JP7271388B2 - Method for removing salt from concrete structure - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for reducing the concentration of chloride ions (salt removal method) in a concrete structure that has been subjected to salt damage before damage due to salt damage (corrosion of steel materials, etc.) becomes apparent. The present invention relates to a method for removing salt from concrete structures that can be carried out with less labor and energy.

If more than a certain amount of chloride ions (Cl ^- ) enter the frame of a concrete structure due to salt damage, etc., the dense structure formed on the surface of steel materials (reinforcing bars embedded inside, PC steel materials, etc.) There is a high risk that the passive film will be destroyed and the steel will corrode. When the steel corrodes in the frame, the volume of the steel expands several times at the corroded portion, which may cause cracks in the concrete along the steel. When cracks occur, the supply of oxygen and water becomes easier, and corrosion progresses at an accelerated rate.

As one of the countermeasures against the intrusion of chloride ions into the frame of a concrete structure due to salt damage, etc., a method of reducing the concentration of chloride ions in the frame (a method of removing salt from concrete structures) is being implemented. It is Various methods have been proposed as methods for removing salt from concrete structures. There is known a method (electrochemical desalination method using electrophoresis) in which a direct current is applied to move the chloride ions that have penetrated into the concrete to the outside of the concrete (electrolyte solution side) by the principle of electrophoresis. It is

JP 2018-199596 A JP 2018-124286 A JP 2009-126728 A JP-A-2000-303700 JP-A-7-89773

However, the electrochemical desalination method using electrophoresis requires a large-scale facility, requiring considerable labor and time for preparation and removal work, as well as consuming a large amount of electrical energy, resulting in construction costs. There is also the problem that the cost is too high. In addition, drilling work or the like is required to access the internal steel material used as the cathode, which may impose a burden on the concrete structure.

The present invention is intended to solve the problems in the conventional technology as described above, and can be easily constructed, labor-saving, energy-saving, and low-cost, and is expected to have a sufficient salt removal effect. An object of the present invention is to provide a method for removing salt from concrete structures.

In the method for removing salt from a concrete structure according to the present invention, the inside of the target concrete is brought into a wet state with a humidity of 98% or more, and a hygroscopic aqueous solution with an equilibrium relative humidity of 95% or less is brought into contact with the surface of the concrete. It is characterized by moving the chloride ions inside the concrete to the outside of the concrete.

As the hygroscopic aqueous solution, it is preferable to use an aqueous solution of lithium nitrite having a concentration of 16% or more. In order to bring the aqueous solution into contact with the concrete surface, it is preferable to attach a water-retentive material containing the aqueous solution to the surface of the concrete, or to include the aqueous solution in the water-retentive material attached to the surface of the concrete. As the water-retaining material, it is preferable to use a material capable of retaining an aqueous solution of 1000 g/m ² or more.

Furthermore, as the water-retaining material, a fibrous substance having water-retaining properties (e.g., pulp, cloth, non-woven fabric, etc.) is formed into a sheet, or a porous material having water-retaining properties (e.g., zeolite, shirasu Balloons, foam beads, etc.) are molded into a board shape (plate shape), or an organic polymer material having water retentivity (for example, a polyacrylic acid-based water-absorbing polymer material) having water permeability It is preferable to use a sheet that is housed in a bag and molded into a sheet.

In addition, as a water-retaining material, a porous material having water-retaining properties (e.g., zeolite, shirasu balloon, or foamed beads), or an organic polymer material having water-retaining properties (e.g., polyacrylic acid-based water-absorbing molecular materials, etc.) may be used and they may be sprayed onto the surface of the concrete to adhere and form a water-retaining layer. Furthermore, it is preferable to use, as the water-retaining material, one whose water-retaining performance does not deteriorate when an aqueous solution of lithium nitrite is used.

