A mass concrete crack monitoring method based on distributed optical fiber technology

Abstract

A kind of mass concrete crack monitoring method based on distributed optical fiber technology, adopt the way of hanging S-shaped ring to fix optical fiber, and from the center of concrete to the edge of the radiation layout optical fiber, through the three equal division loading model of reinforced concrete beam, select 5mm steel strand optical cable as monitoring optical fiber, optical fiber demodulator selects Brillouin optical time domain reflectometer;After the completion of optical fiber layout, establish monitoring station, use AV6419 optical fiber demodulator to connect the optical fiber laid, form monitoring field, real-time monitoring the crack situation generated in the concrete;According to the monitored Brillouin scattering frequency shift, calculate the strain and temperature of each part of optical fiber;Through the obtained optical fiber strain data, calculate the crack width, so as to further judge the development degree of crack, and put forward the corresponding crack control measures. The monitoring method not only can meet the requirement of large-scale monitoring, but also can greatly improve the measurement accuracy, the measurement cost is also greatly reduced, the complex manual data collection process can be saved, and the operation is simple, economical and feasible.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of mass concrete crack monitoring method based on distributed optical fiber technology.The monitoring method can meet the requirement of distributed optical fiber sensing technology for extracting large-scale measurement field information, form mass concrete monitoring field, can also carry out real-time monitoring to the crack generated in the solidification and curing process of mass concrete, can greatly avoid the influence of concrete hydration heat phenomenon on monitoring data, compared with traditional monitoring needing a lot of manpower and material resources, can greatly improve the degree of automation monitoring and monitoring accuracy, realize real-time, accurate monitoring, and can save a lot of cost. BACKGROUND

[0002] After mass concrete is poured, due to its oversized structure, the hydration heat phenomenon will be further intensified, so it is more likely to have temperature crack problems. The existence of cracks will destroy the integrity of the concrete structure, cause the stress state inside the concrete structure to deteriorate sharply, and more likely to cause the concrete structure to break or collapse, thereby forming a serious problem of endangering building safety. Therefore, reliable crack monitoring and control methods are indispensable in engineering construction and structural health monitoring, and have been a hot topic of continuous exploration in engineering and academic circles.

[0003] At present, the monitoring methods for mass concrete cracks mainly include crack width comparison card method, crack microscope method, ultrasonic detection method, infrared detection method, etc. Although these commonly used detection and monitoring technologies can meet the detection or monitoring needs in engineering under normal conditions to some extent, they also have the following shortcomings: (1) the crack width comparison card method is relatively simple to use, but it can only be used for rough measurement and has low testing accuracy; (2) the crack microscope method and ultrasonic detection method have high accuracy, but the testing speed is slow, the testing strength and the manpower and material resources consumed are large, and the data obtained are prone to have large human collection errors; (3) the infrared detection method requires heating the detection site, and has high technical requirements for monitoring personnel.

[0004] The existing conventional detection methods can only be limited to detecting cracks on the surface of concrete, and cannot detect cracks in mass concrete with geometric size of dozens of meters or even hundreds of meters. Moreover, the real-time, parallel and automatic monitoring degree of existing detection technologies is not high, and they are mostly static detection and single-point detection "detection" technologies, rather than "monitoring" technologies, which cannot realize all-weather real-time monitoring of crack evolution in mass concrete. The above-mentioned shortcomings of existing detection technologies affect people's correct judgment and implementation of disaster prevention measures in geotechnical engineering disaster prevention and safety production practice.

[0005] As a new monitoring technology, distributed optical fiber sensing technology has the advantages of continuous arrangement of measuring points along the monitoring object and the ability to realize comprehensive, long-term and real-time automatic monitoring of structural cracks. Therefore, distributed optical fiber sensors have great engineering application value in monitoring the deformation and crack development of concrete structures and are an important tool for new generation structural monitoring. However, for mass concrete, the hydration heat inside is intense, which causes fluctuations in the data monitored by the optical fiber, and the optical fiber bound to the steel bar may not monitor the changes due to the influence of the fluidity of the concrete. Therefore, effective techniques and methods are needed to make up for these deficiencies to meet the requirements of engineering safety monitoring. SUMMARY

[0006] In order to solve the technical problems existing in the prior art, the present application provides a mass concrete crack monitoring method based on distributed optical fiber technology, in order to monitor the cracking phenomenon of concrete in the pouring and curing process in real time and accurately, and take corresponding crack control measures in time.

