Inner formwork for forming hollow spaces in precast concrete members

Abstract

To provide an inner mold form for manufacturing a precast concrete (PCa) member having a hollow part provided with a cotter, the inner mold form capable of supporting a part that forms the cotter and easily removable.SOLUTION: An inner mold form 1 includes: a core member 12; a plate part 13 arranged at the core member 12; and an elastic band 14 to be mounted in an extended and surrounding manner in a circumferential direction to the core member 12 and the plate part 13. The plate part 13 includes a first plate member 17 widened in a removal direction and a second plate member 18 in a shape supplementing the first plate member 17. At a removal from a mold, the core member 12 is pulled out utilizing a gap generated by pulling out the plate part 13, and the contracted elastic band 14 is taken out from a hollow part 5.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an inner formwork for forming a hollow portion in a precast concrete member (hereinafter referred to as "PCa member"). [Background technology]

[0002] Precast concrete members are sometimes used in the construction of architectural or civil engineering structures. For example, in buildings, columns and beams are sometimes made of precast concrete members to shorten construction time and improve quality. Precast concrete members are manufactured in factories, transported to the construction site, and then placed in their designated positions using lifting equipment such as cranes.

[0003] In buildings with large spans between columns, such as logistics facilities, the distance from the lifting machine to the installation position of the PCa members becomes long, making it necessary to either increase the size of the lifting machine or reduce the weight of the PCa members. By providing a hollow section in the PCa member, the PCa member can be made lighter, preventing the need for a larger lifting machine. PCa members with hollow sections are installed in the designated position at the construction site using a lifting machine, and concrete is then poured into the hollow section. To improve the bond between the PCa member and the concrete that will be poured later, cotters are provided on the inner surface that defines the hollow section. Various inner formworks have been proposed that allow the formation of cotters but still be removable.

[0004] For example, Patent Document 1 describes an inner formwork comprising a cylindrical cotter forming formwork, a core member placed inside the cotter forming formwork, and a mat-like air panel disposed between them. Concrete for constructing a PCa member is poured into this inner formwork with the air panel filled with air, and when demolding, the air is released from the air panel to create a gap between the cotter forming formwork and the core member, the air panel and core member are removed, and then the cotter forming formwork is removed.

[0005] Furthermore, Patent Document 2 describes a formwork device that includes a truncated quadrangular pyramid-shaped hollow formwork that tapers toward its tip, and a cotter formwork adhered to the surface of the hollow formwork. When the hollow formwork is pulled out after the concrete has hardened, the cotter formwork peels off from the hollow formwork, and is then removed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-191320 [Patent Document 2] Japanese Patent Publication No. 2022-46053 Summary of the Invention [Problem to be solved by the invention]

[0007] However, with the inner formwork described in Patent Document 1, there was a risk that the support of the cotter forming formwork by the air panels would become unstable. Also, with the inner formwork described in Patent Document 2, the cotter forming formwork tends to remain in the cotter (recess) even after the hollow formwork is removed, so in the case of a relatively long PCa member such as a pillar, it was difficult to remove the cotter formwork remaining in the center of the PCa member in the extension direction.

[0008] In view of the above background, the present invention aims to provide an inner formwork for forming a hollow portion in which a cotter is provided in a PCa member, which can stably support the portion in which the cotter is formed and can be easily removed. [Means for solving the problem]

[0009] In order to solve the above-mentioned problems, one aspect of the present invention is an inner formwork (1) for forming a hollow portion (5) extending along a predetermined axial direction in a precast concrete member (3), the inner formwork (1) comprising: a core member (12) extending along the axial direction and having an outer surface; a plate portion (13) arranged so that its main surface abuts against a placement surface (15) constituting at least a part of the outer surface; and a pair of plates (13) having elasticity in a circumferential direction around the axial direction, arranged in a state stretched in the circumferential direction so as to surround the core member and the plate portion in the circumferential direction, and spaced apart from each other in the axial direction. The core member and the plate portion are configured so that, when demolded, the core member can be displaced in a direction perpendicular to the axial direction by removing the plate portion from the precast concrete member. The plate portion has a width extending in a direction parallel to the placement surface perpendicular to the axial direction, and includes a first plate member (17) whose width becomes wider in the direction of removal from the precast concrete member in the axial direction, and a second plate member (18) which cooperates with the first plate member to cover the entire placement surface. The core member and the plate portion are configured so that, when demolded, the plate portion can be removed so that the core member can be displaced in a direction perpendicular to the axial direction.

