Formwork for forming through holes and method for forming through holes

Abstract

A through-hole is formed within the concrete member that has concrete adhesion performance equivalent to or better than that of a sheath pipe. [Solution] The formwork 10 for forming through holes is installed side-by-side in the formwork space 1 to form through holes with ring-shaped grooves formed in the hole walls, and is pulled out from inside the hardened concrete C after the concrete has hardened, by inserting the beam reinforcement bars to be inserted into the concrete member up to the sleeve S of the grout-filled joint 61. The formwork 10 comprises a cylindrical formwork 11 that forms a substantially circular hole for the through hole, and an O-ring 20 that is detachably attached to the cylindrical formwork 11 so as to form a ring-shaped convex portion 12 on the outer surface of the cylindrical formwork 11. When the cylindrical formwork 11 is pulled out from inside the hardened concrete C, the O-ring 20 peels off from the hole wall inside the through hole, thereby forming a ring-shaped groove in the hole wall.

Description

Technical Field

[0001] The present invention relates to a formwork for forming through-holes and a method for forming through-holes, and more particularly to a formwork for forming through-holes with protrusions for reinforcement such as precast reinforced concrete members and a method for forming such through-holes.

Background Art

[0002] In a precast reinforced concrete (hereinafter simply referred to as precast) construction structure, in order to rationalize the construction of the precast structure, precast members having a composite structure in which precast column-beam joint members and precast beam members are integrated are manufactured. Further, in order to rationally join the composite member and other precast beam members, a construction method for a column-beam joint structure in which a joint member (mechanical joint) for main beam bars is embedded in a column-beam joint member has been proposed (Patent Document 1).

[0003] In the construction method for a column-beam joint structure using the mechanical joint disclosed in Patent Document 1, sheath pipes that function as through-holes are attached between both ends of the mechanical joint disposed in the column-beam joint and the surface of the concrete member. Since each sheath pipe is embedded in the placed concrete, it is necessary to install a sheath pipe for each mechanical joint disposed in the column-beam joint, resulting in a problem of high member cost.

[0004] In order to solve this problem, a formwork for hole formation disclosed in Patent Document 2 has been proposed. This formwork for hole formation is composed of a rubber tube covering around a core pipe over the entire length of the through-hole of the concrete member. This rubber tube has a special shape with irregularities formed on its outer peripheral surface and inner peripheral surface. By placing this formwork for hole formation at a predetermined position within the formwork space for forming the concrete member, pouring concrete, and removing the core pipe after the concrete has hardened, the rubber tube is deformed and removed to form a through-hole in the concrete member.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-167689 [Patent Document 2] Japanese Patent Publication No. 2012-35523 [Overview of the project] [Problems that the invention aims to solve]

[0006] The hole-forming formwork disclosed in Patent Document 2 is designed to create through-holes along the entire length of the concrete member, and therefore cannot accommodate concrete members where mechanical joints are positioned at intermediate locations within the concrete member. Furthermore, the rubber tube has a special shape with irregularities formed on its outer and inner surfaces along its entire length, resulting in high manufacturing costs.

[0007] Incidentally, the applicant has proposed a grout-filling structure for precast members of column-beam joints in reinforced concrete structures, which are provided with main reinforcement holes for the column main reinforcement to pass through the column-beam joint and main reinforcement holes for the beam main reinforcement to pass through and join the beam main reinforcement with a grout-filled joint inside the column-beam joint, in order to reduce labor and manpower in construction and shorten the construction period (Japanese Patent Application No. 2024-73091).

[0008] Figure 6 shows the assembled state of the precast member 51 of the column-beam joint 50 equipped with the grout-filled structure 60 described above. As shown in the figure, the column-beam joint 50 is placed on a column member 53 on which multiple column main reinforcement bars 52 are erected from the top surface, with the column main reinforcement bars 52 passing through it. Furthermore, the precast beam member 55 is pulled laterally and temporarily joined to the side surface 50a of the column-beam joint 50. In addition, as shown in Figure 7, each end of the beam main reinforcement bars 56 of the precast beam member 55 is inserted through beam main reinforcement bar penetration holes 54 to approximately the center of the sleeve S of the grout-filled joint 61, and grout injection pipes 62 are routed from each beam main reinforcement bar penetration hole 54 to the sleeve S of the grout-filled joint 61, with the ends of the pipes connected to the beam main reinforcement bar penetration holes 54 and the sleeve S.