Also, in order to bring the aqueous solution into contact with the surface of concrete, a water tank may be installed so as to cover the surface of concrete, and the aqueous solution may be stored in the water tank to bring the aqueous solution into contact with the surface of concrete. Incidentally, it is preferable to periodically replace the water-retaining material adhered to the concrete surface or the aqueous solution stored in the water tank. Moreover, it is preferable to attach a film or a cover (drying prevention means) so as to cover the outside of the water retaining material or the water tank.

In addition, when the surface of the concrete is painted after the salt removing method as described above is performed, the risk of deterioration due to re-infiltration of salt can be suitably avoided.

INDUSTRIAL APPLICABILITY The method for removing salt from a concrete structure according to the present invention can be implemented very easily, saves labor, saves energy, and can be carried out at low cost. .

FIG. 1 is an explanatory diagram of a container 1, a specimen 2, and the like used in the experiment of Example 1. FIG. FIG. 2 is an explanatory diagram of the container 1, the specimen 2', etc. used in the experiment of Example 1. FIG. FIG. 3 is an explanatory diagram of a method for extracting a sample from the specimen 2 used in the experiment of Example 1. FIG. 4 is a graph showing the measurement results of the chloride ion concentration in the sample obtained in the experiment of Example 1. FIG. FIG. 5 is a graph showing the correlation between the chloride ion concentrations of the outer portion 2d and the inner portion 2e of the specimen IC16 of Example 1 (the specimen immersed in a 16% lithium nitrite aqueous solution). FIG. 6 is an explanatory diagram of one embodiment of the present invention, and is an explanatory diagram of a method of bringing the aqueous solution 3 into contact with the surface of the concrete structure 11 using the water tank 12 .

The "method for removing salt from a concrete structure" according to the present invention is a method for reducing the concentration of chloride ions in the skeleton of a concrete structure. % or more), and a hygroscopic aqueous solution with an equilibrium relative humidity of 95% or less is left in contact with the surface of the concrete to move the chloride ions inside the concrete to the outside of the concrete. It is.

As a specific method for bringing the aqueous solution into contact with the concrete surface, a method using a water-retentive material, a method using a water tank, or the like can be employed. When a water-retaining material is used, for example, a fibrous material having water-retaining properties (pulp, cloth, non-woven fabric, etc.) is formed into a sheet (water-retaining sheet), or an organic polymer material having water-retaining properties. (Polyacrylic acid-based water-absorbing polymer material) is placed in a water-permeable bag and formed into a sheet (water-retaining sheet), impregnated with an aqueous solution, and attached to the surface of concrete. Alternatively, a porous material (zeolite, shirasu balloon, foam beads, etc.) having water retentivity is formed into a board shape (plate shape) (water retentive board), impregnated with an aqueous solution, and placed on the surface of concrete. You may make it contact|adhere to. Alternatively, a water-retaining porous material or a water-retaining organic polymer material is sprayed onto the concrete surface to form a water-retaining layer of the water-retaining material, and the water-retaining layer is made to contain the aqueous solution. can be

As the water-retaining material, it is preferable to use a material having as high a water-retaining property as possible (a material capable of retaining an aqueous solution of 1000 g/m ² or more). For example, when two sheets having a thickness of 6 mm and a water absorption amount of about 5,000 g/m ² are stacked and used, sufficient water retention can be obtained.

On the other hand, when using a water tank, for example, as shown in FIG. come into contact with

Moreover, it is preferable to take measures against dryness for the water-retaining material and the water tank. For example, a water-retaining sheet attached to the surface of concrete, a water-retaining board, a water-retaining layer formed by spraying on the surface of concrete, or a high airtightness (air permeability) so as to cover the outside of the water storage tank Install a film made of plastic (low temperature) or a cover made of metal or synthetic resin (dry prevention means). It should be noted that the drying prevention means must be configured so that the adhered water-retentive sheet or the like can adhere to the surface of the surrounding concrete.

