Prestress releasing method

Abstract

To provide a prestress releasing method capable of safely releasing prestress from a prestress concrete structure.SOLUTION: This prestress releasing method comprises: a cavity boring step of executing cutting from the outer surface of a concrete structure 1 in a direction intersecting the axial direction of a PC steel material 4 and forming a pull-in cavity 7 having a fixed width in the axial direction of the PC steel material 4; and a core boring step of boring a hole 10 so as to cut the edge of a core 6 from the concrete structure 1 from the peripheral edge of a fixing tool 5 in the axial direction of the PC steel material 4. The core 6 moves toward the pull-in cavity 7 to release tension of the PC steel material 4.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a prestress release method for safely releasing the tension from a prestressed concrete body.

Background Art

[0002] As a method of introducing prestress into a concrete structure, there is a post-tensioning method in which tension is applied to PC steel materials after the concrete has hardened. This post-tensioning method is classified into a bonded PC method and an unbonded PC method.

[0003] In the bonded PC method, a PC steel material made of a PC steel strand or the like is inserted into a sheath embedded in a concrete structure into which prestress is to be introduced, the PC steel material is tensioned, and the end of the PC steel material is fixed to the end of the concrete structure. Then, PC grout such as cement milk is filled into the PC steel material through which the sheath has been inserted, and the PC grout is cured and hardened, so that the PC steel material is integrated with the concrete structure through the PC grout and the sheath, and prestress is introduced into the concrete structure (see, for example, Patent Document 1).

[0004] In the unbonded PC method, an unbonded PC steel material in which a grease or the like applied to the outer peripheral surface of a PC steel material such as a PC steel strand is covered with a polyethylene film is used. This unbonded PC steel material is placed in a formwork in the same manner as reinforcing bars, and concrete is placed. After the concrete has hardened, the unbonded PC steel material is tensioned, so that prestress is introduced into the concrete structure (see, for example, Patent Document 2).

[0005] On the other hand, in the demolition of a concrete structure, in the case of a reinforced concrete structure (hereinafter referred to as an RC structure), generally, a large breaker, a hand breaker, a crusher, etc. are used to crush the concrete to a size that can be transported, and internal steel materials such as reinforcing bars and other steel materials embedded in the RC structure are disassembled by gas cutting or the like, and the crushed and disassembled waste materials are transported and removed by a truck or the like.

[0006] In addition, in the demolition of RC structures, there is also a known method of cutting and removing the concrete in a state where internal steel materials are embedded using a wire saw or the like, depending on the situation.

[0007] Also, in the demolition of prestressed concrete structures (hereinafter referred to as PC structures), although PC steel materials with tensile force applied are embedded in the concrete, when the adhesion between the PC steel materials and the surrounding concrete or grout is ensured as in the bonding method, it can be demolished in the same way as an RC structure.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] However, in the conventional technologies as described above, when the adhesion between the PC steel materials and the surrounding concrete or grout is not ensured due to the presence of the unbonding method or unfilled grout portions, etc., if an attempt is made to demolish it by cutting or crushing in the same way as an RC structure, since the PC steel materials arranged in the concrete are suddenly released from tension, there is a risk of the PC steel materials or fixtures popping out and sudden and irregular movement of the PC steel materials within the sheath.

[0010] Actually, a bridge girder with an unfilled grout portion existing at the bent part of the PC steel materials has been confirmed, and there are also cases where prestress is introduced by the unbonding method in the slab part of an apartment building. In such cases, a method for safely releasing the prestress and demolishing the concrete structure is not yet known.

[0011] Therefore, in view of such conventional problems, the present invention has been made for the purpose of providing a prestress release method capable of safely releasing prestress from a prestressed concrete structure.

Means for Solving the Problems

[0012] The feature of the invention according to claim 1 for solving the conventional problems as described above is that after tensioning the PC steel bars arranged in the concrete structure, the end portions of the PC steel bars are fixed to the concrete structure by fixing tools, and in the prestress release method for releasing the tension of the PC steel bars of the concrete structure, cutting is performed from the outer surface of the concrete structure in a direction intersecting the axial direction of the PC steel bars to form a drawing gap portion having a constant width in the axial direction of the PC steel bars, and a core drilling step of drilling the concrete structure so as to cut off the core portion where the PC steel bars are arranged from the periphery of the fixing tool in the axial direction of the PC steel bars, and the core portion moves toward the drawing gap portion to release the tension of the PC steel bars.

[0013] The feature of the invention according to claim 2 is that, in addition to the configuration of claim 1, the drawing gap portion is formed by a water jet.

[0014] The feature of the invention according to claim 3 is that, in addition to the configuration of claim 1 or 2, after the core drilling step, cutting is performed from the outer surface of the concrete structure in a direction intersecting the axial direction of the PC steel bars toward the tip of the core portion to form the drawing gap portion.

