Prediction model for durability life of prestressed concrete pipe in chlorine salt erosion environment

Abstract

The invention relates to the technical field of life prediction, in particular to a model for predicting the durability life of a prestressed concrete pipe in a chlorine salt erosion environment. The method comprises the following steps: calibrating material key parameters required by a prediction model through an accelerated corrosion test of a prototype pipe, calculating a time-varying chloride ion diffusion coefficient based on the material key parameters and collected environmental data, solving the time required by diffusion of chloride ions to the surface of a prestressed tendon and reaching a critical concentration through numerical calculation, and taking the time as the corrosion initiation life of the pipeline. And predicting the failure life. According to the method, on the basis of a full-size prototype test, material science, a transmission theory, electrochemistry and structural mechanics are fused, so that the problems of parameter distortion, mechanism simplification, boundary stiffness, lack of engineering representativeness and the like in a traditional life prediction model are solved; and accurate, reliable, explainable and decision-supportable intelligent prediction of the durability of the large water delivery pipeline in a complex service environment is realized.

Description

Technical Field

[0001] This invention relates to the field of life prediction technology, and more specifically, to a model for predicting the durability life of prestressed concrete pipes under chloride salt corrosion environment. Background Technology

[0002] Prestressed concrete cylinder pipe (PCCP) is a composite pipe composed of a shotcrete protective layer, high-strength prestressed steel wires, a steel cylinder, and a concrete core. It boasts advantages such as high pressure resistance, excellent impermeability, smooth inner wall with low resistance, convenient installation, and low maintenance costs, making it the preferred pipe material for large-diameter, long-distance, high-pressure water transmission pipeline projects both domestically and internationally. However, since PCCP pipelines are mostly buried underground, they inevitably encounter saline-alkali areas. In these areas, corrosive media such as Cl- and SO42- in the soil penetrate the outermost mortar protective layer. On one hand, when Cl- accumulates on the surface of the prestressed steel wires and reaches a critical chloride ion concentration, it damages the passivation film on the surface of the prestressed steel wires, inducing corrosion. On the other hand, SO42- reacts chemically with hydration products to generate expansive products, causing the protective layer to expand and crack, further accelerating the accumulation of chloride ions on the surface of the prestressed steel wires, ultimately leading to corrosion and breakage, and thus causing pipe burst accidents. Pipe burst accidents not only cause huge economic losses and resource waste but also threaten people's lives.

[0003] Therefore, considering stress corrosion and hydrogen embrittlement, a reinforced concrete cylinder pipe (BCCP) was developed. This pipe type inherits the advantages of PCCP such as high working pressure, high impermeability, and high sealing performance. At the same time, it uses cold-rolled ribbed prestressed steel bars with lower stress level to replace high-strength prestressed steel wires, and replaces the outermost 30 mm thick shotcrete protective layer with a 50 mm thick fine stone concrete protective layer. This overcomes the problem of hydrogen embrittlement fracture of high-strength prestressed steel wires in PCCP and improves the corrosion resistance of the pipe itself.

[0004] Both PCCP and BCCP use steam curing for maintenance. On the one hand, this can accelerate the turnover rate of molds and improve production efficiency; on the other hand, it can provide early strength for the pipeline, making it easier to transport the pipeline to the project site as soon as possible. However, due to the violent hydration reaction during steam curing, a large number of bubbles are generated, which leads to an increase in large pores inside the concrete and a deterioration of the pore structure. This weakens the resistance of the concrete protective layer to chloride ion penetration. Chloride ions are the most important factor affecting the durability of reinforced concrete structures and are also the main culprit for the corrosion and breakage of prestressed steel wires in PCCP. When PCCP and BCCP are used in engineering applications, their operating environment is extremely complex. There is a high possibility that a large number of corrosive compounds are distributed around the pipes. In particular, some areas are arid and have little rain all year round, with frequent alternation of dry and wet conditions and widespread saline-alkali soil. Harmful media such as chlorides and sulfates in the soil can penetrate the pipeline, causing corrosion of the prestressed steel wires or cracking and peeling of the protective layer, posing a hidden danger to the safety of the pipeline structure and even the entire water transmission project.

[0005] Therefore, considering the problems encountered in the research and application of PCCP pipelines, this study investigates the transport and damage mechanisms of chloride and sulfate ions in the protective layers of PCCP and BCCP based on the material properties and production processes of PCCP and BCCP, using a combination of experimental and theoretical methods. A theoretical model for chloride and sulfate ion transport in the protective layers of PCCP and BCCP is established, and a model capable of effectively predicting the durability of PCCP and BCCP in chloride and sulfate environments is proposed. This provides a theoretical basis for predicting the durability of in-service PCCP and BCCP pipelines, coating protection, and timely repair or replacement.

