JP7633063B2 - Moisture sensor and moisture amount detection method - Google Patents

Description

The present invention relates to a moisture sensor and a moisture detection method for detecting the amount of moisture on the surface and inside of a hardened cementitious body.

Traditionally, moisture in concrete plays an important role in determining properties such as the hydration reaction of cement, and the progress of concrete shrinkage and creep. Furthermore, moisture that penetrates from the outside can cause corrosion of internal rebars and promote the penetration of chloride ions, so understanding the distribution and state of moisture in concrete is important information that determines the durability of concrete. The moisture state of a concrete structure is also involved in strength development and drying shrinkage, and also has a significant impact on the workability and quality of finishing and waterproofing work. In finishing work, if finishing materials are applied when the material has a high moisture content, sufficient adhesive strength cannot be obtained, and swelling, peeling, cracks, etc. may occur. For these reasons, it is important to understand the amount of moisture in concrete.

As a method for grasping the moisture state in concrete, for example, Patent Document 1 discloses a technique for predicting the moisture content of concrete from the measured value of electrical resistance based on the relationship between the moisture content of concrete and the electrical resistance value (electrical resistance method). Patent Document 2 discloses a technique for predicting the moisture content of concrete from the measured value of electrostatic capacitance based on the relationship between the moisture content of concrete and electrostatic capacitance (electrostatic capacitance method). Other methods for grasping the moisture state in concrete include a method for measuring the humidity inside a small hole provided in the concrete, a method for measuring humidity by attaching an impermeable sheet to the concrete surface, and a method for measuring moisture content and humidity by attaching discoloration paper to the concrete surface. In measuring the moisture content of concrete, it is desirable to use a simple method that does not take much time at building construction sites where the measurement conditions vary.

Patent Document 3 also discloses technology related to a moisture sensor that is installed inside a concrete structure to measure the moisture content of the concrete structure. This moisture sensor uses the mortar material as the dielectric part, on which an opposing electrode is formed, and measures the moisture content inside the concrete by using the electrical characteristics of the dielectric part when an AC field is applied to the moisture sensor as an indicator.

Japanese Patent Application Publication No. 62-087841 JP 2013-250215 A JP 2021-025793 A

However, the methods described in Patent Document 1 and Patent Document 2 are widely used in construction sites such as floor construction because they can measure the moisture content of concrete quickly and easily. However, since the electrodes are embedded in the concrete surface or pressed against the concrete surface to measure the moisture content of concrete, the data is only from the vicinity of the concrete surface, and the moisture content at depths of several centimeters or more from the surface cannot be accurately measured (Fig. 18(a)(b)). In addition, the method described in Patent Document 1 uses embedded electrodes, which not only damages the object to be measured, but also requires repairing the holes in which the electrodes are embedded after measurement (Fig. 18(a)). In addition, the method described in Patent Document 2 measures the moisture content by pressing the electrodes against the concrete surface, so that dirt may adhere to the sensor when measuring a large amount, resulting in poor reliability (Fig. 18(b)). In addition, the method of measuring the humidity inside a small hole provided in concrete, the method of measuring humidity by attaching a moisture-impermeable sheet to the concrete surface, and the method of measuring the moisture content and humidity by attaching discolored paper to the concrete surface take time to measure, so measurements cannot be performed quickly.

In addition, in the moisture sensor described in Patent Document 3, since it is preferable to use a material with a thermal expansion coefficient close to that of concrete or mortar as the material of the electrode part, bulk materials such as precious metals such as gold, platinum, palladium, etc., and stainless steel materials such as SUS304, SUS316, and SUS430 are used. In addition, as a method of forming an electrode in the dielectric part, for example, cement paste, mortar, concrete cement, etc. before hardening are filled into the space of the ring-shaped electrode and hardened to integrate it. However, in this electrode formation method, the cementitious hardened body has the characteristic of shrinking when hardened, and there is a risk of a small gap being generated between the electrode part and the dielectric part. In addition, in actual use in a seasonal environment, there is a possibility that the sensor will be subjected to a thermal history of, for example, from below freezing to over 40°C due to seasonal fluctuations. In such an environment, even if a material with a thermal expansion coefficient close to that of concrete or mortar is used as the material of the electrode part, it is difficult to reliably suppress a small gap between the electrode part and the dielectric part, even if the volume change due to the difference in the thermal expansion coefficient between the electrode part and the dielectric part is small, when considering the load due to repeated temperature fluctuations.