In addition, as the aqueous solution contained in the water-retaining material or stored in the water tank, a lithium nitrite aqueous solution with a concentration of 16% (or 16% or more) (concentration 16%: equilibrium relative humidity 90%) can be used. can. When the aqueous lithium nitrite solution is included in the water-retaining material, the water-retaining material should, of course, be one that does not change in quality upon contact with the aqueous lithium nitrite solution and does not lower its water-retaining performance.

The period during which the aqueous solution is brought into contact with the concrete surface can be appropriately determined according to the degree of salt damage (the amount of chloride ions that have entered) in the target concrete structure. Furthermore, it is preferable to replace the water-retaining material and the aqueous solution with new ones periodically (for example, every four weeks).

Specifically, when a water-retaining sheet or a water-retaining board is used (attached to the concrete surface) as a water-retaining material that allows the aqueous solution to come into contact with the concrete surface, it is removed by peeling it off from the concrete surface. and replace it with a new water-retaining sheet or water-retaining board newly impregnated with the aqueous solution. In addition, when a water-retaining layer is formed on the surface of concrete by spraying a water-retaining material such as a porous material, the water-retaining layer is peeled off and removed, and the water-retaining material is sprayed again to form a new water-retaining layer. , and newly contains the aqueous solution. Also, when the water tank is used, the aqueous solution 3 (see FIG. 6) stored therein is replaced with a new one.

Moreover, it is preferable to paint the surface of the concrete after the above-described salt removal treatment is completed, and in this case, deterioration due to re-intrusion of salt can be preferably avoided.

The results of various experiments conducted by the inventors regarding the effects of the method according to the present invention will be described below as Examples 1 to 4.

An experiment was conducted to confirm the salt removal effect of lithium nitrite aqueous solution. In this experiment, specimens (mortar masses) containing a certain amount of chloride ions were prepared, and the specimens were immersed in an aqueous solution of lithium nitrite and other aqueous solutions for a certain period of time. Chloride ion concentration inside the body was measured and analyzed. In addition, electrochemical desalting treatment was performed on test specimens manufactured under the same conditions, and the effects were compared.

A test body was prepared by kneading sodium chloride into 1:2 mortar with a water-cement ratio (w/c) of 40% and molding it into a cylindrical shape with a diameter of 50 mm and a height of 100 mm. Then, water is applied from the outer surface to make each specimen wet (humidity of 98% or more), and as shown in FIG. The top of the container 1 was sealed and allowed to stand in a room away from direct sunlight for 26 weeks.

As the aqueous solution 3 in which the specimen 2 is immersed, a lithium nitrite aqueous solution with a concentration of 16% (solvent: lime water (calcium hydroxide saturated aqueous solution, pH 12 or higher)) was used. In addition, as a comparative example, lime water and a sufficient amount of anion exchange resin (strongly basic OH ^- type) relative to the lime water (an amount capable of absorbing the total amount of chloride ions contained in the specimen) were added. An aqueous solution was prepared, and the specimen was immersed under the same conditions.

Furthermore, in parallel with the immersion treatment, an electrochemical desalting treatment was performed. Specifically, a cylindrical specimen 2' (conditions such as formulation and chloride ion concentration are the same as those of the specimen 2 in FIG. 1) having a hollow portion with a diameter of 5 mm at the center position was manufactured and shown in FIG. As shown, a carbon rod 4 with a diameter of 5 mm is inserted into the cavity and accommodated in the container 1, and a titanium mesh material 5 is arranged so as to surround the test body 2'. The electrolyte solution 3' was put into the chamber, and the specimen 2' and the mesh material 5 were immersed. Then, the carbon rod 4 and the mesh material 5 are connected to the power supply unit 6, respectively, the carbon rod 4 is used as the cathode, the mesh material 5 is used as the anode, and a direct current of 1 A/m ² is applied from the power supply unit 6 for 8 weeks. An electric current was applied to the sample, and desalting was performed using the principle of electrophoresis. Lime water (pH 12 or higher) was used as the electrolyte solution 3'.

