Manufacturing method of precast concrete, quick hardening material for recast concrete, quick hardening concrete material for precast, and precast concrete

Abstract

To provide a manufacturing method of precast concrete, the method enabling early mold-removal and the precast concrete being desirable in high density and finishing performance.SOLUTION: A manufacturing method of precast concrete, the method injecting by injecting means a concrete material containing a concrete composition and a quick hardening material into a mold frame, includes, while press-feeding the concrete composition, mixing the concrete composition and the quick hardening material immediately before the injection and preparing the concrete material.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing precast concrete, a rapid hardening agent for precast concrete, a rapid hardening concrete material for precast concrete, and precast concrete. [Background technology]

[0002] In recent years, the precast construction method, in which concrete products are manufactured in factories and then transported to the construction site for assembly, has become more common. Since the precast construction method uses precast concrete products manufactured in advance in factories, construction can be carried out efficiently at the construction site.

[0003] In order to further improve the efficiency of the entire precast construction method, the efficiency of the production of precast concrete in factories is being improved. For example, as in Patent Document 1, a method for producing precast concrete has been proposed, which contains an expansive admixture of a specific composition and is characterized by steam curing at a maximum temperature of 40 to 80°C. Alternatively, a method for producing precast concrete using a concrete composition of a specific composition has been proposed, which is characterized by pouring concrete into a formwork, pre-curing at 30 to 50°C for 6 to 9 hours to hasten the initial setting time, and then increasing the curing temperature at a rate of 10 to 20°C / h, steam curing at a maximum temperature of 60 to 90°C for 2 to 4 hours, and then naturally cooling and removing from the form. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2023-66642 A [Patent Document 2] JP 2018-168003 A Summary of the Invention [Problem to be solved by the invention]

[0005] In conventional precast concrete manufacturing, demolding takes time, especially at low temperatures. In addition, when precast concrete is manufactured by mixing concrete in a mixer and pouring it into a formwork, there are cases where air is entrapped, resulting in a decrease in density. Furthermore, when rapid hardening concrete is manufactured continuously in order to shorten the demolding time, there is an issue that the concrete remaining in the concrete mixer hardens during transportation or filling.

[0006] Methods of applying spraying techniques are also being considered, but finishing the concrete surface is difficult, and there are cases in which voids remain behind the rebar, or where filling is incomplete in complex shapes.

[0007] In view of the above, an object of the present invention is to provide a method for manufacturing precast concrete that allows early demolding and has good density and finishability. [Means for solving the problem]

[0008] As a result of intensive research conducted by the inventors in light of the above problems, they discovered that the above problems can be solved by a method for producing precast concrete in which a concrete material containing a concrete composition and a hardening agent is injected into a formwork by a spraying means, and the concrete composition is mixed with the hardening agent just before being injected while being pumped to prepare a concrete material, and thus they have completed the present invention. That is, the present invention is as follows.

[0009] [1] A method for producing precast concrete in which a concrete material containing a concrete composition and a hardening agent is injected into a formwork by a spraying means, and the concrete composition is pumped while the concrete composition is mixed with the hardening agent just before being injected to produce a concrete material. [2] A method for producing precast concrete as described in [1] above, in which the formwork is vibrated with a vibrator after spraying. The liquid quick-setting admixture described in [1] above. [3] A method for producing precast concrete according to [1] or [2] above, wherein the area of ​​the opening of the formwork is 30% or less of the surface area of ​​the precast concrete. [4] A precast concrete quick-hardening additive for use in the manufacturing method for precast concrete according to any one of the above [1] to [3], which has a setting time of 5 to 60 minutes or less after being added to ordinary Portland cement. [5] The rapid hardening agent for precast concrete according to [4] above, which contains calcium aluminate and / or calcium aluminosilicate. [6] A rapid hardening agent for precast concrete according to [4] or [5] above, further comprising gypsum. [7] A rapid hardening material for precast concrete, comprising the rapid hardening agent for precast concrete described in any one of [4] to [6] above. [8] Precast concrete comprising a hardened body of the rapid hardening concrete material for precast concrete described in [7] above. Effect of the Invention

[0010] According to the present invention, a method for producing precast concrete can be provided which enables early demolding and provides good compaction and finishability. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention (the present embodiment) will be described in detail, but the present invention is not limited to the embodiment. In addition, "%" in this specification is based on mass unless otherwise specified.

