A Concrete Saturation Sensor and Monitoring Method Based on the Relative Water-Contact Area of a Fully Polarized Electrode

Abstract

The present invention discloses a concrete water saturation sensor and a monitoring method based on the relative water contact area of completely polarized electrodes. The water saturation sensor includes two identical completely polarized A electrodes and completely polarized B electrodes, which are arranged in an interdigitated form to keep the water contact areas of the two electrodes consistent. The present invention calculates the water saturation of hardened cement paste through the relative water contact area of the water saturation sensor, with good linear relationship, good repeatability, the response curves during the water absorption and dehydration processes not showing obvious separation, slight hysteresis, and slight interference from salts. Generally speaking, the error in calculating the water saturation from the relative water contact area through fitting a linear equation is within ±10%, and the cost of the water saturation sensor and the measurement method is low and the time consumption is short.

Description

Technical Field

[0001] The present invention belongs to the technical field of civil engineering, and relates to a method for monitoring the durability of concrete, specifically to a concrete saturation sensor and a monitoring method based on the relative water contact area of fully polarized electrodes. Background Art

[0002] The water content state of concrete is of great significance to the development of durability damage of steel-concrete structures. First of all, water is a necessary condition for the carbonation reaction of concrete and the electrochemical reaction of steel corrosion, but too high a saturation degree can slow down the diffusion rate of carbon dioxide and oxygen in concrete. Secondly, the water flow caused by the concrete water gradient will increase the invasion speed of erosive media such as chloride ions dissolved in water. Finally, when the concrete saturation degree reaches the critical value, if the temperature drops to the freezing point, the water freezes and expands to generate internal pressure, and the concrete will suffer from freeze-thaw damage, resulting in an increase in the porosity of the concrete, a decrease in strength, and a decrease in the erosion resistance. Therefore, the monitoring of concrete saturation degree should become an important part of the durability monitoring of steel-concrete structures.

[0003] The non-destructive monitoring / detection methods for water in concrete are mainly divided into the relative humidity method, the conductivity method, and the dielectric constant method. The relative humidity method creates a small hole in the concrete and measures the relative humidity of the air in the small hole to infer the water content of the concrete. However, the response curve of air relative humidity to the concrete saturation degree has a significant hysteresis effect, that is, the response curves of the concrete water absorption process and the dehydration process are significantly separated. Therefore, inferring the concrete water content through air relative humidity is seriously interfered by the historical water change. Moreover, the process of establishing the balance between air relative humidity and concrete saturation degree is very slow. Some studies have shown that this process can even take up to several years at most, which further increases the measurement error of the relative humidity method. In addition, the commonly used digital temperature and humidity chip for measuring air relative humidity will be permanently damaged when working in a high humidity environment for a long time. The conductivity method is based on the fact that the conductivity of the concrete pore solution is much greater than that of the concrete solid phase and air, and the four-electrode DC method or the two-electrode AC method is used to measure the conductivity of the concrete. However, it is difficult to decouple the influence of other parameters such as the ion concentration of the pore solution on the conductivity of the concrete from the measurement data. In addition, if the conductivity sensor is set improperly, resulting in the measurement current passing through the steel bar, it will cause significant errors to the test results. The dielectric constant method is based on the fact that the dielectric constant of water is much greater than that of the concrete solid phase and air. However, in order to distinguish the influence of the ion concentration in the concrete pore solution on the dielectric constant, the measurement frequency needs to reach above 1 GHz, and the measurement instrument cost is too high for distributed online monitoring.

[0004] At present, the methods for monitoring the water content of concrete each have different defects. Therefore, it is necessary to develop a new method without significant hysteresis effect, not affected by ion concentration, etc., and taking into account the measurement cost. Summary of the Invention