[0015] The feature of the invention according to claim 4 is that, in addition to the configuration of claim 3, the axial width of the drawing gap portion of the PC steel bars is gradually expanded from the fixing tool side.

[0016] The feature of the invention according to claim 5 is that, in addition to the configuration of claim 1 or 2, after the step of drilling the voids, the core portion is drilled so as to trim the core portion in the axial direction of the PC steel material from the periphery of the fixture to the drawing void portion.

[0017] The feature of the invention according to claim 6 is that, in addition to the configuration of claim 5, after forming a plurality of the drawing void portions at intervals in the axial direction of the PC steel material, the core portion is drilled so as to trim the core portion in the axial direction of the PC steel material until reaching the innermost drawing void.

Advantages of the Invention

[0018] The prestress release method according to the present invention can safely release the prestress applied from the concrete structure by having the configuration described in claim 1.

[0019] Also, in the present invention, by having the configuration described in claim 2, cutting for forming the drawing void portion can be performed while suppressing damage to the PC steel material embedded in the concrete structure, and the work can proceed safely.

[0020] Furthermore, in the present invention, by having the configuration described in claim 3, a drawing void portion can be formed in accordance with the core portion, and the core portion can be drawn in efficiently.

[0021] Also, in the present invention, by having the configuration described in claim 4, the core portion can be moved step by step, preventing the tension of the PC steel material from being suddenly released and allowing the work to proceed safely.

[0022] Also, in the present invention, by having the configuration described in claim 5, since the tension of the PC tendon is not released during the void drilling process, the drawing void portion can be formed safely.

[0023] Furthermore, in the present invention, by providing the configuration according to claim 6, the core part can be moved step by step, preventing the sudden release of the tension of the PC steel material and allowing the work to proceed safely.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0025] Next, embodiments of the prestress release method according to the present invention will be described based on the examples shown in FIGS. 1 to 4.

[0026] In this embodiment, a box girder bridge formed by connecting precast members in the bridge axis direction as the concrete structure 1 will be described as an example. FIG. 1 is an enlarged view of the joint portion between the web portion 2 and the upper floor slab portion 3 of the box girder.

[0027] In this concrete structure 1, PC steel bars 4 are arranged in the web part 2 and the upper floor slab part 3 in the bridge axis direction. After the PC steel bars 4 are tensioned, the ends of the PC steel bars 4 are fixed to the concrete structure 1 by fixing tools 5, and prestress is introduced by the post-tensioning method.

[0028] Further, the PC steel bars 4 are in a state where part or all of the sheath through which the PC steel bars 4 are inserted in the PC steel bars 4 for the unbonded method or the bonded method is not filled with grout, and the PC steel bars 4 and the web part 2 or the upper floor slab part 3 constituting the concrete structure 1 are not adhered to each other.

[0029] To release the prestress (tensile force) applied to this concrete structure 1, a core drilling process of drilling a hole in the concrete structure 1 so as to cut off the core part 6 where the PC steel bars 4 are arranged from the concrete structure 1 in the axial direction of the PC steel bars 4 from the periphery of the fixing tool 5, and a void part drilling process of cutting from the outer surface of the concrete structure 1 in a direction intersecting the axial direction of the PC steel bars 4 to form a drawing void part 7 are performed, and the core part 6 is moved toward the drawing void part 7 so as to release the tensile force of the PC steel bars 4.

[0030] Specifically, first, as a preliminary preparation, the elongation during the release operation of the PC steel bars 4 accompanying the introduction of prestress is estimated.

[0031] For example, in the case of SWPR7BN 12.7·7-strand steel strands frequently used in road bridges, the limit value of the tensile stress immediately after the introduction of prestress is 0.7σ pu (=0.7×1850 = 1295 N / mm 2 ) and considering the reduction of the tensile stress of the PC steel bars 4 due to the drying shrinkage and creep phenomenon of the concrete from immediately after the introduction of prestress, and assuming that the residual tensile stress of the PC steel bars 4 is 0.85 times, the tensile stress of the PC steel bars 4 at the time of release is estimated to be 1100 N / mm 2 or less. From Hooke's law, the residual strain ε of the PC steel bars 4 is ε = 1100 / 2.0×105 It is presumed as follows, and the residual elongation per 10 m of the PC steel material 4 is 10000×550×10 -5 = 55 mm.

[0032] Next, as shown in Fig. 2(a), when the fixture 5 is embedded in the concrete, as shown in Fig. 2(b), the end portion of the concrete structure 1 is chipped off by a chipper or the like until the entire fixture 5 is exposed on the end face portion 8.

[0033] At this time, although not particularly shown, according to the working position, a scaffold is installed, and a shell support is installed so that the concrete shell does not fall to the lower working bench surface.

