Method for cutting the ends of PC steel members using a pretensioning method

Abstract

This invention provides a method for cutting the ends of PC steel members that allows for more reliable cutting of PC steel members at predetermined cutting positions and prevents mechanical damage to the core drill when a core drill is used for cutting. [Solution] A cylindrical hollow member is placed in the formwork with its axis aligned vertically, aligning with a predetermined cutting position close to the unbonded area of ​​the PC steel member, extending from the height of the top surface of the PC member to the height through which the PC steel member passes. Tension is then introduced into the PC steel member. While tension is being introduced into the PC steel member, concrete of a predetermined quality is poured into the formwork and cured. After curing is complete, the tension is released from the PC steel member to introduce prestress into the concrete. After prestress is introduced into the concrete, the center of the core drill is positioned to align with the center of the upper opening of the cylindrical hollow member, and a hole is drilled from the height of the top surface of the PC member to the height through which the PC steel member passes to cut the PC steel member.

Description

Technical Field

[0001] The present invention relates to a method for cutting the end of PC steel bars of PC members by a prestressing method.

Background Art

[0002] At the end face of a PC member immediately after prestress introduction, a part of the PC steel bar protrudes. It is said that this protruding part of the PC steel bar must be cut during finishing and rust prevention treatment must be applied to the cutting location. As a method for rust prevention treatment of PC steel bars, there is a method of cutting the PC steel bar at a depth of 3 mm or more from the concrete surface and applying a rust inhibitor or surface coating thereto (for example, Non-Patent Document 1).

[0003] As another method, in order to ensure the cover of the end of the PC steel bar, a method is known in which a recess is provided on the surface of the PC member, the PC steel bar is cut at a position deeper than the surface of the PC member, and the recess is filled with a filler (for example, Patent Document 1). Further, Patent Document 1 discloses a method of providing a core hole reaching from the surface of a concrete member to the PC steel bar in a PC floor slab, making the part of the PC steel bar from the core hole to the end of the floor slab unbonded, cutting and removing the PC steel bar inside the member using the core hole, and ensuring the cover by filling the core hole and the trace where the PC steel bar has come out with a filler. It is exemplified that a plasma cutting machine is used for cutting the PC steel bar.

[0004] Also, Patent Document 2 discloses a method of adjusting prestress by cutting a tendon of a prestressed PC member or making a tendon in a predetermined section unbonded. Here, as a method for cutting the tendon, heat generation cutting by energization or elongation, and cutting by an electric cutting rod are exemplified.

[0005] Furthermore, Patent Document 3 discloses a two-stage prestress adjustment method for a pretensioned hollow simple girder, in which a certain length portion of a PC steel member on the lower side of the girder end and a certain length portion of the PC steel member on the upper side near the center of the span are left unbonded, and the prestress is adjusted in two stages by cutting the middle portion of the upper PC steel member after the girder is erected. In Patent Document 3, gas cutting is given as an example of a method for cutting PC steel members.

[0006] Incidentally, in recent years, PC (Pressure Concrete) deck slabs used in bridge deck replacement work have become common, and prestress is often introduced by arranging PC steel members in the width direction and using a pretensioning method. In this case, the ends of the PC steel members are exposed on the surface of the concrete members, so measures to further improve durability are required for the purpose of rust prevention and further improvement of durability. In response to this, the technology described in Patent Document 1 involves leaving the PC steel members near the member ends where prestress is not required unbonded, and using the box-out section to cut and remove the ends where tension has not been introduced, thereby removing the parts of the PC steel members that are at high risk of corrosion. Originally, prestress is not required at the ends of PC members, and no tension has been introduced, so there is no risk of the PC steel members flying out or impact during cutting. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2021-017782 [Patent Document 2] Japanese Patent Application Publication No. 40-28419 [Patent Document 3] Japanese Patent Application Publication No. 57-140404 [Non-patent literature]