Therefore, in the moisture sensor described in Patent Document 3, due to the hardening shrinkage of the mortar material and the difference in thermal expansion due to the difference in material between the electrode and mortar, a tiny gap occurs between the electrode and dielectric part as shown in Figure 17. When moisture penetrates into this gap, it becomes difficult to accurately measure the electrical characteristics of the dielectric part, and as a result, there are cases where the exact amount of moisture inside the concrete cannot be detected. Furthermore, when the dielectric part is made of ceramics, the difference in thermal expansion with the electrode part is larger than that of mortar, so the tiny gap becomes more noticeable.

The present invention has been made in consideration of the above circumstances, and aims to provide a moisture sensor and moisture detection method that can suppress the occurrence of minute gaps between the dielectric and the electrodes even when subjected to thermal history due to hardening shrinkage of the cementitious hardened body that forms the dielectric or temperature changes in the real environment, stably acquire the electrical characteristics of the dielectric over a long period of time without destroying the cementitious hardened body to be measured, and can easily and accurately grasp the moisture content on the surface or inside the cementitious hardened body to be measured.

(1) In order to achieve the above object, the present invention provides the following means: That is, the moisture sensor of the present invention is a moisture sensor for detecting the moisture content of a cementitious hardened body, and is characterized in that it comprises a dielectric body formed in a columnar shape, one or more pairs of electrodes provided on the surface or inside of the dielectric body and facing each other, and a current-carrying portion electrically connected to each of the electrodes, each of the pair of electrodes having a plurality of holes, and the electrodes having the holes are in close contact with the dielectric body by a dielectric material filled in the holes.

This makes it possible to suppress the occurrence of minute gaps between the dielectric and the electrodes even when the dielectric is subjected to hardening shrinkage of the cementitious hardened body that forms the dielectric or to thermal history due to temperature changes in the actual environment, and makes it possible to stably obtain the electrical characteristics of the dielectric over a long period of time without destroying the cementitious hardened body to be measured, and to more accurately grasp the moisture content on the surface or inside the cementitious hardened body to be measured simply and with high precision. In addition, since the moisture sensor can be installed at any location on the cementitious hardened body to be measured, it becomes possible to measure not only the surface of the cementitious hardened body to be measured but also deep inside, making it possible to grasp the distribution of moisture throughout the cementitious hardened body to be measured.

(2) In addition, in the moisture sensor of the present invention, the holes are characterized by being mesh-shaped. This allows the electrode to follow the volumetric changes of the dielectric, making it possible to suppress the occurrence of minute gaps between the dielectric and the electrode.

(3) In addition, in the moisture sensor of the present invention, the mesh that constitutes the mesh shape has an opening size of 0.1 to 0.5 mm and a void ratio of 35 to 50%. This allows the electrode to more easily follow the volumetric changes of the dielectric while maintaining its function as an electrode, making it possible to prevent minute gaps from occurring between the dielectric and the electrode.

(4) In addition, in the moisture sensor of the present invention, the dielectric material is a cementitious hardened body or a ceramic. This allows the dielectric material to have the same water absorption as the cementitious hardened body to be measured, making it possible to measure the moisture content with high accuracy.

(5) The moisture content detection method of the present invention is a moisture content detection method that is installed on the surface or inside of a cementitious hardened body and detects the moisture content of the cementitious hardened body, characterized in that the moisture sensor described in any one of (1) to (4) above is installed on the surface or inside of the cementitious hardened body, an AC electric field is applied to the moisture sensor, and the moisture content of the cementitious hardened body is detected based on the correlation between the change in the electrical characteristic value of the moisture sensor and the moisture content of the cementitious hardened body.

