Method for removing salt from concrete structures

Abstract

To provide a salt removal method for a concrete structure which can be implemented with less labor and less energy at low cost, can reduce an environmental load, and can expect sufficient salt removal effects.SOLUTION: A salt removal method for a concrete structure of the present invention is characterized by that an aqueous solution of an alkali metal salt is allowed to contact with a surface of a concrete and is set thereon to be permeated into an inside of the concrete, thereby allowing chloride ions in the inside of the concrete to move to an outside of the concrete. This method can be implemented in a condition of a moisture in the inside of the concrete of 95% or under, and the aqueous solution of the alkali metal salt with an alkali metal salt concentration of 0.3 to 3.0 mol / L can be used.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for reducing the chloride ion concentration in the skeleton of a salt-damaged concrete structure (a salt removal method) before damage due to salt damage (such as corrosion of steel materials) becomes apparent, and in particular to a salt removal method for a concrete structure which can be carried out with reduced labor, energy and cost and which reduces the burden on the environment. [Background technology]

[0002] Chloride ions (Cl) are released into the concrete structure due to salt damage, etc. - ) penetrates beyond a certain level, the dense passive film formed on the surface of steel materials (rebars buried inside, prestressing steel, etc.) is destroyed, increasing the risk of the steel materials corroding. When steel materials corrode inside the structure, the volume of the steel material expands several times in the corroded area, which may cause cracks in the concrete along the steel material. When cracks occur, oxygen and water are easily supplied, accelerating the corrosion process, which may result in concrete falling off or a decrease in the cross-sectional area of ​​the steel material, reducing the strength of the components.

[0003] As one of the countermeasures when chloride ions have penetrated into the skeleton of a concrete structure due to salt damage, etc., a method for reducing the chloride ion concentration inside the skeleton (a method for removing salt from a concrete structure) has been implemented. Various methods have been proposed for removing salt from a concrete structure, and one known method is to place an electrode (anode) on the outer surface of the concrete structure, pass a linear current through the steel material inside the skeleton as the cathode, and move the chloride ions that have penetrated into the concrete to the outside of the concrete by the principle of electrophoresis (an electrochemical desalination method using electrophoresis). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2018-199596 A [Patent Document 2] JP 2018-124286 A [Patent Document 3] JP 2009-126728 A [Patent Document 4] JP 2000-303700 A [Patent Document 5] Japanese Patent Application Publication No. 7-89773 [Patent Document 6] Patent Publication No. 2021-059927 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the electrochemical desalination method using electrophoresis requires large-scale equipment, and therefore requires considerable labor and time for preparation and removal, consumes a large amount of electrical energy, and has problems such as high construction costs. In addition, drilling work is required to access the internal steel material used as the cathode, which may put a strain on the concrete structure.

[0006] The present invention seeks to solve the problems associated with the conventional techniques as described above, and aims to provide a method for removing salt from concrete structures which can be carried out with reduced labor, energy and cost, while also reducing the burden on the environment and providing the expectation of sufficient salt removal effects. [Means for solving the problem]

[0007] The method for removing salt from a concrete structure according to the present invention is characterized in that an aqueous solution of an alkali metal salt is brought into contact with the surface of the concrete, allowed to stand, and allowed to penetrate into the concrete, thereby moving chloride ions inside the concrete to the outside of the concrete. This method can be carried out when the humidity inside the concrete is 95% or less. An aqueous solution of an alkali metal salt having an alkali metal salt concentration of 0.3 to 3.0 mol / L can be used.

[0008] In addition, an aqueous solution of an alkali metal salt in which lithium nitrite and potassium nitrite are dissolved in a molar ratio in the range of 1:1 to 1:3 can be used as the solute, and it is preferable that ions having a higher mobility than lithium ions among the cations and anions in the aqueous solution of the alkali metal salt are allowed to penetrate into the interior of the concrete at 5000 mg / kg or more. Effect of the Invention