[0007] The mass concrete crack monitoring method based on distributed optical fiber technology of the present application, on the one hand, adopts a method of arranging optical fibers radially from the centroid point with the highest hydration heat in the mass concrete to the edge with lower hydration heat; on the other hand, due to the greater fluidity of early-age concrete, the concrete in the range of the circle affected by the ribbed steel bar may be compressed and limited, resulting in a large difference between the data measured by the optical fiber directly bound to the steel bar and the actual value. Therefore, the present application adopts the method of hanging (suspending) multiple S-shaped hangers on the steel mesh and hanging the optical fiber out of the range of 2-3 times the diameter of the steel bar, so as to make the monitoring of the strain and displacement of the raft foundation more accurate.

[0008] Specifically, the technical solution of the present application to solve the above technical problems is:

[0009] A mass concrete crack monitoring method based on distributed optical fiber technology, comprising the following steps in sequence:

[0010] a. Selecting the optical fiber fixing method: adopting the S-shaped hanger suspension fixing method to fix the optical fiber, hanging (suspending) multiple S-shaped hangers (or S-shaped optical fiber suspension devices) on the steel mesh, and hanging the optical fiber out of the range of 2-3 times the diameter of the steel bar;

[0011] b. Laying optical fibers: finding the centroid point with the highest hydration heat of the mass concrete, and then laying optical fibers radially from the centroid point to the edge with lower hydration heat, so as to lay multiple layers of optical fibers in the mass concrete in layers;

[0012] c. Forming monitoring field: after the fiber laying of each layer is completed, the fiber of the layer is flanged with the communication cable, and then connected with the fiber demodulator, so that the fiber monitoring system is formed, the mass concrete crack monitoring workstation is established, and the monitoring field is formed;

[0013] d. Calculating the fiber strain: after the fiber is laid in the mass concrete to be measured, the initial strain curve of the fiber is obtained by testing with the instrument before the deformation of the measured object; when the measured object produces a small deformation Δx, the strain curve of the fiber after the small deformation is measured by the instrument, and the difference between the two strain curves is obtained, that is, the actual strain ε of the fiber;

[0014] e. Calculating the crack width: according to the obtained fiber strain data, the crack width is calculated, and the calculation formula is: In the formula, ε is the fiber strain, L1 is the concrete crack width, L2 is the strain attenuation range of the concrete, and S0 is the thickness of the reinforced concrete protective layer.

[0015] Further, in the method of the present application, the strain and temperature change of each part of the fiber are calculated according to the monitored Brillouin scattering frequency shift amount ν B , and the calculation formula is: In the formula, ε is the fiber strain, ΔT is the temperature change, - temperature coefficient, - strain coefficient.

[0016] The present application judges the cracking condition inside the mass concrete by the strain value of the fiber. When the strain value of the fiber is greater than the specification requirement of the concrete cracking, it is considered that the internal crack appears, and targeted measures can be taken for control according to the position of the crack in the fiber curve.

[0017] Further, in the method of the present application, the fiber demodulator is selected from a Brillouin optical time domain reflectometer (BOTDR).

[0018] Further, in the method of the present application, a 5mm steel strand fiber is selected as the monitoring fiber through the three-equal-part loading model of the reinforced concrete beam.

[0019] Further, in the method of the present application, the fiber is selected from a metal-based cable strain sensing fiber, which comprises a fiber core, a fiber core sheath, a plurality of steel strands and a fiber sheath in sequence from inside to outside, and the plurality of steel strands are uniformly distributed on the periphery of the fiber core sheath along the circumference.