[0005] In order to solve the common problems of serious hysteresis, large measurement errors, and salt interference in the existing methods for measuring the degree of saturation of concrete, starting from the porous characteristics of hardened cement paste, and based on the essence that the degree of saturation is the proportion of water in the hardened cement paste to the pores, the relative water contact area of the electrodes is used to characterize the degree of saturation, and the interfacial capacitance of the completely polarized electrodes that do not undergo electrode reactions is used to calculate the relative water contact area, providing a concrete saturation degree sensor and monitoring method based on the relative water contact area of completely polarized electrodes with high precision, slight hysteresis, and slight salt interference.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] A concrete saturation degree sensor based on the relative water contact area of completely polarized electrodes, including two identical completely polarized A electrodes and completely polarized B electrodes, and the completely polarized A electrodes and completely polarized B electrodes are arranged in an interdigitated form to keep the water contact areas of the two electrodes consistent. Further, the completely polarized A electrodes and completely polarized B electrodes are gold or platinum electrodes, or gold-plated or platinum-plated electrodes of other metal materials.

[0008] A concrete saturation degree monitoring method based on the relative water contact area of completely polarized electrodes includes the following steps:

[0009] Step 1: Burry the saturation degree sensor composed of the above-mentioned completely polarized A electrodes and completely polarized B electrodes into the concrete;

[0010] Step 2: Measure the open-circuit potential between the completely polarized A electrodes and completely polarized B electrodes. If the open-circuit potential is less than 1 mV, proceed to the next step; otherwise, short-circuit the completely polarized A electrodes and completely polarized B electrodes to depolarize the electrodes, and then measure the open-circuit potential again;

[0011] Step 3: Apply a sine-wave potential excitation signal between the completely polarized A electrodes and completely polarized B electrodes, record the current response signal, and calculate the impedance of the saturation degree sensor. The amplitude of the sine-wave potential excitation signal is 10 - 20 mV, so that no electrochemical reaction occurs on the electrode surface, and only ion adsorption occurs;

[0012] Step 4: Calculate the equivalent interfacial capacitance of the saturation degree sensor through an equivalent circuit of a resistor and a capacitor in series;

[0013] Step 5: Calculate the relative water contact area of the saturation degree sensor through the ratio of the equivalent interfacial capacitance of the completely polarized A electrodes and completely polarized B electrodes in the measured object to the equivalent interfacial capacitance of the completely polarized A electrodes and completely polarized B electrodes in the saturated state, and then calculate the saturation degree from the response curve of the saturation degree sensor's relative water contact area to the saturation degree. The calibration process of the response curve of the saturation degree sensor's relative water contact area to the saturation degree includes the following steps:

[0014] (1) Immerse the water saturation sensor into the hardened cement paste, subject the hardened cement paste to water saturation treatment, record its weight, and measure the equivalent interface capacitance in the water-saturated state.

[0015] (2) Encapsulate the remaining surfaces of the hardened cement paste with epoxy resin, leaving only one surface through which moisture enters and exits the hardened cement paste, so that the diffusion direction of moisture is parallel to the surface of the water saturation sensor electrode, preventing calibration errors of the response curve caused by the water saturation gradient of the cement paste in the normal direction of the electrode surface.

[0016] (3) Calculate the water saturation from the measured weight of the hardened cement paste during the drying and moisture absorption processes, calculate the relative water contact area from the ratio of the measured equivalent interface capacitance to the equivalent interface capacitance in the water-saturated state, and calibrate the response curve of the relative water contact area of the water saturation sensor to the water saturation.

[0017] Principle of monitoring the water saturation of concrete:

[0018] A completely polarized electrode refers to an electrode on the surface of which no electrochemical reaction occurs within a certain potential range in an electrolyte solution. Within this potential range, no Faraday current passes through the electrode interface, and the electrode is polarized by ion adsorption. The larger the surface area of the electrode, the larger the amount of charge adsorbed when the electrode is polarized to the same potential, that is, the larger the interface capacitance of the electrode. Concrete is a composite material composed of aggregates such as sand and gravel and hardened cement paste, and moisture mainly exists in the pores of the hardened cement paste. The pores in the hardened cement paste include air pores, capillary pores, and gel pores, with pore diameters continuously distributed from dozens of nanometers to hundreds of micrometers. According to the Kelvin formula, the smaller the pore diameter, the smaller the saturated vapor pressure of water in the pore. When concrete absorbs water, small pores are filled with water prior to large pores, and when concrete dehydrates, large pores dry prior to small pores. Immerse the completely polarized electrode into the concrete. When the concrete is completely saturated with water, all pores on the electrode surface are filled with water, and the water contact area of the electrode reaches the maximum. When the concrete loses some water, some relatively large pores on the electrode surface lose moisture, and the water contact area of the electrode decreases. Therefore, by measuring the interface capacitance of the completely polarized electrode, calculating the water contact area of the electrode, and further obtaining the water saturation of the hardened cement paste in the concrete.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] The present invention calculates the water saturation of the hardened cement paste through the relative water contact area of the water saturation sensor, with good linear relationship, good repeatability, the response curves during the water absorption and dehydration processes not showing obvious separation, slight hysteresis, and slight interference from salts. Generally speaking, the error in calculating the water saturation from the relative water contact area by fitting a linear equation is within ±10%, and the water saturation sensor and the measurement method have low cost and short time consumption. Description of the Drawings

