Concrete saturation degree sensor based on relative water contact area of completely polarized electrodes and monitoring method

Abstract

The invention discloses a concrete saturation degree sensor based on the relative water contact area of completely polarized electrodes and a monitoring method.The concrete saturation degree sensor comprises a completely polarized electrode A and a completely polarized electrode B which are the same, and the completely polarized electrode A and the completely polarized electrode B are arranged in an interdigital mode; therefore, the water contact areas of the two electrodes are kept consistent. According to the method, the saturation degree of the hardened cement paste is calculated through the relative water contact area of the saturation degree sensor, the linear relation is good, the relinearity is good, response curves in the water absorption and dehydration process do not show obvious separation, the hysteresis quality is slight, and the salinity interference is slight. In general, the error of calculating the saturation degree from the relative water contact area by fitting the linear equation is within + / -10%, and the saturation degree sensor and the measurement method are low in cost and short in time consumption.

Description

Technical Field

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

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

[0003] The non-destructive monitoring / detection methods of moisture in concrete are mainly divided into relative humidity method, conductivity method and 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 moisture content of the concrete. However, the response curve of air relative humidity to concrete saturation has a significant hysteresis effect, that is, the response curves of concrete water absorption process and dehydration process are significantly separated. Therefore, the calculation of concrete moisture content by air relative humidity is seriously disturbed by historical moisture changes. In addition, the process of establishing a balance between air relative humidity and concrete saturation is very slow. Studies have shown that this process can even take up to several years, which further increases the error of relative humidity method measurement. In addition, the digital temperature and humidity chip commonly used to measure air relative humidity will be permanently damaged if it works for a long time in a high humidity environment. The conductivity method is based on the fact that the conductivity of concrete pore solution is much greater than that of concrete solid phase and air, and uses a four-electrode DC method or a two-electrode AC method to measure the conductivity of concrete. However, it is difficult to decouple the influence of other parameters such as pore solution ion concentration on concrete conductivity from the measurement data. In addition, if the conductivity sensor is not set up properly, the measured current will pass through the steel bars, which will cause significant errors in 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 effect of ion concentration in the concrete pore solution on the dielectric constant, the measurement frequency needs to reach above 1GHz, and the cost of the measurement instrument is too high for distributed online monitoring.

[0004] At present, the methods used to monitor the moisture content of concrete each have different defects. Therefore, it is necessary to develop a new method that has no significant hysteresis effect, is not affected by ion concentration, etc., and takes into account the measurement cost. Summary of the invention

[0005] In order to solve the common problems of serious hysteresis, large measurement error and salt interference in the existing methods for measuring concrete saturation, the present invention starts from the porous characteristics of hardened cement slurry, based on the fact that the essence of saturation is the ratio of water to pores in hardened cement slurry, uses the relative water contact area of ​​the electrode to characterize the saturation, and uses the interface capacitance of the fully polarized electrode without electrode reaction to calculate the relative water contact area, thereby providing a concrete saturation sensor and monitoring method based on the relative water contact area of ​​the fully polarized electrode with high precision, slight hysteresis and slight salt interference.

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

[0007] A concrete saturation sensor based on the relative water contact area of ​​fully polarized electrodes 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, and furthermore, the fully polarized A electrode and the fully polarized B electrode are gold or platinum electrodes, or gold-plated or platinum-plated electrodes of other metal materials.

[0008] A method for monitoring the saturation of concrete based on the relative water contact area of ​​fully polarized electrodes comprises the following steps:

[0009] Step 1: bury the saturation sensor composed of the above-mentioned fully polarized A electrode and the fully polarized B electrode into concrete;

[0010] 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.

[0011] 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, wherein the amplitude of the sinusoidal 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 interface capacitance of the saturation 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 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, wherein the calibration process of the response curve of the relative water contact area of ​​the saturation sensor to the saturation includes the following steps:

[0014] (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;

[0015] (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;

[0016] (3) During the drying and moisture absorption process, the saturation degree is calculated by measuring the weight of the hardened cement slurry, 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.