[0008] [Non-Patent Document 1] Road Bridge Specifications and Commentary (Precast PC Girders for Salt Damage Based on March 2002), pp. 47-48 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, the gas cutting and plasma cutting methods described above cause a significant temperature rise at the cutting site, raising concerns about adverse effects on the PC steel inside the concrete. In particular, when pretensioning epoxy resin coated PC steel, which is considered to have high rust prevention properties, is used, the softening temperature of the epoxy resin is said to be around 60°C. Therefore, the heat generated during cutting may soften and melt the epoxy resin, causing a loss of adhesion between the PC steel and the concrete, and potentially resulting in a loss of prestress. Another method that could be considered is to drill holes in the concrete with a core drill to cut the PC steel, but even a slight error in the angle at which the core drill is set will have a significant impact on the position of the target, making it difficult to accurately reach the PC steel with the cutting edge, thus increasing the likelihood of cutting errors.

[0010] Furthermore, cutting the PC steel members at the ends of PC floor slabs with a curb at the base of the wall parapet requires drilling to a long depth that includes the height of the curb at the base of the wall parapet, making it increasingly difficult to accurately position the core drill bit at the intended cutting location of the PC steel members.

[0011] In view of the above circumstances, the present invention aims to provide a method for cutting the end of a PC steel member that can more reliably cut the PC steel member at a predetermined cutting position and prevent mechanical damage to the core drill when a core drill is used for cutting. [Means for solving the problem]

[0012] To achieve the above objective, the present invention provides a method for cutting the end of a PC steel member using a pretensioning method, comprising: performing a treatment to separate the PC steel member from the concrete at the unbonded portion of the PC steel member; arranging a cylindrical hollow member in the formwork, aligned vertically with its axis, from the height of the top surface of the PC member to the height through which the PC steel member passes, at a predetermined cutting position close to the unbonded portion of the PC steel member; and introducing tension to the PC steel member. The process includes: pouring concrete of a predetermined quality into the formwork while tension is being applied to the material and curing it; releasing the tension on the PC steel after the curing is complete to introduce prestress into the concrete; filling the cylindrical hollow member with a filler after the prestress has been introduced into the concrete; and after the filler has hardened, positioning the core drill so that its center aligns with the center of the upper opening of the cylindrical hollow member, and drilling a hole from the height of the top surface of the PC member to the height through which the PC steel passes to cut the PC steel.

[0013] According to the PC steel end cutting method of the present invention, when cutting PC steel by drilling a hole using a core drill, for example, from the height of the top surface of the PC member to the height through which the PC steel passes, the movement of the core drill other than in the vertical direction is restricted within the hole created by drilling through the filler material inside the cylindrical hollow member. This allows the core drill to be moved accurately toward the intended cutting position of the PC steel, and prevents axial wobble caused by the cutting edge of the core drill slipping the moment it touches the PC steel. As a result, the PC steel can be cut more reliably at the intended cutting position, and mechanical damage to the core drill can be prevented. Furthermore, according to the PC steel end cutting method of the present invention, adverse effects on the PC steel due to temperature rise at the cut portion are not caused, thus preventing the loss of prestress due to a decrease in the adhesion between the PC steel and concrete.

[0014] In the method for cutting the end of a PC steel bar of the present invention, by using a core drill having a diameter smaller than the inner diameter of the cylindrical hollow member, the drilling by the core drill is performed on the filler filled and hardened in the cylindrical hollow member, so that the drilling load of the core drill can be significantly reduced.