[0009] In this grout-filled structure 60, as shown in Figure 7, numerous sleeves S of grout-filled joints 61 are arranged within the column-beam joint 50. Therefore, instead of the conventional structure in which prefabricated sheath pipes 65 are connected to the main reinforcement beam penetration holes 54 at both ends of each sleeve S, there was a need for the development of a reusable formwork and a method for forming penetration holes using that formwork, which can be used at a low cost and have concrete adhesion performance equivalent to or better than that of a sheath pipe.

[0010] Therefore, the object of the present invention is to resolve the problems of the conventional technology described above and to provide a formwork for forming through holes and a method for forming through holes that can be applied to the developing grout-filled structure. [Means for solving the problem]

[0011] To solve the above-mentioned problems, the present invention provides a formwork for forming through holes, in which reinforcing wires of the formwork concrete member for forming through holes are inserted and positioned to a predetermined position in the concrete, and a formwork for forming through holes in which a ring-shaped groove is formed in the hole wall is installed side by side in the formwork space and is withdrawn from the hardened concrete after the concrete has hardened, comprising a cylindrical formwork that forms a substantially circular hole of the through hole, and an elastic ring member that is detachably attached to the cylindrical formwork so as to form a ring-shaped convex portion on the outer surface of the cylindrical formwork, wherein when the cylindrical formwork is withdrawn from the hardened concrete, the elastic ring member peels off from the hole wall in the through hole, and the ring-shaped groove is formed in the hole wall.

[0012] Preferably, the elastic ring member expands elastically to be attached to the outer surface of the cylindrical formwork, and then shrinks elastically to detach from the hole wall.

[0013] It is preferable that the initial diameter of the elastic ring member is smaller than the outer diameter of the cylindrical mold.

[0014] It is preferable that an O-ring is used as the elastic ring member.

[0015] Preferably, the ring-shaped convex portion is the elastic ring member attached to the outer peripheral surface of the cylindrical formwork.

[0016] Preferably, the cylindrical formwork is installed in the formwork space such that the through hole is formed between the end of the sleeve of the grout filling joint embedded in the concrete member and the concrete surface.

[0017] Preferably, the cylindrical formwork has a frustum shape with a gradually increasing diameter in the range from the end side of the sleeve to the concrete surface.

[0018] As an invention of a method for forming a through hole, an elastic ring member is attached to the outer peripheral surface of a cylindrical formwork to provide a ring-shaped convex portion on the outer peripheral surface. The cylindrical formwork is installed side by side in a formwork space, concrete is placed in the formwork space, and after the concrete is cured, the cylindrical formwork is pulled out from the cured concrete to form a through hole. At the same time, the ring member adhering to the hole wall and remaining in the through hole is peeled off from the hole wall and taken out, and a through hole with a ring-shaped groove formed in the hole wall is formed.

[0019] Preferably, the elastic ring member is elastically expanded in diameter and attached to the outer peripheral surface of the cylindrical formwork, and elastically contracted in diameter to be peeled off from the hole wall.

Advantages of the Invention

[0020] According to the present invention, when forming a through hole in a concrete member, a through hole having a concrete adhesion performance equal to or better than that of a sheath pipe can be formed, and the formwork can be reused.

Brief Description of the Drawings

[0021] [Figure 1] A partially enlarged front view showing the assembled state (a) of the sleeve of the grout filling joint and the formwork for forming a through hole of the present invention in a formwork for constructing a precast member as a column-beam joint shown in FIG. 6 as an example, and the state (b) after concrete placement. [Figure 2] Partial enlarged sectional view showing the through-hole formed in the precast member of Fig. 1, the inside of the sleeve of the grout-filled joint, and the state (a) where the end of the main beam reinforcement is inserted into the through-hole and the sleeve from the member end, and the state (b) where the through-hole and the sleeve are filled with grout material through the grout injection pipe after the main beam reinforcement is arranged. [Figure 3] Perspective view showing a configuration example of the formwork for forming through-holes of the present invention. [Figure 4] Working explanatory diagram showing the working procedure for attaching the ring member to the outer peripheral surface of the cylindrical formwork of the formwork for forming through-holes of the present invention. [Figure 5] Working explanatory diagram showing the working procedure of the method for forming through-holes in a precast member constructed using the formwork for forming through-holes of the present invention. [Figure 6] State explanatory diagram showing the assembled state of a precast member as a column-beam joint portion having a grout-filled structure with a conventional configuration. [Figure 7] State explanatory diagram showing the temporary joining state of the main beam reinforcement in which the main beam reinforcement through-hole is inserted up to approximately the central position in the sleeve of the grout-filled joint in the precast member shown in Fig. 6.