[0017] Concrete saturation monitoring principle:

[0018] A fully polarized electrode refers to an electrode in which no electrochemical reaction occurs on the electrode surface within a certain potential range in an electrolyte solution. No Faraday current passes through the electrode interface within this potential range. The electrode completes polarization through ion adsorption. The larger the surface area of ​​the electrode, the greater the amount of charge adsorbed by the electrode when polarized to the same potential, that is, the greater the interfacial capacitance of the electrode. Concrete is a composite material composed of aggregates such as sand and gravel and hardened cement paste. Water mainly exists in the pores of hardened cement paste. The pores in hardened cement paste include pores, capillary pores and gel pores, and the pore sizes are continuously distributed from tens of nanometers to hundreds of microns. According to the Kelvin formula, the smaller the pore size, the smaller the saturated vapor pressure of water in the pore. When concrete absorbs water, small pores are filled with water before large pores. When concrete is dehydrated, large pores dry before small pores. When a fully polarized electrode is buried in concrete, when the concrete is fully saturated with water, all pores on the electrode surface are filled with water, and the electrode's water contact area reaches the maximum. When the concrete loses some water, a relatively large part of the pores on the electrode surface loses water, and the electrode's water contact area decreases. Therefore, by measuring the interface capacitance of the fully polarized electrode, the electrode contact area is calculated, and then the saturation of the hardened cement paste in the concrete is obtained.

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

[0020] The present invention calculates the saturation of hardened cement paste by the relative water contact area of ​​the saturation sensor, has good linear relationship, good weight linearity, and the response curves of the water absorption and dehydration processes do not show obvious separation, have slight hysteresis, and are slightly disturbed by salt. In general, the error of calculating the saturation by the relative water contact area through fitting the linear equation is within ±10%, and the saturation sensor and the measurement method are low in cost and time-consuming. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

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

[0026] Figure 6 The saturation response curve of the sensor of the present invention in a hardened cement test block containing 1% NaCl;

[0027] Figure 7 The saturation response curve of the sensor of the present invention in a hardened cement test block containing 3% NaCl;

[0028] Figure 8 The saturation response fitting curve of the sensor of the present invention under different salinity levels;

[0029] In the figure, 1. voltage signal generator, 2. voltage measurement module, 3. current measurement module, 4. relay, 5. saturation sensor, 51. fully polarized electrode A, 52. fully polarized electrode B, 6. hardened cement slurry. DETAILED DESCRIPTION

[0030] The technical solution of the present invention is further described below in conjunction with the accompanying drawings, but 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 should be included in the protection scope of the present invention.

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

[0032] Step 1: Prepare the water saturation sensor 5, such as Figure 2 As shown, the water saturation sensor 5 is composed of two identical fully polarized A electrodes 51 and fully polarized B electrodes 52, which are arranged in a forked form. The water saturation sensor 5 is buried in the hardened cement slurry 6. After 24 hours, the hardened cement slurry 6 is demolded and then steam cured at 60°C for 3 days, or standard cured 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 to 10 seconds, and then disconnect them for 5 to 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 electrode A 51 and the fully polarized electrode B 52 through the voltage signal generator 1, use the current measurement module 3 to record the current signal between the fully polarized electrode A 51 and the fully polarized electrode B 52, and calculate the impedance of the saturation sensor 5.

[0036] Step 5: Adoption Figure 3 The measured data are fitted by an equivalent circuit of a resistor R and a capacitor C in series, wherein R represents the concrete resistance between the fully polarized A electrode 51 and the fully polarized B electrode 52 , and capacitor C represents the equivalent interface capacitance between the fully polarized A electrode 51 and the fully polarized B electrode 52 .

[0037] Step 6: Calculate the relative water contact area of ​​the 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 They represent the water contact area and equivalent interface capacitance of the fully polarized A electrode 51 and the fully polarized B electrode 52 respectively when the water saturation is 100%; A d and C d They respectively represent the water contact area and equivalent interface capacitance of the fully polarized A electrode 51 and the fully polarized B electrode 52 at the water saturation to be measured;

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

[0041] Example

[0042] In this embodiment, the water saturation sensor is made of a printed circuit board, such as Figure 1 As shown, the substrate of the sensor is a 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 B electrode 52 are both made of copper-gold-plated materials. Each fork finger is 0.1 mm wide, 17 mm long, and has a spacing of 0.1 mm.