[0009] The method for removing salt from a concrete structure according to the present invention can be carried out extremely easily, with reduced labor and energy consumption and at low cost, while reducing the burden on the environment and providing a sufficient salt removal effect. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is an explanatory diagram of a container 1, a test specimen 2, etc. used in the experiment of Example 1. [Diagram 2] FIG. 2 is a graph showing the calculation results, obtained in the experiment of Example 1, of the salt removal effect ratio in an aqueous solution of multiple types of alkali metal salts. [Diagram 3] FIG. 3 is a graph showing the calculation results, obtained in the experiment of Example 2, of the salt removal effect ratio by concentration in aqueous solutions of multiple types of alkali metal salts. [Figure 4] FIG. 4 is an explanatory diagram of the container 1, the test specimen 2, etc. used in the experiment of Example 3. [Diagram 5] FIG. 5 is an explanatory diagram of a method for taking a sample from the test piece 2 used in the experiment of Example 3. [Figure 6] FIG. 6 is a graph showing the calculation results of the salt removal effect ratio for each concentration of an aqueous lithium nitrite solution, obtained in the experiment of Example 3. [Figure 7] FIG. 7 is a graph showing the calculation results of the salt removal effect ratio for each concentration of the lithium nitrite aqueous solution, obtained in the experiment of Example 3. [Figure 8] FIG. 8 is a graph showing the calculation results of the salt removal effect ratio in the aqueous potassium carbonate solution, obtained in the experiment of Example 4. [Figure 9] FIG. 9 is a graph showing the analysis results of the concentration of ions that have permeated into the test specimens D2-1 and the like obtained in the experiment of Example 4. [Figure 10] FIG. 10 is an explanatory diagram of one embodiment of the present invention, and is an explanatory diagram of a method of bringing an aqueous solution 3 into contact with the surface of a concrete structure 11 using a water tank 12. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The "method of removing salt from a concrete structure" of the present invention is a method of reducing the chloride ion concentration within the framework of a concrete structure, and is characterized in that an aqueous solution of an alkali metal salt is brought into contact with the surface of the concrete and allowed to stand, allowing the aqueous solution to penetrate into the concrete, thereby migrating the chloride ions within the concrete to the outside of the concrete.

[0012] As a specific method for contacting the aqueous solution with the surface of the concrete, a method using a water-retentive material, a method using a water tank, etc. can be adopted. When using a water-retentive material, for example, a fibrous material having water-retentive properties (pulp, cloth, nonwoven fabric, etc.) formed into a sheet (water-retentive sheet), or an organic polymeric material having water-retentive properties (polyacrylic acid-based water-absorbent polymeric material) placed in a water-permeable bag and formed into a sheet (water-retentive sheet) may be impregnated with the aqueous solution and attached to the surface of the concrete, or a porous material having water-retentive properties (zeolite, shirasu balloons, foam beads, etc.) formed into a board (plate) (water-retentive board) may be impregnated with the aqueous solution and attached to the surface of the concrete. In addition, a porous material having water-retentive properties or an organic polymeric material having water-retentive properties may be sprayed and attached to the surface of the concrete to form a water-retentive layer made of the water-retentive material, and the aqueous solution may be impregnated into the water-retentive layer.

[0013] The water-retentive material should preferably have high water-retentive capacity (aqueous solution of 1000 g / m 2For example, a thickness of 6 mm and a water absorption capacity of 5000 g / m 2 If two sheets of this size are stacked together, sufficient water retention can be achieved.

[0014] On the other hand, when using a water tank, for example as shown in FIG. 10, a water tank 12 is installed so as to cover the surface of a concrete structure 11, and the aqueous solution 3 is stored in the water tank 12 so as to come into contact with the surface of the concrete.

[0015] It is also preferable to take measures to prevent drying out of the water-retentive material and the water tank. For example, a water-retentive sheet or board is attached to the surface of the concrete, or a water-retentive layer is formed by spraying it onto the surface of the concrete, or a highly airtight (low air permeability) plastic film or a metal or synthetic resin cover (drying prevention means) is attached to cover the outside of the water tank. The drying prevention means must be configured so that it can adhere closely to the surface of the concrete surrounding the attached water-retentive sheet, etc.

[0016] As the solute of the aqueous solution to be contained in the water-retentive material or stored in the water tank, an alkali metal salt such as lithium nitrite (LiNO2), lithium nitrate (LiNO3), sodium nitrite (NaNO2), potassium nitrite (KNO2), potassium carbonate (K2CO3), etc. The concentration of the aqueous solution is 0.3 to 3.0 mol / L, or 3.0 to 6.0 mol / L.

[0017] When the aqueous solution is contained in a water-retentive material, it is needless to say that the water-retentive material used is one that is not altered or loses its water-retentive performance when it comes into contact with the above-mentioned compound.