[0010] According to this aspect, the core member and the plate portion stably support the elastic band. Furthermore, when the core member and the plate portion are removed, the elastic band tends to contract, causing the elastic band to at least partially detach or become easily detached from the cotter, making it easy to remove the elastic band from the hollow portion.

[0011] In the above aspect, the outer surface includes one or more of the arrangement surfaces (15) each having a substantially planar shape and adjacent to each other continuously in the circumferential direction, and the plate portion (13) includes the first plate member (17) and the second plate member (18) for each of the arrangement surfaces, (i) the outer surface has at least one non-arrangement surface (16) adjacent to the arrangement surface on which the plate portion is not arranged, and the non-arrangement surfaces are configured to approach each other as they extend from both ends of the arrangement surface in the circumferential direction, or (ii) the outer surface may consist only of the plurality of arrangement surfaces. Here, the term "substantially planar shape" for the arrangement surface means not only a completely planar shape but also a shape in which the end is curved during processing, such as when a corner where two planar shapes intersect is curved as a result of bending a steel plate by press forming or the like.

[0012] According to this aspect, the plate portion, the main surface of which contacts the placement surface, can be made to have a substantially planar shape, which makes it easy to form the plate portion.

[0013] In the above aspect, the core member (12) may have a cylindrical shape in which four rectangular flat plates (12a) having rectangular main surfaces are connected to each other at their ends so as to have a rectangular outer shape in a cross section perpendicular to the axial direction, the outer surface may include at least two of the arrangement surfaces (15), and the plate portion may have a flat plate shape.

[0014] According to this aspect, the corners of the core member are formed into a flat plate shape with right-angled intersections, so that no gaps are generated between the core member and the plate portion even if the plate portion is formed into a flat plate shape that is easy to mold.

[0015] In the above aspect, the core member (12) has an approximately rectangular outer shape with curved corners in a cross section perpendicular to the axial direction, the outer surface includes at least two of the arrangement surfaces (15), (i) the plate portion (13) has a flat plate shape, and the inner formwork further includes a filling member (20) that fills the gap between the corner of the core member and the plate portion, or (ii) the plate portion may be curved so as to abut against the corner of the core member.

[0016] According to this aspect, even if commercially available square steel pipes are used as the core members, the infiltration of concrete slag into the gaps between the corners of the core members and the plate portions is suppressed.

[0017] In the above aspect, the second plate members (18) may be provided in pairs so as to sandwich the corresponding first plate members (17) in the width direction.

[0018] According to this aspect, the frictional force between each second plate member and the core member and the concrete portion is smaller than when there is only one second plate member, making it easier to remove the second plate members from the formwork.

[0019] In the above aspect, the placement surface (15) of the core member (12) and the surface of the plate portion (13) that contacts the core member comprise polytetrafluoroethylene.

[0020] According to this embodiment, the frictional force is reduced by polytetrafluoroethylene, making it easier to remove the first plate member and the second plate member when removing them from the mold. [Effects of the Invention]

[0021] According to the above aspect, it is possible to provide an inner formwork for forming a hollow portion in a PCa member in which a cotter is provided, which can stably support the portion in which the cotter is formed and can be easily removed. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a partial cross-sectional side view showing the state in which the inner formwork according to the first embodiment has been erected. [Figure 2] FIG. 2 is a diagram showing a cross section (transverse cross section) perpendicular to the extension direction of the inner form according to the first embodiment (a cross section of the inner form taken along line II-II in FIG. 3). [Figure 3]1 is a partial cross-sectional side view showing a manufacturing process of a PCa member using an inner formwork according to the first embodiment (after concrete is poured, and the illustration of reinforcing bars is omitted). [Figure 4] FIG. 1 is a partial cross-sectional side view showing a manufacturing process of a PCa member using an inner formwork according to the first embodiment (the state shows the flat plate portion being pulled out, and the reinforcing bars are not shown). [Figure 5] FIG. 1 is a partial cross-sectional side view showing a manufacturing process of a PCa member using an inner formwork according to the first embodiment (the state shows the core member being pulled out, and the reinforcing bars are not shown). [Figure 6] FIG. 10 is a cross-sectional view showing a modified example of the inner formwork according to the first embodiment. [Figure 7] FIG. 10 is a cross-sectional view showing a modified example of the core member according to the first embodiment. [Figure 8] 8 is a diagram showing a cross section (VIII-VIII cross section in FIG. 9) perpendicular to the extending direction of the inner form according to the second embodiment. [Figure 9] Partially sectional side view showing the manufacturing process of the PCa member using the inner formwork according to the second embodiment (after concrete is poured) DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Fig. 1 is a partial cross-sectional side view showing a state in which a formwork 2 including an inner formwork 1 according to a first embodiment has been erected. Fig. 2 is a cross-sectional view of the inner formwork 1, and Figs. 3 to 5 are partial cross-sectional side views showing a manufacturing process of a PCa member 3 using the inner formwork 1.

