Manufacturing method of precast concrete

Abstract

To provide a simple method for manufacturing precast concrete that does not require steam curing. [Solution] A method for producing precast concrete, characterized by using a cement composition comprising rapid-strength Portland cement, an expansive agent, and calcium nitrite, and demolding during normal curing at an accumulated temperature in the range of 150 to 300°C·hr. Herein, the cement composition contains 6 to 14 parts by mass of expansive agent and 0.3 to 1.5 parts by mass of calcium nitrite per 100 parts by mass of rapid-strength Portland cement.

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing precast concrete. More specifically, this invention relates to a method for manufacturing precast concrete that has excellent strength reproducibility and can be produced by a simple method that does not require steam curing. [Background technology]

[0002] Typically, precast concrete is manufactured by steam curing, a process in which the concrete composition is cured in a high-temperature steam atmosphere. Therefore, industrially manufacturing precast concrete by steam curing requires steam curing equipment in concrete product factories and a significant amount of energy. To address these challenges, methods for manufacturing precast concrete while suppressing energy costs are being investigated.

[0003] For example, Patent Document 1 discloses a method for producing a cementite hardened product, which includes calcium nitrite, a rapid-strengthening expansive agent, and a high-performance water-reducing agent as admixtures. The problem with Patent Document 1 is that it is difficult to produce a cementite hardened product with a relatively small amount of cement per unit (e.g., 250 kg / m³). 3 The objective is to provide a method for manufacturing precast concrete that allows for a lower steam curing temperature and shorter curing time, and exhibits excellent demolding strength, even under the following conditions. To address this issue, it has been suggested that the calcium nitrite and the high-performance water-reducing agent be added in such a way that the time they are mixed together is less than one hour. However, this document describes a method that requires steam curing and does not mention whether this method can be applied to ordinary curing. Rather, it is intended to reduce energy costs in steam curing, given that it is difficult to achieve sufficient performance with ordinary curing. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2005-306633 [Overview of the project] [Problems that the invention aims to solve]

[0005] As described above, it has been difficult to manufacture precast concrete with excellent properties using conventional curing methods. Therefore, there has been a demand for the development of a method for manufacturing precast concrete with excellent properties using conventional curing methods that can reduce energy costs. [Means for solving the problem]

[0006] As a result of diligent research into the above-mentioned problems, the inventors have discovered a method for producing precast concrete that can achieve excellent strength development by using a cement composition comprising rapid-hardening Portland cement, an expansive agent, and calcium nitrite in specific proportions, and performing normal curing under specific conditions.

[0007] In other words, the present invention is as follows [1] to [5]. [1] A method for producing precast concrete using a cement composition comprising rapid-hardening Portland cement, an expansive agent, and calcium nitrite, The cement composition contains 6 to 14 parts by mass of the expansive agent and 0.3 to 1.5 parts by mass of calcium nitrite per 100 parts by mass of the rapid-hardening Portland cement. A method for manufacturing precast concrete, characterized in that, without steam curing, demolding is performed during normal curing at an accumulated temperature in the range of 150 to 300°C·hr. [2] The method for manufacturing precast concrete according to [1], wherein, before manufacturing the precast concrete, a preliminary test is conducted using a mortar mix obtained by removing coarse aggregate from the precast concrete mix, a calibration curve showing the relationship between cumulative temperature and compressive strength is created, and the cumulative temperature at demolding is determined based on the calibration curve. [3] The aforementioned precast concrete has a restrained expansion coefficient of 150 × 10 at 7 days of age. -6 ~250×10 -6 A method for manufacturing precast concrete according to [1] or [2], wherein the concrete is shrinkage-compensated within the range of [1]. [4] A method for producing precast concrete according to any one of [1] to [3], wherein the content of the rapid-hardening Portland cement relative to the total mass of the precast concrete is 10% by mass or more. [5] A method for manufacturing precast concrete according to any one of [1] to [4], wherein the ordinary curing is performed at a curing temperature of 30°C or lower. [Effects of the Invention]

[0008] According to the present invention, a method for manufacturing precast concrete is provided that enables excellent strength development through ordinary curing, that is, while suppressing energy costs. [Brief explanation of the drawing]

[0009] [Figure 1] A diagram showing the relationship between the cumulative temperature and compressive strength of formulations a, a', and b according to the present invention. [Modes for carrying out the invention]

[0010] The embodiments of the present invention will be described in detail below.