[0018] The period for which the aqueous solution is in contact with the surface of the concrete can be appropriately determined depending on the degree of salt damage (the amount of chloride ions that have penetrated) in the target concrete structure. Furthermore, it is preferable to replace the water-retentive material and the aqueous solution with new ones periodically (for example, every three weeks).

[0019] Specifically, if a water-retentive sheet or board is used (attached to the concrete surface) as the water-retentive material with which the aqueous solution comes into contact with the concrete surface, it is peeled off and removed from the concrete surface and replaced with a new water-retentive sheet or board that has been newly impregnated with the aqueous solution. Also, if a water-retentive layer is formed on the concrete surface by spraying a water-retentive material such as a porous material, this water-retentive layer is peeled off and removed, and the water-retentive material is sprayed again to form a new water-retentive layer that is then saturated with new aqueous solution. Also, if a water tank is used, the aqueous solution 3 (see Figure 10) stored inside is replaced with new one.

[0020] After the above-mentioned salt removal treatment is completed, it is preferable to paint the surface of the concrete. In this case, deterioration due to re-intrusion of salt can be suitably avoided.

[0021] 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. EXAMPLES

[0022] An experiment was conducted to confirm the salt removal effect of each aqueous solution of several types of alkali metal salts. In this experiment, test specimens (mortar blocks) containing a certain amount of chloride ions were prepared and immersed in the aqueous solutions for a certain period of time. After that, each aqueous solution was sampled, and the chloride ion concentration in each solution was measured and analyzed.

[0023] The test specimen was made of mortar with a water-cement ratio (w / c) of 40%, a mixture of cement and sand in a ratio of approximately 1:1.75, and sodium chloride at 10 kg / m 3 The test specimens were made by kneading the above ingredients together and molding them into cubes with sides of 20 mm. The chloride ion concentration inside the specimens was calculated based on the mortar equivalent value (0.6 m3 of volume excluding coarse aggregate). 3 / m 3 (assuming that the 3 It was adjusted so that

[0024] Specifically, as shown in Figure 1, aqueous solution 3 (300 ml) was placed in container 1, test specimens 2 were suspended so that the entire specimen was submerged in aqueous solution 3, the top of container 1 was covered with plastic paraffin film and sealed, and the specimens were immersed for 4 days in a room not exposed to direct sunlight. The container 1 in which the test specimens 2 were immersed was placed on a magnetic stirrer 4, and a stirrer 5 was rotated at a constant speed inside container 1 to continuously stir each aqueous solution 3 from the start to the end of the immersion.

[0025] Incidentally, Patent Document 6 (JP 2021-059927 A) discloses a method for removing salt from a concrete structure, which is characterized in that the inside of the target concrete is brought into a moist state with a humidity of 98% or more, and then a hygroscopic aqueous solution is brought into contact with the concrete. For example, 1500 g / m of water is used. 2 It has been explained that the inside of the concrete can be kept moist with a humidity of 98% or more by applying water to the outer surface of the concrete using a means capable of retaining the water for one day or more. In the experiment of this embodiment, the prepared test specimen was immersed in aqueous solution 3 as it was (without applying water from the outside, with the internal humidity being 95% or less).

[0026] The aqueous solutions 3 in which the test specimens 2 were immersed included 6.0 mol / L aqueous solutions of lithium nitrite, lithium nitrate, sodium nitrite, and potassium nitrite (solvent: purified water). Details of the aqueous solutions used in the experiment are shown in the table below. As a comparative example, purified water was prepared and subjected to the same immersion treatment.

[0027] [Table 1]

[0028] Series I is based on lithium nitrite and has a cation (Li + ) and anions (NO2 - ) was changed (cation: Li + →K + Or Na + , anion: NO2- →NO3 - ) one-component aqueous solutions, series II a combination of lithium nitrite and potassium nitrite (molar ratios 1:1 and 1:3), and series III a combination of lithium nitrite and lithium nitrate (molar ratios 1:1 and 1:3).

[0029] After the immersion, the aqueous solution and the purified water were collected from each container, and the chloride ion concentration in each aqueous solution (I-1 to III-4) and the purified water (Comparative Example) was measured using a chloride ion electrode. The measured chloride ion concentration in each aqueous solution (I-1 to III-4) was divided by the measured chloride ion concentration in the purified water to calculate the improvement rate of the salt removal effect compared to the purified water. The results are shown in Figure 2.

