JP2016211195A - Manufacturing method of prestressed concrete girder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a prestressed concrete girder with a fewer restrictions concerning a work location, capable of suppressing a warp caused by prestress creep deformation as desired, and allowing a warp amount to be adjusted readily.SOLUTION: A manufacturing method of a prestressed concrete girder includes: steps (A, B) in which a concrete girder 10 is formed by casting concrete in a concrete mold 5; the step (B) in which prestress is introduced at a lower part of the concrete girder 10, by applying tensile force of a lower prestressed concrete steel 3 laid at the lower part of the concrete girder 10 at least in a central part of a span and extending in a member axis direction; a step (C) in which prestress is introduced to an upper part of the concrete girder 10, by applying tensile force of an upper prestressed concrete steel 7 laid above a neutral surface of the concrete girder 10 at least in the central part of the span and extending in the member axis direction; and the step (C) in which the prestress by the upper prestressed concrete steel 7 is retained for a prescribed period P, in accordance with a creep deformation caused by the prestress introduced at the lower part of the concrete girder 10.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a method for manufacturing a PC girder in which prestress is introduced by a PC steel material disposed at the lower side at least in the center of the span, and relates to a technique for adjusting the amount of creep deformation due to prestress.

  As a concrete girder, a prestressed concrete girder (hereinafter referred to as a PC girder) in which prestress is introduced by embedding PC steel in a concrete girder in order to increase bending strength is widely used. PC girders are known to be deformed so that the central part of the span is warped by creep due to pre-stress, and normally the design height (the upper and lower surfaces of the girders are flat) at the time of creep convergence. The warpage is offset by lowering the center portion of the span of the bottom plate form by the amount of warpage (see Non-Patent Document 1).

  However, in the structure in which the PC steel material can be arranged only in a straight line and the bottom plate form must be horizontal, such as a pre-tension type PC girder, the warp due to overturning cannot be offset. Therefore, in the case of pre-tension type PC girders, calculate the amount of warp due to creep deformation and adjust the amount of extras to be formed on the upper face of the girders at both ends so that the upper face of the girders will be flat when creep converges. Alternatively, the pavement thickness is adjusted in accordance with the shape of the upper surface of the girder so that the minimum pavement thickness is secured at the intermediate portion of the span that has warped when the creep converges (see Non-Patent Document 2).

“Guide for Construction Plan [T Girder Bridge, Segment T Girder Bridge]”, Prestressed Concrete Construction Association, July 2002, p. 27 “Design and Manufacturing Handbook for Bridges for Road Bridges”, Prestressed Concrete Construction Association, June 2004, p. 90-95

  However, when there are restrictions on the height of the girders and the shape of the upper surface of the girders, such as when widening girders are added to the existing bridge, it may not be possible to cope with the warp by free adjustment of the surplus amount and pavement thickness. is there. In such a case, it is necessary to suppress the creep deformation itself so that the beam does not warp. As a method of suppressing warping due to creep deformation, a method of applying an overload by placing a counterweight on the center part of the PC girder span for a certain period of time, or fixing the gate structure to the ground and fixing the center part of the PC girder span. A method of pressing down from above with a gate structure is conceivable.

  However, in order to place the counterweight on the PC girder, a space for installing a large lifting equipment is required. On the other hand, when a PC girder is pressed down by a gate structure, a strong ground that can exhibit the reaction force of the pressing force is required. That is, in these methods, there are restrictions on the work location.

  In addition, in these methods of applying creep load or a pressing force (hereinafter collectively referred to as a load) to suppress creep deformation, the stress caused by the load necessary to cancel the warpage is the PC girder. May exceed acceptable values. In such a case, it is not possible to obtain a satisfactory warp suppressing effect only by applying a load that satisfies the allowable value.

  Furthermore, the amount of warpage due to creep deformation varies due to the influence of some factors even in the same structure. Therefore, in order to increase the adjustment accuracy of the warpage amount, it is necessary to finely adjust the upper load that cancels the creep deformation. However, in the method of placing the counterweight on the PC girder, a complicated operation using the lifting equipment is necessary to finely adjust the loading load. On the other hand, in the method of pressing the PC girder with the gate structure, it is necessary to consider the creep deformation of the ground due to the load. To finely adjust the pressing force, complicated work such as monitoring the ground deformation is required.