Embodiments for Carrying Out the Invention

[0022] Hereinafter, embodiments of the formwork for forming through-holes and the method for forming through-holes of the present invention will be described with reference to the accompanying drawings.

[0023] Fig. 1(a) shows a grout-filled joint 61 installed at a predetermined position in a formwork space 1 surrounded by a formwork (partition plate 2) for constructing the precast member 51 of the column-beam joint portion 50 illustrated in Fig. 6, and formworks 10 for forming through-holes arranged such that the end faces abut each other at both ends of the grout-filled joint 61. Fig. 1(b) shows the state immediately after concrete is placed in the formwork space 1 shown in Fig. 1(a). Demolding is performed after a predetermined curing period.

[0024] The sleeve S of the grout-filled joint 61 (hereinafter simply referred to as sleeve S) is installed at the position where the main beam reinforcement is arranged in the formwork space 1 (beam main reinforcement joint position) by support means (not shown). Furthermore, at both ends of sleeve S, the formwork 10 for forming through holes according to the present invention (hereinafter simply referred to as through-hole formwork 10) is installed by support means (not shown) in the same way as sleeve S, so as to close the open end of sleeve S with the end face 10a of the formwork. The end 10b of the through-hole formwork 10 opposite to the sleeve S side is held so as to pass through the opening 2a provided in the formwork plate 2 which serves as the formwork for the precast member 51 (Figure 6). As will be described later in Figures 3 and 4, the through-hole formwork 10 is a cylindrical embedded member for forming a beam main reinforcement through-hole 5 (Figure 2(a), hereinafter simply referred to as through-hole 5) that communicates with the end of sleeve S within the precast member 51. As described later, the through-hole formwork 10 is removed during demolding after concrete C is poured into the formwork space 1, and a through-hole 5 is formed in the concrete C. In this embodiment, a grout injection pipe 62 is further connected to the sleeve S and a part of the through-hole formwork 10. This grout injection pipe 62 is an injection pipe for filling the sleeve S and the through-hole 5 with grout material G after the precast member 51, which serves as a column-beam joint 50, is temporarily joined to the precast beam member, etc., and the main beam reinforcement 56 is inserted into the sleeve S through the through-hole 5.

[0025] Figure 2(a) shows the state in which one of the precast beam members 55 is temporarily joined to the precast member 51 of the column-beam joint 50 (Figure 6). In this temporary joining state, the tip of the beam main reinforcement 56 is inserted into one of the sleeves S through the through hole 5 formed in the precast member 51. Furthermore, Figure 2(b) shows the state in which the beam main reinforcement 56 of the precast beam member 55, which is joined from the side, is placed, and grout material G is filled into the through hole 5 and sleeve S in which the beam main reinforcement 56 is placed, through the grout injection pipe 62.

[0026] The grout material G is densely filled into the sleeve S and through-hole 5 so that the main beam reinforcement 56 housed within the sleeve S and through-hole 5 is securely anchored to the precast member 51. To ensure the anchoring effect of the grout material G to the precast member 51 after hardening, narrow ring-shaped grooves 6 are formed at predetermined intervals along the longitudinal direction of the through-hole 5 on the hole wall 5a. When concrete C is poured into the formwork space 1 of the precast member 51 (Figure 1(a)), the ring-shaped protrusions 12, which are provided at predetermined intervals along the longitudinal direction of the cylindrical formwork 11 of the through-hole formwork 10, act as male-shaped grooves 6 formed on the concrete surface of the hole wall 5a of the through-hole 5 (Figure 2(a)).

[0027] Here, the configuration of the cylindrical formwork 11 and the ring member 20 of the through-hole formwork 10 will be explained with reference to the figures in Figure 3. As shown in Figure 3(a), the through-hole formwork 10 consists of a cylindrical formwork 11 of a predetermined diameter and a ring member 20 made of an elastic material that is attached to the outer surface of the cylindrical formwork 11 and functions as a ring-shaped convex portion 12.