[0006] Currently, ion corrosion is one of the main factors causing corrosion and pipe bursting of prestressed steel wires in PCCPs. While domestic and international scholars have achieved many beneficial results in the study of the durability of ordinary reinforced concrete structures, research on the durability of PCCPs or BCCPs is relatively limited. Furthermore, some conclusions are derived from research results on ordinary reinforced concrete structures and the characteristics of prestressed reinforced concrete structures themselves. The experimental data obtained mainly comes from indoor model tests and cannot truly reflect the performance of the pipes themselves. Predicting the chloride ion corrosion life of concrete structures is generally based on Fick's second law, but directly applying this model to PCCPs / BCCPs has significant limitations, including:

[0007] Firstly, almost all existing research data comes from small standard specimens, while PCCP / BCCP is a large-sized composite structure that is steam-cured and has internal stress after winding prestressing. The microstructure, initial defects, and interface state with steel wire of its protective layer (roller-shot mortar for PCCP and cast concrete for BCCP) are fundamentally different from those of small specimens formed by vibration in the laboratory. Predicting pipeline life based on specimen data leads to distorted prediction results.

[0008] Secondly, steam curing is commonly used in the production of PCCP / BCCP. The diffusion capacity of chloride ions in concrete decreases significantly with the continuous hydration of the material. Existing models simplify the diffusion coefficient to a constant, which will seriously overestimate the early erosion rate and result in an overly short predicted life. Alternatively, empirical coefficients are used for "matching", resulting in a lack of specific consideration in the model. Furthermore, the diffusion coefficient of chloride ions in concrete has obvious time-varying characteristics (decreases with age), and many models simplify it to a constant, which will overestimate the early diffusion rate and result in a dangerously biased life prediction.

[0009] In view of this, we propose a model for predicting the durability life of prestressed concrete pipes under chloride salt corrosion environment. Summary of the Invention

[0010] The purpose of this invention is to provide a durability life prediction model for prestressed concrete pipes under chloride salt corrosion environment, so as to solve the problems mentioned in the background art, such as the performance gap between material specimens and engineering entities, the dilemma of universal values ​​of key physical property parameters, and the lack of closed-loop verification of the model.

[0011] To achieve the above objectives, this invention provides a model for predicting the durability life of prestressed concrete pipes under chloride salt corrosion environment, comprising the following steps:

[0012] For the target pipeline type, accelerated corrosion tests on its prototype pipe are conducted to calibrate the key material parameters required for the prediction model using the test data.

[0013] When predicting the target pipeline, the corresponding key parameters are called according to its pipe material type. The key parameters include at least: protective layer thickness, reference chloride ion diffusion coefficient, age decay index, critical chloride ion concentration of prestressed tendons and soil chloride ion concentration of the burial environment to determine the chloride ion concentration on the pipe wall surface.

[0014] Based on key material parameters and collected environmental data, the time-varying chloride ion diffusion coefficient is calculated to characterize the dynamic evolution of chloride ion transport capacity in the pipeline protective layer as a function of time, ambient temperature, and material properties.

[0015] The time-varying chloride ion diffusion coefficient, the protective layer thickness, the surface chloride ion concentration determined based on the environmental chloride ion concentration, and the calibrated critical chloride ion concentration of the prestressing tendon are input into a diffusion analysis model that considers the chemical binding effect of chloride ions. The time required for chloride ions to diffuse to the surface of the prestressing tendon and reach the critical concentration is calculated numerically and used as the pipeline corrosion initiation life.

[0016] Using the corrosion initiation life as the starting point, the corrosion development process of the prestressed tendons is simulated based on the electrochemical corrosion kinetic model. The cross-sectional area loss of the tendons caused by corrosion is calculated. When the cross-sectional area loss reaches the preset failure threshold determined based on the initial tensile stress and material strength, the corresponding total time is the predicted failure life of the pipeline.

[0017] As a further improvement to this technical solution, the pipe type is a prestressed concrete cylinder pipe (PCCP) or a reinforced concrete cylinder pressure pipe (BCCP).

[0018] As a further improvement to this technical solution, the accelerated corrosion test includes the following steps;

[0019] A sealed corrosion chamber was set on the outer wall of a prestressed steel cylinder concrete pipe or a prototype prestressed steel cylinder concrete pipe with steel reinforcement winding. Chloride salt solution was injected and a dry-wet cycle was implemented to simulate accelerated corrosion.

