Method for suppressing alkali-aggregate reaction in cement-based hardened body

Abstract

To provide a method for suppressing the further progress of deterioration due to alkali-aggregate reaction occurring in a cement-based hardened body, by using an inexpensive material that exhibits a sufficient suppressive effect on alkali-aggregate reaction, thereby suppressing such reaction through a simple operation without concerns about economic efficiency or load-bearing capacity.SOLUTION: A method for suppressing alkali-aggregate reaction in a cement-based hardened body containing an aggregate and undergoing an alkali-aggregate reaction includes impregnating the cement-based hardened body with calcium propionate. The impregnation is performed by: immersing the cement-based hardened body in a solution in which calcium propionate is dissolved, under atmospheric pressure or reduced pressure; or applying the solution containing the dissolved calcium propionate onto the cement-based hardened body.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for inhibiting further alkali-aggregate reaction in a cement-based hardened body in which alkali-aggregate reaction has occurred. [Background technology]

[0002] Concrete is composed of cement hydrate, aggregate, and voids, and alkali hydroxides such as sodium hydroxide and potassium hydroxide exist in the voids. These alkali hydroxides have necessary properties, such as inhibiting corrosion of rebar in reinforced concrete, but if the concentration of alkali hydroxide is too high, it is known to cause an alkali-aggregate reaction (ASR), in which reactive aggregate reacts with the alkali hydroxide to produce alkali silicates, which expand when water is supplied to them. Lithium salts are known as ASR inhibitors for suppressing ASR, but because lithium salts are expensive, a method for suppressing ASR has been discovered in which inexpensive calcium propionate is added to cement as an alternative to neutralize the alkali hydroxide (Patent Document 1). It has also been reported that an ASR inhibitory effect was observed when an aqueous calcium propionate solution was injected into hardened concrete that had deteriorated due to ASR (Non-Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-169144 [Non-patent literature]

[0004] [Non-Patent Document 1] Abstracts of the Annual Conference of the Japan Society of Civil Engineers, September 2009, 5th Division, V-110(217-218) Summary of the Invention [Problem to be solved by the invention]

[0005] As described in Non-Patent Document 1, when calcium propionate aqueous solution is injected into concrete undergoing ASR, poor penetration into the concrete results in inconsistent ASR suppression effects. Considering the penetration range of calcium propionate from the internal injection port, the spacing between drilled holes must be narrow, which inevitably increases the number of drilled holes per unit area, making this economically undesirable. Furthermore, when the target object is reinforced concrete, drilling holes must avoid the rebar, necessitating rebar scanning, which is also economically undesirable. Furthermore, there are concerns about the impact of increasing the number of drilled holes on the load-bearing capacity of the concrete. Therefore, there was a need for a method of infiltrating calcium propionate into concrete experiencing ASR that did not require drilling, i.e., that did not require internal injection. Therefore, the present invention aims to provide a method for inhibiting further ASR deterioration in cement-based hardened bodies that have already experienced ASR, using a material that is inexpensive and has a sufficient ASR inhibition effect, without having to worry about the impact on economy or load-bearing capacity, and with simple operations. [Means for solving the problem]

[0006] The present inventors have conducted extensive research to solve the above problems and have discovered that expansion due to ASR can be suppressed by impregnating ASR-deteriorated cement-based hardened body with an aqueous calcium propionate solution, i.e., by bringing the surface of the hardened cement-based body into contact with the aqueous calcium propionate solution. That is, the method for inhibiting alkali-aggregate reaction of the present invention is a method for inhibiting the progress of alkali-aggregate reaction in a cement-based hardened body containing aggregate and undergoing alkali-aggregate reaction, and comprises impregnating the cement-based hardened body with calcium propionate. The impregnation may be carried out by immersing the cement-based hardened body in a solution in which calcium propionate is dissolved under normal pressure or in vacuum, or by applying the solution in which calcium propionate is dissolved to the cement-based hardened body. The impregnation method can be a method of immersing in an aqueous solution of calcium propionate in the atmosphere or in a vacuum for 3 to 14 days, or a method of applying the solution by direct spraying or compressing for 3 to 14 days. [Effects of the Invention]