[0028] The cylindrical formwork 11, which serves as the main body of the through-hole formwork 10, is made up of a straight rod with a circular cross-section. The overall shape of the cylindrical formwork 11 is a frustoconical shape, where the diameter gradually increases from the formwork end face 10a on the sleeve S side to the opposite end face 10b (Figures 1(a), 3(a)). Therefore, when demolding after the poured concrete has hardened, the through-hole formwork 10 can be easily pulled out from the hardened concrete. The diameter and length of the cylindrical formwork 11 are designed based on the inner diameter of the through-hole 5, which is set according to the diameter of the main reinforcement bars 56 of the beam that are arranged in the precast member 51, and the specifications of the sleeve S used for joining, as shown in Figure 1(a). In this embodiment, the cylindrical formwork 11 is a hollow pipe with the end face of a circular steel pipe closed to reduce weight, but a solid cylindrical member made of synthetic resin can also be used.

[0029] The ring member 20, which is attached to the outer surface of the cylindrical formwork 11 and functions as a ring-shaped convex portion 12, will now be described. The ring member 20 is an elastic member made by repurposing a nitrile rubber O-ring, having an inner diameter slightly smaller than the diameter of the cylindrical formwork 11. Therefore, as shown in Figure 3(b), when attaching the ring member 20 to the outer surface of the cylindrical formwork 11, the inner diameter of the ring member 20 is slightly expanded and fitted onto the outer surface of the cylindrical formwork 11, so that it is attached in an elastic, tightly adhering position to the outer surface of the cylindrical formwork 11. Notches 14 are formed for determining the spacing and positioning of the multiple ring members 20 to be attached. As an example, it is preferable that the spacing p of the ring members 20 be 10 times the wire diameter φ of the ring member 20, i.e., p = 10·φ. Since the height of the convex portion 12 of the through-hole formwork 10 formed using the ring member 20 is equal to the groove depth of the ring-shaped groove 6 later formed in the wall of the through-hole 5, the wire diameter φ of the O-ring used as the ring member 20 is preferably about φ = 3 to 5 mm, taking into consideration that the wire diameter becomes slightly thinner due to elastic deformation when the ring diameter is expanded, in order to ensure sufficient anchorage of the beam main reinforcement 56 when the grout material G is filled. There are various materials for the O-ring, but in addition to the nitrile rubber mentioned above, chloroprene rubber, ethylene propylene rubber, urethane rubber, silicone rubber, etc. can be used as alkali-resistant O-rings. Specific examples of the dimensions of each part of the through-hole 5 will be described later.

[0030] Each figure in Figure 4 illustrates the procedure for efficiently mounting the ring member 20, which consists of the O-ring described above, onto the cylindrical formwork 11. As shown in Figure 4(a), a number of notches 14 are formed on a part of the outer circumferential surface of the cylindrical formwork 11. These notches 14 can be used to position and hold the ring member 20 at the mounting position on the outer circumferential surface of the cylindrical formwork 11. As shown in the same figure, a tapered jig 15 for mounting the ring member 20 is attached to one end face of the cylindrical formwork 11. The tapered jig 15 has a roughly frustoconical shape. As shown in Figures 4(b) to 4(c), a predetermined number of ring members 20 with an inner diameter smaller than the outer diameter of the cylindrical formwork 11 are fitted into the tapered jig 15. By sequentially moving the ring members 20 along the tapered surface 15a toward the cylindrical formwork 11, the ring members 20 expand by elastic elongation from the point where the outer diameter of the tapered surface 15a of the tapered jig 15 becomes larger than the inner diameter of the ring members 20 until their diameter is equal to the outer diameter of the outer surface of the cylindrical formwork 11. Then, by moving them to the respective positions of the notches 14 on the outer surface of the cylindrical formwork 11, a ring-shaped convex portion 12 can be formed at predetermined positions on the outer surface of the cylindrical formwork 11, as shown in Figure 4(d). The completed through-hole formwork 10 is then installed in the formwork space 1 of the precast member 51, as shown in each figure of Figure 1.

