Strain monitoring method and prestressed concrete structure

Abstract

To enable monitoring of local distortion.SOLUTION: In a distortion monitoring method for monitoring distortion inside a prestressed concrete structure, an optical fiber sensor is installed between a surface of the prestressed concrete structure and steel material, and the distortion inside the prestressed concrete structure is detected on the basis of a characteristic change of a light wave propagating in the optical fiber sensor. Here, a distance between the surface of the prestressed concrete structure and the optical fiber sensor is 10 mm or more and less than 200 mm.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] This invention relates to a method for monitoring strain inside a prestressed concrete structure, and to a prestressed concrete structure itself. [Background technology]

[0002] In concrete structures such as reinforced concrete (RC) and steel reinforced concrete (SRC), cracking significantly reduces the structural performance of the concrete structure and can lead to spalling of the concrete cover. This can result in damage to third parties. Therefore, detecting concrete damage, including the occurrence of cracks, is extremely important for the maintenance and management of concrete structures. Causes of concrete cracking include rebar corrosion, drying shrinkage, thermal stress, and external forces. In particular, when external forces such as earthquakes or ground subsidence are applied, strain occurs in each part. Under such circumstances, monitoring the strain that occurs in each part of the concrete structure is extremely important.

[0003] Concrete generally has an elastic range, meaning it can contract or expand. However, when a concrete structure is subjected to a large external force exceeding its elastic limit, such as an earthquake or ground subsidence, cracks and other damage occur. The strain generated in each part of the concrete structure does not return to its original state, resulting in significant residual strain. When such significant residual strain occurs in a concrete structure, the structure is considered damaged.

[0004] In concrete structures, damage often manifests as surface cracks, but these are difficult to visually inspect, especially in buildings with finishing materials. Furthermore, many parts of concrete structures, such as internal cracks, cannot be visually inspected.

[0005] Conventionally, methods for measuring strain inside concrete structures include embedded strain gauges and attaching strain gauges to internal reinforcing bars. Patent Document 1 discloses a structural health monitoring system using optical fiber sensors that can easily support the construction management and maintenance management of structures. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2011-132680 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Incidentally, in prestressed concrete structures, a decrease in prestress can easily lead to localized strain near the surface of the prestressed concrete structure and around the PC steel bars. Localized strain refers to the concentration of large strain in certain parts of the concrete due to the heterogeneity of the concrete material, which can cause cracking. Therefore, understanding the occurrence of localized strain is extremely important for maintaining prestressed concrete structures.

[0008] This invention has been made in view of these circumstances and aims to enable the monitoring of local strain. [Means for solving the problem]

[0009] (1) In order to achieve the above objective, the present invention employs the following means: a strain monitoring method for monitoring strain inside a prestressed concrete structure, characterized in that an optical fiber sensor is installed between the surface of the prestressed concrete structure and a steel material, and the strain inside the prestressed concrete structure is detected based on the change in the characteristics of the light wave propagating in the optical fiber sensor.

[0010] In this manner, an optical fiber sensor is set between the surface of the prestressed concrete structure and the steel material. Based on the changes in the characteristics of the light waves propagating through the optical fiber sensor, strain inside the prestressed concrete structure is detected, enabling monitoring of localized strain within the prestressed concrete structure. This improves the efficiency and quality of maintenance of the prestressed concrete structure. Furthermore, because the sensor does not directly contact the steel material, the inclusion of strain from the steel material itself in the measurement values ​​obtained by the optical fiber sensor can be suppressed, improving the accuracy of the measurement values. Therefore, accurate strain monitoring is possible.

[0011] (2) In the strain monitoring method of the present invention, the distance between the surface of the prestressed concrete structure and the optical fiber sensor is 10 mm or more and less than 200 mm.

[0012] Thus, the distance between the surface of the prestressed concrete structure and the optical fiber sensor being 10 mm or more and less than 200 mm allows the optical fiber sensor to detect localized strain occurring inside the prestressed concrete structure. This makes it possible to monitor localized strain.

[0013] (3) In the strain monitoring method of the present invention, the optical fiber sensor is characterized in that it is stretched along the direction in which prestress is applied.

[0014] Local strains in prestressed concrete structures occur along the direction in which the prestress is applied. In this respect, since optical fiber sensors extend along the direction in which the prestress is applied, they can efficiently monitor local strains occurring inside prestressed concrete structures.

[0015] (4) The strain monitoring method of the present invention is characterized in that the distance between the PC steel material that applies prestress to the prestressed concrete structure and the optical fiber sensor is 20 mm or more and 50 mm or less.

[0016] Thus, the distance between the PC steel bar and the optical fiber sensor being between 20 mm and 50 mm makes it possible to detect the strain generated inside the prestressed concrete structure due to prestressing by the PC steel bar. Therefore, during fabrication, it is easy to control the amount of prestress introduced into the prestressed concrete structure via the PC steel bar, while after fabrication, it is possible to detect localized strain generated around the PC steel bar due to the decrease in prestress. In other words, it makes it possible to monitor the strain inside the prestressed concrete structure caused by prestress from both short-term and long-term perspectives.

[0017] (5) In the strain monitoring method of the present invention, the distance between the steel material and the optical fiber sensor is 10 mm or more.

[0018] Thus, because the distance between the steel material and the optical fiber sensor is 10 mm or more, direct contact with the steel material is prevented. This suppresses the inclusion of the strain of the steel material itself in the strain detected by the optical fiber sensor, enabling accurate strain detection.

[0019] (6) In the strain monitoring method of the present invention, the plurality of optical fiber sensors are provided so as to be symmetrical with respect to each other in a cross section perpendicular to the direction of the PC steel material.

[0020] Thus, since a plurality of optical fiber sensors are provided so as to be symmetric with each other in a cross section perpendicular to the direction of the PC steel material, the optical fiber sensors are provided at positions having the same distance from the reference and different directions. When the PC steel material is used as a reference, it becomes easy to grasp the occurrence state of local strain caused by the influence of the PC steel material. Further, when the neutral plane is used as a reference, it becomes easy to compare the differences in the occurrence states of local strain resulting from the differences in stress.

[0021] (7) The prestressed concrete structure of the present invention is a prestressed concrete structure capable of monitoring the strain inside the structure, comprising a prestressed concrete structure body and an optical fiber sensor provided between the surface of the prestressed concrete structure and the steel material.