Next, the specimen 2 (see FIG. 1) subjected to the immersion experiment was taken out from the container 1, and as shown in FIG. position shown), remove the upper part 2a and the lower part 2c to extract the intermediate part 2b, and further divide the intermediate part 2b into the outer part 2d (the part from the outer peripheral surface to the depth of 7 mm toward the center) and the inner portion 2e (the hatched portion in FIG. 3), and samples were taken. The specimen 2' (see FIG. 2) subjected to the electrochemical desalting treatment was also separated into an outer part and an inner part by the same method, and samples were collected. Then, these samples were pulverized individually, and the chloride ion concentration of each sample was measured according to JIS A 1154 "Method for testing chloride ions contained in hardened concrete". Those results are shown in FIG.

In the graph of FIG. 4, "IA" is a specimen immersed in lime water, "IB" is a specimen immersed in an aqueous solution containing an anion exchange resin, and "IC16" is a 16% lithium nitrite aqueous solution. Immersed specimens, "ID" are specimens subjected to electrochemical desalting treatment, and "R" are specimens that were stored as they were after fabrication without immersion or the like. The dashed line (C _lim ) shown ⁱⁿ FIG. 4 is the value ( ^2.86 kg/m ³ ).

As shown in FIG. 4, the chloride ion concentration was significantly reduced both in the outer portion and the inner portion of the specimen ID subjected to the electrochemical desalting treatment. On the other hand, in the specimens IA, IB, and IC16 subjected to immersion treatment, a decrease in chloride ion concentration was confirmed in the outer portion 2d, but no significant decrease in concentration was observed in the inner portion 2e.

In addition, from this experimental result, the salt removal effect (chloride ion concentration reduction effect) that can be expected by performing immersion treatment using an aqueous lithium nitrite solution is effective up to a depth of at least about 7 mm from the outer surface of the specimen. turned out to be. Furthermore, when considering only the outer part (the part from the outer surface to a depth of 7 mm), the lowest chloride ion concentration among the immersion treated specimens IA, IB, and IC16 was the test body IC16 (specimen immersed in 16% lithium nitrite aqueous solution) (more specifically, IC16<IB<IA), therefore, when using 16% lithium nitrite aqueous solution, lime water and anion It was confirmed that a higher salt removal effect can be expected than when using an aqueous solution containing an exchange resin.

Next, an experiment was conducted to confirm the salt removal effect when a water-retaining sheet (water-retaining material) impregnated with an aqueous solution of lithium nitrite was adhered. In this experiment, the same specimen as that used in the immersion experiment (Example 1) was used.

First, water is applied from the outer surface of each specimen to make it wet (humidity of 98% or more). It was sealed and allowed to stand in a room without direct sunlight for 26 weeks, after which the chloride ion concentration inside each specimen was measured.

As the water-retaining sheet, two sheets having a thickness of 6 mm and a water absorption amount of 5000 g/m ² were stacked and used. As the aqueous solution with which the water-retentive sheet is impregnated, a 16% concentration lithium nitrite aqueous solution (solvent: lime water (pH 12 or higher)) was used.

In addition, for one group (specimen S) of the test specimens, a new water-retaining sheet impregnated with the lithium nitrite aqueous solution was provided every four weeks (newly impregnated with the lithium nitrite aqueous solution). A new water-retaining sheet) was replaced with a new water-retaining sheet), and the other group (specimen N) was performed without replacing the water-retaining sheet.

Each test body S, N was taken out from the container, and in the same manner as in Example 1, the intermediate part 2b of the test body was separated into an outer part 2d and an inner part 2e (see FIG. 3), and samples were collected. We measured the concentration of ions. The value of the chloride ion concentration of the outer portion 2d of the specimen S in which the water-retentive sheet was replaced was the value of the outer portion 2d of the specimen IC16 of Example 1 (the specimen immersed in a 16% lithium nitrite aqueous solution). (2.77 kg/m ³ , see FIG. 4) (S<IC16). On the other hand, the value of the chloride ion concentration of the outer portion 2d of the specimen N, in which the water-retentive sheet was not replaced, was higher than the value of the outer portion 2d of the specimen IC16 of Example 1 (S<IC16< N).