[0020] The present application adopts the above-mentioned distributed optical fiber layout method, compared with general straight-line layout, under the condition of generating the same small deformation, a greater strain difference value can be generated, thereby the sensitivity and the measurement precision of the instrument can be greatly improved, and the influence of the hydration heat phenomenon of the concrete on the monitoring data accuracy is reduced.

[0021] The improved method can monitor the cracking deformation evolution of the internal concrete of the mass concrete after pouring in real time, can meet the requirement of large-scale monitoring, and greatly improves the measurement precision.

[0022] The method of the present application is suitable for monitoring the cracks in the internal concrete of the mass concrete, is simple and easy to operate, can realize real-time monitoring of the cracking phenomenon in the pouring and curing process of the concrete, and timely corresponding crack control measures can be taken, compared with the traditional monitoring method, the accuracy of the distributed monitoring is higher, the influence of the hydration heat phenomenon of the concrete on the measurement precision can be better avoided, the measurement cost is greatly reduced, the complicated manual data collection process can be omitted, the operation is simple, economical and feasible, has wide practicability and application prospect in the technical field. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 It is a schematic view of the S-shaped hook fixing mode of the distributed optical fiber in the steel bar net in the present application;

[0024] Figure 2 It is a layout plan of the distributed optical fiber in the present application;

[0025] Figure 3 It is a layout solid drawing of the distributed optical fiber in the present application and a schematic view of the optical fiber monitoring system;

[0026] Figure 4 It is a solid drawing of the metal-based cable strain sensing optical fiber;

[0027] Figure 5 It is a sectional view of the metal-based cable strain sensing optical fiber.

[0028] In the drawing: 1-first layer optical fiber, 2-second layer optical fiber, 3-third layer optical fiber, 4-optical fiber, 5-communication optical cable, 6-S-shaped hanging ring, 7-optical fiber fixing belt, 8-steel bar net, 9-optical fiber sheath, 10-steel strand, 11-fiber core sheath, 12-fiber core. DETAILED DESCRIPTION

[0029] The specific embodiment of the present application will be further described and explained in detail in combination with the drawings.

[0030] The method for monitoring the cracks of the mass concrete based on the distributed optical fiber technology of the present application sequentially comprises the following steps:

[0031] a. Selection of optical fiber fixing method: The existing optical fiber fixing method is generally to directly bind the optical fiber on the steel mesh 8. However, due to the greater fluidity of early-age concrete, the ordinary optical fiber fixing method will cause larger error in monitoring data. In order to avoid this phenomenon, the fixing method of the optical fiber (sensing optical fiber) is changed to an S-shaped ring suspension fixing method, and a plurality of S-shaped rings 6 (or S-shaped optical fiber suspension devices) are hung (suspended) on the steel mesh 8, and the optical fiber 4 is hung out of the range of 2-3 times the diameter of the steel bar, as shown in Figure 1 The S-shaped ring suspension fixing method makes the monitoring of the strain and displacement of the raft foundation more accurate.

[0032] b. Optical fiber layout: The highest centroid point O of the hydration heat of the mass concrete is found by technical means, and the optical fiber 4 is radially laid from the centroid point O to the edge where the hydration heat is lower, as shown in Figure 2

[0033] Taking the three-layer optical fiber layout of the mass concrete as an example, as shown in Figure 3 Firstly, the first layer optical fiber 1 is laid, the optical fiber 4 is attached to the S-shaped ring 6 hung from the steel bar and is bound firmly by the optical fiber fixing belt 7, and then the optical fiber 4 is radially laid from the centroid point O to the edge. Then, the second layer optical fiber 2 and the third layer optical fiber 3 are laid in turn, and the laying method of the second layer optical fiber and the third layer optical fiber is the same as that of the first layer optical fiber.