[0022] Thus, since the optical fiber sensor is provided between the surface of the prestressed concrete structure and the steel material, it becomes possible to monitor the local strain inside the prestressed concrete structure, and the efficiency and quality of the maintenance of the prestressed concrete structure can be improved. Further, since it does not directly contact the steel material, it is possible to suppress the strain of the steel material itself from being included in the measured value by the optical fiber sensor, and the accuracy of the measured value can be improved. Therefore, it is possible to monitor the accurate strain. Also, it is possible to monitor the stress using the static elastic modulus of concrete or coated mortar and the measured strain.

Advantages of the Invention

[0023] According to the present invention, it becomes possible to monitor local strain.

Brief Description of the Drawings

[0024] [Figure 1] A block diagram showing a schematic configuration of a strain monitoring system in the first embodiment. [Figure 2] A configuration diagram showing a schematic configuration of an optical fiber sensor in the first embodiment. [Figure 3] A perspective view showing the configuration of a prestressed concrete structure in the first embodiment. [Figure 4] A cross-sectional view showing the configuration of a prestressed concrete structure in the first embodiment. [Figure 5] An enlarged cross-sectional view showing a portion of the area near the surface of the prestressed concrete structure in the first embodiment. [Figure 6] An enlarged cross-sectional view of a prestressed concrete structure in the first embodiment, with a portion of the area around the PC steel bar enlarged. [Figure 7] A flowchart illustrating the procedure for monitoring strain in the first embodiment. [Figure 8] A perspective view showing the configuration of a prestressed concrete structure in the second embodiment. [Figure 9] A cross-sectional view showing the configuration of a prestressed concrete structure in the second embodiment. [Figure 10] An enlarged cross-sectional view showing a portion of the area near the surface of the prestressed concrete structure in the second embodiment. [Figure 11] A cross-sectional view showing a schematic of the test specimen of Example 1. [Figure 12] A cross-sectional view showing a schematic of the test specimen of Example 1. [Figure 13] A graph showing the results of Example 1. [Figure 14] A cross-sectional view showing an overview of the test specimen of Example 2. [Figure 15] A cross-sectional view showing the cross-section at cross-section A of Example 2. [Figure 16] A graph showing the results of Example 2. [Figure 17] A cross-sectional view showing an overview of the test specimen of Example 3. [Figure 18] A graph showing the results of Example 3. [Figure 19] A perspective view showing the configuration of the prestressed concrete structure in the modified example. [Modes for carrying out the invention]

[0025] The inventors of the present invention focused on the fact that in prestressed concrete structures, where prestress is introduced by PC steel materials, localized strain is likely to occur due to a decrease in prestress. They discovered a method for monitoring localized strain by embedding optical fiber sensors between the surface of the prestressed concrete structure and the steel materials, leading to the present invention. Embodiments of the present invention will be described below.

[0026] First, let's explain the areas where localized strain is likely to occur. Localized strain is caused by a decrease in prestress due to relaxation or creep, resulting in concentrated, large strain in certain parts of the interior of a prestressed concrete structure. Localized strain occurs because concrete is a heterogeneous material. Localized strain is particularly likely to occur near the surface of the prestressed concrete structure and around PC steel. Near the surface of the prestressed concrete structure, localized strain is likely to occur because strong tensile or compressive forces are likely to be applied. Around PC steel, localized strain is likely to occur due to the difference in deformation increase coefficients between the concrete and the steel.

[0027] The strain monitoring method according to this embodiment involves installing an optical fiber sensor between the surface of the prestressed concrete structure and the steel material, and detecting the strain inside the prestressed concrete structure based on the change in the characteristics of the light waves propagating in the optical fiber sensor. The steel material refers to a member provided inside the prestressed concrete structure 300 and made mainly of steel, and includes reinforcing bars 31 and PC steel materials 32.

[0028] The optical fiber sensor 10 provided between the surface of the prestressed concrete structure 300 and the steel material is an optical fiber sensor 10 provided in a position that is not exposed from the surface of the prestressed concrete structure 300 and does not directly contact the steel material, and other steel materials may be provided further between the optical fiber sensor 10 and the surface of the concrete structure 300.

[0029] By installing optical fiber sensors between the surface of the prestressed concrete structure and the steel members, it is possible to suppress the detection of strain in the steel members themselves and to detect localized strain occurring inside the prestressed concrete structure. This makes it possible to estimate the amount of prestress introduced inside the prestressed concrete structure from the occurrence of localized strain, and to perform maintenance according to the deterioration status of the prestressed concrete structure. [First Embodiment]

[0030] Referring to Figures 1 and 2, a first embodiment of the strain monitoring system, prestressed concrete structure, and strain monitoring method will be described in order. In this embodiment, the case where the prestressed concrete structure is a beam will be described. [Configuration of the strain monitoring system]

[0031] First, we will explain the configuration of the strain monitoring system, referring to Figure 1.

[0032] The strain monitoring system 200 installs an optical fiber sensor 10 between the surface of the prestressed concrete structure 300 and the steel material, and detects strain inside the prestressed concrete structure 300 based on changes in the characteristics of the light waves propagating in the optical fiber sensor 10.

[0033] The strain monitoring system 200 detects the strain caused by the prestress introduced into the prestressed concrete structure 300 during its fabrication, contributing to the selection of the optimal prestress. Furthermore, after fabrication, it monitors the strain inside the prestressed concrete structure 300, improving its maintainability. In other words, the strain monitoring system 200 can monitor localized strains caused by prestress both in the short and long term.

[0034] As shown in Figure 1, the strain monitoring system 200 comprises an optical fiber sensor 10 embedded in the prestressed concrete structure 300, a strain monitoring device 20, and a display unit 201.

[0035] Figure 2 shows a schematic configuration of the optical fiber sensor 10. The optical fiber sensor 10 has a detection unit 2 that detects strain on an optical fiber cable 1 that transmits light waves. The optical fiber sensor 10 is also covered around the detection unit 2 by a covering part 11. The material of the covering part 11 is, for example, a cement-based material or resin. Furthermore, the optical fiber sensor 10 does not need to be covered by the covering part 11; it may be embedded inside the prestressed concrete structure 300 in an uncovered state, i.e., in its raw wire state. In this embodiment, the case in which the optical fiber sensor 10 is an FBG sensor will be described.