[0034] Regarding this end chipping work, considering safety, protective equipment should be used properly, and the work should be carried out from above or the side of the fixture 5, and it is desirable not to enter the rear in the axial direction of the PC steel material 4 of the fixture 5.

[0035] When the fixture 5 is exposed, a core drill is used to drill a hole from the concrete structure 1 so as to cut off the core portion 6 in which the PC steel material 4 is arranged in the axial direction of the PC steel material 4 from the peripheral edge of the fixture 5 (core drilling process).

[0036] In the core drilling process, as shown in Fig. 2(c), while rotating the cylindrical core bit 9 having a cutting edge 9a at the tip, a hole is drilled axially from the rear in the axial direction of the PC steel material 4 from the end face portion 8, and the fixture 5 is arranged at the end by the drilled hole 10, and the cylindrical core portion 6 in which the PC steel material 4 is arranged in the center is cut off from the concrete structure 1.

[0037] When spiral bars 11 are arranged in the concrete structure 1, the diameter of the drilled hole is set to be larger than the outer diameter of the spiral bars 11 and larger than the outer periphery of the fixture 5.

[0038] In addition, the depth of the hole drilling is set to reach the planned formation position of the drawing-in void portion 7 described later. More specifically, it is formed deeper than the total length of the spiral ribs 11 so as not to interfere with the spiral ribs 11 when forming the drawing-in void portion 7.

[0039] Next, as shown in FIG. 3(d), as a preliminary step for forming the drawing-in void portion 7, the concrete structure 1 is cut away to a predetermined depth from the outer surface of the concrete structure 1 in a direction intersecting the axial direction of the PC steel material 4 by a chipper or the like to form a recess 12, exposing the internal reinforcing bars 13, 13... Then, as shown in FIG. 3(e), the reinforcing bars 13, 13... in the bridge axis direction are cut using a gas cutting machine.

[0040] As shown in FIG. 1, the cutting position is such that for the slab portion 3, it is from above and below (from above the upper slab portion 3 in this embodiment), and for the web portion 2, it is from both the left and right side surfaces of the web (the following operations will be described taking the web portion 2 as an example).

[0041] After the cutting of the reinforcing bars 13, 13... is completed, next, cutting is performed from the outer surface of the concrete structure 1 in a direction intersecting the axial direction of the PC steel material 4 to form a drawing-in void portion 7 having a constant width in the axial direction of the PC steel material 4 (void portion hole drilling step).

[0042] The drawing-in void portion 7 is set such that the height and width are larger than the diameter of the core portion 6, and the axial width is wider than the above-described estimated elongation (55 mm per 10 m of the PC steel material length).

[0043] In the void portion hole drilling step, in order to prevent the PC steel material 4 from breaking due to the hole drilling operation, it is desirable to cut the concrete by spraying water with a water jet.

[0044] The procedure of the void part drilling process is as follows: First, divide the area to be drilled into a plurality of drilling sections 71 to 74 in the vertical direction and the axial direction of the PC steel bar 4. As shown in FIGS. 3(f) to 4(h), high-pressure water is sprayed from both side surfaces of the front part (fixing tool 5 side) in the axial direction of the PC steel bar 4 in the order of the upper and lower parts 71a, 71b farthest from the PC steel bar 4 to its inner parts 71c, 71d, and the central part 71e. While crushing the concrete until the PC steel bar 4 is exposed, the crushed shell is removed.

[0045] In addition, every time the concrete in one section divided in the vertical direction is crushed, the operation is temporarily interrupted, and the management engineer checks the situation. If there is no abnormality, the operation is continued, the next range is crushed, and if there is an abnormality, the operation is carried out by instructing changes in the crushing range, etc.

[0046] And when the drilling of the front part (fixing tool 5 side) in the axial direction of the PC steel bar 4 divided into a plurality is completed, the core part 6 is separated from the concrete structure 1 and moves to the drawing void part 71 side by the tensile force of the PC steel bar 4.

[0047] Next, the remaining concrete parts 72, 73, 74 divided in the axial direction are drilled step by step in the axial direction of the PC steel bar 4 by repeating the operation of water jet drilling from the upper and lower parts farthest from the PC steel bar 4 in the order from the front side and the operation of moving the core part 6 in the same procedure as described above.

[0048] And when the drawing void part 7 reaches a predetermined width in the axial direction, as shown in FIG. 4(i), the core part 6 moves into the drawing void part 7 by the elongation amount of the PC steel bar 4. Along with this, the tension of the PC steel bar 4 is released and the prestress is released.

[0049] After the prestress is released, the concrete structure 1 can be demolished and removed by crushing the concrete structure 1 in the same way as the demolition of a general reinforced concrete structure.