  In view of such a background, it is an object of the present invention to provide a method for manufacturing a PC girder that has few restrictions on work locations, can suppress warping due to creep deformation, and can easily adjust the amount of warping.

  In order to solve the above problems, the present invention provides a step (FIGS. 4 (A) and 4 (B)) of placing concrete in a formwork (5) to form a concrete girder (10), and the concrete girder. The lower PC steel (3) which is arranged at the lower part of the concrete girder and extends in the axial direction of the member at least in the central part of the span of the span (between the fulcrums 2) is acted on the lower part of the concrete girder. A step of introducing stress (FIG. 4 (B)), and an upper PC steel material disposed above the neutral surface of the concrete girder and extending in the member axial direction at least in the central part between the diameters of the concrete girder ( The step of introducing prestress into the upper part of the concrete girder by applying the tension of 7) (FIG. 4C) and the amount of creep deformation due to prestress introduced into the lower part of the concrete girder Flip, a configuration and a step (FIG. 4 (C)) for holding the pre-stress by the upper PC steel over time (P).

  Here, the creep generated by the pre-stress in the axial direction of the member introduced by the lower PC steel material includes the contraction in the axial direction of the member and the warp caused by this, but the amount of creep deformation refers to the amount of warping. Yes.

  According to this invention, the upper PC steel material extended in the member axial direction of the concrete girder is tensioned using a small tension device, and the prestress is introduced into the upper part of the concrete girder, thereby warping due to creep deformation. Since it can be suppressed as desired, it is possible to save space and work regardless of the ground strength. In addition, the amount of warp due to creep deformation can be easily adjusted by adjusting the tension of the upper PC steel and the period during which the pre-stress is retained according to the amount of warp due to pre-stress introduced under the concrete girder. It can be carried out.

  Moreover, in said invention, after hold | maintaining the prestress by the said upper PC steel material over the said predetermined period, it can be set as the structure which further releases the prestress by the said upper PC steel material (FIG.4 (D)). .

  According to this configuration, almost no prestress is introduced into the upper part of the PC girder, so that the bending strength of the PC girder can be prevented from being lowered.

  In the above invention, before placing the concrete, the upper part in the formwork extends in the member axial direction at least at the central part between the diameters of the concrete girders, and the concrete form girder (5c And a step (FIG. 4 (A)) of installing the sheath (6) leading to the above, and in the step (FIG. 4 (C)) of introducing prestress into the upper part of the concrete girder, the sheath inserted through the sheath It can be set as the structure which makes the tension | tensile_strength of an upper PC steel material act.

  According to this structure, the prestress introduction | transduction operation | work to the upper part of a concrete girder by the tension | tensile_strength of an upper PC steel material can be made easy by installing the sheath which can insert an upper PC steel material in PC girder.

  Moreover, in said invention, after releasing the prestress by the said upper side PC steel material, it is set as the structure which further removes the said upper side PC steel material and fills the said sheath with grout (FIG.4 (D)). it can.

  According to this configuration, since no defective portion is generated inside the PC girder, the reliability of the PC girder can be improved.

  As described above, according to the present invention, it is possible to provide a PC girder manufacturing method in which there are few restrictions on the work location, warp due to creep deformation can be suppressed as desired, and the amount of warp can be easily adjusted.

Side view of PC girder according to embodiment II-II sectional view in FIG. III-III sectional view in FIG. The figure for demonstrating an example of the manufacturing method of the PC girder shown in FIG. The graph which shows the behavior of the warp of the PC girder shown in FIG.

  Hereinafter, a PC girder 1 and a method for manufacturing the same according to the present invention will be described in detail with reference to the drawings with reference to the drawings.

  As shown in FIGS. 1 to 3, the PC girder 1 is a T-girder having a T-shaped constant cross section that is long in the member axial direction, and is a support that serves as a fulcrum 2 on the lower surface 1 a near both ends in the member axial direction. Prestress is introduced by the tension of the lower PC steel material 3 (3i, 3o) embedded in the axial direction of the member inside the reinforced concrete simple girder in which is formed. In order to avoid complication of the figure, the reinforcing bars 4 (FIGS. 2 and 3) protruding from the PC girder 1 are omitted in FIG. 1, and the main bars are not shown in FIGS. Yes.