[0036] Returning to Figure 1, the strain monitoring device 20 calculates the strain inside the prestressed concrete structure 300 based on the changes in the physical properties of the light waves transmitted from the optical fiber sensor 10. The strain monitoring device 20 comprises an optical processing unit 21, a storage unit 22, a control unit 23, and an operation unit 24.

[0037] The optical processing unit 21 converts a value corresponding to the intensity of the light wave transmitted from the optical fiber sensor 10 into an electrical signal. The storage unit 22 stores the electrical signal values ​​converted by the optical processing unit 21 in chronological order, associating them with the measurement date and time.

[0038] The control unit 23 calculates strain from the characteristic changes of the electrical signal converted by the optical processing unit 21. The control unit 23 may also calculate strain considering temperature changes, in which case a thermometer may be provided near the optical fiber sensor 10. This allows for more detailed strain measurement by calculating a value that subtracts the strain due to temperature changes from the strain when the optical fiber sensor 10 is subjected to an external force.

[0039] The control unit 24 allows the user to input arbitrary values. The control unit 24 can be, for example, a keyboard, touch panel, or mouse. The user can change parameters and calculation formulas used when calculating strain from electrical signal values ​​using the control unit 24.

[0040] The display unit 201 displays the analysis results transmitted from the strain monitoring device 20. The display unit 201 is, for example, a monitor or a tablet.

[0041] The strain monitoring device 20 only needs to be able to calculate the strain inside the prestressed concrete structure 300 based on the changes in the physical properties of the light waves transmitted from the optical fiber sensor 10. As shown in Figure 1, the optical processing unit 21, storage unit 22, control unit 23, and operation unit 24 may be configured as an integrated unit, or they may be configured separately. For example, the strain monitoring device 20 equipped with the optical processing unit 21 and the storage unit 22 may store the measured values ​​from the optical fiber sensor 10, and the strain may be calculated from the measured values ​​by the strain monitoring device 20 having other configurations. [Structure of prestressed concrete structures]

[0042] Next, the structure of the prestressed concrete structure will be explained with reference to Figures 3 to 6.

[0043] Figure 3 shows a prestressed concrete structure 300 having a portion of the concrete 30 that has been peeled off to expose the reinforcing bars 31. As shown in Figure 1, the prestressed concrete structure 300 comprises concrete 30, reinforcing bars 31 including main reinforcement 31A and shear reinforcement 31B, PC steel bars 32, fixing devices 301 for fixing the PC steel bars 32, and eight optical fiber sensors 10. The prestressed concrete structure body is defined as the concrete 30, reinforcing bars 31 and PC steel bars 32. The number of optical fiber sensors 10 is not limited to eight; it may be eight or more, or eight or fewer. The number of optical fiber sensors 10 is determined appropriately according to the shape of the prestressed concrete structure 300, the position of the steel bars installed inside, the part where strain is to be detected, etc.

[0044] In the prestressed concrete structure 300, prestress is applied to the concrete 30 by pulling on both ends of the PC steel members 32. At this time, the prestress is applied along the prestress direction PB, which is the axial direction of the PC steel members 32. The axial direction of the PC steel members 32 is also called the PC steel member direction. The fixing device 301 fixes the PC steel members 32 while maintaining the state in which both ends are pulled. As a result, prestress is applied to the inside of the prestressed concrete structure 300, and its load-bearing capacity is improved.

[0045] The optical fiber sensor 10 is positioned to extend along the prestress direction PB. That is, the optical fiber sensor 10 is positioned in a straight line parallel to the prestress direction PB. Prestressed concrete structures 300 are prone to localized strain along the prestress direction PB, which is the direction in which prestress is applied. Therefore, by positioning the optical fiber sensor 10 along the prestress direction PB, it becomes possible to efficiently detect localized strain.

[0046] Figure 4 is a cross-sectional view of a prestressed concrete structure 300 including shear reinforcement bars 31B. As shown in Figure 4, the shear reinforcement bars 31B are provided along the outer edge of the prestressed concrete structure 300, for example, with a concrete cover of 30 mm. The shear reinforcement bars 31B are rectangular in shape with rounded corners, and main reinforcement bars 31A are provided inside each corner.

[0047] With the neutral plane of the prestressed concrete structure 300 as the reference point, the area above the neutral plane is defined as the upper part, and the area below the neutral plane is defined as the lower part. In the lower part of the prestressed concrete structure 300, the PC steel member 32 is provided in the center in the left-right direction. Optical fiber sensors 10 are provided between the surface of the prestressed concrete structure 300 and the shear reinforcement bars 31B, and between the shear reinforcement bars 31B and the PC steel member 32. In this case, the optical fiber sensors provided between the surface of the prestressed concrete structure 300 and the shear reinforcement bars 31B are designated as the first optical fiber sensors 10A1 and 10B1, and the optical fiber sensors provided between the shear reinforcement bars 31B and the PC steel member 32 are designated as the second optical fiber sensors 10A2 and 10B2.

[0048] The first optical fiber sensors 10A1 and 10B1 are provided for the purpose of detecting local strain occurring near the surface of the prestressed concrete structure 300, and are installed between the surface of the prestressed concrete structure 300 and the steel material closest to the surface. The second optical fiber sensors 10A2 and 10B2 are provided for the purpose of detecting local strain occurring around the PC steel material 32, and are installed around the PC steel material 32 so as not to be in direct contact with it.

[0049] The first optical fiber sensors 10A1 and 10B1, and the second optical fiber sensors 10A2 and 10B2, are installed symmetrically in the upper and lower parts of a cross section perpendicular to the direction of the PC steel members of the prestressed concrete structure 300. That is, in the upper and lower parts of the prestressed concrete structure 300, the four optical fiber sensors 10 are installed symmetrically in a cross section perpendicular to the direction of the PC steel members, with respect to a perpendicular line CL that passes through the center of the PC steel members 32. The optical fiber sensors 10B1 and 10B2 located in the lower part are further aligned in a straight line with the PC steel members 32.

