A prestressed duct friction test detection method and system

Abstract

The application provides a prestressed hole friction test detection method and system, a weak grating is integrated in a prestressed steel strand and is unevenly distributed according to a hole linear shape; a demodulator is provided with an independent temperature compensation channel and a zero-load baseline is established; double-end synchronization / alternation staged tensioning is performed, and along-path strain is collected in a stable window; geometric identification obtains discrete segment length l and angle θ; multi-stage full-measuring-point data weighted nonlinear least squares are combined to solve friction coefficient μ and deviation coefficient k, and residual error / sensitivity / confidence interval is output; residual error exceeding a threshold triggers segmented identification and returns a control instruction, forming a loading-measuring-identifying-controlling closed loop. The system is composed of optical fiber sensing, tensioning execution, demodulation, control and data processing modules, and can generate segmented μ, k and along-path friction distribution and leave marks for archiving.

Description

Technical Field

[0001] This invention relates to the field of prestressed construction technology, specifically to a method and system for testing the friction resistance of prestressed ducts. Background Technology

[0002] With the increasing span of highway and railway bridges, including the construction of elevated intercity subway lines, the application of prestressed concrete beams, capable of adapting to complex working conditions, is becoming increasingly widespread. To ensure the structural safety and durability of the beams during service, accurately determining the friction coefficient μ and duct deviation coefficient k of the prestressed ducts has become a crucial aspect of construction control and design verification. The current "Test Method for Friction of Post-Tensioned Prestressed Concrete Beams for Railways" Q / CR 566-2017 provides unified regulations for friction testing: using a method of anchoring at one end and tensioning at the other, with alternating graded loading, the friction parameters are derived by measuring the tension loss.

[0003] However, existing friction testing methods still have certain limitations: First, this method relies on pressure measurements at the tensioning and anchoring ends, and is limited by the accuracy of the pressure sensor and the stability of the hydraulic jack's return oil. Influencing factors include: 1. Construction sites often involve working at heights, requiring sensors, jacks, and other equipment to be installed using hoisting methods, leading to some misalignment between the equipment and the duct; 2. Since no working anchor is installed during the duct friction test, the steel strand cannot be pulled to the design control stress using a "reverse jacking" method. For some large-span beams (over 60m), the long duct length poses a significant safety risk to the test (jacks need to be installed in series). Secondly, traditional methods can only obtain the overall friction characteristics of the duct, but cannot obtain the friction distribution at different locations inside the duct, making it difficult to reflect the real situation of long or curved ducts. When the duct shape is complex or there are multiple curves, the calculation assumptions of traditional methods differ from the actual situation, resulting in insufficient parameter representativeness. This makes it impossible to truly guide on-site prestressed construction operations, which may lead to potential risks to the durability of the beam. Finally, traditional tests often require multiple alternating loading stages, making the testing process cumbersome and requiring a large number of instruments, which increases the difficulty and cost of detection.

[0004] Therefore, how to propose a new testing and detection method and system that can realize distributed monitoring of the entire channel, improve the accuracy of friction parameter calculation, and have good adaptability and automation level, based on the principles of Q / CR 566-2017 standard, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention provides a novel testing and detection method and system that can achieve distributed monitoring of the entire channel, improve the accuracy of friction parameter calculation, and has good adaptability and automation.

[0006] The present invention solves the above-mentioned technical problems through the following technical solution: A method for testing the friction of prestressed ducts includes: A) Integrate weak grating optical fibers into prestressed steel strands to form fiber-optic prestressed steel strands, and arrange the sensing points of the weak grating optical fibers at non-uniform intervals according to the channel line shape, so that the spacing between measuring points in the curvature / bend section is smaller than that in the straight section. B) The prestressed steel strand is threaded through the test duct and tensioning and anchoring devices are installed at both ends; C) Connect the weak grating fiber to the demodulator, set up an independent temperature compensation channel and establish a zero-load baseline; D) The control system performs synchronous or alternating graded tensioning at both ends, and synchronously collects strain along the process and converts it into distributed tension during each stability window; E) The data processing unit automatically acquires or reverses the duct path parameters, including the discrete segment lengths li,i+1 and the bend angles θi,i+1 between adjacent measuring points; F) Establishing a system based on full-point measurement data with multi-level loading. The equations are solved by weighted nonlinear least squares to obtain the full-channel friction coefficient μ and deviation coefficient k, and the residual plot, sensitivity and confidence interval are output. G) When the residual exceeds the threshold in the curvature / angle concentrated segment, segment identification is triggered to obtain segment μ and k, and control commands are sent back, thus forming a closed-loop process of loading-measurement-identification-control.