[0007] The ASR suppression method of the present invention does not require drilling, and therefore can suppress ASR in cement-based hardened bodies without worrying about the impact on economy or load-bearing capacity. Furthermore, as mentioned above, calcium propionate does not have excellent permeability into hardened cementitious materials, so it is expected that the ASR suppression effect will be limited when calcium propionate aqueous solution is brought into contact with undeteriorated hardened cementitious materials. However, if cracks have already appeared on the surface of the hardened cementitious material due to ASR, the calcium propionate aqueous solution will penetrate more easily due to capillary action. In other words, further improvements in economic efficiency can be made by bringing calcium propionate aqueous solution into contact only with parts of the hardened cementitious material that have surface cracks caused by ASR. As mentioned above, ASR deterioration progresses when aggregate that has produced alkali silicates absorbs water and expands, so deterioration is accelerated in an environment where water is supplied from outside. If cracks caused by ASR already exist on the surface of the cement-based hardened body, capillary action will allow water to penetrate further into the body, accelerating the rate of deterioration due to ASR. In this invention, an aqueous solution of calcium propionate, which has an ASR-inhibiting effect, is infiltrated from the surface, so calcium propionate can be effectively delivered to locations where water is easily infiltrated, i.e., where ASR deterioration is likely to occur. DETAILED DESCRIPTION OF THE INVENTION

[0008] (Cement-based hardened body) The cement-based hardened product containing aggregate (hereinafter simply referred to as cement-based hardened product) of the present invention is not particularly limited to any known product. The cement-based hardened product can be produced by any known method without any limitation, and can be produced by kneading cement, aggregate, water, and an admixture, which is added as needed, and then hardening the mixture. Examples of cement include ordinary Portland cement, moderate heat Portland cement, and low heat Portland cement. The aggregate in the present invention refers to both fine aggregate and coarse aggregate. The effects of the present invention are particularly pronounced when reactive aggregate is used as the aggregate. Examples of admixtures include air-entraining agents, air-entraining water-reducing agents, water-reducing agents, high-performance air-entraining water-reducing agents, superplasticizers, separation-reducing agents, foaming agents, foaming agents, setting / hardening regulators, and accelerators.

[0009] (ASR inhibitor and impregnation method using it) The ASR inhibitor used in the ASR suppression method of the present invention is preferably calcium propionate in the form of an aqueous solution. The concentration of the aqueous calcium propionate solution is preferably 5 to 26 mass %, more preferably 10 to 25 mass %, and even more preferably 10 to 20 mass %. The ASR suppression method of the present invention is used by impregnating a cement-based hardened body with an aqueous calcium propionate solution. The impregnation can be carried out by applying the aqueous calcium propionate solution to the cement-based hardened body, or by immersing the cement-based hardened body in the aqueous calcium propionate solution under atmospheric pressure or in a vacuum. The impregnation treatment can be carried out on the cement-based hardened body not only once, but also multiple times.

[0010] In the impregnation method by immersion, the immersion time is preferably 3 to 14 days. The immersion temperature may be 5 to 35°C, and preferably 15 to 30°C. The amount of calcium propionate aqueous solution used for immersion may be sufficient to completely immerse the cementitious hardened body, but is generally about 200 times the volume of the cementitious hardened body, assuming that the volume of the cementitious hardened body is 100. When immersing in a vacuum, the cementitious hardened body and the calcium propionate aqueous solution are placed in an airtight container, and the internal pressure is set to 0.05 MPa or less. Furthermore, the impregnation method by application is not limited to general spraying or brushing, but can also be carried out by applying a cloth or water-absorbent polymer sheet soaked in the aqueous solution to the surface of the cement-based hardened body as a surface compress. The application is carried out so that the surface of the cement-based hardened body is wet. In such a method, moisture usually evaporates over time, so it is preferable to prevent moisture evaporation by using a sheet curing method or the like. [Example]

[0011] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. The methods for producing, storing, and measuring the change in length of the cementitious hardened bodies (also called mortar bar specimens) in the examples and comparative examples were basically carried out in accordance with JIS A 1146-2007 (Testing methods for alkali-silica reactivity of aggregates (mortar bar method)).