[0050] This allows for a comparison of strain on the right and left sides of the prestressed concrete structure, using the reference line CL as a reference. Therefore, it is possible to understand the differences between the left and right sides of the prestressed concrete structure. Note that the reference line CL is not limited to a vertical line; it can also be a horizontal line. In this case, it becomes easier to compare the upper and lower parts of the prestressed concrete structure.

[0051] Figure 5 is a partially enlarged cross-sectional view showing a portion of the area near the surface of the prestressed concrete structure, with the right side of the page representing the surface of the prestressed concrete structure 300. The optical fiber sensors 10A1 and 10A2 located on the upper part of the prestressed concrete structure 300 will be described below, but the same applies to the optical fiber sensors 10B1 and 10B2 located on the lower part.

[0052] As shown in Figure 5, a first optical fiber sensor 10A1 and a second optical fiber sensor 10A2 are provided around the shear reinforcement bar 31B. The first optical fiber sensor 10A1 is provided such that the overlap length OF is between it and the surface of the prestressed concrete structure 300, and the length between it and the shear reinforcement bar 31B is equal to the steel length SF. The second optical fiber sensor 10A2 is provided such that the length between it and the shear reinforcement bar 31B is equal to the steel length SF.

[0053] The concrete cover length OF is the distance from the surface of the prestressed concrete structure 300 to the optical fiber sensor 10, and should be between 10 mm and 200 mm, but is particularly preferable to be 20 mm or less. A concrete cover length OF of 10 mm or more prevents the optical fiber sensor 10 from being exposed from the concrete surface 30 and enables the detection of strain inside the prestressed concrete structure 300. A concrete cover length OF of less than 200 mm allows the optical fiber sensor 10 to detect localized strain inside the prestressed concrete structure 300.

[0054] Having a cover length OF of 20 mm or less allows the optical fiber sensor 10 to be placed in areas where localized strains that cause cracks are likely to occur, enabling efficient detection of localized strains. In addition, in many reinforced concrete structures, the minimum cover when reinforcing bars are installed is often around 30 mm. That is, the distance between the surface of the prestressed concrete structure 300 and the steel material closest to the surface is often at least around 30 mm. Therefore, having a cover length OF of 20 mm or less also prevents direct contact with the steel material itself.

[0055] The steel length SF is the distance between each steel material contained within the prestressed concrete structure 300 and the optical fiber sensor 10. Specifically, it is the distance between the main reinforcement 31A, shear reinforcement 31B, and PC steel material 32 and the optical fiber sensor 10, and it should be 10 mm or more. The distance between each steel material and the optical fiber sensor 10 may also be different. The optical fiber sensor 10 is installed so that the distance to all steel materials contained within the prestressed concrete structure is 10 mm or more. This suppresses the detection of strain in the steel material itself by the optical fiber sensor 10.

[0056] Figure 6 is a partially enlarged cross-sectional view showing a portion of the area around the PC steel members of a prestressed concrete structure. As shown in Figure 6, a second optical fiber sensor 10B2 is provided so as to be aligned with the PC steel members 32 in the left-right direction. The second optical fiber sensor 10B2 is provided such that its distance from the PC steel members 32 is equal to the length of the PC steel members PF, and its distance from the shear reinforcement bars 31B is equal to the length of the steel members SF.

[0057] The PC steel length PF is the distance between the PC steel 32 and the optical fiber sensor 10, and is sufficient if it is 10 mm or more, but more preferably 20 mm or more and 50 mm or more. A PC steel length PF of 10 mm or more suppresses the detection of strain in the PC steel 32 itself. A PC steel length PF of 20 mm or more allows for the detection of local strain due to prestress introduced by the PC steel 32 while suppressing the detection of strain in the PC steel 32 itself. A PC steel length PF of 50 mm or less allows for the detection of local strain due to prestress introduced by the PC steel 32. [Methods for monitoring strain]

[0058] Next, we will explain the method for monitoring strain, referring to Figure 7.

[0059] First, an optical fiber sensor 10 is installed inside the prestressed concrete structure 300 (S1). At this time, the prestressed concrete structure 300 is manufactured such that at least one optical fiber sensor 10 is positioned between the surface of the prestressed concrete structure 300 and the steel material.

[0060] Next, the optical fiber sensor 10 detects changes in the characteristics of the light wave (S2). First, the optical fiber sensor 10 appropriately transmits the detected light wave to the strain monitoring device 20. The transmitted light wave is converted into an electrical signal by the optical processing unit 21. The converted electrical signal is recorded in the storage unit 22.

[0061] Next, the strain is calculated based on the changes in the characteristics of the light wave (S3). Specifically, the control unit 23 calculates the strain from the changes in the characteristics of the electrical signal converted by the optical processing unit 21.

[0062] Next, the calculated strain is displayed on the display unit 201 (S4). The control unit 23 creates display data including the calculated strain and transmits it to the display unit 201 via wireless or wired communication. The display unit 201 displays the data transmitted from the strain monitoring device 20. This makes it possible for the user to monitor the strain detected by one or more optical fiber sensors 10 installed inside the prestressed concrete structure 300.

[0063] The display data is data processed to be suitable for display by the display unit 201. The display data may be strain waveforms or numerical values ​​from each optical fiber sensor 10, or it may be data visualizing the strain distribution inside the prestressed concrete structure 300. This makes it possible to visually grasp the strain distribution inside the prestressed concrete structure 300, and makes it easier to understand the conditions inside the prestressed concrete structure 300.

[0064] As described above, the following effects can be obtained according to the first embodiment.

[0065] (1-1) Since the first optical fiber sensors 10A1 and 10B1 and the second optical fiber sensors 10A2 and 10B2 are provided between the surface of the prestressed concrete structure 300 and the steel material, it is possible to monitor local strain occurring inside the prestressed concrete structure 300.

[0066] (1-2) The first optical fiber sensors 10A1 and 10B1 are installed such that the cover length OF is 10 mm or more and 20 mm or less, making it possible to monitor local strain occurring near the surface of the prestressed concrete structure 300.

[0067] (1-3) The second optical fiber sensors 10A2 and 10B2 are installed such that the PC steel length PF is between 20 mm and 50 mm, making it easier to detect the strain caused by the prestress applied to the inside of the prestressed concrete structure 300 by the PC steel 32. Furthermore, it becomes possible to estimate the strength of the prestress applied to the inside of the prestressed concrete structure 300, and to understand the conditions inside the prestressed concrete structure 300 in more detail.