[0043] The hardened cement slurry was prepared using standard cement and deionized water with a water-cement ratio of 0.4. Three different hardened cement slurries were prepared with NaCl accounting for 0%, 1% and 3% of cement mass. The cement slurry was cast in a silicone mold, demolded after 24 hours, and cured for three days at a temperature of 60°C and a relative humidity of 98%. The test block was then immersed in water for three days and weighed. The equivalent interface capacitance C of the sensor in the saturated state was measured according to the steps in the monitoring method of the present invention. s The sides and bottom of the test block are then encapsulated with epoxy resin to make the water diffusion direction parallel to the electrode surface of the sensor to prevent the hardened cement slurry from having a water gradient in the normal direction of the electrode, which would lead to an erroneous response curve. The test block is placed in a desiccator for dehydration, and then weighed and the equivalent interface capacitance C is measured. d , repeat the steps of drying, weighing and measuring the equivalent interface capacitance. When the test block is dried to a saturation of about 20%, stop drying to prevent the test block from cracking and causing measurement errors. Then place the test block in an environment of 100% relative humidity to absorb moisture. Similar to the measurement steps in the drying process, repeat the steps of absorbing moisture, weighing and measuring the equivalent interface capacitance.

[0044] The impedance measurement range of the saturation sensor is 10000Hz~10Hz, the amplitude is 20mV, and the ... Figure 3 The RC series circuit shown in the figure is fitted, and the equivalent interface capacitance of the saturation sensor is calculated using formula (2):

[0045]

[0046] Among them, C dl is the equivalent interface capacitance of the saturation sensor, f is the measurement frequency, Z im is the imaginary part of impedance at frequency f. Typical test results are as follows Figure 4 As shown in the figure, under the same measurement frequency, the higher the water saturation, the higher the equivalent interface capacitance of the sensor; under the same water saturation condition, the lower the measurement frequency, the larger the equivalent interface capacitance value measured. And by Figure 4 It can be seen that the interface capacitance values ​​of the test blocks with saturation of 14% to 53% at high frequency are close and difficult to distinguish. The equivalent interface capacitance measured at a frequency of 10 Hz is selected to calculate the relative water contact area of ​​the sensor.

[0047] The equivalent interface capacitance of the saturation sensor at the maximum water contact area measured under the saturated state of the test blocks containing different amounts of NaCl is shown in Table 1. The saturation sensor is not significantly disturbed by the NaCl concentration. The saturation sensor has good reproducibility, with an average saturated capacitance of 12.97μF and a maximum deviation of ±0.48μF.

[0048] Table 1 Equivalent interface capacitance of saturation sensor

[0049]

[0050]

[0051] The corresponding curve of the relative water contact area and saturation of the saturation sensor is as follows: Figure 5 , Figure 6 and Figure 7 As shown in the figure, the test blocks with NaCl accounting for 0%, 1% and 3% of cement mass are respectively represented. The relative water contact area is calculated according to formula (1) by the ratio of the equivalent interface capacitance to the equivalent interface capacitance under saturated state. The saturation sensor has good reproducibility, and the response curves of the water absorption process and the dehydration process have a high degree of overlap, without showing obvious hysteresis. Figures 5 to 7 It can be seen that the water saturation of the test block is calculated from the relative water contact area through the linear fitting formula, and the error is within ±10%. Figure 8 The linearly fitted sensor response curves in test blocks containing 0%, 1% and 3% NaCl were summarized. The NaCl concentration had no significant effect on the response curve, and the deviation of the saturation calculated by the relative water contact area under different salinity contents was within ±8%.

Claims

1. A concrete saturation sensor based on the relative water contact area of ​​fully polarized electrodes, characterized in that 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.

2. The concrete saturation sensor based on the relative water contact area of ​​the fully polarized electrode according to claim 1 is 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. 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 saturation sensor through an equivalent circuit of a resistor and a capacitor in series; 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 piece 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 response curve of the relative water contact area of ​​the calibrated saturation sensor to the saturation.

4. The method for monitoring the saturation of concrete based on the relative water contact area of ​​fully polarized electrodes according to claim 3, 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.

5. The method for monitoring the saturation of concrete based on the relative water contact area of ​​fully polarized electrodes according to claim 3, characterized in that The equivalent interface capacitance calculation formula of the water saturation sensor is as follows: Among them, C dl is the equivalent interface capacitance of the saturation sensor, f is the measurement frequency, Z im is the imaginary part of impedance at frequency f.

6. The method for monitoring the saturation of concrete based on the relative water contact area of ​​fully polarized electrodes according to claim 3, characterized in that The relative water contact area calculation formula of the water saturation sensor is as follows: Where: RWA is the relative water contact area of ​​the saturation sensor; A s and C s Respectively represent the water contact area and equivalent interface capacitance of the fully polarized A electrode and the fully polarized B electrode 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.

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

8. The method for monitoring the saturation of concrete based on the relative water contact area of ​​fully polarized electrodes according to claim 3, 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) During the drying and moisture absorption process, the saturation degree is calculated by measuring the weight of the hardened cement slurry, 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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