  Inside the PC girder 1 is embedded a lower PC steel material 3 that is disposed below the PC girder 1 and extends in the member axial direction in the center portion of the span of FIG. Here, the lower portion of the PC girder 1 means a portion below the neutral surface in the strong axis direction of the PC girder 1 (a continuous surface of the neutral axis that extends horizontally in the cross-section and that corresponds to the vertical load). Yes. The lower PC steel 3 may be placed in a tensioned state in the mold 5 (FIG. 4A) before placing the concrete of the PC girder 1 by a pre-tension method. After installation, it may be inserted into a sheath (not shown) that has been buried and tensioned by a post-tension method. When the lower PC steel material 3 is installed by the post-tension method, the grout is filled into the sheath after the tension and fixing of the lower PC steel material 3.

  In the present embodiment, 4 rows × 4 tiers, a total of 16 lower PC steel materials 3 are installed by the pretension method. Two rows (eight) of lower PC steel members 3o arranged on the outside are arranged linearly from one end to the other end in the member axial direction of the PC girder 1 and parallel to the lower surface 1a of the PC girder 1 ing. Here, it is assumed that the lower surface 1a and the upper surface 1b of the PC girder 1 are arranged horizontally. Accordingly, the lower PC steel material 3o on the outside extends horizontally. On the other hand, two rows (eight) of the lower PC steel members 3i arranged on the inner side are outside the length of about one third of the span between the spans of the span of the PC girders 1 (between the fulcrums 2). A horizontal part (not indicated) that extends straight and parallel to the lower two rows of lower PC steel 3o at the same height as the lower PC steel 3o of the two rows, and a PC girder 1 at both ends of the horizontal portion From a pair of inclined portions (not shown) that bend upward and toward the end of the PC girder 1 and extend obliquely and parallel to the lower surface 1a of the PC girder 1 until reaching the end of the PC girder 1 It is configured.

In addition, a sheath 6 is embedded in the PC girder 1 so that the upper part of the PC girder 1 extends in the member axial direction at the center of the span in FIG. Here, the upper part of the PC beam 1 means a part above the neutral plane of the PC beam 1 in the strong axis direction. That is, the sheath 6 is disposed so as to extend in the member axial direction through the upper side of the neutral surface of the PC girder 1 at least in the center portion of the span. Inside the sheath 6, the upper PC steel material 7 is inserted and tensioned by a post-tension method at a predetermined time (after time t _{4 to} t 5 in FIG. ₅ ) after placing the concrete, and after a predetermined period described later. The tension of the upper PC steel material 7 is maintained over P (FIG. 5). After the elapse of the predetermined period P, the upper PC steel material 7 is pulled out, and the sheath 6 is filled with the injected grout. That is, the upper PC steel material 7 is used as a temporary PC steel material.

  In the present embodiment, three sheaths 6 are installed, and the upper PC steel material 7 is inserted into each sheath 6. The sheath 6 and the upper PC steel material 7 are arranged side by side at a position as high as possible over the length of approximately 60% to 80% of the span between the spans of the span of the PC girders 1 (between the fulcrums 2). A horizontal portion (not shown) extending linearly in parallel with the upper surface 1b of the PC girder 1 and parallel to each other, and after gently curving from both ends of the horizontal portion, downward toward the end of the PC girder 1 and The PC girder 1 is composed of a pair of inclined portions (not labeled) extending obliquely toward the neutral surface in the weak axis direction.

  A plurality of PC girders 1 are arranged side by side and bridged between the piers 8 and 8 (FIG. 1). As shown in FIGS. 2 and 3, the floor slab concrete 9a constructed integrally at the upper portion and the upper surface thereof are applied. Supports the pavement 9b and the like.

  Next, a method for manufacturing the PC girder 1 configured as described above will be described with reference to FIG. In FIG. 4, the PC girder 1, the concrete girder 10 in the previous stage, the lower PC steel material 3, the sheath 6, the upper PC steel material 7 and the like are schematically shown.

  As shown to (A), the reinforcement 4 (not shown) and the formwork 5 (5a, 5b, 5c) of the PC girder 1 are assembled in the place where the manufacturing facilities of PC products, such as a yard of a PC factory, are prepared. The bottom mold 5a does not need to be lowered or raised, and is installed horizontally (flat). In addition, the concrete striking surface (the upper end of the side mold 5b in this embodiment) which is the upper surface 1b of the PC girder 1 is also horizontal (straight line).