[0024] As shown in FIG. 5, the PCa member 3 to be manufactured has a hollow portion 5 in which a cotter 4 is provided. The PCa member 3 is, for example, a member used to construct a pillar of a building. The PCa member 3 extends along a predetermined axial direction, but the axial direction of the PCa member 3 during manufacture may differ from the axial direction (vertical direction) when the PCa member 3 is installed in a building; in the illustrated example, it is horizontal. The hollow portion 5 extends along the axial direction and is open at both ends in the axial direction. The cotter 4 is a recess provided on the inner surface of the hollow portion 5, and multiple cotters 4 are provided spaced apart from each other in the axial direction, and each cotter 4 extends around the entire circumference in the circumferential direction relative to the axial direction. As shown in FIGS. 1 and 5, the PCa member 3 includes reinforcing bars 6 and a concrete portion 7.

[0025] As shown in FIG. 1, the formwork 2 includes an outer formwork 8 for defining the outer shape of the concrete section 7 (see FIG. 5) and an inner formwork 1 for defining the hollow section 5 (see FIG. 5). Note that in FIG. 1, only the outer formwork 8 is shown in vertical cross section, and the other members are shown in side view. The outer formwork 8 is, for example, a steel formwork in the shape of a rectangular parallelepiped box with the entire top surface open. The reinforcing bars 6 include main reinforcements 9 extending in the axial direction and hoops 10 extending circumferentially so as to intersect with the main reinforcements 9. One axial end of the main reinforcements 9 is joined to a mechanical joint 11 located within the outer formwork 8, and the other end protrudes from the outer formwork 8.

[0026] As shown in Figures 2 and 3, the inner formwork 1 comprises a core member 12 extending along the axial direction, a plate portion 13 arranged so that its main surface abuts at least a portion of the outer surface of the core member 12, and an elastic band 14 that is elastic in the circumferential direction and is attached so as to surround the core member 12 and the plate portion 13 in the circumferential direction.

[0027] The core member 12 is preferably made of steel, and is, for example, a cylindrical member formed by connecting four flat steel plates 12a, each having a rectangular main surface, at their ends by welding or the like so as to form a rectangular outer shape in a cross section perpendicular to the axial direction. The core member 12 has side surfaces including two arrangement surfaces 15 on which the plate portions 13 are arranged and two non-arrangement surfaces 16 on which the plate portions 13 are not arranged. The two arrangement surfaces 15 are adjacent to each other. Two core members 12 are preferably provided adjacent to each other in the axial direction (see FIG. 1), and the plate portions 13 and elastic bands 14 are arranged relative to each of the core members 12.

[0028] The outer shape of the cross section of the core member 12 perpendicular to the axial direction may be a polygon other than a rectangle or may include partial curves, but it is preferable that the placement surface 15 and the non-placement surface 16 are parallel to the axial direction, and it is preferable that each placement surface 15 is planar.

[0029] The plate portion 13 includes one first plate member 17 for each arrangement surface 15 on which it is arranged, and a pair of second plate members 18 arranged to sandwich the first plate member 17. The first plate member 17 and the second plate member 18 are flat plate-like members extending in the axial direction, having a width extending in a direction parallel to the arrangement surface 15 on which it is arranged, perpendicular to the axial direction, and a thickness extending in a direction perpendicular to the arrangement surface 15 on which it is arranged. The first plate member 17 and the second plate member 18 are preferably made of steel plate. Two plate portions 13 arranged on adjacent arrangement surfaces 15 are arranged so that their ends abut each other.

