of 300 mm, and a cable 170 is connected thereto. A waterproof gauge 172 is installed on the specimen 166 at a position facing the copper plate electrode 168.

  In addition, the concrete made 60% of water cement ratio using the early strong Portland cement. After placing the concrete, the casting surface was covered with a poultice, cured at 20 ° C. for 24 hours, and then demolded. Thereafter, wet curing was performed for 22 days.

  FIG. 10 is a diagram showing an outline of the polished steel bar 160 around which the optical fiber sensor 161 is wound. Of the polished steel bar 160, the 20 mm portions at both ends are outside the concrete, and the other is in the concrete. A portion in the concrete is divided into sections 1 to 8, and the strain measurement by the optical fiber sensor 161 is performed at the measurement points corresponding to the sections 1 to 8 in each section. That is, in the polished steel bar 160, the portion in the concrete is 310 mm from (350 mm-40 mm).

  The method of winding the optical fiber sensor 161 is as follows. That is, winding is started from a portion 20 mm from the end in the concrete, and is wound so as to advance 25 mm in the longitudinal direction of the polished steel bar 160 every round. The FBG sensor was wound with both ends of the measurement unit fixed with an adhesive so that each FBG sensor was positioned at the center of 25 mm. Each section has an interval of 10 mm. As a result, in the steel bar 160 in the concrete, there are two 20 mm portions from both ends, eight 25 mm portions around which the optical fiber sensor is wound once, and seven 10 mm portions as intervals between the sections, for a total of 310 mm. It has become. Further, the FBG sensors were individually connected to the measuring machine without being arranged in series so that all the FBG sensors would not be able to be measured when the optical fiber was disconnected or measurement was impossible.

  In the state shown in FIGS. 9 and 10, a constant current output device is connected to the cables 162 and 170, and a current of 0.2 A is applied. In addition, the strain is measured by the optical fiber sensor 161. The measurement was performed until the age of 26 days when the surface of the concrete was cracked by corrosion of the reinforcing bars.

  FIG. 11A and FIG. 11B are diagrams showing the elapsed time of the electric corrosion test and the strain at each measurement point. As described above, it is also possible to measure the process in which the volume expands due to the progress of corrosion of the steel material by the detection method according to the present invention. Furthermore, at the age of 25.7 days when cracks were generated due to corrosion of the reinforcing bars, the strain curve rapidly changed and the bending point appeared, and the occurrence of cracks could be captured.

  Cracks first occurred on the surface side of the measurement point, and then the crack length increased. In FIG. 11, the strain amount was the largest at the measurement point, and the crack width was the largest in visual observation after the crack. There were many other measurement points where the strain increased sharply, and conversely, there were some measurement points that began to decrease, but in each case a bending point was confirmed. Inflection points appeared almost simultaneously at the measurement points around 30 cm around where cracks occurred. As shown in FIGS. 12A and 12B, even at the measurement point 7 and the measurement point 8 where the strain variation seemed to be smallest before and after the crack, the gradient of the strain curve showed a difference of 10% or more before and after the crack. In addition, by installing a plurality of optical fiber sensors in the vicinity, only one bending point appears, such as the measurement point 2 at the age of 20.5 days and the measurement point 1 at the age of 22.8 days in FIG. If not, it can be determined that it is not a through crack.

In addition, when detecting the occurrence of cracks by performing numerical processing, for example, in this embodiment, a regression equation of a strain curve is created at every 10 measurement points, and the crack is detected when the gradient of the regression equation fluctuates by 10% or more. It can be detected as an occurrence. At this time, it can be detected more accurately by using a state in which there is no strain fluctuation due to temperature and external force, or a value corrected by a dummy sensor or the like. The amount of strain varies depending on the situation because the degree of strain gradient fluctuation before and after the crack varies depending on the degree of restraint of the measurement point, concrete strength, cover thickness, distance from the cracking location, measurement interval, etc. What is necessary is just to set the conditions of the general process.

  As described above, according to the present embodiment, early detection of concrete cracks is possible. In addition, it is possible to measure the strain up to cracking.

40 Specimen 50 Steel bar 51 Optical fiber sensor 52 Cement hardened body (mortar)
60 Round Steel 62 Optical Fiber Sensor 65 Deformed Reinforcement 66 Rib 67 Node 69 Bonding Point 72 Covering Layer 74 Covering Portion 160 Polished Bar Steel 161 Optical Fiber Sensor 162 Cable 164 Concrete 166 Specimen 168 Copper Plate Electrode 169 Container 170 Cable 172 Waterproof Gauge

Claims (6)

A crack detection method for detecting cracks in concrete,
Fixing the optical fiber sensor to the surface of the steel material in the concrete;
A step of measuring strain of the steel material based on a characteristic change of a light wave propagating in the optical fiber sensor;
A crack detection method characterized by detecting cracks in concrete based on the characteristics of the measured strain over time.   The crack detection method according to claim 1, wherein the bending point appearing in the time-dependent change in the measured strain is defined as a crack in the concrete.   The crack detection method according to claim 1 or 2, wherein the cracks in the concrete are caused by corrosion expansion of a steel material in the concrete.   The crack detection method according to any one of claims 1 to 3, wherein a plurality of the optical fiber sensors are fixed in a certain range of the measurement target.   A dummy sensor in which the optical fiber sensor is fixed is installed inside the concrete at a place where the steel material is not corroded, and the dummy sensor detects strain caused by a factor other than corrosion of the steel material, and the dummy The crack detection method according to any one of claims 1 to 4, wherein the strain of the steel material is corrected using strain detected by a sensor. A crack sensor applied to the crack detection method according to any one of claims 1 to 5,
An optical fiber sensor;
A steel material for holding the optical fiber sensor,
A crack sensor characterized in that a change occurs in characteristics of light waves propagating through the optical fiber sensor due to the occurrence of the crack.