[0068] (1-4) Each optical fiber sensor 10 is installed such that the length SF of the steel material relative to each steel material is 10 mm or more, so that it is possible to suppress the detection of strain in the steel material itself, including the reinforcing bars 31 and PC steel materials 32, and to detect the strain accurately.

[0069] (1-5) The optical fiber sensors 10A1 and 10A2 located at the top and the optical fiber sensors 10B1 and 10B2 located at the bottom are respectively installed in a cross section perpendicular to the direction of the PC steel members of the prestressed concrete structure 300, so as to be symmetrical with respect to the PC steel members 32. That is, they are installed so as to be symmetrical with respect to the reference line CL. This makes it possible to compare the strain on the right and left sides of the interior of the prestressed concrete structure with respect to the reference line CL. Therefore, it is possible to grasp the difference between the left and right sides inside the prestressed concrete structure 300.

[0070] (1-6) The optical fiber sensors 10A1 and 10A2 located at the top and the optical fiber sensors 10B1 and 10B2 located at the bottom are arranged so as to be vertically aligned in the vertical direction. This makes it possible to install the optical fiber sensors 10 at positions where the positional relationship other than height is the same in the upper and lower parts. Therefore, it becomes easy to compare the strain in the upper and lower parts and to grasp the difference in strain between the upper and lower parts.

[0071] (Example of modification of the first embodiment)

[0072] Furthermore, the first embodiment described above can also be implemented with the following modifications.

[0073] The first optical fiber sensors 10A1 and 10B1 may be installed near the surface of the bottom or top surface of the prestressed concrete structure. More specifically, they may be installed between the surface of the bottom surface of the prestressed concrete structure and the reinforcing bars 31, or between the surface of the top surface and the reinforcing bars 31. In particular, since local strain is likely to occur near the surface of the bottom surface due to tensile stress, installing the optical fiber sensor 10 near the surface of the bottom surface makes it possible to detect local strain that could lead to cracking at an early stage. [Second Embodiment]

[0074] A second embodiment of the prestressed concrete structure will be described with reference to Figures 8 and 9. The difference between the first embodiment and the second embodiment is that the prestressed concrete structure in the first embodiment was a beam, while in the second embodiment it is a floor slab. The following description will focus on the differences between the second and first embodiments, and components similar to those in the first embodiment will be denoted by the same reference numerals and their descriptions will be omitted.

[0075] Figure 8 shows a prestressed concrete structure 300 having a portion of the concrete 30 that has been peeled off to expose the reinforcing bars 31. The optical fiber sensor 10 is omitted from the diagram. As shown in Figure 8, in the prestressed concrete structure of the second embodiment, the reinforcing bars 31, consisting of main reinforcement bars 31A and support reinforcement bars 31C, are arranged in a grid pattern on the top and bottom surfaces, respectively.

[0076] Figure 9 is a cross-sectional view of the prestressed concrete structure 300, cut perpendicular to the direction of the PC steel bars. As shown in Figure 9, the optical fiber sensors 10 are arranged in four horizontal rows inside the prestressed concrete structure 300. In addition, they are arranged in two rows each above and below the neutral plane, with the neutral plane as the reference point.

[0077] In this case, the optical fiber sensors 10 provided near the top and bottom surfaces of the prestressed concrete structure 300 in the upper and lower parts are designated as first optical fiber sensors 10A1 and 10B, and the optical fiber sensors 10 provided between the reinforcing bars 31 on the top and bottom sides of the prestressed concrete structure 300 are designated as second optical fiber sensors 10A2 and 10B2. The first optical fiber sensors 10A1 and 10B1 are provided between the surface of the prestressed concrete structure 300 and the reinforcing bars 31. The second optical fiber sensors 10A2 and 10B2 are provided between the PC steel members 32 and are arranged alternately with the PC steel members 32.

[0078] Furthermore, each optical fiber sensor 10 is positioned to be symmetrical with respect to the reference line CL. This allows for comparison of strain between the left and right sides of the prestressed concrete structure 300.

[0079] Furthermore, each optical fiber sensor 10 is arranged perpendicular to the others. This allows for comparison of strain between the upper and lower parts of the prestressed concrete structure 300. In addition, each optical fiber sensor 10 is positioned symmetrically with respect to the neutral plane. Moreover, as mentioned above, each optical fiber sensor 10 is positioned symmetrically with respect to the reference line CL, so they are evenly distributed within the prestressed concrete structure 300. This makes it easier to analyze the strain distribution within the prestressed concrete structure 300.

[0080] Figure 10 is a partially enlarged cross-sectional view showing a portion of the area near the surface of the prestressed concrete structure in the second embodiment, with the right side of the paper being the surface of the prestressed concrete structure 300. The optical fiber sensors 10A1 and 10A2 located on the upper part of the prestressed concrete structure 300 will be described below, but the same applies to the optical fiber sensors 10B1 and 10B2 located on the lower part.

[0081] As shown in Figure 10, not only the first optical fiber sensor 10A1 and the second optical fiber sensor 10A2, but also optical fiber sensors 10 provided near the surface of the side surface must be considered in the design as having an overlap length OF due to the distance from the side surface.

[0082] The first optical fiber sensor 10A1 has an overlap length OF at the top or bottom surface. The second optical fiber sensor 10A2 has a distance of PC steel length PF and steel length SF at the alternately arranged PC steel bars 32. In addition, not limited to the first optical fiber sensor 10A1 and the second optical fiber sensor 10A2, the distance to the reinforcing bar 31 is the steel length SF.

[0083] (Example of modification of the second embodiment)

[0084] As described above, according to the second embodiment, in addition to (1-1) to (1-6), the following effects can be obtained.

[0085] (2-1) Optical fiber sensors 10 are provided so as to be arranged alternately with each PC steel material 32. This makes it possible to detect local strain occurring around each PC steel material 32.

[0086] The second embodiment can also be implemented with the following modifications.