  Next, the lower PC steel 3 is disposed in the form 5 of the assembled PC girder 1 so as to extend in the member axial direction, and the sheath 6 is disposed so as to extend in the member axial direction. The inner lower PC steel material 3i that is bent and arranged is locked with the bending support 11 installed at two intermediate points in the member axial direction of the PC girder 1 so that it does not float when tension is applied. To. All the lower PC steel materials 3 have at least the lower part (the lower part of the PC girder 1) in the mold 5 between the two bending supports 11 and 11 including the center part between the diameters of the PC girder 1 in the member axial direction. Arrange to extend to. All the sheaths 6 are arranged at least in the center between the diameters of the PC girders 1 and are fixed to the reinforcing bars 4 in the upper part of the mold 5 (upper part of the PC girders 1). The sheath 6 is disposed so that both ends reach the wife formwork 5c.

  Thereafter, all the lower PC steel materials 3 are tensioned, and fresh concrete is placed in the mold 5 in this state. When the fresh concrete is hardened, a concrete girder 10 is formed as shown in (B). After a predetermined curing period (for example, 1 day) elapses, the mold is removed and the locking to the lower PC steel material 3 of the bending support 11 is released. Moreover, after reaching the age (for example, 1 day) in which the strength necessary for fixing the lower PC steel material 3 was reached, the lower PC steel material 3 was added to the lower PC steel material 3 by releasing the tension. Tension is applied to the concrete girder 10 and prestress is introduced to the lower part of the concrete girder 10. When prestress is introduced to the lower part of the concrete girder 10, the concrete girder 10 (which has already been prestressed and has become the PC girder 1) warps the center part of the span as indicated by the white arrow. Deforms to rise. Unnecessary portions of the lower PC steel material 3 are cut.

  After demolding, as shown in (C), the upper PC steel material 7 is inserted into each of the sheaths 6 and the fixtures 12 are attached to both end surfaces of the concrete girder 10 in the member axial direction. The fixture 12 may be integrally fixed to the concrete girder 10 by, for example, supporting concrete 13 struck on the end face of the concrete girder 10, or fixed to the concrete girder 10 by an anchor member (not shown) arranged on the end face of the concrete girder 10. Also good. Or you may use both the support concrete 13 and the anchor member which were struck.

  Next, the upper PC steel material 7 is tensioned by a tension tool (not shown) installed so as to apply a reaction force to the fixing tool 12, and both ends of the upper PC steel material 7 are locked by a fixing tool (not shown) to be fixed to the fixing tool 12. Thus, the tension force applied to the upper PC steel material 7 is applied to both end faces of the concrete girder 10, and prestress is introduced into the upper part of the concrete girder 10. When prestress is introduced into the upper part of the concrete girder 10, the concrete girder 10 is deformed in a direction to return the warping of the center part of the span (the direction of warping down) as indicated by the white arrow. In the present embodiment, the tension force of the upper PC steel material 7 is set so that prestress is introduced in such a size that the center portion of the span is slightly warped compared to immediately after the concrete is placed. Thereafter, the prestress due to the upper PC steel material 7 is maintained for a predetermined period P (FIG. 5, for example, half a year to two years) according to the amount of creep deformation due to the prestress introduced into the lower part of the concrete girder 10. In the present embodiment, creep due to prestress progresses during this predetermined period P, and the amount of warpage (minus warpage) of the PC girder 1 becomes larger than immediately after the introduction of prestress by the upper PC steel material 7.

  After the elapse of the predetermined period P, the prestress due to the upper PC steel material 7 is released, and the fixture 12 is removed as shown in (D). When the prestress by the upper PC steel material 7 is released, the concrete girder 10 is deformed in a direction in which the center portion of the span warps as shown by the white arrow. In the present embodiment, the warping amount at this time is set to be warped (positive warping amount). When the amount of warping is larger than expected and plus, the upper PC steel material 7 may be tensioned again to introduce prestress into the concrete girder 10, and creep due to this prestress may be advanced. If unnecessary, the upper PC steel material 7 is pulled out from the sheath 6 and removed, and the sheath 6 is filled with grout injected into the sheath 6. The PC girder 1 in which the warp due to the creep deformation is suppressed by holding the prestress due to the upper PC steel material 7 over a predetermined period P according to the amount of creep deformation due to the prestress introduced into the lower part of the concrete girder 10 in this way. Manufactured.