[0012] (Production of cement-based hardened bodies containing aggregates and undergoing alkali-aggregate reaction) (1) Blending and kneading The mortar was formulated with a mass ratio of 1 part cement, 0.5 parts water, and 2.25 parts aggregate. The cement used was ordinary Portland cement (NC), and since the Na2O equivalent was 0.55 mass%, NaOH was added to make it 1.2 mass%. Chert from Gifu Prefecture was used as the reactive aggregate. The amounts mixed per mixing were as follows: NC 600g Aggregate 1350g Water + 1mol / L-NaOH 300g Mixing was carried out using a Hobart mixer. Cement and fine aggregate were added to the mixer and mixed for 30 seconds. Water was then added and mixed for 30 seconds, followed by a 20-second pause. During the pause, any mortar adhering to the mixing bowl and paddle was scraped off with a spoon. The mixture was then mixed for 120 seconds. (2) Molding A triple formwork measuring 40 x 40 x 160 mm was used. After mixing, the mortar was immediately poured into the formwork. The mortar was poured in two layers, filling half the height of the formwork at a time, and each layer was tamped approximately 15 times per specimen using a tamping rod so that the end was 5 mm in. Finally, the excess was carefully scraped off so as not to damage the specimen, and the top surface was smoothed. (3) Initial curing and demolding, measurement of initial values After molding, the kneaded product was covered with a glass plate for 24 hours from kneading to prevent evaporation of water, and stored at 20°C. After 24 hours, the mold was removed and the initial value was measured, taking care not to let the specimen dry out. (4) Storage and measurement The specimens were stored in sealed containers at a temperature of 40±2°C and a humidity of 95% or more. When the specimens reached the specified age, they were removed from the storage room or box together with the container, and after keeping them at 20±3°C for 16 hours or more, the container was opened and the change in length at that age was measured. (5)Measurement method The change in length was measured according to JIS A 1129-3 (dial gauge method).

[0013] (ASR suppression test by impregnation of ASR inhibitor) When the expansion rate exceeded 0.05% at the age of 50 days, the mortar bar specimens were immersed in aqueous solutions of calcium propionate adjusted to various concentrations for 7 days (Examples 1 to 3). The immersion was carried out by placing the specimen in a plastic container and adding approximately 2 liters of calcium propionate aqueous solution of various concentrations so that the entire surface of the specimen was immersed. A spacer was placed below the specimen to ensure that the specimen was also impregnated from the bottom. The container was covered with a lid to prevent water from evaporating from the calcium propionate aqueous solution during immersion. After immersion, the specimen was returned to an environment of 40±2°C and a humidity of 95% or more, and storage and measurements were continued for 238 days. The results obtained are shown in Table 1.

[0014] [Table 1]

[0015] Comparative Example 1 shows the expansion rate when the concrete was not immersed in a calcium propionate aqueous solution. The expansion rate at 50 days was 0.076%, and at 238 days it was 0.146%, indicating that expansion continued even after 50 days if no suppression measures were taken. Example 1 shows the expansion rate when immersed in a 10% by mass aqueous solution of calcium propionate. The expansion rate at 50 days, before immersion, was 0.068%, and the expansion rate at 238 days was 0.095%, indicating that expansion was suppressed compared to the comparative example. Example 2 shows the expansion rate when immersed in an 18% by mass aqueous solution of calcium propionate. The expansion rate at 50 days (before immersion) was 0.069%, and the expansion rate at 238 days was 0.092%, indicating that expansion was suppressed compared to the comparative example. Example 3 shows the expansion rate when immersed in a 25% by mass aqueous solution of calcium propionate. The expansion rate at 50 days (before immersion) was 0.070%, and the expansion rate at 238 days was 0.092%, indicating that expansion was suppressed compared to the comparative example.

Claims

1. A method for inhibiting the progress of alkali-aggregate reaction in a cement-based hardened body containing aggregate and undergoing alkali-aggregate reaction, comprising: A method for inhibiting alkali-aggregate reaction, comprising impregnating the cement-based hardened body with calcium propionate.

2. 2. The method according to claim 1, wherein the impregnation is carried out by immersing the cement-based hardened body in a solution containing dissolved calcium propionate under atmospheric pressure or in a vacuum.

3. 2. The method according to claim 1, wherein the impregnation is carried out by applying a solution of calcium propionate to the cement-based hardened body.

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