Brief Description of the Drawings

[0015] [Figure 1] It is a perspective view of a PC floor slab with a ground covering part at the base of a wall railing, which is an example of a PC member. [Figure 2] It is a cross-sectional view showing the structure of the PC floor slab 1 before cutting the end of the PC steel bar. [Figure 3] It is a cross-sectional view showing a state where the filler 17 is filled in the cylindrical hollow member 15 of the PC floor slab 1 in FIG. 2. [Figure 4] It is a side view showing a state where the core drill 19 is positioned in accordance with the cutting planned positions P of the PC steel bars 11A and 11B of the PC floor slab 1 in FIG. 3. [Figure 5] It is a cross-sectional view showing a state of drilling by the core drill 19. [Figure 6] It is a cross-sectional view showing a state of drilling by the core drill 19 following FIG. 5. [Figure 7] It is a cross-sectional view showing a completed state of drilling by the core drill 19 and cutting of the PC steel bars 11A and 11B. ]> [Figure 8] It is a cross-sectional view showing an example where the filler is filled only in the upper part of the cylindrical hollow member 15. [Figure 9] It is a cross-sectional view for explaining another embodiment of the method for cutting the end of a PC steel bar of a prestressed concrete member by the prestressing method of the present invention. [[ID=]>[0000090]>

Embodiments for Carrying Out the Invention

[0016] [[ID=-40]]Hereinafter, embodiments of the method for cutting the end of a PC steel bar of a prestressed concrete member by the prestressing method of the present invention will be described in detail based on the drawings. Figures 1 to 7 are diagrams for explaining a method of cutting the end of PC steel bars of a prestressed concrete member (hereinafter referred to as "PC member") by a pretension method.

[0017] Figure 1 is a perspective view of a PC floor slab with a wall railing, which is an example of a PC member. On both sides in the width direction of the PC floor slab 1, there are provided covering parts 2 corresponding to the base parts of the wall railings. After the laying of the PC floor slab 1, for example, the upper part of the wall railing (not shown) with a cast-in-place concrete structure is added on the covering part 2 of the base part of the wall railing. On the top end surface 2a of the covering part 2 of the base part of the wall railing, a group of reinforcing bars (reference numeral 5 in Figure 2) for fixing the upper part of the wall railing with a cast-in-place concrete structure protrude. Inside the PC floor slab 1, PC steel bars (reference numerals 11A and 11B in Figure 2) for introducing prestress in the width direction are arranged, for example, in two upper and lower rows. On the top end surface 2a of the covering part 2 of the base part of the wall railing, there are repair traces 3 of holes formed by drilling holes from the top end surface 2a of the covering part 2 of the base part of the wall railing by a core drill (reference numeral 19 in Figures 4 to 7) when cutting the PC steel bars 11A and 11B. Due to the cutting of the PC steel bars 11A and 11B, the part outside the width direction from the cutting position becomes an unbonded part. On the side surface 2b of the covering part 2 of the base part of the wall railing, there are repair traces 4 of two upper and lower rows of holes formed after extracting the cut pieces of the PC steel bars 11A and 11B from the side surface 2b of the covering part 2 of the base part of the wall railing.

[0018] Next, the cutting procedure for the end of the PC steel bar will be described. Figure 2 is a cross-sectional view showing the structure of the PC floor slab 1 before cutting the end of the PC steel bar. [Step 1.] Prepare a formwork (not shown) for manufacturing the PC member. [Step 2.] Assemble and fix the reinforcing bars in the formwork. [Step 3.] PC steel members 11A and 11B are placed and fixed within the formwork. Here, the section from the planned cutting position of PC steel members 11A and 11B to one end in the width direction will be an unbonded section, so PVC pipes 13, 13 are installed to separate it from the concrete. Note that PVC pipes 13, 13 will be removed after the PC steel members 11A and 11B are cut.