[0021] Figure 1 This is the physical diagram of the water saturation sensor of the present invention;

[0022] Figure 2 This is the schematic diagram of the water saturation monitoring method of the present invention;

[0023] Figure 3 This is the equivalent circuit of the water saturation sensor of the present invention;

[0024] Figure 4 This is the equivalent interface capacitance of the water saturation sensor of the present invention under different water saturations and measurement frequencies;

[0025] Figure 5 This is the water saturation response curve of the sensor of the present invention in a hardened cement specimen containing 0% NaCl;

[0026] Figure 6 This is the water saturation response curve of the sensor of the present invention in a hardened cement specimen containing 1% NaCl;

[0027] Figure 7 This is the water saturation response curve of the sensor of the present invention in a hardened cement specimen containing 3% NaCl;

[0028] Figure 8 This is the fitting curve of the water saturation response of the sensor of the present invention under different salt contents;

[0029] In the figure, 1. Voltage signal generator, 2. Voltage measurement module, 3. Current measurement module, 4. Relay, 5. Water saturation sensor, 51. Fully polarized A electrode, 52. Fully polarized B electrode, 6. Hardened cement paste. Detailed implementation manners

[0030] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention shall be covered by the protection scope of the present invention.

[0031] The present invention provides a method for monitoring the water saturation of concrete based on the relative water contact area of fully polarized electrodes, and the method includes the following steps:

[0032] Step 1: Prepare the water saturation sensor 5, as Figure 2 shown. The water saturation sensor 5 is composed of two identical fully polarized A electrodes 51 and fully polarized B electrodes 52. The fully polarized A electrode 51 and the fully polarized B electrode 52 are arranged in a finger-like form. The water saturation sensor 5 is buried in the hardened cement paste 6. After 24 hours, the hardened cement paste 6 is demolded, and then steam-cured at 60°C for 3 days or cured under standard conditions for 28 days.

[0033] Step 2: Control the relay 4 to short-circuit the fully polarized A electrode 51 and the fully polarized B electrode 52 for 5 - 10 seconds, and then disconnect for 5 - 10 seconds.

[0034] Step 3: Use the voltage measurement module 2 to measure the open-circuit potential difference between the fully polarized A electrode 51 and the fully polarized B electrode 52. If the open-circuit potential difference is less than 1 mV, proceed to Step 4; otherwise, return to Step 2.

[0035] Step 4: Apply a sine wave signal with an amplitude of 20 mV between the fully polarized A electrode 51 and the fully polarized B electrode 52 through the voltage signal generator 1. Use the current measurement module 3 to record the current signal between the fully polarized A electrode 51 and the fully polarized B electrode 52, and calculate the impedance of the water saturation sensor 5.

[0036] Step 5: Use the Figure 3 equivalent circuit of the series connection of the shown resistor R and capacitor C to fit the measurement data, where R represents the concrete resistance between the fully polarized A electrode 51 and the fully polarized B electrode 52, and the capacitor C represents the equivalent interface capacitance of the fully polarized A electrode 51 and the fully polarized B electrode 52.