[0034] c. Forming a monitoring field: As shown in Figure 3 After the optical fiber of each layer is laid, the optical fiber of the layer is flange-connected with the communication optical cable 5, and then is connected with the AV6419 optical fiber demodulator. The second layer and the third layer are the same as the first layer. In this way, the optical fiber monitoring system is composed, the mass concrete crack monitoring workstation is established, and the monitoring field is formed.

[0035] d. Calculating the optical fiber strain: After the optical fiber is laid in the mass concrete (measured object) to be measured, the initial strain curve of the optical fiber is obtained by testing with the instrument before the measured object deforms. When the measured object produces a small deformation Δx, the strain curve of the optical fiber after the small deformation is measured by the instrument, and the difference between the two strain curves is the actual strain ε of the optical fiber.

[0036] e. Calculating the crack width: The crack width is calculated according to the obtained optical fiber strain data, and the calculation formula is In the formula, ε is the actual strain of the distributed optical fiber, L1 is the crack width of the mass concrete, L2 is the strain attenuation of the mass concrete within the range of L2, and S0 is the thickness of the protective layer of the mass concrete.

[0037] ​The strain value of the optical fiber is collected by a distributed optical fiber collector, data analysis is carried out after being connected to a computer, the crack width is calculated according to the strain value of the optical fiber, so that the cracking condition in the mass concrete is judged. When the strain value of the optical fiber is greater than the cracking specification of the concrete, it is considered that the crack appears in the interior, and the position of the crack in the optical fiber strain curve can be used to take targeted measures for control.

[0038] In the method of the present application, when the object to be measured produces a small deformation Δx, the contrast image measured on the instrument will have a strain difference. The larger Δx is, the more obvious the strain difference of the measured image is in theory. The degree of differentiation of the strain difference can be defined by the discrimination. When the discrimination is poor, it can be considered that the object to be measured has not been displaced; on the contrary, when the discrimination is high, it is beneficial to timely judge the deformation, which is beneficial to the monitoring of the project. When the strain curve obtained after deformation and the initial strain curve are consistent or the hierarchical feeling is not strong, and the discrimination is poor, it can be judged that no deformation has occurred; on the contrary, if there is a clear hierarchical distinction between the two, that is, the discrimination is good, it can be judged that deformation has occurred. By further judging the development degree of the crack, corresponding control measures can be proposed.

[0039] In the method of the present application, the optical fiber is a metal-based cable-shaped strain sensing optical fiber. Figures 4-5 As shown in the figure, the metal-based cable-shaped strain sensing optical fiber includes a core 12, a core sheath 11, a plurality of steel strands 10 and an optical fiber sheath 9 in sequence from inside to outside, wherein the plurality of steel strands 10 are uniformly distributed on the periphery of the core sheath 11 along the circumference.

[0040] By embedding the optical fiber in the mass reinforced concrete beam, using the three-point concentrated loading method, the strains of different types of optical fibers are tested and compared with the strain values measured by the strain gauges attached to the steel bars. The results show that compared with the polyurethane optical fiber, the steel strand optical fiber has better cooperative deformation ability because its elastic modulus is closer to the concrete medium, so the monitoring data of the steel strand optical fiber is more accurate and less affected by external factors. Therefore, the present application selects a 5mm steel strand optical fiber as the optical fiber for monitoring the cracks in the concrete.

[0041] In the method of the present application, the optical fiber demodulator is preferably a Brillouin optical time domain reflectometer (BOTDR). At the same time, the radial arrangement of the distributed optical fiber and the S-shaped ring fixing method still belong to the field of distributed optical fiber sensing technology, and provide a new method and idea for the engineering application of the BOTDR technology.

[0042] The optical fiber is radially arranged from the centroid to the edge in three layers, and the optical fiber is fixed on the steel mesh of the raft foundation by adopting the S-shaped hanging ring, and the measurement accuracy can reach 1mm, and the monitoring accuracy requirement of most projects can be met. It can be seen that the application can monitor the crack situation generated in the internal curing and maintenance process of mass concrete in real time and effectively, and can improve the monitoring sensitivity and measurement accuracy.