This makes it possible to suppress the occurrence of minute gaps between the dielectric and the electrodes even when the dielectric is subjected to hardening shrinkage of the cementitious hardened body that forms the dielectric or to thermal history due to temperature changes in the actual environment, and makes it possible to stably obtain the electrical characteristics of the dielectric over a long period of time without destroying the cementitious hardened body to be measured, and to more accurately grasp the moisture content on the surface or inside the cementitious hardened body to be measured simply and with high precision. In addition, since the moisture sensor can be installed at any location on the cementitious hardened body to be measured, it becomes possible to measure not only the surface of the cementitious hardened body to be measured but also deep inside, making it possible to grasp the distribution of moisture throughout the cementitious hardened body to be measured.

According to the present invention, it is possible to easily and accurately grasp the distribution of moisture throughout the hardened cementitious body to be measured without destroying the hardened cementitious body to be measured.

1 is a diagram showing a schematic configuration of a moisture sensor according to an embodiment of the present invention; 2A and 2B are front and side views of a dielectric and electrodes of the moisture sensor according to the embodiment. FIG. 1 is a diagram showing the permeation of moisture into a dielectric. 1A to 1C are diagrams illustrating steps in a method for manufacturing a moisture sensor. FIG. 2 is a diagram showing a schematic configuration of a moisture sensor used in Examples 1 to 4 and Comparative Examples 1 to 3. 3 is a photograph of the outer electrode used in Example 1. 1 is a photograph of a metal mesh (material: SUS304) used as an electrode in Examples 1 to 3. FIG. 1 is a diagram showing a schematic configuration of a conventional moisture sensor. 1 is a graph showing the relationship between moisture content and capacitance at frequencies of 100 Hz, 1 kHz, and 100 kHz for a moisture sensor using the electrode of Example 1. 13 is a graph showing the relationship between the moisture content and the capacitance at frequencies of 100 Hz, 1 kHz, and 100 kHz for a moisture sensor using the electrode of Example 2. 13 is a graph showing the relationship between moisture content and capacitance at frequencies of 100 Hz, 1 kHz, and 100 kHz for a moisture sensor using the electrode of Example 3. 13 is a graph showing the relationship between moisture content and capacitance at frequencies of 100 Hz, 1 kHz, and 100 kHz for a moisture sensor using the electrode of Example 4. 13 is a graph showing the relationship between moisture content and capacitance at frequencies of 100 Hz, 1 kHz, and 100 kHz for a moisture sensor using the electrode of Comparative Example 2. 13 is a graph showing the relationship between moisture content and capacitance at frequencies of 100 Hz, 1 kHz, and 100 kHz for a moisture sensor using the electrode of Comparative Example 3. 13 is a graph showing the relationship between moisture content and capacitance at frequencies of 100 Hz, 1 kHz, and 100 kHz for a moisture sensor using a conventional electrode of Comparative Example 4. 13 is a photograph showing a cross section (part of the outer electrode and dielectric) of the moisture sensor of Example 3. 1 is a photograph showing the state between a dielectric and an electrode of a conventional moisture sensor. FIG. 1 is a diagram showing an outline of moisture amount measurement using a conventional moisture sensor.

The inventors have noticed that when a moisture sensor is installed on the surface or inside of a cementitious hardened body to be measured, it is subjected to thermal history due to the hardening shrinkage of the cementitious hardened body or temperature changes in the actual environment, which causes a minute gap to form between the dielectric and the electrode, making it impossible to accurately measure the moisture content. They have discovered that by providing a hole in the electrode, it is possible to prevent the formation of a minute gap between the dielectric and the electrode, and to stably obtain the electrical characteristics of the dielectric over a long period of time without destroying the cementitious hardened body to be measured, thereby making it possible to more accurately grasp the moisture content on the surface or inside of the cementitious hardened body to be measured simply and with high accuracy, and have thus come up with the present invention.

This embodiment will be described with reference to the drawings. A cementitious hardened body is a hardened body that contains at least cement, and is a concept that includes cement paste, mortar, and concrete.