[0031] Figures 5(a) to 5(f) show the procedure for forming a through-hole 5 in a precast member 51 using a through-hole formwork 10, which has a ring-shaped convex portion 12 formed on the outer surface of the cylindrical formwork 11 described above. The procedure for manufacturing a precast member 51 in which a sleeve S for joining the main beam reinforcement 56 is embedded inside the concrete C will be explained below with reference to each figure. The sleeve S and the through-hole formwork 10 are installed at a predetermined position in the formwork space 1 surrounded by the formwork plates 2 for constructing the precast member 51, so as to be in a straight line with the sleeve S in the axial direction (Figure 5(a)). At this time, the outer end of the through-hole formwork 10 is supported by the opening formed in the formwork plates 2, so that the through-hole 5 and the sleeve S can be kept in a straight line even when a concrete load is applied during concrete pouring.

[0032] Next, concrete C is poured into the formwork space 1, and after a predetermined curing period, the formwork is removed, and the cylindrical formwork 11 of the through-hole formwork 10 is pulled out and removed from within the hardened concrete C (Figures 5(b), (c)). To further facilitate the removal of the cylindrical formwork 11, which has a slightly frustoconical shape, it is preferable to apply a release agent to the outer surface of the cylindrical formwork 11 or to coat the outer surface of the cylindrical formwork 11 with a fluororesin coating. It goes without saying that the cylindrical formwork 11 may also be a straight cylinder if it facilitates demolding after the concrete hardens.

[0033] When the cylindrical formwork 11 is withdrawn, most of the ring members 20 attached to the outer surface of the cylindrical formwork 11 are embedded in the concrete C of the hole wall 5a of the through hole 5. Therefore, the ring members 20 remain in the concrete C and only the cylindrical formwork 11 is withdrawn (Figure 5(d)). When the cylindrical formwork 11 is withdrawn and the ring members 20 remain in the concrete C, they attempt to elastically deform from the expanded diameter state caused by the cylindrical formwork 11 back to their initial diameter. At this time, the elastic deformation force of the ring members 20 is greater than the resistance of adhesion between the ring members 20 embedded in the concrete C and the resistance of pulling them out from their embedded state in the concrete C. Therefore, each ring member 20 peels off and separates from the hole wall 5a (concrete C) of the through hole 5. The through hole 5 is completed by removing each ring member 20 that has peeled off from the concrete C using a hook-shaped recovery jig 8 made of wire (Figures 5(e), (f)).

[0034] [Specific examples of through-hole formwork configurations] A specific example of the configuration of the through-hole formwork 10 within the precast member 51 of the column-beam joint 50 will be briefly described. For example, if the column width of the column-beam joint 50 is 1,000 mm and the main reinforcement size of the beam joined within the column-beam joint 50 is D32, the sleeve length will be 370 mm. Therefore, the length of one side of the through-hole 5 in this embodiment will be 315 mm. Thus, if the ring members 20 attached to the cylindrical formwork 11 have a wire diameter φ of φ = 4 mm and are attached at intervals of 10 times the wire diameter φ, then seven ring members will be provided at predetermined intervals within a predetermined range of the cylindrical formwork 11.

[0035] In this invention, a commercially available O-ring is used as the ring member 20 described above, but points to consider are: (1) Relationship between the diameter of the cylindrical mold 11 and the appropriate size of the O-ring (2) What is the required pulling force when removing the cylindrical formwork 11 fitted with the O-ring from the hardened concrete? (3) Whether the O-ring left inside the through hole can be easily removed from the concrete. This will be examined below based on the relationship between the elastic properties of the O-ring and the concrete strength of the concrete.

[0036] (Size of the O-ring to be installed) Using a through-hole (D=50mm) where the main beam reinforcement (D32) is placed as an example, we calculate the stress acting on the O-ring. Assuming that the bond strength between the concrete and the O-ring is 5% of the compressive strength of the concrete, the concrete strength of the precast member 51 at the time of demolding is 12 N / mm². 2 With this setting, the adhesion strength is P = 0.6 N / mm 2 Therefore, when the O-ring (wire diameter φ=4mm) is expanded to a diameter (inner diameter) of 50mm, the tensile stress σ required to detach the O-ring from the concrete is σ = P·D / 2t. s , and P=0.6N / mm 2 Therefore, σ = 3.75 N / mm 2 This is the result. At this time, the tensile force generated in the cross-section of the O-ring is 47 N. Here, the tensile stress of an O-ring made of nitrile rubber when stretched to 100% is 5.2 N / mm². 2 Therefore, it can be seen that to generate the tensile stress necessary for demolding the O-ring, an elongation of about 60% is sufficient. Thus, if the diameter of the cylindrical mold 11 is 50 mm, an O-ring with an initial diameter of 30 mm should be used.