[0020] At multiple different corrosion ages, cylindrical core samples penetrating the protective layer thickness were drilled from the corrosion chamber area. The core samples were then layered from the outer surface to the inner surface, and the free chloride ion concentration in each layer was measured to obtain a series of chloride ion concentration distribution curves at different time points as a function of depth.

[0021] As a further improvement to this technical solution, the key material parameters required for calibrating the prediction model further include diffusion parameter inversion and critical concentration determination, comprising the following steps:

[0022] For each of the aforementioned distribution curves, iterative inversion calculations are performed based on the unsteady-state diffusion theory to solve for the power exponent of the decay of the chloride ion diffusion coefficient of the pipeline protective layer material with time, as well as the reference chloride ion diffusion coefficient at the reference time point; simultaneously, by analyzing the relationship between the total chloride ion and free chloride ion concentrations at the same location, the chloride ion chemical binding parameters of the material are calibrated.

[0023] Simulated specimens embedded with real prestressed tendons were prepared, and the electrochemical state of the tendons inside the specimens was monitored simultaneously during the accelerated corrosion process.

[0024] When a sudden change in electrochemical parameters indicating passivation film rupture is detected, the experiment is immediately terminated and the concentration of free chloride ions in the protective layer at the interface of the reinforcing material is measured. The critical chloride ion concentration value is determined by statistical analysis of multiple sets of experiments.

[0025] As a further improvement to this technical solution, the specific steps for performing the iterative inversion calculation are as follows:

[0026] Set an initial assumed value for the age decay index and calculate the equivalent diffusion time corresponding to each corrosion time in the test;

[0027] Using the chloride ion concentration distribution test data at each corrosion age, the apparent chloride ion diffusion coefficient corresponding to each age is derived based on Fick's second law. The power function relationship between the apparent chloride ion diffusion coefficient and the equivalent diffusion time is fitted to obtain the fitted reference chloride ion diffusion coefficient and the fitted age decay index.

[0028] Determine whether the difference between the fitted age decay index and the initial assumed value is less than the preset tolerance;

[0029] If so, the final age decay index is output as the fitted age decay index, and the reference chloride ion diffusion coefficient is output as the fitted reference chloride ion diffusion coefficient.

[0030] If not, the fitted age decay index is used as the new initial assumption, and the iteration calculation is repeated until the tolerance requirement is met.

[0031] As a further improvement to this technical solution, the range of the age decay index is as follows:

[0032] For PCCP pipes, the age decay index ranges from 0.559 to 0.676.

[0033] For BCCP pipes, the age decay index ranges from 0.616 to 0.791.

[0034] The calibrated range of the critical chloride ion concentration, calculated as a percentage of the protective layer material by mass, is as follows:

[0035] For PCCP pipes, the critical chloride ion concentration ranges from 0.149% to 0.155%.

[0036] For BCCP pipes, the critical chloride ion concentration ranges from 0.051% to 0.06%.

[0037] As a further improvement to this technical solution, when determining the chloride ion concentration on the pipe wall surface based on the soil chloride ion concentration in the buried environment, if the stable value of the soil chloride ion concentration is known, then the stable value is directly taken as the surface chloride ion concentration.

[0038] Otherwise, by testing the chloride ion concentration on the pipe wall surface at different time points, and fitting the result using an exponential function model, the surface chloride ion concentration function that changes over time can be obtained.

[0039] As a further improvement to this technical solution, the calculation of the time-varying chloride ion diffusion coefficient is expressed as the product of the reference chloride ion diffusion coefficient and a series of correction coefficients, wherein the correction coefficients include at least:

[0040] The following correction factors are used to quantify the long-term impact of steam curing on the microstructure of the protective layer: a first correction factor to characterize the decay of diffusion capacity with increasing exposure time; a third correction factor to reflect the slowing of the diffusion process by the chemical binding of chloride ions by the material; and a fourth correction factor to quantify the effect of ambient temperature on ion migration rate.

[0041] The second correction factor is expressed as a power function of the ratio of the reference time to the current exposure time, and the exponent of the power function is the age decay index.

[0042] The fourth correction coefficient is expressed as an exponential function with the natural constant as the base. Its exponential term is directly proportional to the chloride ion diffusion activation energy and inversely proportional to the reciprocal of the ambient absolute temperature.

[0043] As a further improvement to this technical solution, the step of numerically calculating the time required for chloride ions to diffuse to the surface of the prestressed tendon and reach a critical concentration includes the following steps:

[0044] A partial differential equation with the concentration of free chloride ions as the only unknown variable is established as the mathematical framework for the diffusion analysis model, wherein the effective diffusion capacity of chloride ions is determined by the product of the time-varying chloride ion diffusion coefficient and a chloride ion binding capacity influence function.