[Moisture sensor configuration]
FIG. 1 is a diagram showing a schematic configuration of a moisture sensor 100 according to this embodiment. FIGS. 2(a) and 2(b) are a front view and a side view of a dielectric 3 and electrodes 5 (5a, 5b) of the moisture sensor 100 according to this embodiment. The moisture sensor 100 includes a dielectric 3 formed in a columnar shape, a pair or more of electrodes 5 (5a, 5b) provided on the surface or inside of the dielectric 3 so as to face each other, and a lead wire 7 connected to each electrode 5 (5a, 5b) constituting a current-carrying part, and detects electrical characteristic values such as capacitance, impedance, and dielectric loss tangent that change according to the moisture content of the dielectric. The lead wire 7 is bonded to the electrodes 5 (5a, 5b) by solder. The lead wire 7 is connected to a measuring instrument 9, and an AC electric field is applied to detect the electrical characteristic values of the dielectric 3.

The dielectric 3 is a cylinder. In this embodiment, a cylinder shape is used as an example, but is not limited to this. For example, it may be a rectangular prism with a polygonal base, or a ring-shaped hollow cylinder (annular cylinder) with a hollow center.

The diameter of the dielectric 3 should be sufficient to contain the electrode 5, and is preferably 15 mm or more and 110 mm or less. The thickness of the dielectric 3 is preferably 3 mm or more and 50 mm or less. If the thickness of the dielectric 3 is less than 3 mm, it may be weak and may be damaged, and if it is more than 50 mm, it may take a long time for water to penetrate, which may reduce responsiveness.

The electrode 5 is a hollow cylinder. The diameter of the electrode 5 may be smaller than the diameter of the dielectric 3, but is preferably at least 10 mm smaller than the diameter of the dielectric 3. The height of the electrode 5 may be the same as the thickness of the dielectric 3, but is preferably at least 2 mm smaller. This is to prevent the electrode 5 from being exposed from the dielectric.

The material used for the dielectric 3 may be any material that contains water, and it is preferable to use a material that has the same water absorption as the cementitious hardened body to be measured. For example, it is preferable to use a cementitious hardened body such as cement paste, mortar, or concrete, or ceramics, as is the case with the measurement object. It is also possible to use a cementitious hardened body partially impregnated with resin.

The cement type of the cementitious hardened body may be any type, such as ordinary Portland cement or early-strength cement, but from the viewpoint of the hydration reaction, it is preferable to use an early-strength cement. The age of the cementitious hardened body is preferably 28 days or more, at which time the hydration reaction is almost complete.

When mortar is used for the dielectric 3, it is preferable to use a mixture having the same water absorption as the cementitious hardened body to be measured. Specifically, the water/cement ratio (W/C) is 15-70%, and the ratio of cement to fine aggregate is preferably 1 cement to 2-3 fine aggregate. If the water/cement ratio is less than 15%, the amount of pores in the mortar becomes too small, and if it is more than 70%, the amount of water absorption increases, so the amount of water does not approximate that of the cementitious hardened body, which is not preferable. If the ratio of fine aggregate is less than the cement ratio of 2, the amount of pores in the mortar becomes too small, and if it is more than 3, it is susceptible to the influence of the water absorption of the fine aggregate, so the amount of water does not approximate that of the cementitious hardened body, which is not preferable. In addition, since the fine aggregate is susceptible to the influence of the water absorption of the fine aggregate, the physical properties of the fine aggregate are preferably a water absorption rate of 3.0% or less and a specific surface area of 400 cm ² /g or less. For example, dense artificial sand is good as the fine aggregate.

When concrete is used as the dielectric 3, it is preferable to use a composition that has the same water absorption as the concrete structure to be measured.

By using a cementitious hardened body or ceramics for the dielectric 3, it is possible to increase adhesion to the cementitious hardened body being measured, and it is less likely to affect the strength and durability of the cementitious hardened body being measured.