[0019] [Step 4.] In line with the planned cutting positions P on the plane of the PC steel members 11A and 11B, a cylindrical hollow member 15 is placed and fixed within the formwork with its axis aligned in the vertical direction, extending from the height of the top surface 2a of the base of the wall parapet to the height through which the lowest PC steel members 11A and 11B pass. Here, the vertical direction may be the direction of the thickness of the PC slab 1. The cylindrical hollow member 15 is provided with PC steel member through-holes 16, 16 through which the upper and lower PC steel members 11A and 11B pass, and it is necessary to pass the PC steel members 11A and 11B through the PC steel member through-holes 16, 16 of the cylindrical hollow member 15 before fixing the PC steel members 11A and 11B. For example, a spiral sheath or a cylindrical body of a similar shape can be used for the cylindrical hollow member 15. Here, to prevent concrete from entering the cylindrical hollow member 15 when concrete is poured into the formwork, the height of the upper opening 15a of the cylindrical hollow member 15 is adjusted to be the same as the height of the top surface 2a of the wall parapet 2, or slightly higher than the height of the top surface 2a of the base of the wall parapet 2. Note that the placement and fixing of this cylindrical hollow member 15 does not necessarily have to be done after step 3, but can be done between step 1 and step 4, when tension is introduced to the PC steel members 11A and 11B. The cylindrical hollow member 15 must be fixed with sufficient strength so as not to move under the pressure of the concrete poured into the formwork, and the cylindrical hollow member 15 itself must also have sufficient strength. Ideally, the position where the center of the upper opening 15a of the cylindrical hollow member 15 is moved downward in the thickness direction of the PC slab 1 after concrete pouring coincides with the planned cutting position P of the PC steel members 11A and 11B.

[0020] [Step 5.] Next, tension is introduced into the PC steel members 11A and 11B by connecting their ends to hydraulic jacks or the like. [Step 6.] Concrete of the specified quality is poured into the formwork and cured under specified conditions. After curing is complete, the tension of the PC steel bars 11A and 11B is released to introduce prestress into the concrete. [Step 7.] Disassemble and remove the formwork.

[0021] [Step 8.] Next, as shown in Figure 3, a filler material 17, such as non-shrink mortar, is filled into the cylindrical hollow member 15 through the upper opening 15a. Here, the filler material 17 may be filled along the entire length of the cylindrical hollow member 15, or it may be filled only in the upper part of the cylindrical hollow part 15. For example, as shown in Figure 8, the filler material 17 may be filled only in the upper part of the cylindrical hollow part 15 so that the core drill 19 can restrict movement in directions other than vertical while cutting the PC steel material 11B furthest from the top surface of the base of the wall parapet with the core drill 19. The height range in which the filler material 17 is filled into the cylindrical hollow part 15 should be selected according to the length of the core drill 19. This makes it possible to reduce the amount of filler material required and lower costs.

[0022] [Step 9.] After the filler 17 has hardened, the core drill 19 is positioned to match the planned cutting position P on the plane of the PC steel members 11A and 11B, as shown in Figure 4. At this time, the position of the upper opening 15a of the cylindrical hollow member 15 can be easily confirmed by visual inspection, so by aligning the center of the tip of the core drill 19 with the center of the circular upper opening 15a of the cylindrical hollow member 15, the position of the core drill 19 can be easily aligned with the planned cutting position P on the plane of the PC steel members 11A and 11B. The diameter of the core drill 19 may be the same as the inner diameter of the upper opening 15a of the cylindrical hollow member 15, or slightly smaller than the inner diameter of the upper opening 15a of the cylindrical hollow member 15. However, since drilling with the core drill 19 is less stressful when performed on the filler that has hardened inside the cylindrical hollow member 15 than when performed on the concrete surrounding the cylindrical hollow member 15, it is preferable that the diameter of the core drill 19 be slightly smaller than the inner diameter of the cylindrical hollow member 15. For example, a cylindrical hollow member 15 with an outer diameter of 42 mm was used, and the core drill used had an outer diameter larger than the diameter of the PC steel materials 11A and 11B, and smaller than the inner diameter of the cylindrical hollow member 15. In order to ensure more reliable cutting of the PC steel materials 11A and 11B, it is desirable to use a core drill with an outer diameter larger than the diameter of the PC steel materials 11A and 11B.