[0030] As shown in the graph in Figure 2, the salt removal effect of all the aqueous solutions used in the experiment was 1.4 times or more that of purified water. In addition, among the aqueous solutions in Series I to III, the salt removal effect of the aqueous solution in Series II (Aqueous Solutions II-1 and II-2), which combines lithium nitrite and potassium nitrite, was 3.0 times or more that of purified water.

[0031] The above experimental results confirmed that the movement of chloride ions inside the concrete can be promoted by contacting the surface of the concrete with an aqueous solution of alkali metal salts and allowing it to penetrate into the concrete. In addition, the absolute mobility of each ion is as follows: lithium ion (Li + ) < Sodium ion (Na + ) < Nitrate ion (NO3 - ) (≒ Nitrite ion (NO2 - ))<Potassium ion (K + ) and it was confirmed that the above experimental results show that the movement of chloride ions inside concrete can be more effectively promoted by using an aqueous solution of alkali metal salts that contains ions with high mobility. EXAMPLES

[0032] Four types of aqueous solutions of alkali metal salts (lithium nitrite aqueous solution, lithium nitrate aqueous solution, sodium nitrite aqueous solution, and potassium nitrite aqueous solution) with four different concentrations (0.1; 0.3; 1.0; 3.0 mol / L) were prepared, and an experiment was conducted to confirm the salt removal effect of each concentration. In this example, a test specimen manufactured under the same conditions as the test specimen in Example 1 was immersed under the same conditions as Example 1 (without water from the outside and with an internal humidity of 95% or less). However, in this example, the solvent was lime water (calcium hydroxide aqueous solution) in order to maintain the pH of the aqueous solution at 12 or more. As a comparative example, the test specimen was also immersed in lime water.

[0033] After the immersion, the aqueous solutions (alkali metal salt aqueous solution and lime water) were collected from each container, and the chloride ion concentration in each aqueous solution was measured using an ion chromatograph. The chloride ion concentration in the alkali metal salt aqueous solution was then divided by the chloride ion concentration in the lime water to calculate the improvement in the salt removal effect of the alkali metal salt aqueous solution compared to the lime water. The results are shown in Figure 3.

[0034] As shown in the graph of Fig. 3, it was confirmed that for any of the lithium nitrite aqueous solution, lithium nitrate aqueous solution, sodium nitrite aqueous solution, and potassium nitrite aqueous solution, if the concentration is 0.3 mol / L or more, a salt removal effect equal to or greater than that of lime water can be obtained. Furthermore, from a comparison with the experimental results of Example 1, it was confirmed that when carrying out the salt removal method of the present invention using an aqueous solution of an alkali metal salt, in order to obtain a sufficient salt removal effect, the concentration of the aqueous solution used should be 0.3 mol / L or more.

[0035] It is expected that the equilibrium relative humidity of an aqueous solution of an alkali metal salt increases as the concentration decreases. When the equilibrium relative humidity of the aqueous solutions used in the experiment of this embodiment was measured, the equilibrium relative humidity of the aqueous solution of lithium nitrite with a concentration of 1.47 mol / L (8 wt%) was 98%, and the equilibrium relative humidity of the aqueous solution of sodium nitrite with a concentration of 1.52 mol / L (10 wt%) was 95%. Therefore, it was confirmed that the movement of chloride ions can be promoted even when an aqueous solution of an alkali metal salt with an equilibrium relative humidity of 95% or more is used. EXAMPLES

[0036] Aqueous solutions of lithium nitrite with different concentrations were prepared, and an experiment was conducted to confirm the salt removal effect when the amount of the aqueous solution was changed. In this experiment, test specimens containing a certain amount of chloride ions were immersed in the aqueous solution or the aqueous solution was applied to the surface of the test specimen, and after a certain period of time, the chloride ion concentration inside each test specimen was measured and analyzed.

[0037] The test specimen was made of mortar with a water-cement ratio (w / c) of 45%, a mixture of cement and sand in a ratio of approximately 1:1.75, and sodium chloride at 10 kg / m 3 The test specimens were mixed and molded into cylindrical shapes with a diameter of 50 mm and a height of 100 mm. The chloride ion concentration inside the specimens was calculated based on the mortar equivalent value (0.6 m3 of volume excluding coarse aggregate). 3 / m 3 (assuming that the 3 It was adjusted so that

[0038] In this experiment, the test specimens were immersed in or coated with a 16% lithium nitrite solution (LN16) and a 40% lithium nitrite solution (LN40). Lime water (LN0) was used as a comparative example. The details of the solutions and immersion conditions used in the experiment are shown in the table below.