In this way, by controlling the composition of the cementitious hardened body in the dielectric 3, it is possible to improve the consistency between the moisture sensor 100 and the moisture content inside the cementitious hardened body to be measured. In other words, by making the water absorption rate of the cementitious hardened body used in the dielectric 3 equivalent to that of the cementitious hardened body to be measured, it is possible to properly grasp the moisture content of the cementitious hardened body to be measured.

The pair of electrodes 5 (5a, 5b) are provided inside the dielectric 3 formed into a cylindrical body so that their central axes are the same and face each other, forming concentric cylindrical electrodes. Here, the electrode provided on the outer periphery of the inside of the dielectric 3 is referred to as the outer electrode 5a, and the electrode provided on the inner periphery of the inside of the dielectric 3 is referred to as the inner electrode 5b. The distance between the outer electrode 5a and the inner electrode 5b, i.e., the interelectrode distance, is preferably 3 mm or more and 50 mm or less. If the interelectrode distance is shorter than 3 mm, there is a possibility of a short circuit, and if it is longer than 50 mm, there is a possibility that uneven water content will affect the measurement accuracy.

The electrode 5 has a plurality of holes. In this embodiment, the plurality of holes is formed by a mesh (woven net) structure, but it is not limited to this as long as there are a plurality of holes. For example, the holes may be formed by punching. In addition, the mesh (woven net) structure may be a plain weave, a twill weave, a plain tatami weave, a patterned tatami weave, etc., and any weaving method may be used.

The inner electrode 5b may be formed as a dense electrode and provided inside the dielectric 3. Since compressive stress is applied due to the difference in thermal expansion, even if the inner electrode 5b is a dense electrode, there is a low possibility of a minute gap occurring between the dielectric and the electrode. Here, dense refers to a shape in which no holes are formed on the surface. In addition, when the dielectric 3 is a hollow cylinder, the inner electrode 5b may be provided so as to be in close contact with the inner surface of the dielectric 3.

Metal formed in a mesh shape has a lower rigidity than metal formed in a bulk shape, and can follow minute volume changes caused by hardening and shrinking of the dielectric. Furthermore, by providing a mesh-shaped electrode inside the dielectric, the surface of the electrode adheres closely to the dielectric material, and the inner surface that forms the area that becomes the void of the electrode hole adheres closely to the dielectric material filled in the hole. This allows the electrode and the dielectric to be completely integrated. Therefore, it is possible to prevent minute gaps from occurring between the dielectric and the electrode.

In addition, when the shape of the dielectric 3 is a rectangular parallelepiped or a cylindrical column with no hollow space and a polygonal base, parallel plate electrodes may be formed on the top and bottom surfaces. In the case of parallel plate electrodes, it is preferable that the pair of opposing electrodes have the same shape and size. Also, from the viewpoint of measurement accuracy, it is preferable that the distance between the electrodes is constant.

Figures 3(a) and (b) are diagrams showing the permeation of moisture into a dielectric. As shown in Figure 3(b), in the case of parallel plate electrodes, the sides are open, which may result in uneven permeation of moisture. On the other hand, as shown in Figure 3(a), in a structure in which the dielectric is shaped as a hollow cylinder and electrodes are formed on the curved surfaces, the sides of the dielectric are closed, which is the most preferable, as it ensures uniformity of moisture permeating into the dielectric. Note that some surfaces of the exposed part of the dielectric of the moisture sensor may be covered with resin or the like.

Since the moisture sensor is used by embedding it inside the cementitious hardened body, the electrode 5 is preferably made of a corrosive material. Furthermore, in order to improve the ability of the electrode 5 to follow minute volume changes caused by the hardening shrinkage of the dielectric and thermal history in the actual environment, it is preferable that the electrode 5 be made of a flexible material with a thermal expansion coefficient close to that of concrete or mortar. For example, precious metals such as gold, platinum, and palladium, as well as stainless steel materials such as SUS304, SUS316, and SUS430 are preferred.