Therefore, when a water-retaining sheet impregnated with a 16% lithium nitrite aqueous solution is attached to the outer surface of the specimen, and the water-retaining sheet is periodically replaced, the test specimen is immersed in the 16% lithium nitrite aqueous solution. It was also confirmed that a high salt removal effect can be expected.

The same experiment was conducted using lithium nitrite aqueous solutions with a concentration of 4% and 8% in addition to the aqueous solution of lithium nitrite with a concentration of 16% as the aqueous solution to be impregnated in the water-retaining sheet. It was confirmed that the higher the concentration of lithium nitrate, the higher the salt removal effect can be expected.

In order to verify the salt removal mechanism of lithium nitrite, the equilibrium relative humidity of the lithium nitrite aqueous solution was measured, and in order to confirm the effectiveness as a preventive maintenance measure, Nitrite ions were analyzed.

The inventors of the present invention hypothesize that the difference in humidity between the interior and exterior of the concrete facilitates desalinization (migration of chloride ions from the interior of the concrete with high humidity to the exterior with low humidity). As a result of the above experiments (Examples 1 and 2), it was confirmed that a certain salt removal effect was obtained. Then, as shown in Table 1, it was confirmed that the equilibrium relative humidity of the 16% concentration lithium nitrite aqueous solution used in the experiment was 90% in an environment at a temperature of 20°C. This fact agrees with the above assumption. However, other effects were also identified between equilibrium relative humidity and salinity removal rate.

FIG. 5 is a graph showing the correlation between the chloride ion concentrations in the outer portion 2d and the inner portion 2e of the specimens of Examples 1 and 2 (excluding specimens IB and ID). As shown in this graph, both chloride ion concentrations showed a negative correlation. That is, as the chloride ion concentration of the outer portion 2d decreases, the chloride ion concentration of the inner portion 2e tends to increase from the initial chloride ion concentration (see specimen R in FIG. 4). .

At the stage when the nitrite ions in the aqueous solution brought into contact with the specimen permeate the interior of the specimen (the outer part 2d and the inner part 2e), part of the chloride ions present in the outer part 2d of the specimen , moved to the outside of the test body, while another part moved to the inner part 2e of the test body. In other words, it is inferred that both "equilibrium theory" and "kinetics" were involved in the factor that the coexistence of nitrite ions and chloride ions affected the movement speed of chloride ions.

The analysis results of nitrite ions in the specimen IC of Example 1 are shown in the table above. The NO ₂ ^- /Cl ^- molar ratio in the specimen IC was 2.39 in the outer part 2d and 0.50 in the inner part 2e. Nitrite ions have the effect of repairing the passive film of steel destroyed by chloride ions and suppressing corrosion reactions. According to previous studies, it has been reported that a corrosion inhibitory effect is high when the molar ratio of nitrite ions to chloride ions in concrete is 0.6 or more.

In the specimen IC16 with a diameter of 50 mm, it was confirmed that nitrite ions penetrated to the inner part 2e. It is thought that nitrite ions can be expected to have a corrosion inhibitory effect within a depth range of approximately 25 mm, and it was therefore confirmed that this can be effective as a preventive maintenance measure.