[0039] [Table 2]

[0040] The test specimens A1-2 and A3-2 in series A shown in the above table were placed in a container 1 shown in Fig. 4(1) containing an aqueous solution (400 ml, 25 L / m 2 ) so that the depth of water (the distance from the outer circumferential surface of the specimen 2 to the inner circumferential surface of the vessel 1) was 20 mm. The specimens B1-1 and B1-2 of series B were immersed in an aqueous solution (100 ml, 6.5 L / m 2 ) to a depth of 5 mm. On the other hand, the outer periphery of the specimen C1-1 in series C was immersed in an aqueous solution (320 ml, 0.3 L / m 2 ) was applied (film thickness: 0.25 mm) and placed in the container 1 in FIG. 4(3).

[0041] The top of each container 1 was covered with a plastic paraffin film and sealed, and the containers were left to stand for 182 days in a room away from direct sunlight. For the test specimen B1-1 of series B, the aqueous solution was replaced with a new one periodically (every 3 weeks) during the experiment.

[0042] After the period had elapsed, each test specimen 2 was removed from the container 1 (see FIG. 4) and, as shown in FIG. 5, cut horizontally at a position 25 mm below the top end and at a position 25 mm above the bottom end (positions shown by dashed lines in FIG. 5). The upper and lower parts 2a and 2c were removed to extract the middle part 2b. The middle part 2b was then separated into an outer part 2d (a portion extending from the outer periphery to a depth of 8 mm toward the center) and an inner part 2e (a portion shown by diagonal lines in FIG. 5), and samples were taken.

[0043] Next, the samples were individually pulverized and the chloride ion concentration of each sample was measured according to JIS A 1154 "Test method for chloride ions contained in hardened concrete". The measured chloride ion concentrations in the test specimens A3-2 to C1-1 immersed in or coated with lithium nitrite aqueous solution (concentration 16%, 40%) were divided by the measured chloride ion concentration in the test specimen A1-2 immersed in lime water (comparative example) to calculate the improvement rate of salt removal effect compared to lime water. The results are shown in Figures 6 and 7.

[0044] As shown in the graph of FIG. 6, the method carried out on the test specimen C1-1 (a 40% lithium nitrite aqueous solution of 0.3 L / m 2 The salt removal effect of the test specimen A1-2 with 25 L / m of lime water was compared with that of the test specimen A1-2 with 25 L / m 2 The method used for specimen B1-1 (immersion in 6.5 L / m2 of a 16% lithium nitrite aqueous solution) was approximately 0.1 times that used for specimen B1-2. 2 The salt removal effect of the method carried out on the test specimen A3-2 (immersion in 25 L / m2 of a 16% lithium nitrite aqueous solution) was about 1.3 times that of the comparative example. 2 The salt removal effect in the case of immersion in a 16% lithium nitrite solution was about 1.9 times that of the comparative example. Therefore, when using a 16% lithium nitrite solution, the amount of solution is 6.5 L / m 2 By setting the above, the lime water is 25L / m 2 It was confirmed that a higher salt removal effect could be expected than in the comparative example in which

[0045] In addition, as shown in the graph of FIG. 7, the method carried out on the test specimen B1-2 (a 16% lithium nitrite aqueous solution at 6.5 L / m 2 The salt removal effect of the method performed on specimen B1-1 (immersion in 6.5 L / m2 of 16% lithium nitrite aqueous solution, with replacement of the aqueous solution) was compared with that of the method performed on specimen B1-2 (immersion in 6.5 L / m2 of 16% lithium nitrite aqueous solution, with replacement of the aqueous solution). 2 This is an improvement over the method carried out for specimen A3-2 (immersion in 25 L / m2 of a 16% lithium nitrite aqueous solution, without replacing the aqueous solution). 2 The results were similar to those of the comparative example (immersion in water, without replacing the aqueous solution) (approximately 1.9 times that of the comparative example).