[0011] The method for producing precast concrete according to the present invention involves obtaining precast concrete by using a specific cement composition and curing it under specific conditions.

[0012] In the method according to the present invention, the cement composition comprises rapid-hardening Portland cement, an expansive agent, and calcium nitrite.

[0013] Portland cement is classified into ordinary Portland cement, early-strength Portland cement, super-early-strength Portland cement, medium-heat Portland cement, low-heat Portland cement, sulfate-resistant Portland cement, etc. In the present invention, early-strength Portland cement is used. Early-strength Portland cement exhibits high strength at an early stage compared to the commonly used ordinary Portland cement. Generally, early-strength Portland cement can achieve the strength of ordinary Portland cement cured for about 7 days in 3 days of curing.

[0014] An expansive agent is an effective component for exhibiting expansive performance by hydration and suppressing shrinkage cracking, and generally, an expansive agent used in concrete can be used. Specifically, quicklime-based expansive agents, CSA (calcium sulfoaluminate) - based expansive agents, etc. can be mentioned. In particular, from the perspective of compatibility with calcium nitrite described later, quicklime-based expansive agents are preferred.

[0015] The fineness of the expansive agent is preferably 2000 - 6000 cm 2 / g. In order to enhance the hydration reactivity and increase the initial heat of hydration, it is more preferably 3000 - 5000 cm 2 / g.

[0016] From the perspective of crack suppression and strength development, the blending amount of the expansive agent needs to be 6 - 14 parts by mass, and preferably 8 - 13 parts by mass, based on 100 parts by mass of early-strength Portland cement.

[0017] Calcium nitrite has a function of promoting the hardening of cement. In the present invention, by blending an appropriate amount of calcium nitrite together with the expansive agent, sufficient strength can be obtained even under normal curing. The blending amount of calcium nitrite needs to be 0.3 - 1.5 parts by mass, and preferably 0.6 - 1.2 parts by mass, based on 100 parts by mass of early-strength Portland cement in order to obtain sufficient strength development even under normal curing.

[0018] The method according to the present invention uses a cement composition that contains the above-mentioned components as essential, but this cement composition may also contain general additives that are blended into cement compositions, to the extent that they do not impair the effects according to the present invention.

[0019] The type of water used in the precast concrete of this invention is not particularly limited, but tap water or well water can be used.

[0020] Furthermore, the aggregate used in precast concrete can be fine aggregate or coarse aggregate as needed. Fine aggregates can include land sand, river sand, crushed sand, sea sand, slag fine aggregate, light fine aggregate, heavy fine aggregate, recycled fine aggregate, or mixtures thereof. Coarse aggregates can include river gravel, mountain gravel, sea gravel, crushed stone, slag coarse aggregate, light coarse aggregate, heavy coarse aggregate, recycled coarse aggregate, or mixtures thereof.

[0021] The precast concrete of the present invention can be mixed with various commonly used admixtures. Examples of common admixtures include water-reducing agents, AE water-reducing agents, high-performance water-reducing agents, and high-performance AE water-reducing agents. These admixtures may include melamine sulfonic acid-based, naphthalene sulfonic acid-based, and polycarboxylic acid-based components.

[0022] In addition to water-reducing agents, other admixtures such as air-entraining agents, antifoaming agents, defoaming agents, foaming agents, thickeners, rust inhibitors, shrinkage-reducing agents, and pigments may be added, provided that they do not impair strength or workability.