[0007] In a specific embodiment, the temperature compensation channel uses an isolated grating channel or a bare grating for ΔT correction. Strain-temperature separation is performed before tensile force conversion and parameter identification. The fiber optic demodulator is equipped with an independent temperature compensation channel to measure the temperature along the path and... Strain-temperature separation is performed.

[0008] In a specific embodiment, the stability window is determined jointly by the variance of the strain / tension time series and the gradient threshold of the end force curve, and the extinguished grid and abnormal noise measurement points are automatically shielded.

[0009] In one specific embodiment, the outer strands of the prestressed steel strand are provided with a semi-enclosed shallow groove along the spiral direction, the weak grating optical fiber is placed in the shallow groove and fixed by encapsulation with structural adhesive with an elastic modulus of 1–3 GPa, the groove depth is 2%–8% of the strand diameter, and the groove is covered with a protective layer with a thickness of 0.3–1.0 mm.

[0010] A prestressed duct friction testing system includes: Fiber optic sensing module with fiber optic prestressed steel strand and non-uniformly spaced measuring points. Tensioning execution module, double-end alternating graded tensioning; Fiber optic demodulator, including temperature compensation channel; Data processing unit, configuration: a. Geometric recognition module, automatically acquires / inversely calculates l, θ; b. Quality control module: stable window identification, anomaly shielding, and end-point verification; c. Parameter identification module: performs weighted nonlinear least squares on multi-level full measurement point data, outputs μ, k, residual / sensitivity / confidence interval, and performs segmented identification when residuals aggregate; The control system communicates bidirectionally with the tensioning execution module and the data processing unit to issue hold / upgrade / retest commands based on the recognition results, thereby completing the closed-loop test.

[0011] In a specific embodiment, the system further includes a data processing module for establishing a four-level read-only data repository for engineering → components → ducts → tests. This module implements read-only archiving and access control for raw test data, processing results, version information, and parameter settings, and records tamper-proof logs. It also supports one-click generation of raw records containing curves, tables, and layer overlay results.

[0012] In a specific embodiment, the following test release criteria are set: the equivalent loss deviation at the end is calculated based on the difference between the equivalent loss measured at the end and the equivalent loss calculated by the model. When the deviation is ≤10% and the determination coefficient R² of the global fit is ≥0.95, the advanced loading or the parameter identification convergence is allowed; if the conditions are not met, the retest, abnormal measurement point shielding and parameter re-estimation process is automatically triggered before the determination is made.

[0013] This invention provides a method and system for testing the friction of prestressed ducts, which has the following advantages compared to the prior art: This method significantly improves the sensitivity and spatial resolution of frictional changes in complex sections such as bends by using linearly "non-uniform" point placement and automatic geometric identification (discrete segment length li,i+1, bend angle θi,i+1), avoiding undersampling errors from uniform point placement. An independent temperature compensation channel and zero-load baseline separate the temperature term from its source, followed by tensile force conversion and parameter identification, giving μ and k a more stable physical orientation.

[0014] At the data and control level, a dual-end synchronous / alternating graded tensioning method is introduced, with sampling using a "stable window" as the admission criterion. Combined with anomaly shielding and end-point mutual verification, reliable data input is provided. The weighted nonlinear least squares solution is used for multi-level full measurement points to output residuals, sensitivity, and confidence intervals. When the residual exceeds the threshold, segmented identification is triggered and control commands are sent back, forming a closed loop of loading-measurement-identification-control, which shortens the test time and improves convergence determinism.