[0030] The first plate member 17 and the second plate member 18 cooperate to cover the entire corresponding placement surface 15. The width of the first plate member 17 increases in the axial direction in the direction in which the first plate member 17 is removed during demolding. The first plate member 17 and the pair of second plate members 18 have equal lengths in the extension direction. The pair of second plate members 18 complement the first plate member 17 and cover the placement surface 15. The outer shape of the first plate member 17 as viewed in the thickness direction is preferably a trapezoid or triangle with a base at the end in the axial direction, and the outer shape of each second plate member 18 as viewed in the thickness direction is preferably a trapezoid or triangle with a hypotenuse abutting the first plate member 17 and a side parallel to the axial direction.

[0031] The placement surface 15 of the core member 12 and both thickness-wise surfaces of the first plate member 17 and the second plate member 18 preferably contain polytetrafluoroethylene (PTFE) to reduce the coefficient of friction. Furthermore, the non-placement surface 16 of the core member 12 and / or other surfaces of the first plate member 17 and the second plate member 18 may also contain polytetrafluoroethylene.

[0032] The elastic bands 14 are arranged in a circumferentially stretched state and spaced apart from each other in the axial direction so as to circumferentially surround the core member 12 and the plate portion 13. The elastic force of the elastic bands 14 in the circumferential direction detachably fixes the plate portion 13 to the core member 12, and also detachably fixes the elastic bands 14 themselves to the core member 12 and the plate portion 13. The elastic bands 14 include, for example, rubber.

[0033] A method for manufacturing the PCa member 3 using the inner formwork 1 will be described.

[0034] As shown in Figure 1, workers assemble the reinforcing bars 6, place the inner formwork 1 in a state where the core member 12, plate portion 13, and elastic band 14 are combined with one another inside the cylindrical assembly of the reinforcing bars 6, and then assemble the outer formwork 8. The inner formwork 1 may be inserted into the assembly of the reinforcing bars 6 from the axial direction, or may be inserted from above before attaching the upper parts of the ties 10 to the main reinforcement 9, and then the upper parts of the ties 10 may be attached to the main reinforcement 9.

[0035] Next, as shown in FIG. 3, workers pour concrete into the outer form 8 so that the inner form 1 is buried.

[0036] Next, when the concrete portion 7 has hardened to the extent that it can be removed from the form, the worker removes the outer formwork 8, and as shown in FIG. 4, pulls out the first plate member 17 and then the second plate member 18.

[0037] Next, as shown in FIG. 5, the worker pulls out the core member 12 and then removes the elastic band 14 from the hollow portion 5.

[0038] The function and effect of the inner form 1 will be described with reference to FIGS.

[0039] Because the placement surface 15 and non-placement surface 16 of the core member 12 are parallel to the axial direction, commercially available steel pipes can be used as the core member 12, thereby reducing material costs. Furthermore, because the first plate member 17 and the second plate member 18 have a flat plate shape, they can be easily produced by processing commercially available steel plates, etc.

[0040] Since the elastic band 14 is attached to the steel core member 12 and plate portion 13, the support of the elastic band 14 by the core member 12 and plate portion 13 is stable.

[0041] Because the plate portion 13 is divided into the first plate member 17 and the second plate member 18, the frictional force between the core member 12 and the concrete portion 7 is smaller than when the plate portion 13 is made up of a single plate member, making it easier to pull out when removing the form. Similarly, because two second plate members 18 are provided for one first plate member 17, the frictional force between each second plate member 18 and the core member 12 and the concrete portion 7 is smaller than when there is only one second plate member 18, making it easier to pull out when removing the form.

[0042] Furthermore, since the first plate member 17 is tapered toward the direction of removal in the axial direction, the second plate member 18 does not hinder the removal of the first plate member 17.

[0043] If the surfaces of the first plate member 17 and the second plate member 18 and the placement surface 15 of the core member 12 contain polytetrafluoroethylene, the amount of friction between the core member 12 and the concrete portion 7 is reduced, making it even easier to pull out the first plate member 17 and the second plate member 18.

[0044] The core member 12 can be easily pulled out because it is pulled out by utilizing the gap between the plate portion 13 and the concrete portion 7 that is created when the plate portion 13 is pulled out.

[0045] When the core member 12 is pulled out, the elastic band 14 tends to shrink due to its elasticity, and therefore at least partially comes off or is easily released from the recessed portion of the cotter 4. This makes it easy to remove the elastic band 14 from the hollow portion 5.