[0087] The first optical fiber sensors 10A1 and 10B1 and the second optical fiber sensors 10A2 and 10B2 do not necessarily have to be positioned perpendicular to each other. In this case, the first optical fiber sensor 10A1 located at the top and the optical fiber sensor 10B1 located at the bottom may be positioned perpendicular to each other. The same applies to the second optical fiber sensors 10A2 and 10B2. [Example 1]

[0088] The inventors evaluated the results of detecting local strain using an optical fiber sensor 10 placed between the surface of a prestressed concrete structure 300 and a steel material. In this example, strain detection by the optical fiber sensor 10 was evaluated using a beam-shaped prestressed concrete structure. In this example, strain was induced inside the concrete test specimen by a bending test, and the strain detected by the optical fiber sensor 10 placed inside the test specimen and the strain detected by the strain gauge 400 placed on the surface of the test specimen were compared. Example 1 will be described in detail below. (Concrete test specimen)

[0089] The materials used in the concrete test example related to this embodiment are shown in the table below. In this table, symbol C represents ordinary Portland cement with a density (surface dry) of 3.16 g / m³. 3 ) Also, symbol S represents mountain sand from Kakegawa City, with a density (surface dry) of 2.58 (g / m³). 3 ) Also, symbol G is crushed stone from Sakuragawa City, and its (surface dry) is 2.65 (g / m³). 3 ) [Table 1]

[0090] The concrete mix for this embodiment is shown in the table below. As shown in this table, "s / a" is the "fine aggregate ratio (%)" and "W" is the "tap water (kg / m³)" 3 )" and "C" is "unit cement content (kg / m 3 )" and "S" stands for "Unit fine aggregate quantity (kg / m 3 )" and "G" stands for "Unit Coarse Aggregate Content (kg / m 3 )” [Table 2]

[0091] Figures 11 and 12 are cross-sectional views showing an overview of the concrete test specimen according to this embodiment. In Figure 12, the main reinforcement bars 31A located behind the shear reinforcement bars 31B are indicated by a double dashed line. The air content of the concrete in this embodiment was 2%, and the slump flow was 67 cm. The design strength was 50 N / mm². 2 In a concrete test specimen measuring 254 mm × 300 mm × 1900 mm, shear reinforcement bars 31B were placed at a position where the concrete cover was 30 mm. Furthermore, with the distance from the top surface of the test specimen as the height, and the total height of the test specimen as 1, a PC steel bar was placed in the center in the left-right direction at a height of 2 / 3 (H=2 / 3).

[0092] As shown in Figure 11, an optical fiber sensor 10 was installed around the PC steel bar 32. The optical fiber sensor 10 is positioned parallel to the PC steel bar 32 at a position where the PC steel bar length PF is 50 mm, the steel bar length SF relative to the shear reinforcement bar 31B is 15 mm, and H = 2 / 3. Furthermore, the optical fiber sensor 10 was not covered by the covering portion 11 and was installed along the prestress direction PB. That is, the optical fiber sensor 10 was installed in a straight line in its strand state so as to remain parallel to the prestress direction PB.

[0093] As shown in Figure 12, the detection unit 2 is installed at positions where L=1 / 2 and L=1 / 4 are determined when the position halfway along the length of the main reinforcement 31A in the extending direction of the concrete test specimen is defined as L=1 / 2, and the position halfway from L=1 / 2 to the end of the main reinforcement 31A is defined as L=1 / 4.

[0094] Returning to Figure 11, the strain gauge 400 (PL-60: manufactured by Tokyo Sokki) was attached to the surface of the concrete test specimen so that it was at the same height as the optical fiber sensor 10.

[0095] Prestressing was introduced at 7 days of age using the post-tensioning method. A hydraulic jack was used to apply a prestress of 550 kN to the concrete specimen. (Bending test)

[0096] For the bending test, a load cell (CLP-NB: manufactured by Tokyo Sokki) and a hydraulic jack were used to apply a load of 5.8 kN in stages to the concrete test specimen according to this embodiment, while monitoring the applied load and the occurrence of cracks. (Measurement method)

[0097] The measurement interval in the detection unit 2 was set to 0.1 seconds, and the wavelength data per 1000 Hz was averaged to obtain the measured value of the optical fiber sensor 10. Then, the strain was calculated using equation (1). Here, ε is the strain (μ), λ is the wavelength (mm), and α is a coefficient due to the center wavelength.

number

[0098] Figure 13 shows the results of Example 1. The horizontal axis represents the magnitude of the load applied to the test specimen by the bending test machine, and the vertical axis represents the magnitude of the strain measured by the optical fiber sensor 10 or strain gauge 400.

[0099] The strain gauge 400, located at L=1 / 2, detected a behavior in which strain decreased as the actual load increased between 200kN and 300kN. On the other hand, the optical fiber sensors 10, located at L=1 / 2 and L=1 / 4, detected a behavior in which strain increased with increasing load even between 200kN and 300kN. From this, it can be said that at L=1 / 2, the strain gauge 400 has difficulty detecting local strain inside the concrete when a large load is applied, but the optical fiber sensor 10 can accurately detect local strain even under large loads.

[0100] Furthermore, as the load increased, the strain measured by the optical fiber sensor 10 located inside the concrete test specimen was several hundred micrometers greater than that measured by the strain gauge 400 located on the surface. Therefore, the optical fiber sensor 10 makes it possible to detect localized strain occurring inside the concrete test specimen.

[0101] Note that the optical fiber sensor 10 was provided at a position where the length PF of the PC steel material was 50 mm and the length SF of the steel material with respect to the shear reinforcement 31B was 15 mm. That is, even when the length PF of the PC steel material is 50 mm and the length SF of the steel material is 15 mm, it is possible to detect local strain. [Example 2]

[0102] Next, using a floor slab-shaped prestressed concrete structure, the strain detection by the optical fiber sensor was evaluated. In this example, strain was generated inside the concrete test specimen by a prestress introduction test, and the strain detected by the optical fiber sensor provided inside the test specimen and the embedded strain gauge was compared. Hereinafter, Example 2 will be described in detail. (Concrete test specimen)

[0103] Regarding the materials used in the concrete test example and the concrete mix, they are the same as those in Example 1.

[0104] FIG. 14 is a cross-sectional view showing the outline of the concrete test specimen according to this example, and FIG. 15 is a cross-sectional view at cross-section A. The air content of the concrete according to this example was 2%, and the slump flow was 67 cm. Also, the design standard strength was 50 N / mm 2 With that as the standard, in a 5500 mm × 2900 mm × 700 mm concrete test specimen, the reinforcing bar 31 was arranged at a position where the cover was 30 mm, and the PC steel material 32 was arranged at intervals of 100 mm at the positions of 1 / 3 and 2 / 3 of the height.