If the holes in the electrode 5 are too large, the electrode area is insufficient, and the electrical characteristics cannot be measured accurately, which is not preferable. If the holes are too small, the material forming the dielectric cannot enter the holes, and integration and adhesion are reduced, which is not preferable. Therefore, the size of the holes in the electrode 5 is preferably mesh 40 (#40) or more from the viewpoint of functioning as an electrode. Also, mesh 100 (#100) or less is preferable from the viewpoint of integration with the dielectric. Here, mesh refers to the number of lines or meshes per 25.5 mm (1 inch), and mesh size is an index that indicates the combination of line diameter and mesh. For stainless steel mesh, a size that satisfies a line diameter of 0.1 to 0.25 mm and a mesh size of 0.1 to 0.5 mm is preferable.

[Manufacturing method of moisture sensor]
4 is a diagram showing steps in a method for manufacturing a moisture sensor. First, cylindrical rings that will become the outer and inner electrodes are made of mesh-shaped metal (S1). The inner electrode may be dense. In addition, in the case of parallel plate electrodes rather than concentric cylindrical electrodes, it is preferable to make both electrodes from mesh-shaped metal.

Next, lead wires are attached to the two electrodes (outer electrode and inner electrode) by soldering (S2). Next, a dielectric is fabricated (S3). The two electrodes (outer electrode and inner electrode) to which the lead wires fabricated in (S2) are attached are arranged concentrically on a formwork for the dielectric. Then, a material for forming the dielectric, such as cement paste, mortar, or concrete, is poured into the formwork and hardened. For example, when mortar is used as the material for the dielectric, the mortar poured into the formwork has a cement:fine aggregate ratio of 1:3, and the cement is ordinary Portland cement, and the fine aggregate is artificial sand with a specific surface area of 300 cm ² /g. The W/C is 55%, and the material age is preferably 28 days. As described above, the material used for the dielectric is not limited to mortar. Cementitious hardened bodies other than mortar (cement paste, concrete), ceramics, etc. may also be used. The hardened dielectric is removed from the formwork, and a moisture sensor is fabricated.

[How to install the moisture sensor]
The moisture sensor is installed on the surface or inside of the cementitious hardened body to be measured. When installing, the lead wire connected to the electrode is extended to the outside from the side of the cementitious hardened body to be measured. The lead wire is connected to a measuring device such as an LCR meter. The moisture sensor may also be installed by fixing it with a bolt to the surface of the cementitious hardened body to be measured.

[Calculation of moisture content]
The moisture sensor according to this embodiment detects the moisture content of the cementitious hardened body to be measured from the correlation between the capacitance (C) of the dielectric and the moisture content under an AC voltage and a constant frequency. It is also possible to detect the moisture content of the cementitious hardened body to be measured from the correlation between the relative dielectric constant ( _εr ), impedance (Z), dielectric tangent (tanδ), and the moisture content.

In addition, the capacitance (C) of the dielectric is measured in the frequency range of 10 Hz to 100 MHz, and the moisture content of the cementitious hardened body to be measured is detected from the correlation between the slope of each measured value (logarithm) versus frequency ( _logarithm ). Similarly, the moisture content of the cementitious hardened body to be measured can also be detected from the correlation between the slope of each measured value (logarithm) versus frequency (logarithm) for the relative dielectric constant (εr).

The latter detection method derives the slope from measurement data over a wide range of frequencies, which provides greater measurement accuracy than measurements at a constant frequency.

[Example]
[1] Preparation of Moisture Sensor A moisture sensor was actually prepared and the moisture content inside a concrete test piece was measured. Fig. 5 is a diagram showing the schematic configuration of the moisture sensor used in Examples 1 to 4 and Comparative Examples 1 to 3.

(1) Preparation of electrodes In Examples 1 to 4 and Comparative Examples 1 to 3, an outer electrode having a diameter of 28 mm and a thickness of 10 mm was prepared, and an inner electrode having a diameter of 10 mm and a thickness of 10 mm was prepared. Fig. 6 is a photograph of the outer electrode used in Example 1. Figs. 7(a) to 7(c) are photographs of metal (SUS304 material) meshes used as electrodes in Examples 1 to 3. As shown in Fig. 6, the outer and inner electrodes prepared in each Example and Comparative Example were cylindrical ring-shaped electrodes (hereinafter referred to as mesh electrodes) made of SUS304 material.