[0046] The plate portion 13 is intended to provide a gap between the core member 12 and the concrete portion 7 that allows the core member 12 to be displaced. Therefore, for a core member 12 that has a rectangular outer shape in a cross section perpendicular to the axial direction, three or four of the four side surfaces may be used as the placement surfaces 15. If there is only one placement surface 15 among the four side surfaces, or if only two parallel surfaces are used as the placement surfaces 15, even if a gap occurs, the core member 12 will be difficult to displace due to the adhesion and friction between the two parallel non-placement surfaces 16 that are perpendicular to the placement surface 15 and the concrete portion 7. Therefore, for a core member 12 that has a rectangular outer shape in a cross section perpendicular to the axial direction, it is necessary that two or more adjacent side surfaces be used as the placement surfaces 15.

[0047] Furthermore, when the outer shape of the core member 12 in a cross section perpendicular to the axial direction is a polygon other than a rectangle, or when the non-positioning surfaces 16 are curved, (i) the outer surface of the core member 12 includes at least one non-positioning surface 16, and the non-positioning surfaces 16 are configured to approach each other as they extend from both circumferential ends of the positioning surface 15, or (ii) the outer surface of the core member 12 does not include the non-positioning surface 16 and consists only of the positioning surface 15. Here, the term "approaching each other" does not include the case where they are parallel to each other. By satisfying this condition, the core member 12 can displace toward the gap between the core member 12 and the concrete portion 7 created by the removal of the plate portion 13, making it easier to remove the core member 12. For example, when the outer shape of the core member 12 in a cross section perpendicular to the axial direction is a regular n-gon (n is an integer of 3 or greater), at least n / 2 consecutively adjacent positioning surfaces 15 (rounded down to the nearest whole number) are provided.

[0048] The above condition (i) will be explained using a specific example. FIG. 6(A) shows a modified example in which the core member 12 has an equilateral triangular outer shape in a cross section perpendicular to the axial direction, after the plate portion 13 (see FIG. 4) has been removed. Similarly, FIG. 6(B) shows a modified example in which the core member 12 has a regular pentagonal outer shape in a cross section perpendicular to the axial direction. By removing the plate portion 13, a gap 19 is formed between the core member 12 and the concrete portion 7. As shown in the figure, the non-positioning surfaces 16 are positioned so that they approach each other as they extend from both ends of the positioning surface 15. The direction of the core member 12 toward the gap 19 (the direction of the arrow in the figure) is the direction in which the non-positioning surfaces 16 peel off from the concrete portion 7. Therefore, the core member 12 peels off from the concrete portion 7, making it easier to remove the core member 12. If the non-positioning surfaces 16 had portions that moved away from each other as they extended from both ends of the positioning surface 15, those portions would get caught on the concrete portions 7, and the core member 12 would not be able to displace toward the gap 19. Furthermore, if the non-positioning surfaces 16 had portions that became parallel to each other as they extended from both ends of the positioning surface 15 (for example, if the core member 12 had a rectangular outer shape in a cross section perpendicular to the axial direction, and had one positioning surface 15 and three non-positioning surfaces 16), the adhesive and frictional forces between those portions and the concrete portions 7 would make it difficult for the core member 12 to displace toward the gap 19. Therefore, when non-positioning surfaces 16 are provided, the above condition (i) is required in order to easily pull out the core member 12.

[0049] FIG. 7 shows a modified example of the core member 12. The core member 12 has a substantially rectangular shape with curved corners in a cross section perpendicular to the axial direction. For example, the core member 12 is a square steel pipe manufactured by press-forming a single steel plate and connecting both circumferential ends by welding or the like. When the steel plate is bent by press-forming, the corners are curved. In this case, as shown in FIG. 7(A), compensation members 20 are placed at the locations where the placement surfaces 15 and non-placement surfaces 16 intersect with each other and at the locations where one of the placement surfaces 15 intersects with each other to fill the gaps between the curved corners of the core member 12 and the flat plate portion 13. For example, a commercially available R-faced lumber (a member that extends linearly in a predetermined direction and has a cross section perpendicular to the extension direction in which the hypotenuse of a right triangle is curved inward) may be used as the compensation member 20. In this way, the gap between the curved corner of the core member 12 and the second plate member 18 of the flat plate portion 13 is filled with the filling member 20, which prevents concrete slag from getting into the gap, hardening, and impeding demolding. Also, as shown in Figure 7(B), the second plate portion of the plate portion 13 may be curved so as to abut against the curved corner of the core member 12, thereby preventing the creation of a gap between the curved corner of the core member 12 and the plate portion 13 into which slag can get.