[0037] (The tensile force required to remove the cylindrical formwork, and the tensile stress required for the O-ring to separate it from the concrete) It was found that the elastic stress causing the O-ring to return to its initial diameter significantly exceeds the force required to remove the cylindrical formwork 11, indicating that the O-ring easily detaches from the concrete. Furthermore, each O-ring is tightened with a cross-sectional tensile force of 47 N. Assuming a friction coefficient of 1.0 between the O-ring attached to the outer surface of the cylindrical formwork and the outer surface of the cylindrical formwork, it can be seen that the force required to pull the cylindrical formwork out of the concrete while leaving the O-ring in the concrete is approximately 329 N.

[0038] In the above explanation, we described each configuration assuming a precast member constituting a column-beam joint as an example of application of through-hole formwork. However, this through-hole formwork can also be used to form through-holes for various precast concrete products such as PC segments, which use sheath pipes to insert PC cables in prestressed concrete structures, allowing for the insertion of various reinforcing bars and steel reinforcing wires such as PC cables.

[0039] It should be noted that the present invention is not limited to the embodiments described above, and various modifications are possible within the scope of each claim. In other words, embodiments obtained by combining technical means that have been appropriately modified within the scope of the claims are also included in the technical scope of the present invention. [Explanation of Symbols]

[0040] 1 Formwork space 5 Beam main reinforcement through hole (through hole) 5a hole wall 6. Ring-shaped grooves 10. Formwork for forming through holes (through hole formwork) 11 Cylindrical formwork 12 Ring-shaped convex part 20 Ring Member 50 Column beam joint 51 Precast members C Concrete S Grout-filled fitting sleeve (sleeve)

Claims

1. A formwork for forming through holes, which is installed side-by-side within a formwork space and is withdrawn from the hardened concrete after the concrete has hardened, in order to insert and position reinforcing wires of a concrete member into a predetermined position within the concrete, and to form through holes with ring-shaped grooves formed in the hole walls, The system comprises a cylindrical formwork that forms a substantially circular hole for the through-hole, and an elastic ring member that is detachably attached to the cylindrical formwork so as to form a ring-shaped convex portion on the outer circumferential surface of the cylindrical formwork. The elastic ring member is characterized in that, when the cylindrical formwork is withdrawn from the hardened concrete, it peels off from the hole wall within the through-hole, and the ring-shaped groove is formed on the hole wall.

2. The formwork for forming a through hole according to claim 1, wherein the elastic ring member expands elastically to be attached to the outer surface of the cylindrical formwork and expands elastically to be detached from the hole wall.

3. The formwork for forming through holes according to claim 2, wherein the elastic ring member has an initial diameter smaller than the outer diameter of the cylindrical formwork.

4. The formwork for forming through holes according to any one of claims 1 to 3, wherein an O-ring is used as the elastic ring member.

5. The through-hole forming formwork according to claim 1, wherein the ring-shaped convex portion is the elastic ring member attached to the outer circumferential surface of the cylindrical formwork.

6. The formwork for forming through holes according to claim 1, wherein the cylindrical formwork is installed in the formwork space such that the through hole is formed between the end of the sleeve of the grout-filled joint embedded in the concrete member and the concrete surface.

7. The formwork for forming through holes according to claim 6, wherein the cylindrical formwork has a frustoconical shape in which the diameter gradually increases from the end side of the sleeve to the concrete surface.

8. A method for forming a through-hole, characterized by attaching an elastic ring member to the outer surface of a cylindrical formwork to provide a ring-shaped convex portion on the outer surface, arranging the cylindrical formworks side by side in a formwork space, pouring concrete into the formwork space, withdrawing the cylindrical formwork from the hardened concrete after the concrete has hardened to form a through-hole, and removing the ring member that was attached to the wall of the through-hole and remained in the through-hole by peeling it off from the wall of the hole, thereby forming a through-hole with a ring-shaped groove formed in the wall of the hole.

9. The method for forming a through hole according to claim 6, wherein the elastic ring member is elastically expanded in diameter and attached to the outer surface of the cylindrical formwork, and then elastically reduced in diameter and detached from the hole wall.

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