[0037] Step 6: Calculate the relative water contact area of the water saturation sensor 5 according to formula (1):

[0038]

[0039] where: RWA is the relative water contact area of the water saturation sensor 5; A s and C s respectively represent the water contact area and the equivalent interface capacitance of the fully polarized A electrode 51 and the fully polarized B electrode 52 when the water saturation is 100%; A d and C d respectively represent the water contact area and the equivalent interface capacitance of the fully polarized A electrode 51 and the fully polarized B electrode 52 under the water saturation to be measured;

[0040] Step 7: Measure the relative water contact area of the water saturation sensor 5 under different water saturations, calibrate the response curve, and calculate the water saturation from the response curve.

[0041] Example

[0042] In this example, the water saturation sensor is made of a printed circuit board. As Figure 1 shown, the substrate of the sensor is an FR-4 epoxy glass cloth laminate with a length of 25 mm and a width of 20 mm. Electrodes are arranged on both sides of the substrate. The fully polarized A electrode 51 and the B electrode 52 are both made of copper-gilded material, with each finger width of 0.1 mm, a length of 17 mm, and a spacing of 0.1 mm.

[0043] The hardened cement paste was prepared using reference cement and deionized water with a water-cement ratio of 0.4. Three different hardened cement pastes with 0%, 1%, and 3% NaCl by mass of the cement were prepared. The cement paste was cast in a silica gel mold and demolded after 24 hours. It was cured for three days in an environment with a temperature of 60°C and a relative humidity of 98%. Then, the specimens were immersed in water for three days and weighed, and the equivalent interface capacitance C of the sensor in the saturated water state was measured according to the steps in the monitoring method of the present invention. s Then, the sides and bottom of the specimens were encapsulated with epoxy resin to make the water diffusion direction parallel to the electrode surface of the sensor, preventing the existence of a water gradient in the normal direction of the electrode in the hardened cement paste, which could lead to an incorrect response curve. The specimens were placed in a dryer for dehydration treatment, and then the specimens were weighed and the equivalent interface capacitance C was measured. d The steps of drying, weighing, and measuring the equivalent interface capacitance were repeated. When the specimens were dried to a water saturation of about 20%, further drying was stopped to prevent measurement errors caused by specimen cracking. Then, the specimens were placed in an environment with a relative humidity of 100% for moisture absorption, and similar measurement steps as in the drying process were carried out, repeating the moisture absorption, weighing, and measuring the equivalent interface capacitance.

[0044] The impedance measurement range of the water saturation sensor was 10000 Hz to 10 Hz, with an amplitude of 20 mV. At each frequency, fitting was performed using the RC series circuit shown in Figure 3 . The equivalent interface capacitance of the water saturation sensor was calculated using Equation (2):

[0045]

[0046] where C dl is the equivalent interface capacitance of the water saturation sensor, f is the measurement frequency, and Z im is the imaginary part of the impedance at frequency f. Typical test results are shown in Figure 4 . At the same measurement frequency, the higher the water saturation, the higher the equivalent interface capacitance of the sensor; under the condition of the same water saturation, the lower the measurement frequency, the larger the measured value of the equivalent interface capacitance. And as can be seen from Figure 4 , at high frequencies, the measured interface capacitance values of specimens with a water saturation of 14% to 53% are close and difficult to distinguish. The relative water contact area of the sensor was calculated using the equivalent interface capacitance measured at a frequency of 10 Hz.

[0047] The equivalent interface capacitances of the water saturation sensors at the maximum water contact area of specimens with different NaCl contents in the saturated water state are shown in Table 1. The water saturation sensors were not significantly interfered by the NaCl concentration. The reproducibility of the water saturation sensors was good, with an average saturated water capacitance of 12.97 μF and a maximum deviation of ±0.48 μF.

[0048] Table 1 Equivalent interface capacitance of water saturation sensors

[0049]

[0050]

[0051] The corresponding curves of the relative water contact area and water saturation of the water saturation sensor measured are as Figure 5 , Figure 6 and Figure 7 shown, representing specimens with 0%, 1%, and 3% NaCl by mass of cement respectively. The relative water contact area is calculated according to Equation (1) by the ratio of the equivalent interface capacitance to the equivalent interface capacitance in the saturated water state. The water saturation sensor has good reproducibility, and the response curves during the water absorption process and the dehydration process have a high degree of overlap, without showing obvious hysteresis. As can be seen from Figures 5 to 7 , the water saturation of the specimen is calculated from the relative water contact area through the linear fitting formula, and the error is within ±10%. Figure 8 summarizes the sensor response curves of linear fitting in specimens containing 0%, 1%, and 3% NaCl. The NaCl concentration has no significant effect on the response curve, and the deviation of the water saturation calculated by the relative water contact area under different salt contents is within ±8%.