[0105] As shown in FIG. 14, an optical fiber sensor (FBG sensor) and a thermocouple-embedded strain gauge (KM-100BT: manufactured by Tokyo Sokki Kenkyujo Co., Ltd.) were respectively embedded at cross-section A and cross-section B.

[0106] As shown in Figure 2, the optical fiber sensor 10 is covered around the detection unit 2 by a covering portion 11. The covering portion 11 is made of a cement-based material similar to concrete 30 and is provided to cover the entire detection unit 2. The covering portion 11 is a cylinder with a diameter of 20 mm and a length of 50 mm.

[0107] Sections A and B are cross-sections obtained by cutting the PC steel material 32 in a direction equal to the axial direction. Section A is a cross-section at a position where the lengths from both sides are equal, and section B is a cross-section located midway between section A and the side located on the left side of the paper. In other words, section A is at a position where the horizontal length is halved, and section B is at a position where the horizontal length is 1 / 4.

[0108] In each cross-section, an optical fiber sensor 10 and an embedded strain gauge 500 are provided. In cross-section A, optical fiber sensors 10 are provided at positions 180 mm and 200 mm from the top surface of the concrete test specimen, and an embedded strain gauge 500 is provided at a position in contact with the optical fiber sensor 10 located at 200 mm.

[0109] In section B, the optical fiber sensor 10 is provided only at a position 180 mm from the top surface of the concrete test specimen, and the embedded strain gauge 500 is provided at a position parallel to the embedded strain gauge 500 in section A. The detection unit 2 of the optical fiber sensor 10 provided in sections A and B is provided at a position 400 mm from the side, as shown in Figure 15. (Prestress induction trial)

[0110] Prestress was introduced using the pretensioning method at 1 day after steam curing. Prestress was introduced using a hydraulic jack. Prestress was introduced intermittently until it reached 3800 kN, with a speed of 1 kN per second, and the process was temporarily stopped for 60 seconds each time the prestress increased by 500 kN for the concrete test specimen in this embodiment. (Measurement method)

[0111] This is the same as in Example 1. (Results of Example 2)

[0112] Figure 16 shows the results of Example 2. The horizontal axis represents elapsed time, and the vertical axis represents the magnitude of strain measured by the optical fiber sensor 10 or the embedded strain gauge 500.

[0113] The embedded strain gauge 500 detected strains around -10μ regardless of the cross-section. This was also true for the optical fiber sensor 10 located 200mm from the top surface in cross-section A. On the other hand, the optical fiber sensor 10 located 180mm from the top surface detected large strains in either the tensile or compressive direction, depending on the cross-section.

[0114] In section A, the optical fiber sensor 10 detects a negative value, indicating that it is detecting strain in the compressive direction. In section B, the optical fiber sensor 10 detects a positive value, indicating that it is detecting strain in the tensile direction. From the above, it can be concluded that the optical fiber sensor 10 enables the detection of localized strain occurring inside the concrete test specimen.

[0115] Furthermore, the distance from the top surface to the optical fiber sensor 10 is the shortest distance from the surface of the concrete test specimen. In other words, the distance from the top surface of the test specimen to the optical fiber sensor 10 is the cover length OF. While strain could not be detected with the optical fiber sensor 10 at 200 mm from the top surface, strain could be detected with the optical fiber sensor 10 at 180 mm. From this, it can be said that strain can be detected if the cover length OF is less than 200 mm.

[0116] Furthermore, since the optical fiber sensor 10, which is 200 mm from the top surface, was in direct contact with the steel material, it can be said that strain cannot be detected when it is in direct contact with the steel material. Therefore, it is necessary to install the optical fiber sensor 10 so that it does not come into direct contact with the steel material. [Example 3]

[0117] Next, the accuracy of the strain detected by the optical fiber sensor 10 was evaluated using a beam-shaped prestressed concrete structure. In this example, strain was induced inside the concrete test specimen by a bending load test, and the strain detected by the optical fiber sensor 10 installed inside the test specimen and the strain gauges 400 installed on both sides were compared. Example 3 will be described in detail below. (Concrete test specimen)

[0118] In this embodiment, the concrete test specimen, as shown in Figure 17, has strain gauges 400 on both sides compared to the test specimen in Embodiment 1. Furthermore, when the right side of the paper is considered the front and the left side is considered the back, the strain gauge 400 on the front is designated as the front strain gauge 400A, and the strain gauge 400 on the back is designated as the back strain gauge 400B. In other words, regarding the detection unit 2, only the data from the detection unit 2 located at the position L=1 / 2 in Figure 12 was evaluated. (Bending load test)

[0119] For the bending test, a load cell (CLP-NB: manufactured by Tokyo Sokki) and a hydraulic jack were used to apply load to the concrete specimen in three cycles while monitoring the cracking condition. In the first cycle, the load was applied up to 110kN, the load at which the tensile strength becomes 0 according to the design and just before cracking occurs. In the second cycle, the load was applied up to 150kN, the load at which cracking occurs according to the design. In the third cycle, the load was applied up to 256kN, the load at which the failure load is designed, then the load was removed to 100kN, and then the load was applied again up to 256kN. (Measurement method)

[0120] This is the same as in Example 1. (Results of Example 3)

[0121] Figure 18 shows the results of Example 3. Figure 18 shows the relationship between load and strain in the third cycle, with the horizontal axis representing load and the vertical axis representing strain.

[0122] In Figure 18, the strain gauge 400A on the surface showed a tendency for the strain to increase in the compression direction around 150kN. This can be attributed to the fact that the strain gauge 400A is installed on the concrete surface. On the other hand, the strain gauge 400B on the back showed an increase in the tensile direction around 150kN. This can be attributed to the fact that cracks were propagating on the back side, causing the strain on the surface side to shift to the compression side. Furthermore, since there is only one PC steel bar 32 in the test specimen, the introduced prestress is eccentric, and the strain distribution in the cross-section is also considered to be a contributing factor. On the other hand, the optical fiber sensor 10 detected more tensile strain than the strain gauge 400A on the surface. This is because it is located inside the concrete test specimen, and therefore more tensile strain is occurring inside the optical fiber sensor than inside the surface. In other words, it can be said that it accurately detects the strain inside the concrete test specimen.