[0050] Furthermore, when the core member 12 is formed from a cylindrical steel pipe, and the outer shape of the core member 12 is circular in a cross section perpendicular to the axial direction, the placement surface 15 is arranged to form an arc with a central angle of 180° or more in a cross section perpendicular to the axial direction, and the plate portion 13 is curved so that the main surface as a whole abuts against the core member 12.

[0051] Next, an inner form 21 according to a second embodiment of the present invention will be described with reference to Figures 8 and 9. In the description, the same components as those in the first embodiment will be denoted by the same reference numerals and description thereof will be omitted.

[0052] The inner formwork 21 comprises a cylindrical cotter forming member 22 extending along the axial direction and having an outer surface with unevenness, a core member 23 inserted into the cotter forming member 22 and extending along the axial direction, and a plurality of air tubes 24 inserted between the cotter forming member 22 and the core member 23.

[0053] The cotter-forming member 22 includes four sheathing plates 25 arranged to form a rectangular cylindrical shape and multiple protrusions 26 fixed to the outer surface of each sheathing plate 25. The sheathing plates 25 are flat and formed, for example, from steel plates. Two adjacent sheathing plates 25 are releasably connected. The multiple protrusions 26 are spaced apart from one another in the axial direction. Each protrusion 26 is arranged along the surface of the sheathing plate 25 in a direction perpendicular to the axial direction. Each protrusion 26 may extend across the entire width of the sheathing plate 25 or partially across the entire width. In the illustrated example, the protrusions 26 on different sheathing plates 25 are aligned circumferentially, but they do not need to be aligned. When the protrusions 26 extend to the middle of the width of each sheathing plate 25 as shown in the figure, the cotter 4 of the PCa member 3 is formed in a portion of the inner circumferential surface of the hollow portion 5 other than the corners. The ridges 26 are formed, for example, from a steel plate having a predetermined thickness. By fixing the ridges 26 to the surface of the flat sheathing 25, unevenness is formed on the outer surface of the cotter forming member 22. It is preferable that two cotter forming members 22 are provided adjacent to each other in the axial direction, and a core member 23 and an air tube 24 are provided for each cotter forming member 22.

[0054] The core member 23 is a cylindrical or columnar member having a predetermined rigidity and is preferably made of steel, for example, a square steel pipe. The outer surface of the core member 23 includes two adjacent opposing surfaces 27 that face the inner surface of the cotter-forming member 22 with a gap between them, and two adjacent placement surfaces 28 that abut the inner surface of the cotter-forming member 22.

[0055] The air tube 24 is a member that extends in the axial direction, expands radially when air is injected, and contracts radially when air is discharged. The outer diameter of the air tube 24 when air is injected is equal to or greater than the gap between the opposing surface 27 of the core member 23 and the opposing inner surface of the cotter-forming member 22 when not restrained by other members. A plurality of air tubes 24 are arranged between each opposing surface 27 of the core member 23 and the opposing inner surface of the cotter-forming member 22 so that they are in close contact with each other when filled with air.

[0056] The opposing surface 27 of the core member 23 in the second embodiment may be curved, unlike the arrangement surface 15 (see FIG. 2) of the core member 12 in the first embodiment. Similarly to the plate portion 13 (see FIG. 2) in the first embodiment, the air tube 24 is a member that, when removed, creates a gap that allows the core member 23 to be displaced. Therefore, in a cross section perpendicular to the axial direction, if the outer shape of the core member 23 is a polygon other than a rectangle or is curved, (i) (a) the arrangement surfaces 28 are configured to approach each other as they move away from the opposing surface 27, or (b) the arrangement surface 28 is configured by only one flat surface, or (ii) the outer surface of the core member 23 consists of only the opposing surface 27.

[0057] A method for manufacturing the PCa member 3 using the inner formwork 21 will be described.

[0058] First, a worker assembles the reinforcing bars 6 (see FIG. 1) and the inner formwork 21. The inner formwork 21 is assembled by inserting a core member 23 and a deflated air tube 24 into a cotter forming member 22, and then injecting air into the air tube 24. Thereafter, the worker places the inner formwork 21 inside the cylindrical assembly of the reinforcing bars 6, and assembles the outer formwork 8, as in the first embodiment.

[0059] Next, workers pour concrete into the outer form 8 so that the inner form 21 is buried.