[0023] The precast concrete of the present invention may further contain commonly used admixtures. Examples of admixtures, in addition to the expansive materials mentioned above, include fly ash, blast furnace slag powder, silica fume, volcanic glass powder, and limestone powder. However, in the present invention, it is preferable that the binder, which is the sum of the rapid-hardening Portland cement and the admixtures, contains a sufficient amount of rapid-hardening Portland cement, and that the admixture content is limited. Specifically, the rapid-hardening Portland cement content in the binder is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more.

[0024] In this invention, it is preferable that the content of the rapid-hardening Portland cement relative to the total mass of the precast concrete is 10% by mass or more, and more preferably 12% by mass or more. The higher the content of rapid-hardening Portland cement, the more pronounced the effects of the present invention tend to be.

[0025] The method according to the present invention involves curing concrete containing the above-mentioned cement composition under specific conditions to obtain precast concrete. The method for producing precast concrete according to an embodiment of the present invention will be described below. The method for manufacturing precast concrete according to embodiments of the present invention is: (1) A process of mixing high-early-strength Portland cement, expansive agent, calcium nitrite, aggregate and water to mix concrete. (2) The process of pouring the mixed concrete into the formwork, (3) The process of curing the poured concrete by ordinary curing, (4) After the concrete reaches the predetermined cumulative temperature, the process of removing it from the formwork. Includes.

[0026] In step (1), the concrete is mixed by adding the predetermined amounts of rapid-hardening Portland cement, expansive agent, calcium nitrite, aggregate, and water to a mixer. At this time, it is preferable that the calcium nitrite is added as an aqueous solution dissolved in water. Alternatively, it is preferable to add the aqueous solution of calcium nitrite after mixing the other constituent materials and then mix further. Admixtures may be added to this concrete as needed. Mixers such as forced twin-shaft, pan-type, and tilting-drum mixers used in ready-mix concrete plants can be used.

[0027] In step (2), the mixed concrete is poured into a formwork of the desired shape. It is preferable to pour the concrete less than one hour after the mixing is completed.

[0028] After the concrete pouring is completed in step (2), curing is carried out by ordinary curing in step (3). One feature of the method of the present invention is that steam curing is not performed. That is, treatments such as heating or steam application are unnecessary, and no such devices or equipment are required. In the method of the present invention, ordinary curing is generally performed at room temperature, and it is preferable to perform it at a temperature of 30°C or lower. On the other hand, in order to prevent concrete freezing, it is preferable to perform ordinary curing at a temperature above 0°C, and it is more preferable to perform it at a temperature of 5°C or higher.

[0029] In step (4), after reaching a predetermined cumulative temperature, the concrete is removed from the formwork. The predetermined cumulative temperature in this invention is in the range of 150 to 300°C·hr, preferably in the range of 200 to 280°C·hr. Within this range of cumulative temperature, the target demolding strength is 10 to 15 N / mm 2 This can be obtained.

[0030] In this invention, the cumulative temperature is the cumulative value of the curing temperature (temperature of the curing space) and curing time during the manufacture of mortar or concrete. For the specific calculation of the cumulative temperature, the following formula was used, with 0°C as the reference value. [Number] Here, T n : Curing temperature (°C) at the Nth measurement T n+1 : Curing temperature (°C) at the (N + 1)th measurement Δ t : Time difference (hr) between the Nth measurement and the (N + 1)th measurement That is.

[0031] The properties of the precast concrete obtained by the method according to the present invention are set according to the application, and the curing conditions (cumulative temperature) until demolding are set according to the concrete mix. In the present invention, precast concrete having excellent strength development can be manufactured. Furthermore, the restrained expansion rate at 7 days of age is 150×10 -6 ~250×10 -6 Shrinkage-compensating concrete within the range can be obtained.

[0032] Precast concrete can be manufactured by the method as described above. However, in demolding the product from the mold, it is preferable to grasp the relationship between the curing temperature, the cumulative temperature, and the compressive strength in advance and determine the optimal curing conditions. To determine such curing conditions, before manufacturing the precast concrete, a mortar mix excluding coarse aggregate from the precast concrete mix is used, the cumulative temperature is changed, a preliminary test is conducted, and a calibration curve showing the relationship between the cumulative temperature in normal curing and the compressive strength of the obtained precast concrete is created, and it is preferable to determine the cumulative temperature at demolding based on the calibration curve.