[0045] The computer numerical simulation demonstrates the dynamic penetration process of chloride ions in the protective layer. The chloride ion concentration on the surface of the steel bar is continuously monitored. When the simulation shows that the concentration at that location has reached the critical concentration for corrosion triggering, the corresponding cumulative time is the corrosion initiation life of the pipeline, and the corrosion initiation life of the pipeline is output.

[0046] As a further improvement to this technical solution, the electrochemical corrosion kinetic model is established based on Faraday's law of electrolysis, and the corrosion rate is calculated by corrosion current density, and then the corrosion amount is obtained by integration;

[0047] When the cross-sectional area loss of the prestressed tendons reaches the preset failure threshold, the corresponding total service time is the predicted failure life of the pipeline.

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] In this model for predicting the durability and lifespan of prestressed concrete pipes under chloride salt corrosion, experimental data are obtained through accelerated corrosion tests on the prototype pipe. This allows the experimental data to calibrate the key material parameters required for the prediction model, changing the traditional model that relies on small specimens, solving the problem of scale distortion, and improving the representativeness and reliability of parameters. Furthermore, high-resolution chloride ion concentration gradient data are obtained through multi-age layered grinding and dual-parameter synchronous inversion mechanisms, supporting unsteady diffusion modeling, avoiding sparse sampling errors, ensuring physical consistency, and enhancing model stability.

[0050] Introducing online electrochemical monitoring to identify the moment of passivation film rupture, providing an early warning compared to macroscopic observation, improving the accuracy of rust initiation judgment, and integrating four major factors—maintenance history, time decay, chemical bonding, and temperature response—to achieve a true reproduction of diffusion behavior;

[0051] Therefore, based on full-scale prototype testing, and integrating materials science, transport theory, electrochemistry, and structural mechanics, a full life-cycle prediction framework from "microscopic mass transfer" to "macroscopic failure" was constructed. This not only solves the problems of parameter distortion, mechanism simplification, boundary rigidity, and lack of engineering representativeness in traditional life prediction models, but also achieves accurate, reliable, interpretable, and decision-support-enabled intelligent prediction of the durability of large water pipelines under complex service environments by combining high-fidelity data-driven and time-varying physical modeling. Attached Figure Description

[0052] Figure 1 This is an overall flowchart of the present invention;

[0053] Figure 2 This is a schematic diagram of the pipe type of the present invention;

[0054] Figure 3 This is a flowchart of the iterative inversion calculation performed according to the present invention. Detailed Implementation

[0055] The technical solutions in 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.

[0056] Example 1

[0057] Please see Figures 1-3 As shown, this embodiment provides a model for predicting the durability life of prestressed concrete pipes under chloride salt corrosion environment, including the following steps:

[0058] Step 1: Calibrating key material parameters based on the prototype pipe (aiming to obtain real and reliable material property parameters for the target pipe type, rather than using general or empirical values ​​based on standard specimens):

[0059] For the target pipe type, namely prestressed concrete cylinder pipe (PCCP) or reinforced concrete cylinder pressure pipe (BCCP), the test data is obtained through accelerated corrosion test of the prototype pipe, and the test data is used to calibrate the key material parameters required for the prediction model.

[0060] When predicting the target pipeline, the corresponding key parameters are called according to its pipe material type. The key parameters include at least: protective layer thickness, reference chloride ion diffusion coefficient, age decay index, critical chloride ion concentration of prestressed tendons and soil chloride ion concentration of the burial environment to determine the chloride ion concentration on the pipe wall surface.

[0061] Because traditional methods use standard small specimens, the material structure and curing conditions of which are very different from those of large-diameter prototype pipes, the parameters obtained are not representative enough. Therefore, by conducting full-scale accelerated corrosion tests on prototype pipes, the accelerated corrosion test is improved. The accelerated corrosion test includes the following steps.

[0062] A sealed corrosion chamber is set up on the outer wall of a prestressed steel cylinder concrete pipe or a prototype prestressed steel cylinder concrete pipe with steel reinforcement. Chloride salt solution is injected and a dry-wet cycle is implemented to simulate accelerated corrosion. This is beneficial for obtaining chloride ion intrusion data of the actual protective layer (concrete or mortar) of the pipeline under real process conditions.

[0063] At multiple corrosion ages, cylindrical core samples penetrating the protective layer thickness were drilled from the corrosion chamber area. The core samples were then layered from the outer surface to the inner surface, and the free chloride ion concentration in each layer was measured. This yielded a series of chloride ion concentration distribution curves at different time points as a function of depth. Layered grinding and measurement can accurately obtain the gradient distribution curve of chloride ion concentration with the depth of the protective layer (concentration-depth curve). The distribution curves at multiple time points constitute the core dataset describing the unsteady transport process of chloride ions. This dataset is the only reliable data source for all subsequent inversion analyses and parameter calibrations, avoiding information loss and errors caused by overall measurement or sparse sampling.