  Hereinafter, the behavior of the warp of the PC girder 1 according to the present embodiment will be described with reference to FIG. 5 while comparing with the case where the upper PC steel material 7 is not used. The horizontal axis in FIG. 5 indicates the time point, and the vertical axis indicates the amount of warpage. The horizontal axis indicates a certain point in the manufacturing process at regular intervals, and the distance between the points does not indicate time. Here, the warpage amount will be described with specific numerical values.

Time t ₁ shows a concrete punch設時(after ink設直), since the upper end of the side mold 5b to be Uchidome surface of the concrete is a straight line, the warp amount at this time is 0 mm. Time t ₂ is required for fixing the lower PC steel 3 material age (e.g., one day) after that led to show a prestress introduced immediately before by the lower PC steel 3. Warpage is not possible to change from the time _{t 1,} it is 0mm also at time _{t 2.} In the lower PC steel 3 time t ₃ immediately after solving tensions were prestressed in the lower part of the concrete girder 10, distortion concrete girder 10 by tension of the lower PC steel 3, warpage becomes 18 mm. In addition, since the curvature amount is plus, the concrete girder 10 is in a deformed state so that the central portion of the span is warped up. Creep begins by tension in the lower PC steel 3 from the time t _3.

The time point t ₄ indicates that 7 days have elapsed since the time point t ₃ and immediately before the prestress is introduced by the upper PC steel material 7. In this point t _4, the creep progresses by tension of the lower PC steel 3 during seven days, the amount of warpage becomes 22 mm. Time t ₅ shows immediately prestress introduced by the upper PC steel 7. In this point _{t 5,} strain concrete girder 10 by the upper PC steel 7 tension, warpage becomes -5 mm. Since the warpage amount is negative, the concrete girder 10 is deformed so that the center portion of the span is warped down. Creep begins by the tension of the upper PC steel 7 from the time point t _5.

On the other hand, in the comparative example shown by the broken line, because of not using the upper PC steel 7, warpage of time t ₅ remains 22mm not vary naturally from time t _4. Time will be described above for the behavior of the warp of the comparative example, creep progresses by tension of the lower PC steel 3 from time t _3, time t _4, t ₅ from the predetermined period P (one year in this case) has elapsed At t ₆ (immediately before releasing the prestress by releasing the tension of the lower PC steel 3 relative to the PC girder 1 according to the present invention) and at time t ₇ (just after releasing the prestress by the lower PC steel 3), warping The amount increases to 47mm. At time t ₆ and t ₇ to 30 days is horizontal writing Engineering construction after the time t ₈ has elapsed, deflection digit by the load of the horizontal writing engineering is applied, the amount of warpage becomes 39mm decreases 8 mm, further 30 days in elapsed bridge surface load after application time t _9, the amount of warpage bends digits more becomes 38mm decreases 1 mm. Elapsed from the time _{t 9} is 20 years, the creep completely completion timing t10, warpage becomes 41mm increasing 3 mm.

  On the other hand, in the PC girder 1 according to the present invention, creep (deformation of the concrete girder 10 in the direction in which the center portion of the span warps down) by the tension of the lower PC steel material 3 and the upper PC steel material 7 proceeds from time t5. Then, at a time t6 immediately after the predetermined period P has elapsed from the time t5 and the tension of the lower PC steel material 3 is released and the prestress is released, the amount of warpage becomes −18 mm. At the time t7, which is substantially the same as the time t6 and immediately after releasing the prestress by releasing the tension of the lower PC steel material 3, the concrete girder 10 is distorted again in the direction in which the center portion of the span warps and the warpage amount is 6 mm. become.

  After the time t7, the PC digit 1 shows the same warp behavior as in the comparative example. That is, at the time t8 after the horizontal assembly work after 30 days from the time t7, the PC girder 1 is bent by the load of the horizontal assembly, the warp amount is reduced by 8 mm to -2 mm, and the bridge after 30 days have passed. At time t9 after the surface load is applied, the beam is further bent and the amount of warpage is reduced by 1 mm to -3 mm. At the time t10 when 20 years have elapsed from the time t9 and the creep is completely completed, the amount of warpage increases by 3 mm to 0 mm.