[0012] [Precast concrete manufacturing method] The method for producing precast concrete according to this embodiment is a method for producing precast concrete in which a concrete material containing a concrete composition and a hardening agent is injected into a formwork by a spraying means, and the concrete composition is pumped while being mixed with the hardening agent just before being injected to produce a concrete material.

[0013] The spraying means used in the present invention produces precast concrete by pumping concrete material with compressed air and injecting it into a formwork from an injection nozzle, etc. By filling the formwork with concrete material by injection, the action of the injected air can reduce the amount of entrapped air in the concrete material, improving the density of the precast concrete produced.

[0014] The concrete material used in the present invention includes a concrete composition and a hardening agent. A concrete composition usually contains cement and aggregate, and hardens to form concrete by reacting with water. When the concrete material to be poured into a formwork includes a concrete composition and a hardening agent, the hardening agent acts to promote the setting of the concrete material, allowing the precast concrete to be produced to be quickly removed from the form, resulting in good finishability.

[0015] The concrete material is made by mixing the concrete composition pumped by a pump or the like with a quick-hardening agent just before being injected. Just before being injected means within 5 m of the concrete material being injected into the formwork. It is preferable that the quick-hardening agent is mixed with the concrete composition by merging with a quick-hardening agent transported from a different route into the concrete composition's transport route. By mixing the concrete composition and the quick-hardening agent just before being injected, it is possible to prevent the concrete material from hardening within the transport route.

[0016] The concrete composition is pumped under the following conditions: air flow rate is 0 to 10 m 3 / min, and 1 to 7 m 3 / min is more preferable, and 2 to 4 m 3 / min. The air pressure is preferably 0 to 0.7 MPa, more preferably 0.2 to 0.65 MPa, and even more preferably 0.3 to 0.5 MPa. By keeping the pumping conditions of the concrete composition within the above ranges, the concrete composition can be well mixed with the rapid hardening agent.

[0017] The hardening agent is preferably pumped by a pump or the like, similar to the concrete composition. The pumping conditions for the hardening agent are an air flow rate of 0.1 to 10 m 3 / min, and 1 to 7 m 3 / min is more preferable, and 2 to 4 m 3 / min. The air pressure is preferably 0.2 to 0.5 MPa, more preferably 0.25 to 0.45 MPa, and even more preferably 0.3 to 0.4 MPa. By keeping the pressure-feeding conditions of the hardening agent within the above range, the mixing property with the concrete composition is improved.

[0018] The cement used in the concrete composition is not particularly limited, but includes various Portland cements such as normal, high-early-strength, super high-early-strength, moderate-heat, and low-heat, filler cements obtained by mixing these Portland cements with blast furnace slag, fly ash, and limestone fine powder, and mixed cements such as environmentally friendly cements (eco-cements) manufactured using municipal waste incineration ash and sewage sludge incineration ash as raw materials, and these can also be used in fine powder form. The ratio of the mixture and cement in these mixed cements is not particularly limited, and those mixed at a ratio greater than that specified by JIS can also be used. One or more of the above cements may be used. If necessary, the cement may further contain commercially available cement admixtures (materials) such as water-reducing agents, air-entraining agents, polymer emulsions, thickeners, shrinkage-reducing agents, rust inhibitors, antifreeze agents, clay minerals, and silica fume, within the range that does not impair performance.

[0019] The aggregate used in the concrete composition is not particularly limited, but fine aggregate and coarse aggregate can be used. As fine aggregate, river sand, mountain sand, sea sand, lime sand, silica sand, etc. can be used. As coarse aggregate, river gravel, mountain gravel, lime gravel, etc. can be used. Crushed sand and crushed stone can also be used.

[0020] The hardening accelerator used in the present invention can contain, as a setting accelerator, organic acid calcium salts typified by calcium formate, aluminum salts typified by aluminum sulfate and alum, silicates typified by water glass, aluminates typified by calcium aluminate (calcium aluminate), aluminosilicates typified by calcium aluminosilicate (calcium aluminosilicate), as well as nitrates, sulfates, carbonates, thiocyanates, amines, maleic anhydride, etc. Among the setting accelerators described above, the hardening accelerator preferably contains an aluminate and / or an aluminosilicate, more preferably further contains gypsum, and more specifically, it is more preferable that the hardening accelerator is a hardening accelerator for precast concrete, which will be described later.