Experiments were carried out to confirm the method of moistening the inside of concrete by supplying water from the outer surface. In this experiment, a concrete member (3.7 m in length and 1.7 m in height) that had been removed due to salt damage was prepared as a test piece. Area: 30 × 30 cm) are selected at four locations, and the following four methods are individually applied to each test surface (one method for one test surface) to give water (tap water), It was allowed to stand still for one day while covered with plastic wrap to prevent drying.
Method a Sprinkling (500 g/m ² of water is sprayed)
Method B Attaching a water-retentive sheet (thickness 2 mm, water content 1,500 g/m ² ) Method C Attaching a water-retentive sheet (thickness 4 mm, water content 3,000 g/m ² ) Method D Water-retentive sheet (thickness 6 mm, water content 5,000 g /m ² ) pasting

After that (after 1 day), the wrap (the wrap and the water-retentive sheet in methods a to e) was removed. A hole was drilled inward from each test surface, a humidity sensor was fixed at a depth of about 20 mm, and the internal humidity was measured with the drilled hole sealed. The experiment was carried out indoors, and the temperature at the time of measurement was 24° C. and the humidity was 65%.

As a result of the measurement, the humidity inside the test surface subjected to method a was 88%, and the humidity inside the test surface subjected to methods b to e was all 98% or higher. From this experimental result, it was confirmed that the inside of concrete can be kept in a wet state (humidity of 98% or more) by using means capable of retaining water of 1500 g/m ² or more for one day or more.

1: container,
2, 2': test body,
2a: top,
2b: middle part,
2c: bottom,
2d: outer part,
2e: inner part,
3: aqueous solution,
3′: electrolyte solution,
4: carbon rod,
5: mesh material,
6: power supply unit,
11: Concrete structure,
12: water tank

Claims (13)

Make the inside of the target concrete a wet state with a humidity of 98% or more,
A hygroscopic aqueous solution with an equilibrium relative humidity of 95% or less is brought into contact with the surface of the concrete and allowed to stand still to move chloride ions inside the concrete to the outside of the concrete,
A method for removing salt from a concrete structure, wherein an aqueous solution of lithium nitrite is used as the aqueous solution. 2. The method for removing salt from a concrete structure according to claim 1, wherein the concentration of lithium nitrite in said lithium nitrite aqueous solution is 16% or more. By attaching a water-retentive material impregnated with the aqueous solution to the surface of the concrete, or by allowing the water-retentive material attached to the surface of the concrete to include the aqueous solution, the aqueous solution is added to the concrete. 3. The method for removing salt from a concrete structure according to claim 1, wherein the salt is brought into contact with the surface. 4. The method for removing salt from a concrete structure according to claim 3, wherein the water-retaining material is a material capable of retaining 1000 g/m ^<2> or more of the aqueous solution. 5. The method according to claim 4, wherein as the water-retaining material, a fibrous material having water-retaining properties is formed into a sheet, or a porous material having water-retaining properties is formed into a board. A method for desalinating a concrete structure as described. 5. The concrete structure according to claim 4, wherein an organic polymer material having water retentivity is contained in a bag having water permeability and formed into a sheet shape as the water retentivity material. Method for removing salt from materials. A water-retaining porous material or a water-retaining organic polymer material is used as the water-retaining material, and the material is sprayed onto the surface of the concrete to form a water-retaining layer. 5. The method for removing salt from a concrete structure according to claim 4. 5. The method for removing salt from a concrete structure according to claim 4, wherein the water-retaining material is one whose water-retaining performance does not deteriorate when the aqueous solution is used. Periodically replacing the water-retaining material with a water-retaining material newly containing the aqueous solution, or periodically replacing the water-retaining material with a new one and newly containing the aqueous solution. The method for removing salt from a concrete structure according to any one of claims 3 to 8, characterized by: Install a water tank so as to cover the surface of the concrete,
3. The method for removing salt from a concrete structure according to claim 1, wherein the aqueous solution is stored in the water tank so as to be brought into contact with the surface of the concrete. 11. The method for removing salt from a concrete structure according to claim 10 , wherein said aqueous solution in said water tank is periodically replaced. The method for removing salt from a concrete structure according to any one of claims 3 to 9 , characterized in that a film or cover is attached as drying prevention means so as to cover the outside of the water retaining material. A method for repairing a concrete structure, which comprises coating the surface of the concrete after performing the method for removing salt from a concrete structure according to any one of claims 1 to 12.

Images