[0064] Furthermore, the key material parameters required for calibrating the prediction model include diffusion parameter inversion and critical concentration determination, comprising the following steps:

[0065] For each distribution curve (concentration-depth curve), iterative inversion calculations are performed based on unsteady-state diffusion theory to solve for the power exponent of the decay of the chloride ion diffusion coefficient of the pipeline protective layer material over time, as well as the reference chloride ion diffusion coefficient at the reference time point. Simultaneously, by analyzing the relationship between the total chloride ion and free chloride ion concentrations at the same location, the chloride ion chemical binding parameters of the material are calibrated, where:

[0066] The specific steps for performing iterative inversion calculations are as follows:

[0067] Set an initial assumed value for the age decay index and calculate the equivalent diffusion time corresponding to each corrosion time in the test;

[0068] Using the chloride ion concentration distribution test data at each corrosion age, the apparent chloride ion diffusion coefficient corresponding to each age was derived based on Fick's second law. The power function relationship between the apparent chloride ion diffusion coefficient and the equivalent diffusion time was fitted to obtain the fitted reference chloride ion diffusion coefficient and the fitted age decay index.

[0069] Determine whether the difference between the fitted age decay index and the initial assumed value is less than the preset tolerance;

[0070] If so, the final age decay index is the fitted age decay index, and the reference chloride ion diffusion coefficient is the fitted reference chloride ion diffusion coefficient.

[0071] If not, the fitted age decay index is used as the new initial assumption value, and the iteration calculation is repeated until the tolerance requirement is met. Therefore, through iterative approximation, two core parameters are determined simultaneously: the age decay index and the reference chloride ion diffusion coefficient. The age decay index quantifies the rate at which the pipeline protective layer's ability to resist chloride ion intrusion "ages" (actually, it is continuous hydration and densification) over time. The age decay index obtained through prototype pipe tests reflects the true long-term performance of the material. For example, it was found that the age decay index of BCCP concrete is usually higher than that of PCCP mortar, indicating that the BCCP protective layer has a stronger long-term densification ability and better resistance to chloride ion penetration, providing a direct quantitative basis for engineering selection and durability design.

[0072] Then, simulated specimens embedded with real prestressed tendons were prepared. During the accelerated corrosion process, the electrochemical state of the tendons inside the specimens was monitored simultaneously. This is equivalent to a precise physical examination of the "permeability" and "the rate of performance aging over time" of this specific pipe material.

[0073] When a sudden change in electrochemical parameters, indicating the rupture of the passivation film, is detected (a sudden change in the electrochemical signal of the steel wire, the instant when corrosion begins), the test is immediately terminated and the concentration of free chloride ions in the protective layer at the interface of the reinforcing steel is measured. By statistically analyzing multiple sets of tests, the critical chloride ion concentration value is determined, which helps to more accurately capture the critical point of corrosion. This is more accurate and reliable than the traditional method of judging by the naked eye or estimating by theory based on corrosion products. It is the key threshold for dividing the "safe period" and the "corrosion period".

[0074] It is worth noting that the range of values ​​for the age-related decline index is as follows:

[0075] For PCCP pipes, the age decay index ranges from 0.559 to 0.676.

[0076] For BCCP pipes, the age decay index ranges from 0.616 to 0.791.

[0077] The calibrated range of critical chloride ion concentration, calculated as a percentage of the protective layer material by mass, is as follows:

[0078] For PCCP pipes, the critical chloride ion concentration ranges from 0.149% to 0.155%.

[0079] For BCCP pipes, the critical chloride ion concentration ranges from 0.051% to 0.06%.

[0080] Step 2: Service environment data acquisition and surface chloride ion concentration determination (aimed at providing accurate environmental boundary conditions for the prediction model):

[0081] When determining the chloride ion concentration on the pipe wall surface based on the soil chloride ion concentration in the buried environment, if the stable value of the soil chloride ion concentration is known, then that stable value can be directly taken as the surface chloride ion concentration.

[0082] Otherwise, by testing the chloride ion concentration on the pipe wall surface at different time points, an exponential function model is used for fitting to obtain the surface chloride ion concentration function that changes over time. The surface chloride ion concentration is used as the upper boundary condition of the model to make the driving potential (concentration difference) of the diffusion process conform to the actual situation. The introduction of the dynamic surface concentration function enables the model to adapt to more complex service environments, such as scenarios affected by the seasonality of de-icing agents or the gradual aggravation of soil pollution, thus improving the adaptability of the model.