[0050] In the above-described embodiments, the case where the void portion drilling process is performed after the core drilling process has been described. However, as shown in FIGS. 5 and 6, the core drilling process may be performed after the void portion drilling process has been performed first. Note that the same components and processes as those in the above-described embodiments will be described with the same reference numerals.

[0051] In this case, first, after exposing the fixing tool 5 in the same manner as in the above-described embodiments, as shown in FIG. 5(a), a plurality of drawing void portions 20, 21, 22 are formed at intervals in the axial direction of the PC steel material 4.

[0052] The axial width of each drawing void portion 20, 21, 22 is set such that the sum of the axial widths of all the drawing void portions 20, 21, 22 is wider than the above-described estimated elongation (55 mm per 10 m).

[0053] Next, as shown in FIGS. 5(b) to 6(e), a core drill is used to drill a hole so as to cut off the core portion 6 in which the PC steel material 4 is disposed from the concrete structure 1 in the axial direction of the PC steel material 4 from the peripheral edge of the fixing tool 5.

[0054] Then, as shown in FIG. 5(c), when the core drilling reaches the drawing void portion 20 on the near side, the core portion 6 is separated from the concrete structure 1 and moves to the drawing void portion 20 side by the tensile force of the PC steel material 4, and a part of the tensile force of the PC steel material 4 is released.

[0055] Next, when the core drilling further proceeds and reaches the second drawing void portion 21 as shown in FIG. 6(d), the concrete 23 between the drawing void portions 20 and 21 is separated from the concrete structure 1, and the core portion 6 moves to the second drawing void portion 21 side by the tensile force of the PC steel material 4 while pushing in the separated concrete 23, and the tensile force of the PC steel material 4 is further released.

[0056] Furthermore, as the core drilling progresses and reaches the third drawing gap 22 as shown in FIG. 6(e), the concrete 24 between the drawing gaps 21 and 22 is separated from the concrete structure 1. While the core part 6 pushes in the separated concrete 24, it moves toward the third drawing gap 22 side due to the tensile force of the PC steel material 4, and all the tension of the PC steel material 4 is released.

[0057] In addition, in the above-described embodiment, the box girder bridge has been described as an example, but the form of the concrete structure 1 is not limited thereto. For example, it may be a case where there is an unfilled grout part in a portion where the PC steel material 4 is bent in a bridge, or a slab part of an apartment building to which prestress is applied by an unbonded method.

[0058] Moreover, the drawing gaps 7, 20, 21, and 22 are not particularly limited in shape as long as the core part 6 can be drawn in.

[0059] Furthermore, in the above-described embodiment, the case where the drawing gaps 7, 20, 21, and 22 are formed using a water jet has been described, but a chipper or a drilling machine may be used as long as the PC steel material 4 is not broken.

Explanation of Reference Numerals

[0060] 1 Concrete structure 2 Web part 3 Upper floor slab part 4 PC steel material 5 Fixture 6 Core part 7 Drawing gap 71 - 73 Drilling section 8 End face part 9 Core bit 10 Drilling 11 Spiral bar 12 Recess 13 Reinforcing bar 20 - 22 Drawing gap 23, 24 Concrete

Claims

1. In a prestress release method for releasing the prestress of a PC steel bar in a concrete structure, after tensioning the PC steel bar disposed in the concrete structure, the end of the PC steel bar is fixed to the concrete structure by a fixture, comprising: A void cutting step of cutting from the outer surface of the concrete structure in a direction intersecting the axial direction of the PC steel bar to form a drawing void portion having a constant width in the axial direction of the PC steel bar; A core cutting step of cutting a core portion in which the PC steel bar is disposed from the concrete structure so as to cut the periphery of the fixture in the axial direction of the PC steel bar; and The prestress release method, characterized in that the prestress of the PC steel bar is released by moving the core portion toward the drawing void portion.

2. The prestress release method according to claim 1, wherein the drawing void portion is formed by a water jet.

3. The prestress release method according to claim 1 or 2, wherein after the core cutting step, cutting is performed from the outer surface of the concrete structure in a direction intersecting the axial direction of the PC steel bar toward the tip of the core portion to form the drawing void portion.

4. The prestress release method according to claim 3, wherein the axial width of the drawing void portion of the PC steel bar is gradually expanded from the fixture side.

5. The prestress release method according to claim 1 or 2, wherein after the void cutting step, cutting is performed from the periphery of the fixture to the drawing void portion in the axial direction of the PC steel bar so as to cut the core portion.

6. The prestress release method according to claim 5, wherein after forming a plurality of the drawing void portions at intervals in the axial direction of the PC steel bar, cutting is performed from the periphery of the fixture to the innermost drawing void in the axial direction of the PC steel bar so as to cut the core portion.

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