[0015] In terms of sensing links and quality control, the shallow trench-filled fiber-steel strand integration ensures efficient strain transfer while keeping the load-bearing capacity impact within an acceptable range; setting release criteria (end equivalent loss deviation ≤10%, global fitting R²≥0.95) unifies convergence standards and retesting paths; and with read-only data processing and one-click reporting for "engineering → component → duct → test", results are traceable and standardized. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0017] Figure 1 This is a general block diagram of the system. Figure 2 This is a flowchart of the method. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1: A method for testing the friction of prestressed ducts A prestressed duct with a single bend was selected as the object. The fiber-optic prestressed steel strand adopted an integrated structure with semi-closed shallow grooves in the spiral direction of the outer strands. The weak grating fiber was embedded in the groove and fixed with structural adhesive with an elastic modulus of 1–3 GPa. The groove depth was 2%–8% of the strand diameter, and a 0.3–1.0 mm protective layer was applied. After the strands were threaded, tensioning and anchoring devices were configured at both ends as usual, and the fiber lead-out end was reliably terminated with the demodulator. Before leaving the test bench, the strain transfer coefficient and temperature coefficient were obtained as the basic data for release upon arrival.

[0020] Non-uniform measurement point layout was implemented along the channel line: points were placed at 0.2–0.5 m intervals within the bends and ±2 m ranges, and at 0.5–1.0 m intervals along straight sections, with appropriate densification at locations suspected of sudden frictional changes. An independent temperature compensation channel was set up on the demodulator to establish a zero-load baseline under no-load conditions; strain-temperature separation (εmeas=εf) was performed before on-site data acquisition. kT·ΔT, εmeas: effective strain after temperature correction, used for subsequent tensile / parameter identification; εf: apparent strain originally measured by the fiber grating; kT: temperature coefficient of the fiber / demodulation link; ΔT: temperature change relative to the zero-load baseline), and then the strain along the path is converted into distributed tensile force to provide temperature-corrected data for subsequent identification.

[0021] The loading process is organized by the control system through synchronous or alternating staged tensioning at both ends. After each stage reaches the target value, it is held for a period of time to form a stable window. The quality control logic determines whether it has entered the stable window based on the variance of the strain / tension time series along the friction and the gradient threshold of the end force curve. Only the data within this window is accepted, and the grid extinguishing and abnormal noise measurement points are automatically blocked. The end oil pressure / displacement and the friction-inverted tension are used for equivalent loss verification, and the verification deviation will be one of the factors in the subsequent weighting.

[0022] The data processing unit automatically acquires or inversely calculates the discrete segment lengths li,i+1 and bend angles θi,i+1 between adjacent measurement points. When the design geometric data is incomplete, a usable discrete geometric sequence is provided by using thread counting combined with end scanning / endoscopic methods to ensure the completeness and consistency of the parameterized input of the exponential decay model.

[0023] Based on the full measurement point data loaded at multiple levels, construct Discrete system of equations, Where Pi: the axial force of the prestressed steel strand at the i-th measuring point; Pi+1: The axial force at the next adjacent measuring point i+1; μ: Pipe friction coefficient; θi,i+1: The cumulative rotation angle of the duct from measuring point i to i+1; k: Deviation coefficient; li,i+1: The length of the centerline between measurement point i and i+1; A weighted nonlinear least squares approach is used for joint solution. The weights are derived from the variance assessment within the stability window and the cross-validation bias of the equivalent loss at the ends. The solution simultaneously outputs a residual plot, sensitivity index, and 95% confidence interval, serving as a direct quantification of the reliability of the results.

[0024] When the residuals accumulate in concentrated sections of curvature or bends and exceed a threshold, the system triggers segmented identification, dividing the channel into several identification segments, calculating μ and k for each segment, and generating a friction distribution curve along the path. The identification results are overlaid with the global fit and residual distribution in a layered manner, facilitating on-site verification and report generation.

[0025] The identification results are transmitted back to the control system in real time, generating a closed-loop command of "hold / advance / retest". The test release criteria are: the equivalent loss deviation at the end point is ≤10%, and the global fitting determination coefficient R² is ≥0.95. If the conditions are met, advancement is allowed or convergence is determined; if not, retesting, outlier masking, and parameter reestimation are automatically triggered until the release criteria are met.

[0026] Finally, the data processing module archives the raw data, processing version, and parameter configuration in a four-level structure of "engineering → component → duct → test" in read-only mode and records logs, generating a report containing curves, tables, and layer overlay results with one click. Through the above process, this embodiment completes the closed loop of "loading-measurement-identification-control" within one test cycle, obtaining μ and k parameters with higher spatial resolution, complete statistical data, and traceability.