[0021] The concrete material preferably contains water. The concrete material and water may be mixed in advance by mixing the concrete composition or the hardening agent with the water, or the concrete composition or the hardening agent may be mixed with the water during transportation, or the concrete material may be mixed with the water during transportation.

[0022] The formwork used in the present invention is suitable for the shape of the precast concrete to be produced, and various shapes can be used, but the area of ​​the opening of the formwork is preferably 35% or less of the surface area of ​​the precast concrete, more preferably 30% or less, even more preferably 25% or less, and even more preferably 20% or less. There is no particular lower limit as long as the concrete can be sprayed, but a lower limit of 10% or more is preferable. By keeping the area of ​​the opening of the formwork within the above range, the labor required for finishing work can be reduced and the finished surface can be improved.

[0023] In the method for producing precast concrete according to the present embodiment, it is preferable to vibrate the formwork with a vibrator after the concrete is poured in. The vibrator that can be used is not particularly limited, but an example of a concrete vibrator is a concrete vibrator manufactured by Makita Corporation, product number: VR350DRGX. By vibrating the formwork with a vibrator after the concrete is poured in, the filling property of the concrete material in the formwork can be improved further until the poured concrete material sets, and the density of the precast concrete produced can be further improved.

[0024] [Accelerated hardening material for precast concrete] The precast concrete quick-setting admixture according to this embodiment is a precast concrete quick-setting admixture used in the manufacturing method of precast concrete of the present invention, and preferably has a setting time of 5 to 60 minutes after being added to ordinary Portland cement. If the setting time of the precast concrete quick-setting admixture after being added to ordinary Portland cement is 5 to 60 minutes, the setting time of the precast concrete to be manufactured is easily set to an appropriate time, and early demolding is easily possible. The setting time after being added to ordinary Portland cement can be adjusted by adjusting the composition and particle size of the precast concrete quick-setting admixture and the amount of setting regulator added. The setting time after adding the precast concrete accelerator to ordinary Portland cement can be measured in accordance with the method specified in JSCE-D 102-2018 "Quality standard for accelerators for sprayed concrete (mortar) (draft)" Annex 3 (regulations) "Method of measuring setting time by penetration resistance of mortar with accelerator added." Note that the above ordinary Portland cement is ordinary Portland cement specified in JIS R 5210:2009 "Portland cement."

[0025] The hardening agent for precast concrete preferably contains calcium aluminate and / or calcium aluminosilicate. When the hardening agent for precast concrete contains calcium aluminate and / or calcium aluminosilicate, it becomes possible to quickly demold the precast concrete to be produced.

[0026] Calcium aluminate is a compound of CaO and Al 2 O 3 It is a general term for compounds that have hydration activity and are mainly composed of CaO and / or Al. 2 O 3 Compounds in which a part of the above has been replaced with alkali metal oxides, alkaline earth metal oxides, titanium oxide, iron oxide, alkali metal halides, alkaline earth metal halides, alkali metal sulfates, and alkaline earth metal sulfates, etc., or CaO and Al 2 O 3 Calcium aluminate is a substance that contains a small amount of these as solid solutions in the main components, and can be either crystalline or amorphous. To distinguish it from calcium aluminosilicate, calcium aluminate is listed as containing SiO 2 does not include.

[0027] As a specific example of a crystalline substance, CaO is C, Al 2 O 3 A, R 2 O(Na 2 OK 2 O, Li 2 O) is R, then C 3 A and its alkali metal solid solution C 14 RA 5 , C.A., C. 12 A 7 Or C 11 A 7 ·CaF 2 , C 4 A Fe 2 O 3 , and C 3 A 3 ·CaSO 4Among them, amorphous calcium aluminate is preferred because of its good quick setting property. In the case of amorphous calcium aluminate, the vitrification rate is preferably 80% or more.

[0028] Calcium Aluminate CaO / Al 2 O 3 The molar ratio is not particularly limited, but from the viewpoints of short-term strength development and suppression of elution of hexavalent chromium, the molar ratio is preferably from 1.3 to 3.0, and more preferably from 1.6 to 2.7.