[0083] Step 3: Calculation of time-varying chloride ion diffusion coefficient (transforming the chloride ion diffusion coefficient in the protective layer from a constant or simple time-varying parameter into a dynamic variable influenced by multiple factors):

[0084] Based on key material parameters and collected environmental data, the time-varying chloride ion diffusion coefficient is calculated to characterize the dynamic evolution of chloride ion transport capacity in the pipeline protective layer as a function of time, ambient temperature, and material properties.

[0085] Specifically, the time-varying chloride ion diffusion coefficient is calculated by multiplying a reference chloride ion diffusion coefficient by a series of correction factors, which include at least:

[0086] The following correction factors are used to quantify the long-term impact of steam curing on the microstructure of the protective layer: a first correction factor to characterize the decay of diffusion capacity with increasing exposure time; a third correction factor to reflect the slowing of the diffusion process by the chemical binding of chloride ions by the material; and a fourth correction factor to quantify the effect of ambient temperature on ion migration rate.

[0087] The second correction factor is expressed as a power function of the ratio of the reference time to the current exposure time, and the exponent of the power function is the age decay index.

[0088] The fourth correction factor is expressed as an exponential function with the natural constant as the base. Its exponential term is directly proportional to the chloride ion diffusion activation energy and inversely proportional to the reciprocal of the absolute temperature of the environment. The specific expression is as follows:

[0089]

[0090] in, The time-varying chloride ion diffusion coefficient is... The reference chloride ion diffusion coefficient is used, which is also the fitted reference chloride ion diffusion coefficient. The diffusion coefficient of steam curing process was determined through comparative experiments (the ratio of the diffusion coefficient of steam-cured specimens to that of standard-cured specimens). Steam curing can reduce porosity, typically... <1 (e.g., 0.7-0.9), This is a time decay term, with reference time. Typically, 28 days is used, consistent with the test conditions for the reference chloride ion diffusion coefficient, for service life. It needs to be converted to the same unit as the reference time (such as days or years), Kira - Age Decay Index It is obtained by fitting the relationship between the apparent chloride ion diffusion coefficient and the equivalent diffusion time, reflecting the mitigating effect of the continuous densification of concrete hydration on the diffusion coefficient. The coefficient representing the influence of chloride ion binding is a nonlinear binding parameter obtained by fitting the relationship curve between bound chloride ion concentration and free chloride ion concentration. The stronger the chemical / physical binding effect of concrete on chloride ions, The smaller, The environmental temperature influence coefficient is used to quantify the impact of temperature changes on the chloride ion diffusion rate, enabling the model to automatically correct for differences in diffusion rates caused by environmental temperature changes in different regions and seasons, greatly improving the model's universality and prediction accuracy.

[0091] Step 4: Calculation of corrosion initiation life (calculating the time required for chloride ions to diffuse from the outer surface of the pipe to the surface of the prestressed tendons and accumulate to reach a critical concentration, i.e., the corrosion initiation life):

[0092] The time-varying chloride ion diffusion coefficient, the protective layer thickness, the surface chloride ion concentration determined based on the environmental chloride ion concentration, and the calibrated critical chloride ion concentration of the prestressing tendon are input into the diffusion analysis model that considers the chemical binding effect of chloride ions. The time required for chloride ions to diffuse to the surface of the prestressing tendon and reach the critical concentration is calculated numerically and used as the pipeline corrosion initiation life.

[0093] The process of numerically calculating the time required for chloride ions to diffuse to the surface of the prestressed tendon and reach a critical concentration includes the following steps:

[0094] A partial differential equation with the concentration of free chloride ions as the only unknown variable is established as the mathematical framework for the diffusion analysis model. Specifically, it is: (rate of change of free chloride ions) = (net inflow rate caused by diffusion) - (rate "lost" due to chemical bonding). The diffusion analysis model is a mathematical model of chloride ion transport. Through high-precision numerical simulation by computer, the dynamic penetration process of chloride ions in the protective layer is demonstrated day by day and year by year, like a movie. The computer continuously monitors the chloride ion concentration at the surface of the steel reinforcement. When the simulation shows that the concentration at that location reaches the "corrosion triggering critical concentration" calibrated in the first step, the corresponding cumulative time is the corrosion initiation life of the pipeline. This marks the end of the pipeline's "health period" and the beginning of the "corrosion period", and the corrosion initiation life of the pipeline is output.