Claims

1. A method for monitoring the saturation of concrete based on the relative water contact area of ​​fully polarized electrodes, characterized in that The method comprises the following steps: Step 1: burying a water saturation sensor in concrete, wherein the water saturation sensor comprises two identical fully polarized A electrodes and a fully polarized B electrode, wherein the fully polarized A electrode and the fully polarized B electrode are arranged in a forked finger form to keep the water contact areas of the two electrodes consistent; Step 2: Measure the open circuit potential between the fully polarized electrode A and the fully polarized electrode B. If the open circuit potential is less than 1 mV, proceed to the next step. Otherwise, short-circuit the fully polarized electrode A and the fully polarized electrode B to depolarize the electrodes and measure the open circuit potential again. Step 3: Apply a sinusoidal potential excitation signal between the fully polarized A electrode and the fully polarized B electrode, record the current response signal, and calculate the impedance of the saturation sensor; Step 4: Calculate the equivalent interface capacitance of the water saturation sensor through an equivalent circuit of a resistor and a capacitor in series. The calculation formula of the equivalent interface capacitance of the water saturation sensor is as follows: in, C dl is the equivalent interface capacitance of the saturation sensor, f To measure frequency, Z im for f The imaginary part of impedance at frequency; Step 5: Calculate the relative water contact area of ​​the saturation sensor by the ratio of the equivalent interface capacitance of the fully polarized A electrode and the fully polarized B electrode in the tested object to the equivalent interface capacitance of the fully polarized A electrode and the fully polarized B electrode in the saturated state, and then calculate the saturation by the calibrated response curve of the relative water contact area of ​​the saturation sensor to the saturation. The calculation formula of the relative water contact area of ​​the saturation sensor is as follows: in: RWA is the relative water contact area of ​​the saturation sensor; A s and C s They represent the water contact area and equivalent interface capacitance of the fully polarized A electrode and the fully polarized B electrode respectively when the water saturation is 100%; A d and C d They respectively represent the water contact area and equivalent interfacial capacitance of the fully polarized A electrode and the fully polarized B electrode at the measured water saturation.

2. The method for monitoring the saturation of concrete based on the relative water contact area of ​​fully polarized electrodes according to claim 1, characterized in that The fully polarized A electrode and the fully polarized B electrode are gold electrodes, platinum electrodes, gold-plated electrodes or platinum-plated electrodes.

3. The method for monitoring the saturation of concrete based on the relative water contact area of ​​fully polarized electrodes according to claim 1, characterized in that The amplitude of the sinusoidal wave potential excitation signal is 10-20 mV.

4. The method for monitoring the saturation of concrete based on the relative water contact area of ​​fully polarized electrodes according to claim 1, characterized in that The calibration process of the response curve of the water saturation sensor relative to the water contact area to the water saturation comprises the following steps: (1) The saturation sensor is buried in the hardened cement slurry, the hardened cement slurry is saturated with water, its weight is recorded, and the equivalent interface capacitance in the saturated state is measured; (2) The remaining surfaces of the hardened cement slurry are encapsulated with epoxy resin, leaving only one surface through which water enters and leaves the hardened cement slurry, so that the diffusion direction of water is parallel to the electrode surface of the saturation sensor, thereby preventing the saturation gradient of the cement slurry in the normal direction of the electrode surface from causing calibration errors in the response curve; (3) The saturation degree is calculated by the measured weight of the hardened cement slurry during the drying and moisture absorption process, and the relative water contact area is calculated by the ratio of the measured equivalent interface capacitance and the equivalent interface capacitance in the saturated state. The response curve of the saturation sensor relative to the water contact area is calibrated.

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