[0029] The method of obtaining calcium aluminate is to use CaO raw materials such as calcium carbonate and calcium hydroxide, and Al raw materials such as bauxite. 2 O 3 For example, the raw materials are mixed in a predetermined ratio, heated and melted in an electric furnace, and then quenched by contacting with compressed air or water. By adjusting the ratio of each raw material, the above CaO / Al 2 O 3 The molar ratio can be adjusted, and the vitrification rate can be adjusted by the temperature during heating and melting and the cooling method.

[0030] The Blaine specific surface area value (Blaine value) of calcium aluminate is not particularly limited, but is usually 3,000 to 9,000 cm 2 / g is preferable, and 4,000 to 8,000 cm 2 / g is more preferable. By the Blaine value being within the above range, the handleability can be improved. The Blaine specific surface area value is measured based on the specific surface area test described in JIS R 5201:2015 "Physical test method for cement".

[0031] The content of calcium aluminate in the hardening agent for precast concrete is preferably 2 mass% or more, more preferably 4 to 80 mass%, and even more preferably 40 to 75 mass%. When the content of calcium aluminate in the hardening agent for precast concrete is within the above range, it becomes easy to enable early demolding of the precast concrete.

[0032] Calcium aluminosilicate is a compound of CaO and Al 2 O 3 and SiO 2 It is a general term for compounds that have hydration activity and are mainly composed of CaO, Al, 2 O 3 and / or SiO 2 Compounds in which a part of the above has been replaced with alkali metal oxides, alkaline earth metal oxides, titanium oxide, iron oxide, alkali metal halides, alkaline earth metal halides, alkali metal sulfates, and alkaline earth metal sulfates, etc., or CaO and Al 2 O 3 and SiO 2 The calcium aluminate may be either crystalline or amorphous, but from the viewpoint of reactivity, it is preferably amorphous.

[0033] The method for obtaining calcium aluminosilicate is to use CaO raw materials such as calcium carbonate and calcium hydroxide, and Al raw materials such as bauxite. 2 O 3 Raw materials, etc. and SiO such as silica 2 The raw materials may be heat-treated in a rotary kiln, an electric furnace, etc. Specifically, the raw materials may be mixed in a predetermined ratio, heated and melted in an electric furnace, etc., and then quenched by contacting with compressed air or water, etc.

[0034] The Blaine value of calcium aluminosilicate is not particularly limited, but is usually 3,000 to 9,000 cm 2 / g is preferable, and 4,000 to 8,000 cm 2When the Blaine value is within the above range, the handleability can be improved.

[0035] The content of calcium aluminosilicate in the hardening agent for precast concrete is preferably 2 mass% or more, more preferably 4 to 80 mass%, and even more preferably 40 to 75 mass%. When the content of calcium aluminosilicate in the hardening agent for precast concrete is within the above range, it becomes easy to enable early demolding of the precast concrete.

[0036] The precast concrete quick hardening agent according to the present embodiment preferably further contains gypsum. As the gypsum, anhydrous gypsum, hemihydrate gypsum, and dihydrate gypsum can be used, and anhydrous gypsum is preferable. Anhydrous gypsum is calcium sulfate anhydrite, CaSO 4 Hemihydrate gypsum is a general term for compounds with the molecular formula CaSO 4 1 / 2H 2 It is a general term for compounds with the molecular formula CaSO 4 2H 2 It is a general term for compounds represented by the molecular formula O. When the precast concrete quick hardening agent further contains gypsum and is used in combination with calcium aluminate and / or calcium aluminosilicate, it tends to enable the precast concrete to be quickly demolded.

[0037] The Blaine value of gypsum is not particularly limited, but is usually 3,000 to 9,000 cm 2 / g is preferable, and 4,000 to 8,000 cm 2 When the Blaine value is within the above range, the handleability can be improved.

[0038] The content of gypsum in the hardening agent for precast concrete is preferably 50 to 300 parts by mass, and more preferably 100 to 200 parts by mass, per 100 parts by mass of the total amount of calcium aluminate and calcium aluminosilicate. When the content of gypsum in the hardening agent for precast concrete is within the above range, it becomes easier to enable early demolding of the precast concrete.