Table 1 summarizes the mesh, wire diameter, mesh size, and void ratio of the mesh electrodes used in each example and comparative example.

(2) Preparation of dielectric Lead wires were soldered to the outer and inner electrodes prepared in (1), and the two electrodes were arranged concentrically in a formwork with an outer diameter of 38 mm and a thickness of 10 mm, as shown in Fig. 5. Mortar was then poured into the formwork and allowed to harden. The mortar poured into the formwork had a cement:fine aggregate ratio of 1:3, with ordinary Portland cement as the cement and artificial sand with a specific surface area of 300 ^cm2 /g as the fine aggregate. The W/C ratio was 55%, and the material age was 28 days.

(3) Finishing After the mortar hardened, it was removed from the formwork and a moisture sensor was prepared.

(4) Fabrication of a conventional moisture sensor FIG. 8 is a diagram showing a schematic configuration of a conventional moisture sensor. As Comparative Example 4, a conventional moisture sensor was fabricated in which the electrodes were formed of a metal plate without holes. In the moisture sensor of Comparative Example 4, a ring-shaped outer electrode 15a with an outer diameter of 32 mm and an inner electrode 15b with an outer diameter of 8.2 mm, a thickness of 1.0 mm, a width of 10 mm, and an outer diameter of 8.2 mm were fabricated using a metal plate made of SUS304, a material of SUS304, and a thickness of 2.0 mm. The inner electrode 15b was placed at the center of the outer electrode 15a. Next, mortar was filled in the space formed between the two electrodes 15 (15a, 15b) to form a dielectric 13. The filled mortar was the same as the mortar used in Examples 1 to 4 and Comparative Examples 1 to 3, and after the mortar hardened, lead wires were attached to the two electrodes by soldering.

[2] Correlation between the moisture content and electrical characteristics of the moisture sensor Next, the correlation between the moisture content and electrical characteristics of the moisture sensor will be described. Correlation data between the moisture content and electrical characteristics of the moisture sensor was collected under the following conditions.

(i) The moisture sensor was dried, and the weight obtained by subtracting the weight of the electrodes and lead wires from the total weight of the moisture sensor in the dried state was determined as the initial weight (W0). Next, the moisture sensor was immersed in water in a vacuum environment, and the inside of the dielectric was saturated with water. It was then left for 24 hours in an environment with a temperature of 20°C and a humidity of 60%, and moisture content 1 (W1) was measured. It was then left for 12 hours in the same environment, and moisture content 2 (W2) was measured, and it was confirmed that W1 = W2, confirming that an equilibrium state had been reached.

(ii) Next, the moisture content (A60) in the dielectric was calculated by content 2 (W2) / initial weight (W0) × 100. The capacitance (C) of this moisture sensor was measured at frequencies of 100 Hz, 1 kHz, and 100 kHz using a measuring instrument (LCR meter) connected to the lead wires.

As in the case of 60% humidity, steps (i) and (ii) were repeated in an environment with a constant temperature (20°C) and humidity of 20%. As a result, the moisture content of the moisture sensor in a bone dry state (A0), a 20% humidity environment (A20), a 60% humidity environment (A60), and a saturated moisture state (A100) were approximately 0%, 2.0 wt%, 4.0 wt%, and 6.0 wt%, respectively.

Next, the capacitance of the moisture sensor was measured at frequencies of 100 Hz, 1 kHz, and 100 kHz for each moisture content. Figures 9 to 15 are graphs showing the relationship between moisture content and capacitance at frequencies of 100 Hz, 1 kHz, and 100 kHz for moisture sensors using electrodes of Examples 1 to 4 and Comparative Examples 2 to 4, respectively. As shown in Figures 9 to 15, in Examples 1 to 4 and Comparative Examples 2 to 4, the moisture content and capacitance of the moisture sensor are proportional, indicating that accurate moisture content can be measured. Furthermore, the capacitance could not be measured for Comparative Example 1 (mesh 30). This is thought to be because the mesh openings in the mesh electrode were too large and it did not function as an electrode.