Example

[0033] Formulations a, a' and b Mortar mixes a and b were prepared using two concrete mixes A and B, which have different design strengths, as described later, as base mixes. The mortar mixes were prepared by removing the coarse aggregate from the concrete mixes. In addition, mortar mix a' was prepared by removing calcium nitrite from mix a and designing the strength using only the expansive agent. To suppress fluctuations in air content, the process was carried out using a formulation that included the addition of an antifoaming agent. Each mortar mix is ​​shown in Table 1. Details of the raw materials used are shown in Table 2.

[0034] [Table 1]

[0035] [Table 2]

[0036] For each prepared mortar mix, mixing was performed at 10°C, followed by normal curing at a constant temperature of 10°C or 20°C. The test ages were 16 hours and 24 hours, and demolding was performed when the cumulative temperature during curing reached 160, 240, 320, and 480°C·hr, respectively. The resulting samples were molded into Φ5 × 10 cm molds and subjected to compression tests.

[0037] Table 3 shows the results of the compressive strength tests for each sample measured. Figure 1 shows the relationship between cumulative temperature and compressive strength for mortar mixes a, a', and b. These results show that the mixes without calcium nitrite did not provide sufficient effect, and the combined effect of the expander and the accelerator calcium nitrite was confirmed compared to the expander alone. A correlation between cumulative temperature and compressive strength was also confirmed. At a cumulative temperature of 150-300°C·hr, the compressive strength was 10-15 N / mm, which is the target demolding strength. 2 It can be seen that it reaches this point. Furthermore, it can be seen that good strength development can be obtained even at 28 days of age.

[0038] [Table 3]

[0039] Formulations A and B Two types of precast concrete mixes, A and B, with different design strengths, were prepared according to the mix designs shown in Table 4. The details of the raw materials used in these concretes are shown in Table 2.

[0040] [Table 4]

[0041] For each of the prepared concrete samples, normal curing was carried out under the same conditions as for mixes a and b, and the compressive strength at each accumulated temperature was evaluated. The results are shown in Table 5.

[0042] [Table 5] Even when precast concrete is manufactured using the cement composition of the present invention, the compressive strength is 10 to 15 N / mm at an accumulated temperature of 150 to 300°C·hr. 2 It can be seen that it will reach this point.

Claims

1. A method for producing precast concrete using a cement composition comprising rapid-hardening Portland cement, an expansive agent, and calcium nitrite, The cement composition contains 6 to 14 parts by mass of the expansive agent and 0.3 to 1.5 parts by mass of the calcium nitrite, with respect to 100 parts by mass of the rapid-hardening Portland cement. A method for manufacturing precast concrete, characterized in that, without steam curing, demolding is performed during normal curing when the cumulative temperature is in the range of 150 to 300°C·hr.

2. The method for manufacturing precast concrete according to claim 1, wherein, before manufacturing the precast concrete, a preliminary test is conducted using a mortar mix obtained by removing coarse aggregate from the precast concrete mix, a calibration curve showing the relationship between cumulative temperature and compressive strength is created, and the cumulative temperature at demolding is determined based on the calibration curve.

3. The aforementioned precast concrete has a restrained expansion coefficient of 150 × 10 at 7 days of age. －６ ~250 x 10 －６ A method for manufacturing precast concrete according to claim 1 or 2, wherein the concrete is shrinkage-compensated within the range of the specified area.

4. A method for producing precast concrete according to claim 1 or 2, wherein the content of the rapid-hardening Portland cement relative to the total mass of the precast concrete is 10% by mass or more.

5. The method for manufacturing precast concrete according to claim 1 or 2, wherein the ordinary curing is performed at a curing temperature of 30°C or lower.

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