[0060] When the concrete section 7 has hardened to the extent that it can be removed from the form, the worker removes the outer formwork 8, expels the air from the air tube 24, and pulls out the air tube 24 and core member 12 from the cotter forming member 22. Next, the worker releases the connections between the sheathing boards 25 in the cotter forming member 22 and removes each sheathing board 25 from the hollow section 5 of the PCa member 3. Because the protrusions 26 are fixed to the sheathing boards 25, they are removed from the hollow section 5 together with the sheathing boards 25.

[0061] The function and effect of the inner formwork 21 will be described.

[0062] A commercially available steel pipe can be used as the core member 23. The cotter forming member 22 can be easily produced by processing a commercially available steel plate or the like.

[0063] The core member 23 can be easily pulled out from the cotter forming member 22 because a gap is created between the core member 23 and the cotter forming member 22 when the air tube 24 is deaerated and shrunk or when the shrunk air tube 24 is removed.

[0064] Since multiple air tubes 24 are arranged between each of the sheathing boards 25 and the core member 23, the deformation of the air tubes 24 is smaller than when one large plate-shaped air tube 24 is arranged, and the support of the sheathing boards 25 by the core member 23 via the air tubes 24 is more stable.

[0065] Although the description of specific embodiments has been completed above, the present invention is not limited to the above-described embodiments and modifications, and can be implemented in a wide variety of modifications. The PCa member 3 may be a part of an architectural or civil engineering structure other than a column, such as a beam. In the second embodiment, the air tube 24 is not fixed to the cotter-forming member 22 and the core member 23 but is merely in contact with them when the inner formwork 21 is assembled, but may be fixed to only one of the cotter-forming member 22 and the core member 23. The core members 12, 23, the first plate member 17, and the second plate member 18 may be removed by pushing them out from the opposite side instead of pulling them out. [Explanation of symbols]

[0066] 1: Inner formwork 3: Precast concrete members (PCa members) 4: Cotter 5:Hollow part 8: Outer formwork 12: Core member 13: Plate part 14: Elastic band 15: Placement surface 16: Non-placement surface 17:First plate member 18:Second plate member 20: Compensation material

Claims

1. An inner form for forming a hollow portion extending along a predetermined axial direction in a precast concrete member, a core member extending along the axial direction and having an outer surface; a plate portion arranged so that a main surface thereof abuts on an arrangement surface constituting at least a part of the outer surface; a plurality of elastic bands that have elasticity in a circumferential direction around the axial direction, are arranged in a stretched state in the circumferential direction so as to surround the core member and the plate portion in the circumferential direction, and are arranged at intervals in the axial direction; The plate portion is a first plate member having a width extending in a direction perpendicular to the axial direction and parallel to the placement surface, the width of which increases in the axial direction in a direction in which the first plate member is removed from the precast concrete member; a second plate member that cooperates with the first plate member to cover the entire arrangement surface, The core member and the plate portion are configured so that, when demolded, the core member can be displaced in a direction perpendicular to the axial direction by removing the plate portion.

2. The outer surface includes one or more of the arrangement surfaces each having a substantially planar shape and adjacent to each other in the circumferential direction, the plate portion includes the first plate member and the second plate member for each of the placement surfaces, An inner formwork as described in claim 1, wherein (i) the outer surface has at least one non-positioning surface adjacent to the positioning surface on which the plate portion is not positioned, and the non-positioning surfaces are configured to approach each other as they extend from both circumferential ends of the positioning surface, or (ii) the outer surface consists only of a plurality of the positioning surfaces.

3. the core member includes a cylindrical shape in which four rectangular flat plates, each having a rectangular main surface, are connected to each other at their ends so as to have a rectangular outer shape in a cross section perpendicular to the axial direction, the outer surface includes at least two of the placement surfaces; The inner formwork according to claim 2 , wherein the plate portion has a flat plate shape.

4. the core member has a substantially rectangular outer shape with curved corners in a cross section perpendicular to the axial direction, the outer surface includes at least two of the placement surfaces; An inner formwork as described in claim 2, wherein (i) the plate portion has a flat plate shape and the inner formwork further comprises a filling member that fills the gap between the corner portion of the core member and the plate portion, or (ii) the plate portion is curved so as to abut against the corner portion of the core member.

5. The inner formwork according to any one of claims 2 to 4, wherein the second plate members are provided in pairs so as to sandwich the corresponding first plate members in the width direction.

6. The inner form according to claim 1 , wherein the placement surface of the core member and the surface of the plate portion that abuts against the core member comprise polytetrafluoroethylene.

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