[3] Integrity of the dielectric and the outer electrode Next, each moisture sensor was placed in a constant temperature and humidity bath, and the temperature was raised and lowered 100 times at a rate of 10°C/hr in the temperature range of -5 to 50°C. The electrode interface of each moisture sensor was then observed with an optical microscope. FIG. 16 is a photograph showing a cross section (part of the outer electrode and the dielectric) of the moisture sensor of Example 3. As shown in FIG. 16, even after the thermal history was applied, no minute gap was generated between the dielectric and the outer electrode, and it was confirmed that the dielectric and the outer electrode were in close contact. In addition, in Examples 1, 2, 4, and Comparative Example 1, as in Example 3, even after the thermal history was applied, no minute gap was generated between the dielectric and the outer electrode, and it was confirmed that the dielectric and the outer electrode were in close contact.

In the moisture sensors of Examples 1 to 4 and Comparative Example 1, even after a thermal history was applied, the dielectric and the electrode were completely integrated, whereas in the moisture sensors of Comparative Examples 2 to 4, minute gaps were observed. In other words, even if the outer electrode has holes, as in the moisture sensors of Comparative Examples 2 and 3, if the openings are too small, the material that forms the dielectric cannot penetrate into the holes, and integration and adhesion are reduced, resulting in minute gaps as a result of the thermal history.

Furthermore, when the capacitance was measured using the same method as in [2], the moisture sensors of Examples 1 to 3 showed no change in value before and after the application of thermal history, whereas the moisture sensors of Comparative Examples 2 to 4 showed a difference. In other words, the moisture sensors of Comparative Examples 2 to 4 were unable to measure the capacitance accurately because a tiny gap was created between the dielectric and the electrode, allowing moisture to get into the tiny gap. In other words, it was found that Examples 1 to 4 have mesh openings of 0.1 to 0.5 mm and a void ratio of 35 to 50%, and the size and proportion of the holes in the electrode are suitable for close contact with the dielectric.

As described above, by installing the moisture sensor according to this embodiment on the surface or inside of the cementitious hardened body to be measured and monitoring the change in the electrical characteristic value, it becomes possible to measure the moisture content on the surface and inside of the cementitious hardened body to be measured easily and with high accuracy. In addition, since the moisture sensor can be installed at any location inside the cementitious hardened body to be measured, it becomes possible to measure not only near the surface of the cementitious hardened body to be measured but also deep inside, and it becomes possible to grasp the distribution of moisture content throughout the entire cementitious hardened body to be measured.

100 Moisture sensor 3, 13 Dielectric 5, 15 Electrode 5a Outer electrode 5b Inner electrode 7 Lead wire, current-carrying part 9 Measuring instrument, LCR meter 15a Outer electrode 15b Inner electrode

Claims (5)

A moisture sensor for detecting the moisture content of a cementitious hardened body,
A dielectric material formed in a columnar shape;
A pair or more of electrodes provided on a surface or inside of the dielectric material and facing each other;
a current-carrying portion electrically connected to each of the electrodes,
Each of the pair of electrodes has a plurality of holes;
A moisture sensor, characterized in that the electrode having the hole is in close contact with the dielectric material by means of a dielectric material filled in the hole. The moisture sensor according to claim 1, characterized in that the multiple holes are in a mesh shape. The moisture sensor according to claim 2, characterized in that the mesh forming the mesh shape has an opening size of 0.1 to 0.5 mm and a void ratio of 35 to 50%. The moisture sensor according to any one of claims 1 to 3, characterized in that the dielectric is a cementitious hardened body or a ceramic. A moisture detection method for detecting a moisture content of a cementitious hardened body by installing a moisture sensor on a surface or inside the cementitious hardened body, comprising:
A moisture sensor according to any one of claims 1 to 4 is installed on a surface or inside of the cementitious hardened body,
Applying an AC electric field to the moisture sensor;
A moisture detection method comprising the steps of: detecting a moisture content of the cementitious hardened body based on a correlation between a change in an electrical characteristic value of the moisture sensor and the moisture content of the cementitious hardened body.