  As described above, in the PC girder 1 according to the embodiment of the present invention, the prestress due to the upper PC steel material 7 is maintained for a predetermined period P according to the amount of creep deformation caused by the prestress introduced in the lower part of the concrete girder 10. Thus, warping due to creep deformation can be suppressed as desired. Further, when prestress is introduced into the lower part of the concrete girder 10, the upper PC steel material 7 may be tensioned using a small tension device, so that the work can be performed in a small space regardless of the ground strength. . Furthermore, the amount of warpage due to creep deformation can be easily adjusted by adjusting the period during which the tension and prestress of the upper PC steel material 7 are maintained.

  In the present embodiment, after the prestress by the upper PC steel material 7 is maintained over a predetermined period P, the prestress by the upper PC steel material 7 is released. Therefore, almost no prestress is introduced into the upper part of the PC girder 1 and the bending strength of the PC girder 1 is not lowered.

  In the present embodiment, before placing the concrete, a sheath 6 is provided that extends in the member axial direction at the upper part of the mold 5 at least in the center portion of the span and reaches the end mold 5c of the concrete girder 10. When the prestress is introduced into the upper part of the concrete girder 10, the tension of the upper PC steel material 7 inserted through the sheath 6 is applied. Therefore, the prestress introduction work to the upper part of the concrete girder 10 is easy.

  In this embodiment, after releasing the pre-stress due to the upper PC steel material 7, the upper PC steel material 7 is removed and the sheath 6 is filled with grout. The reliability of 1 is not lowered.

Although the description about specific embodiment is finished above, PC girder 1 concerning the present invention and its manufacturing method are not limited to the above-mentioned embodiment. For example, in the above embodiment, the PC girder 1 has a constant cross section, but may have a form in which the cross sectional shape changes. Moreover, in the said embodiment, although the sheath 6 is installed in the upper part (inside) of the PC girder 1 and the upper PC steel material 7 is installed in the inside of the sheath 6, the upper PC steel material 7 is pivoted on the upper part of the PC girder 1. If the direction prestress can be introduced, it is not necessary to be installed inside the PC girder 1. In the above embodiment, during a predetermined time period P from the time t ₅ to time t _6, but did not change the tension of the upper PC steel 7, a form for changing the tension of the upper PC steel 7 in the middle You can also. Furthermore, in the said embodiment, although the upper side PC steel material 7 is installed by the post tension system, if there is room in the compressive strength of concrete, you may install by the pre tension system. In addition, the PC girder 1 of the above embodiment is a simple girder made of precast concrete, but may be a form or continuous girder constructed on site. In addition, the specific configuration, shape, arrangement, quantity, material, procedure, and the like of each member and part can be changed as appropriate without departing from the spirit of the present invention. On the other hand, all the elements and procedures shown in the above embodiment are not necessarily essential, and may be appropriately selected.

DESCRIPTION OF SYMBOLS 1 PC girder 2 Support point 3 Lower PC steel 5 Formwork 5c End form 6 Sheath 7 Upper PC steel 10 Concrete girder P Predetermined period

Claims (4)

Placing concrete in a formwork to form a concrete girder;
Introducing pre-stress in the lower part of the concrete girder by applying the tension of the lower PC steel disposed in the lower part of the concrete girder and extending in the axial direction of the member at least in the central part between the diameters of the concrete girder When,
Prestress is applied to the upper part of the concrete girder by applying the tension force of the upper PC steel material disposed above the neutral surface of the concrete girder and extending in the member axial direction at least in the central portion between the diameters of the concrete girder. The steps of introducing
Holding the prestress by the upper PC steel material for a predetermined period according to the amount of creep deformation caused by the prestress introduced into the lower part of the concrete girder.   The method for manufacturing a PC girder according to claim 1, further comprising the step of releasing the prestress caused by the upper PC steel after the prestress caused by the upper PC steel is maintained for the predetermined period. Before placing the concrete, the step of installing a sheath extending in the member axial direction at the upper part in the mold at least in the central part between the diameters of the concrete girders and reaching the wife formwork of the concrete girders In addition,
The method of manufacturing a PC girder according to claim 2, wherein in the step of introducing prestress to the upper part of the concrete girder, a tension force of the upper PC steel material inserted through the sheath is applied.   The method of manufacturing a PC girder according to claim 3, further comprising the step of removing the upper PC steel material after the prestress caused by the upper PC steel material is released and filling the sheath with grout.

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