[0043] In summary, the monitoring method disclosed by the application can meet the requirements of large-scale monitoring, greatly improve the measurement accuracy, and greatly avoid the measurement data errors caused by the hydration heat phenomenon of concrete. Therefore, the application can meet the requirement of the distributed optical fiber sensing technology for accurately extracting the distributed information of a large-scale measurement field, and achieve the real-time monitoring of the internal cracks of concrete, so that corresponding control measures can be taken for the internal cracks of concrete in time.

Claims

1. A method for monitoring cracks in large-volume concrete based on distributed optical fiber technology, comprising the following steps: a. Select the fiber fixing method: Use the S-shaped hanging ring to fix the fiber (4), and hang multiple S-shaped hanging rings (6) on the steel mesh (8) to hang the fiber (4) out of a range of 2-3 times the diameter of the steel bar; b. Fiber optic cable layout: Find the centroid point with the highest heat of hydration in the large volume concrete, and then lay out the fiber optic cable radially from the centroid point to the edge with lower heat of hydration (4). In this way, multiple layers of fiber optic cable are laid out in the large volume concrete. c. Forming a monitoring field: After the fiber optic cables of each layer are laid out, the fiber optic cables of that layer are connected to the communication optical cable (5) by flange connection, and then connected to the fiber optic demodulator. In this way, a fiber optic monitoring system is formed, a large-volume concrete crack monitoring workstation is established, and a monitoring field is formed. d. Calculating fiber optic strain: After the fiber optic cable is installed in a large volume of concrete, it is necessary to test it with an instrument before the object under test deforms to obtain the initial strain curve of the fiber optic cable; when the object under test undergoes slight deformation... Then, the strain curve of the optical fiber after undergoing a slight deformation is measured by the instrument. The strain difference obtained by subtracting the two strain curves is the actual strain of the optical fiber. ; e. Calculate the crack width: Based on the obtained fiber strain data, calculate the crack width using the following formula: In the formula, -Fiber optic strain, - Concrete crack width, - The range of strain attenuation in concrete at cracks -Thickness of reinforced concrete cover; f. Data Analysis: The strain values ​​of optical fibers are collected by a distributed optical fiber acquisition instrument, connected to a computer for data analysis, and the crack width is calculated based on the strain value of the optical fiber to determine the cracking situation inside the large volume of concrete. When the strain value of the optical fiber is greater than the concrete cracking specification requirements, it is considered that cracks have appeared inside the concrete. Based on the location of the cracks in the optical fiber strain curve, targeted measures are taken for control.

2. The method for monitoring cracks in large-volume concrete based on distributed optical fiber technology according to claim 1, characterized in that: When cracks form inside the concrete, the monitored Brillouin dispersion frequency shift is used as a reference. The strain and temperature changes at various points along the optical fiber were calculated using the following formula: In the formula, -Fiber optic strain, - Temperature change -Thermosensitive coefficient, - Strain coefficient.

3. The method for monitoring cracks in large-volume concrete based on distributed optical fiber technology according to claim 2, characterized in that: The fiber optic demodulator is a Brillouin optical time domain reflectometer.

4. The method for monitoring large-volume concrete cracks based on distributed optical fiber technology according to claim 2 or 3, characterized in that: Using a reinforced concrete beam with a three-part loading model, 5mm steel stranded optical fiber was selected as the monitoring optical fiber.

5. The method for monitoring cracks in large-volume concrete based on distributed optical fiber technology according to claim 4, characterized in that: The optical fiber (4) is a metal-based cable strain sensing optical fiber.

6. The method for monitoring cracks in large-volume concrete based on distributed optical fiber technology according to claim 5, characterized in that: The metal-based cable-like strain sensing optical fiber includes, from the inside out, a fiber core (12), a fiber core sheath (11), multi-strand steel strands (10), and an optical fiber sheath (9). The multi-strand steel strands (10) are evenly distributed around the periphery of the fiber core sheath (11).

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