[0027] Example 2: A prestressed duct friction testing system The system in this embodiment consists of an optical fiber sensing module, a tensioning execution module, an optical fiber demodulator, a data processing unit, and a control system. All modules are interconnected via an industrial Ethernet and serial bus to form a low-latency closed-loop channel. The optical fiber sensing module uses fiber-optic prestressed steel strands as the carrier, and the weak gratings are arranged non-uniformly according to the channel shape: the gratings are denser in the angled and high-curvature areas to 0.2–0.5 m, and in the straight areas to 0.5–1.0 m. To ensure strain transfer and durability, semi-enclosed shallow grooves are opened in the spiral direction of the outer strands of the fiber-optic prestressed steel strands, and the optical fibers are coated with medium-high modulus structural adhesive and covered with a protective layer.

[0028] The tensioning execution module integrates a double-ended jack with a force / displacement sensor, supporting both synchronous and alternating loading modes. The control system issues target force, holding time, and cascading strategies, and retrieves end measurements in real time. The fiber optic demodulator is equipped with an independent temperature compensation channel, synchronously acquiring strain and temperature sequences along the path. After system startup, a zero-load baseline is established. During operation, temperature values ​​are converted using a kT coefficient to eliminate the influence of temperature on wavelength drift at the source, before outputting temperature-corrected strain data for calculation.

[0029] The data processing unit comprises three software modules: geometric recognition, quality control, and parameter recognition. The geometric recognition module automatically imports the designed duct profile. If data is missing, it combines thread counting and end-scan / endoscopic results to inversely calculate discrete segment lengths and bend angles, generating l and θ sequences corresponding one-to-one with the measurement points. The quality control module performs a "stability window" determination on the data stream at each loading stage. The rules are jointly determined by the variance of the strain / tension sequence along the path and the gradient threshold of the end force curve. Data outside the stability window is not included in the calculation. Simultaneously, it automatically masks grid extinguishing and abnormal noise points, and writes the equivalent loss deviation between the end oil pressure / displacement and the inverted tension into the weight dictionary for later use in the solution.

[0030] After obtaining the full measurement point data from multi-level loading, the parameter recognition module proceeds according to... A discrete system of equations is established and solved jointly using weighted nonlinear least squares. The weights are derived from the stability window variance, end-point cross-verification bias, and measurement point quality markings. The solution simultaneously outputs residual plots, sensitivity matrices, and 95% confidence intervals, and performs spatial clustering analysis on the residuals. When the residuals exceed the threshold in the curvature or bend angle concentrated segment, the system automatically switches to segmented recognition mode, calculates the segmented μ and k respectively, and generates a friction distribution curve along the path. The results are displayed on the front end in a layered overlay manner, showing the global fit, segmented parameters, and residual heat, facilitating on-site verification.

[0031] The control system maintains bidirectional communication with the tensioning execution module and data processing unit, using the identification results for process decision-making: when the equivalent end loss deviation does not exceed 10% and the global determination coefficient R² is not less than 0.95, it automatically issues an upgrade or convergence command; if these conditions are not met, it issues a retest command and lowers the sampling step size or extends the holding time to improve steady-state quality, while simultaneously triggering abnormal measurement point re-evaluation and weight update. Through this closed-loop strategy, the system shortens the test cycle and improves parameter convergence determinism without increasing the burden of manual judgment.

[0032] The data processing module implements read-only archiving of raw data, processed versions, parameter configurations, and release criteria according to a four-level structure of "engineering → component → duct → test," and generates an immutable audit log. After the test, the system can output a standardized report with one click. The report includes global and segmental μ, k, residual and sensitivity maps, end-point cross-verification results, and the release decision process. It supports dual-track archiving of PDF and data packages, meeting the traceability requirements for review and verification.

[0033] In field applications, the system can operate under different duct alignments, such as concrete T-beams, box girders, or continuous beams. Through the synergy of non-uniform point distribution, independent temperature compensation, stable window admission, and weighted joint solution, it achieves engineering output that is "accurate in measurement, controllable, and clear in explanation." Even under conditions where design data is incomplete or construction deviations are large, it can still obtain μ and k parameters with confidence through geometric inverse calculation and segmented identification, and ensure the efficiency and consistency of the test organization through closed-loop control.