[0046] Therefore, it was confirmed that when carrying out the salt removal method of the present invention using a lithium nitrite aqueous solution (aqueous solution of an alkali metal salt), the movement of chloride ions can be more effectively promoted by continuously retaining at least a certain amount of the aqueous solution on the surface of the target object (concrete) and periodically replacing the aqueous solution. EXAMPLES

[0047] An experiment was conducted to confirm the salt removal effect of each aqueous solution using potassium carbonate as the target. In this experiment, a test specimen containing a certain amount of chloride ions (the same test specimen as in Example 3) was immersed in a potassium carbonate aqueous solution (KC29) with a concentration of 29%, and after a certain period of time, the chloride ion concentration inside the test specimen was measured and analyzed. The details of the aqueous solutions used in the experiment and the immersion conditions are shown in the table below.

[0048] [Table 3]

[0049] The test specimen D2-1 of series A shown in the above table was dissolved in water (400 ml, 25 L / m 2 ) so as to be immersed in water to a depth of 20 mm. The top of the container 1 was then covered and sealed with plastic paraffin film, and the container was left to stand for 182 days in a room not exposed to direct sunlight.

[0050] After the period had elapsed, the specimen 2 was removed from the container 1, and a sample was taken, pulverized, and the chloride ion concentration was measured in the same manner as in Example 3. The measured chloride ion concentration in the specimen D2-1 was then divided by the measured chloride ion concentration in the comparative example of Example 3 (the specimen A1-2 immersed in lime water) to calculate the improvement rate of the salt removal effect relative to the lime water. The results are shown in Figure 8. For comparison, Figure 8 also shows the results of the specimen A3-2 obtained by the experiment in Example 3.

[0051] As shown in the graph of FIG. 8, the method carried out on the test specimen D2-1 (25 L / m of potassium carbonate solution with a concentration of 29%) 2 The salt removal effect in the experimental test (immersion in 29% potassium carbonate solution) was approximately 1.4 times that of the comparative example. Therefore, it was confirmed that the movement of chloride ions inside the concrete can be promoted even when a 29% potassium carbonate solution is used as the aqueous solution of alkali metal salts.

[0052] Furthermore, in order to confirm the ions that had permeated into the test specimen D2-1 from the potassium carbonate aqueous solution, potassium ions (K + In order to compare the concentration of the ions that penetrated the specimens A3-2 and C1-1 used in the experiment of Example 3, the nitrite ion (NO2 - The results are shown in Figure 9.

[0053] As shown in the graph in Figure 9, it can be seen that ions penetrated into the immersed test specimens in both the cases where the potassium carbonate aqueous solution and the lithium nitrite aqueous solution were used. Therefore, it was confirmed that a salt removal effect can be expected even when a potassium carbonate aqueous solution with a concentration of 29% is used as the aqueous solution of the alkali metal salt.

[0054] In addition, the ion concentration that penetrated into the concrete of C1-1 was 4100mg / kg, and in this case, the salt removal effect from the lime water was not obtained. Therefore, it was confirmed that the ion concentration that penetrates into the concrete needs to be at least 5000mg / kg. [Explanation of symbols]

[0055] 1: Container, 2: Test specimen, 2a: Upper part, 2b: middle part, 2c: Lower part, 2d: outer part, 2e: medial part, 3: Aqueous solution, 4: Magnetic stirrer, 5: Stirrer, 11: Concrete structures, 12: Water tank

Claims

1. A method for removing salt from a concrete structure, comprising contacting an aqueous solution of an alkali metal salt containing lithium nitrite, lithium nitrate, sodium nitrite, potassium nitrite, or potassium carbonate as a solute with the surface of the concrete and allowing it to permeate into the concrete, while the humidity inside the concrete is 95% or less, thereby moving chloride ions inside the concrete to the outside.

2. 2. The method for removing salt from a concrete structure according to claim 1, wherein an aqueous solution of an alkali metal salt having a concentration of 0.3 to 3.0 mol / L is used.

3. 3. The method for removing salt from a concrete structure according to claim 1 or 2, characterized in that an aqueous solution of an alkali metal salt in which lithium nitrite and potassium nitrite are dissolved in a molar ratio in the range of 1:1 to 1:3 is used as a solute.

4. 2. The method for removing salt from a concrete structure according to claim 1, wherein ions having a higher mobility than lithium ions among cations and anions in an aqueous solution of an alkali metal salt are allowed to penetrate into the interior of the concrete in an amount of 26,000 mg / kg or more.

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