[0095] Step 5: Failure Life Prediction

[0096] Taking the corrosion initiation life as the starting point, the corrosion development process of the prestressed tendon is simulated based on the electrochemical corrosion kinetic model, and the cross-sectional area loss of the tendon caused by corrosion is calculated. When the cross-sectional area loss reaches the preset failure threshold determined according to the initial tensile stress and material strength, the corresponding total time is the predicted failure life of the pipeline.

[0097] Specifically, based on an electrochemical corrosion kinetic model and Faraday's law of electrolysis, the corrosion rate is calculated using the corrosion current density. This corrosion current density can be obtained from empirical values, electrochemical test data, or online pipeline monitoring data. The model considers three stages of corrosion development: the initial stage, the stable propagation stage, and the accelerated stage.

[0098] The corrosion rate is integrated over time to calculate the cross-sectional area loss of the prestressed tendons due to corrosion.

[0099] When the cross-sectional area loss of the prestressed tendons reaches the preset failure threshold, the corresponding total service time is the predicted failure life of the pipeline. This failure threshold needs to be determined comprehensively based on the initial tensile stress of the pipeline, the remaining material strength, and the structural safety factor. This is beneficial to directly link the durability life with the structural mechanical performance, realizing a comprehensive prediction from "material corrosion" to "structural failure". It provides a complete time frame from "starting to rust" to "potential failure" for pipeline maintenance, repair, and replacement decisions, and has stronger guiding significance.

[0100] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A model for predicting the durability life of prestressed concrete pipes under chloride salt corrosion environment, characterized in that, Includes the following steps: For the target pipeline type, accelerated corrosion tests on its prototype pipe are conducted to calibrate the key material parameters required for the prediction model using the test data. When predicting the target pipeline, the corresponding key parameters are called according to its pipe material type. The key parameters include at least: protective layer thickness, reference chloride ion diffusion coefficient, age decay index, critical chloride ion concentration of prestressed tendons and soil chloride ion concentration of the burial environment to determine the chloride ion concentration on the pipe wall surface. Based on key material parameters and collected environmental data, the time-varying chloride ion diffusion coefficient is calculated to characterize the dynamic evolution of chloride ion transport capacity in the pipeline protective layer as a function of time, ambient temperature, and material properties. The time-varying chloride ion diffusion coefficient, the protective layer thickness, the surface chloride ion concentration determined based on the environmental chloride ion concentration, and the calibrated critical chloride ion concentration of the prestressing tendon are input into a diffusion analysis model that considers the chemical binding effect of chloride ions. The time required for chloride ions to diffuse to the surface of the prestressing tendon and reach the critical concentration is calculated numerically and used as the pipeline corrosion initiation life. Using the corrosion initiation life as the starting point, the corrosion development process of the prestressed tendons is simulated based on the electrochemical corrosion kinetic model. The cross-sectional area loss of the tendons caused by corrosion is calculated. When the cross-sectional area loss reaches the preset failure threshold determined based on the initial tensile stress and material strength, the corresponding total time is the predicted failure life of the pipeline.

2. The prestressed concrete pipe durability life prediction model under chloride salt corrosion environment according to claim 1, characterized in that: The pipe type is a prestressed steel cylinder concrete pipe or a steel cylinder concrete pressure pipe with reinforcing bars.

3. The prestressed concrete pipe durability life prediction model under chloride salt corrosion environment according to claim 2, characterized in that: The accelerated corrosion test, Includes the following steps; A sealed corrosion chamber was set on the outer wall of a prestressed steel cylinder concrete pipe or a prototype prestressed steel cylinder concrete pipe with steel reinforcement winding. Chloride salt solution was injected and a dry-wet cycle was implemented to simulate accelerated corrosion. At multiple different corrosion ages, cylindrical core samples penetrating the protective layer thickness were drilled from the corrosion chamber area. The core samples were then layered from the outer surface to the inner surface, and the free chloride ion concentration in each layer was measured to obtain a series of chloride ion concentration distribution curves at different time points as a function of depth.

4. The prestressed concrete pipe durability life prediction model under chloride salt corrosion environment according to claim 3, characterized in that: The key material parameters required for calibrating the prediction model further include diffusion parameter inversion and critical concentration determination, comprising the following steps: For each of the aforementioned distribution curves, iterative inversion calculations are performed based on the unsteady-state diffusion theory to solve for the power exponent of the decay of the chloride ion diffusion coefficient of the pipeline protective layer material with time, as well as the reference chloride ion diffusion coefficient at the reference time point; simultaneously, by analyzing the relationship between the total chloride ion and free chloride ion concentrations at the same location, the chloride ion chemical binding parameters of the material are calibrated. Simulated specimens embedded with real prestressed tendons were prepared, and the electrochemical state of the tendons inside the specimens was monitored simultaneously during the accelerated corrosion process. When a sudden change in electrochemical parameters indicating passivation film rupture is detected, the experiment is immediately terminated and the concentration of free chloride ions in the protective layer at the interface of the reinforcing material is measured. The critical chloride ion concentration value is determined by statistical analysis of multiple sets of experiments.