[0034] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0035] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for testing the friction of prestressed ducts, characterized in that, include: A) Integrate weak grating optical fibers into prestressed steel strands to form fiber-optic prestressed steel strands, and arrange the sensing points of the weak grating optical fibers at non-uniform intervals according to the channel line shape, so that the spacing between measuring points in the curvature / bend section is smaller than that in the straight section. B) The prestressed steel strand is threaded through the test duct and tensioning and anchoring devices are installed at both ends; C) Connect the weak grating fiber to the demodulator, set up an independent temperature compensation channel and establish a zero-load baseline; D) The control system performs alternating, graded tensioning at both ends, and synchronously collects strain along the path and calculates distributed tension during each stability window; E) The data processing unit automatically acquires or reverses the duct path parameters, including the discrete segment lengths li,i+1 and the bend angles θi,i+1 between adjacent measuring points; F) Establishing a system based on full-point measurement data with multi-level loading. The equations are solved by weighted nonlinear least squares to obtain the full-channel friction coefficient μ and deviation coefficient k, and the residual plot, sensitivity and confidence interval are output. G) When the residual exceeds the threshold in the curvature / angle concentrated segment, segment identification is triggered to obtain segment μ and k, and control commands are sent back, thus forming a closed-loop process of loading-measurement-identification-control.

2. The method for testing the friction of prestressed ducts according to claim 1, characterized in that, The temperature compensation channel uses an isolated grating channel or a bare grating for ΔT correction. Strain-temperature separation is performed before tensile force conversion and parameter identification. The fiber optic demodulator is equipped with an independent temperature compensation channel to measure the temperature along the path and... Perform strain-temperature separation; Where εmeas: effective strain after temperature correction, used for subsequent tensile force / parameter identification; εf: The apparent strain originally measured by the fiber grating; kT: Temperature coefficient of the fiber optic / demodulation link; ΔT: Temperature change relative to the zero-load baseline.

3. The method for testing the friction of prestressed ducts according to claim 1, characterized in that, The stability window is determined by the combined variance of the strain / tension time series and the gradient threshold of the end force curve, and automatically shields the extinguished grid and abnormal noise measurement points.

4. The method for testing the friction of prestressed ducts according to claim 1, characterized in that, The prestressed steel strand has a semi-enclosed shallow groove along the spiral direction on the outer strands. The weak grating optical fiber is placed in the shallow groove and fixed by encapsulation with structural adhesive with an elastic modulus of 1–3 GPa. The groove depth is 2%–8% of the strand diameter, and the groove is covered with a protective layer with a thickness of 0.3–1.0 mm.

5. A prestressed duct friction test system, characterized in that, include: Fiber optic sensing module with fiber optic prestressed steel strand and non-uniformly spaced measuring points. Tensioning execution module, double-end alternating graded tensioning; Fiber optic demodulator, including temperature compensation channel; Data processing unit, configuration: a. Geometric recognition module, automatically acquires / inversely calculates l, θ; b. Quality control module: stable window identification, anomaly shielding, and end-point verification; c. Parameter identification module: performs weighted nonlinear least squares on multi-level full measurement point data, outputs μ, k, residual / sensitivity / confidence interval, and performs segmented identification when residuals aggregate; The control system communicates bidirectionally with the tensioning execution module and the data processing unit to issue hold / upgrade / retest commands based on the recognition results, thereby completing the closed-loop test.

6. The prestressed duct friction test system according to claim 5, characterized in that, The system also includes a data processing module, which is used to establish a four-level read-only data repository for engineering → components → ducts → tests. It implements read-only archiving and access control for raw test data, processing results, version information and parameter settings, and records tamper-proof logs. It supports one-click generation of raw records containing curves, tables and layer overlay results.

7. The prestressed duct friction test system according to claim 5, characterized in that, The system also sets the following test release criteria: the equivalent loss deviation at the end is calculated based on the difference between the equivalent loss measured at the end and the equivalent loss calculated by the model. When the deviation is ≤10% and the determination coefficient R² of the global fit is ≥0.95, the advanced loading or the parameter identification convergence is allowed; if the conditions are not met, the retest, abnormal measurement point shielding and parameter re-estimation process is automatically triggered before the judgment is made.

Images