[0039] The hardening agent for precast concrete may contain, as other components, calcium salts of organic acids such as calcium formate, aluminum salts such as aluminum sulfate and alum, silicates such as water glass, other nitrates, sulfates, carbonates, thiocyanates, amines, maleic anhydride, etc.

[0040] [Rapid hardening concrete material for precast concrete] The rapid-hardening concrete material for precast concrete according to this embodiment includes the rapid-hardening additive for precast concrete of the present invention. By including the rapid-hardening concrete material for precast concrete in the rapid-hardening concrete material for precast concrete, the material hardens in an appropriate setting time, enabling the precast concrete to be quickly demolded.

[0041] The precast rapid hardening concrete material essentially contains cement and aggregate, and the types of cement and aggregate may include the above-mentioned cement types and aggregates.

[0042] The content of the quick-hardening admixture for precast concrete in the quick-hardening concrete material for precast concrete is preferably 3 to 30 mass % relative to the cement, more preferably 5 to 25 mass %, and even more preferably 10 to 20 mass %.

[0043] [Precast concrete] The precast concrete according to this embodiment is made of the hardened product of the rapid hardening concrete material for precast concrete of the present invention. The rapid hardening concrete material for precast concrete is filled into a formwork and hardened, so that precast concrete can be quickly removed from the formwork and has good density and finish. EXAMPLES

[0044] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples as long as it does not deviate from the gist of the present invention.

[0045] [Experimental Example 1] The concrete composition and quick hardening material A, which contain the materials shown below in the ratios shown in Table 1, were pumped and mixed together just before spraying to form the concrete material, which was then poured into formwork A (dimensions: 3 m × 1 m × 0.3 m, opening area: 0.9 m). 2 ) to produce precast concrete. The concrete composition pump air flow rate was 3m 3 / min, air pressure is 0.4MPa, air flow rate of the pump for hardening materials is 3m 3 The air pressure was set at 0.4MPa and the content of the quick hardening agent in the concrete material was adjusted to 20% by mass relative to the cement. After filling the first formwork, construction was suspended for one hour, and then the same formwork was replaced, and the formwork was sprayed a total of five times in the same manner, and each was evaluated using the evaluation method described below. The results are shown in Table 2.

[0046] <Materials used> Cement: Ordinary Portland cement, commercially available, Blaine value 3,200 cm 2 / g. Water: Tap water. Fine aggregate: Himekawa River sand from Itoigawa City, Niigata Prefecture, maximum dimension 5mm or less, density 2.62g / cm 3 . Coarse aggregate: Niigata Prefecture Itoigawa No. 6 crushed stone, maximum dimension 15 mm, density 2.67 g / cm 3 . Calcium aluminosilicate: CaO / Al2 O 3 Amorphous calcium aluminosilicate made by melting calcium carbonate, aluminum oxide and silica in an electric furnace and quenching them so that the molar ratio is 2.0. The vitrification rate is 97% (measured by powder X-ray diffraction) and the Blaine value is 5,500g / cm. 3 , amorphous, silica content 20%. Gypsum: Natural anhydrous gypsum, Blaine value 5,000g / cm 3 .

[0047] <Condensing time> For concrete materials, the start and end times of setting were measured each time the formwork was changed in accordance with the method specified in JSCE-D 102-2018 "Quality standard for quick-setting agents for sprayed concrete (mortar) (draft)" Appendix 3 (regulations) "Method of measuring setting time by penetration resistance of mortar containing added quick-setting agents."

[0048] <Compressive strength> The precast concrete produced was measured for compressive strength at 1 hour and 1 day according to the method specified in JSCE-G 561-2010 "Test method for initial strength of shotcrete by pull-out method", and for compressive strength at 7 days and 28 days according to the method specified in JIS A 1107:2022 "Method of core extraction from concrete and test method of compressive strength". 2 If the above was true, it was determined that demolding was possible.

[0049] <Finishability> After filling the formwork, the concrete surface of the opening was finished with a trowel. If the trowel finishing was completed in less than 5 minutes, it was marked as ○, if it was completed in 5 to 8 minutes, it was marked as △, and if it took more than 8 minutes or could not be finished, it was marked as ×.

[0050] <Unit volume mass> The mass of the test specimen for compressive strength was measured and divided by the volume calculated from the diameter and height of the specimen to calculate the unit mass. The higher the unit mass value, the higher the density of the precast concrete.