[0123] (Modifications of Embodiments 1 and 2)

[0124] Furthermore, each of the above embodiments can also be implemented with the following modifications.

[0125] The optical fiber sensor 10 does not have to be installed in a straight line. The optical fiber sensor 10 may be embedded in a curved or spiral shape. The optical fiber sensor 10 does not necessarily have to be installed in a straight line; it is sufficient if it is installed between the surface of the prestressed concrete structure and the steel material.

[0126] When the optical fiber sensor 10 is installed in a spiral shape, for example, as shown in Figure 10, one method is to wind the optical fiber sensor 10 around a cylindrical concrete member and then place the cylindrical concrete member with the optical fiber sensor 10 wound around it into a formwork. Alternatively, the optical fiber sensor 10 may contain a shape-memory material and be wound around a cylindrical member to form a spiral shape, after which only the spiral optical fiber sensor 10 may be placed into the formwork. Furthermore, a cylindrical concrete member with the optical fiber sensor 10 installed inside the concrete may be manufactured, and the cylindrical concrete member containing the optical fiber sensor 10 may be placed into the formwork.

[0127] In any of the methods described above, the optical fiber sensor 10, which has been placed in the formwork, is set up to be in the desired position, and then concrete is poured. This embeds the spiral optical fiber sensor 10 inside the prestressed concrete structure 300.

[0128] Furthermore, the optical fiber sensor 10 does not need to be wound at equal intervals, and there are no limitations on the winding intervals. Also, the cross-sectional shape of the cylindrical concrete member may be circular or rectangular.

[0129] By arranging the optical fiber sensor 10 in a spiral pattern, it is easier to detect strain at various positions compared to arranging it in a straight line, making it easier to understand the conditions inside the prestressed concrete structure 300.

[0130] The optical fiber sensors 10A1 and 10A2 located at the top and the optical fiber sensors 10B1 and 10B2 located at the bottom do not necessarily have to be positioned perpendicular to each other. In other words, the cover length OF and the length SF of the steel members relative to each other may differ between the optical fiber sensors 10A1 and 10A2 located at the top and the optical fiber sensors 10B1 and 10B2 located at the bottom.

[0131] In the first place, the stress behavior inside the prestressed concrete structure 300 differs between the upper and lower parts, with the neutral plane as the boundary. That is, the strain generation conditions that arise in response to the stress also differ. Therefore, it is desirable to place the optical fiber sensors 10 at positions in the upper and lower parts where strain is likely to occur, and as a result, the optical fiber sensors 10 may or may not be arranged vertically.

[0132] The optical fiber sensors 10 do not necessarily have to be installed symmetrically. It is preferable to install the optical fiber sensors 10 in positions where strain is likely to occur, and they should be installed appropriately according to the structure of each prestressed concrete structure 300. In other words, considering the strain generation situation that can be predicted in advance, the optical fiber sensors 10 may be installed only in parts where localized strain is likely to occur.

[0133] The second optical fiber sensors 10A2 and 10B2 can be installed anywhere around the PC steel material 32. For example, the second optical fiber sensors 10A2 and 10B2 can be installed on the upper or lower side of the PC steel material 32. This makes it possible to detect strain on the upper and lower sides of the PC steel material 32. In particular, localized strain is likely to occur on the lower side of the PC steel material 32 due to tensile stress. Therefore, installing the optical fiber sensor 10 on the lower side of the PC steel material 32 makes it possible to detect localized strain that could lead to cracking at an early stage.

[0134] The optical fiber sensors 10 can be installed in any number of rows inside the prestressed concrete structure 300. Furthermore, they do not need to be installed in a single horizontal row. Reducing the number of optical fiber sensors 10 installed inside the prestressed concrete structure 300 suppresses a decrease in the load-bearing capacity of the prestressed concrete structure 300. Conversely, installing a large number of optical fiber sensors 10 leads to a more detailed understanding of the conditions inside the prestressed concrete structure 300. [Explanation of symbols]

[0135] 1. Fiber optic cable 2. Detection unit 10 Fiber Optic Sensors 10A Optical fiber sensor located at the top 10A1 First optical fiber sensor located at the top 10A2 Second optical fiber sensor located at the top 10B Optical fiber sensor located at the bottom 10B1 First optical fiber sensor located at the bottom 10B2 Second fiber optic sensor located at the bottom 11 Covering part 20. Strain monitoring device 21 Optical Processing Unit 22 Memory section 23 Control Unit 24 Control section 30 Concrete 31 Reinforcement bars 31A Main reinforcement 31B Shear reinforcement 31C Reinforcement 32 PC steel material 200 Strain Monitoring System 201 Display section 300 Prestressed concrete structures 301 Fixtures 400 strain gauges 500 Embedded strain gauges CL Reference Line OF Head length PC steel length PB prestress direction SF steel length

Claims

1. A strain monitoring method for monitoring strain inside a prestressed concrete structure, comprising: an optical fiber sensor is installed between the surface of the prestressed concrete structure and a steel material; A strain monitoring method, comprising detecting strain within the prestressed concrete structure based on changes in the characteristics of light waves propagating through the optical fiber sensor.

2. 2. The strain monitoring method according to claim 1, wherein the distance between the surface of the prestressed concrete structure and the optical fiber sensor is 10 mm or more and less than 200 mm.

3. The strain monitoring method according to claim 1 , wherein the optical fiber sensor extends along the direction in which prestress is applied.

4. 4. A strain monitoring method according to claim 1, wherein the distance between the PC steel member that applies prestress to the prestressed concrete structure and the optical fiber sensor is 20 mm or more and 50 mm or less.

5. 5. The strain monitoring method according to claim 1, wherein the distance between the steel material and the optical fiber sensor is 10 mm or more.

6. 6. The strain monitoring method according to claim 1, wherein the plurality of optical fiber sensors are arranged symmetrically with respect to each other in a cross section perpendicular to the direction of the PC steel member.

7. A prestressed concrete structure capable of monitoring strain inside the structure, a prestressed concrete structure body; A prestressed concrete structure having an optical fiber sensor between the surface of the prestressed concrete structure and a steel member.

Images