[0023] [Step 10.] Next, as shown in Figures 5 to 7, the core drill 19 is operated to drill from the height of the top surface 2a of the base of the wall parapet to a height slightly deeper than the height where the lowest PC steel member 11B is located, thereby cutting the respective PC steel members 11A and 11B. [Step 11.] Cut pieces of PC steel members 11A and 11B in the unbonded section are removed from the side surface 2b of the base of the wall railing 2, and the holes where the cut pieces were removed are repaired with repair material.

[0024] In the above embodiment, a cylindrical hollow member 15 is placed in the range from the height of the top surface 2a of the curb portion 2, which is set higher than the upper surface of the PC floor slab 1, to the height through which the lowest PC steel member 11B passes. A filler material 17 is filled into the cylindrical hollow member 15, and a core drill 19 is used to drill a hole from the top surface 2a of the curb portion 2 of the base of the wall parapet to a height slightly deeper than the height at which the lowest PC steel member 11B is located, thereby cutting the PC steel members 11A and 11B. However, the present invention is not limited to this. For example, as shown in Figure 9, it is possible to similarly cut the PC steel members 11A and 11B in a PC floor slab 1' where the height of the curb portion 2 of the base of the wall parapet is equal to the height of the upper surface of the PC floor slab 1'.

[0025] According to the PC steel end cutting method of this embodiment, when cutting PC steel materials 11A and 11B by using a core drill 19 to cut from, for example, a height position slightly deeper than the height at which PC steel materials 11A and 11B exist, the movement of the core drill 19 in directions other than the drilling direction is restricted within the drilled hole 21 created by drilling through the filler material 17 inside the cylindrical hollow member 15 with the core drill 19. As a result, the core drill 19 can be lowered toward the planned cutting position P of the PC steel materials 11A and 11B, and the cutting edge of the core drill 19 is less likely to slip and cause axial wobble the moment it touches the PC steel materials 11A and 11B.

[0026] In recent years, the use of pretensioned epoxy resin coated PC steel has been increasing for purposes such as salt damage countermeasures. However, when pretensioned epoxy resin coated PC steel is used, if the epoxy resin softens or melts due to heat, the adhesion between the PC steel and the concrete decreases, and there is a risk of loss of prestress. According to the PC steel end cutting method of this embodiment, the PC steel 11A and 11B can be cut at the target position with the core drill 19. Therefore, the pretensioned epoxy resin coated PC steel can be cut without affecting the pretensioned epoxy resin coated PC steel with heat. [Explanation of Symbols]

[0027] 1 PC floor slab 2. Base of the wall railing 2a Top surface of the wall railing 11A,11B PC steel material 15. Cylindrical hollow member 17 Filling material 19 Core Drill

Claims

1. A method for cutting the end portion of a PC steel member using a pretensioning method, To separate the PC steel from the concrete in the areas where it is planned to be unbonded, To align a predetermined cutting position adjacent to the unbonded portion of the PC steel material, a cylindrical hollow member extending from the height of the top surface of the PC member to the height through which the PC steel material passes is placed within the formwork with its axis aligned in the vertical direction, Introducing tension to the aforementioned PC steel material, While tension is being applied to the PC steel material, concrete of a predetermined quality is poured into the formwork and cured. After the curing process is complete, the tension on the PC steel is released to introduce prestress into the concrete. After introducing prestress to the concrete, the cylindrical hollow member is filled with a filler material. After the filler has hardened, the core drill is positioned so that its center aligns with the center of the upper opening of the cylindrical hollow member, and a hole is drilled from the height of the top surface of the PC member down to the height through which the PC steel material passes, thereby cutting the PC steel material. A method for cutting the end of a PC steel material, including the method described above.

2. A method for cutting the end of a PC steel material according to claim 1, As the core drill, a core drill with a diameter smaller than the inner diameter of the cylindrical hollow member is used. PC steel end cutting method.

3. A method for cutting the end of a PC steel material according to claim 2, After cutting the PC steel material, the cut pieces are removed. Includes PC steel end cutting method.

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