[0051] [Table 1]

[0052] [Table 2]

[0053] [Experimental Example 2] Using the concrete composition and hardening agent A shown in Table 1, the concrete material was mixed for 2 minutes using a two-shaft concrete mixer so that the content of hardening agent A in the concrete material was 20% by mass relative to the cement, and the concrete was poured into a formwork of the same type as in Experimental Example 1 to produce precast concrete. After the first formwork was filled, construction was suspended for 1 hour, and then the same formwork was replaced, and the concrete was poured into the formwork a total of 5 times in the same manner, and each was evaluated using the evaluation method shown below. The results are shown in Table 3. [Table 3]

[0054] [Experimental Example 3] Precast concrete was produced in the same manner as in Experimental Example 2, except that 0.4 parts by mass of a retarder (sodium citrate) was further added per 100 parts by mass of cement in the concrete composition when mixing the concrete composition with the hardening agent A. The results are shown in Table 4.

[0055] [Table 4]

[0056] [Experimental Example 4] In addition to formwork A, formwork B (dimensions: 0.6m x 1m x 2m, opening area: 2m 2 ), Formwork C (dimensions: 0.5m x 0.3m x 4m, opening area: 1.2m 2 ) and formwork D (dimensions: 0.3m x 2m x 2m, opening area: 4m 2) was used, and after the concrete material was injected into each formwork, each formwork was vibrated using a vibrator (Makita Corporation: product number VR350DRGX). Precast concrete was produced in the same manner as in Experimental Example 1. The results are shown in Table 5.

[0057] [Table 5]

[0058] [Experimental Example 5] The setting time of the hardening accelerator A with the composition shown in Table 1 and the hardening accelerators B to G with the composition shown in Table 6 was measured after adding them to ordinary Portland cement in accordance with the method specified in JSCE-D 102-2018 "Quality standard for accelerators for sprayed concrete (mortar) (draft)" Annex 3 (regulations) "Method of measuring setting time by penetration resistance of mortar with added accelerator". The results are shown in Table 6. Further, precast concrete was produced for the first time only in the same manner as in Experimental Example 1, except that the hardening agents B to G were used instead of the hardening agent A. The results are shown in Table 7.

[0059] <Materials used> Calcium aluminate: CaO / Al 2 O 3 Amorphous calcium aluminate was prepared by melting calcium carbonate and aluminum oxide in an electric furnace and quenching them to obtain a molar ratio of 2.3. The vitrification rate was 97% (measured by powder X-ray diffraction) and the Blaine value was 5,800 cm. 2 / g, amorphous. Calcium formate: Commercially available product, manufactured by Asahi Chemical Industry Co., Ltd., dry product. Aluminum sulfate: Commercially available product, manufactured by Taimei Chemical Industry Co., Ltd., dried product, octahydrate.

[0060] [Table 6]

[0061] [Table 7] [Industrial Applicability]

[0062] The method for producing precast concrete of the present invention enables early demolding and provides good density and finish, thereby improving the efficiency of precast construction methods.

Claims

1. A method for producing precast concrete, comprising spraying a concrete material containing a concrete composition and a hardening agent into a formwork by a spraying means, A method for producing precast concrete, comprising the steps of: pumping a concrete composition while mixing the concrete composition with the hardening agent just before injection to produce a concrete material.

2. 2. The method for producing precast concrete according to claim 1, wherein the formwork is vibrated with a vibrator after the concrete is poured.

3. 3. The method for producing precast concrete according to claim 1, wherein the area of ​​the opening of the formwork is 30% or less of the surface area of ​​the precast concrete.

4. 3. A precast concrete quick-hardening additive for use in the method for producing precast concrete according to claim 1 or 2, the quick-hardening additive having a setting time of 5 to 60 minutes after being added to ordinary Portland cement.

5. The hardening agent for precast concrete according to claim 4, which contains calcium aluminate and / or calcium aluminosilicate.

6. The quick hardening agent for precast concrete according to claim 4, further comprising gypsum.

7. A rapid hardening concrete material for precast concrete, comprising the rapid hardening agent for precast concrete according to claim 4.

8. A precast concrete comprising a hardened body of the rapid hardening concrete material for precast concrete according to claim 7.

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