5. The prestressed concrete pipe durability life prediction model under chloride salt corrosion environment according to claim 4, characterized in that: The specific steps for performing the iterative inversion calculation are as follows: Set an initial assumed value for the age decay index and calculate the equivalent diffusion time corresponding to each corrosion time in the test; Using the chloride ion concentration distribution test data at each corrosion age, the apparent chloride ion diffusion coefficient corresponding to each age is derived based on Fick's second law. The power function relationship between the apparent chloride ion diffusion coefficient and the equivalent diffusion time is fitted to obtain the fitted reference chloride ion diffusion coefficient and the fitted age decay index. Determine whether the difference between the fitted age decay index and the initial assumed value is less than the preset tolerance; If so, the final age decay index is output as the fitted age decay index, and the reference chloride ion diffusion coefficient is output as the fitted reference chloride ion diffusion coefficient. If not, the fitted age decay index is used as the new initial assumption, and the iteration calculation is repeated until the tolerance requirement is met.

6. The prestressed concrete pipe durability life prediction model under chloride salt corrosion environment according to claim 5, characterized in that: The range of values ​​for the age decay index is: For PCCP pipes, the age decay index ranges from 0.559 to 0.

676. For BCCP pipes, the age decay index ranges from 0.616 to 0.

791. The calibrated range of the critical chloride ion concentration, calculated as a percentage of the protective layer material by mass, is as follows: For PCCP pipes, the critical chloride ion concentration ranges from 0.149% to 0.155%. For BCCP pipes, the critical chloride ion concentration ranges from 0.051% to 0.06%.

7. The prestressed concrete pipe durability life prediction model under chloride salt corrosion environment according to claim 1, characterized in that: When determining the chloride ion concentration on the pipe wall surface based on the soil chloride ion concentration in the buried environment, if the stable value of the soil chloride ion concentration is known, then that stable value is directly taken as the surface chloride ion concentration. Otherwise, by testing the chloride ion concentration on the pipe wall surface at different time points, and fitting the result using an exponential function model, the surface chloride ion concentration function that changes over time can be obtained.

8. The prestressed concrete pipe durability life prediction model under chloride salt corrosion environment according to claim 5, characterized in that: The time-varying chloride ion diffusion coefficient is calculated by expressing it as the product of the reference chloride ion diffusion coefficient and a series of correction coefficients, wherein the correction coefficients include at least: The following correction factors are used to quantify the long-term impact of steam curing on the microstructure of the protective layer: a first correction factor to characterize the decay of diffusion capacity with increasing exposure time; a third correction factor to reflect the slowing of the diffusion process by the chemical binding of chloride ions by the material; and a fourth correction factor to quantify the effect of ambient temperature on ion migration rate. The second correction factor is expressed as a power function of the ratio of the reference time to the current exposure time, and the exponent of the power function is the age decay index. The fourth correction coefficient is expressed as an exponential function with the natural constant as the base. Its exponential term is directly proportional to the chloride ion diffusion activation energy and inversely proportional to the reciprocal of the ambient absolute temperature.

9. The prestressed concrete pipe durability life prediction model under chloride salt corrosion environment according to claim 8, characterized in that: The method of numerically calculating the time required for chloride ions to diffuse to the surface of the prestressed tendon and reach a critical concentration includes the following steps: A partial differential equation with the concentration of free chloride ions as the only unknown variable is established as the mathematical framework for the diffusion analysis model, wherein the effective diffusion capacity of chloride ions is determined by the product of the time-varying chloride ion diffusion coefficient and a chloride ion binding capacity influence function. The computer numerical simulation demonstrates the dynamic penetration process of chloride ions in the protective layer. The chloride ion concentration on the surface of the steel bar is continuously monitored. When the simulation shows that the concentration at that location has reached the critical concentration for corrosion triggering, the corresponding cumulative time is the corrosion initiation life of the pipeline, and the corrosion initiation life of the pipeline is output.

10. The prestressed concrete pipe durability life prediction model under chloride salt corrosion environment according to claim 9, characterized in that: The electrochemical corrosion kinetic model is established based on Faraday's law of electrolysis. The corrosion rate is calculated by the corrosion current density, and then the corrosion amount is obtained by integration. When the cross-sectional area loss of the prestressed tendons reaches the preset failure threshold, the corresponding total service time is the predicted failure life of the pipeline.

Images