JP5541493B2 - Hybrid FRP reinforcement and prestress introduction method - Google Patents

Description

  The present invention relates to a hybrid FRP reinforcing bar having a hybrid structure made of composite fibers and a prestress introduction method using the hybrid FRP reinforcing bar.

  Conventionally, Nefmac (registered trademark) is known as an FRP material (FRP reinforcing bar) for concrete reinforcement (see, for example, Non-Patent Document 1). Nefmac is a single-piece high-performance continuous fiber such as glass fiber, carbon fiber, or aramid fiber that is integrally molded in a grid while impregnating a resin with excellent chemical resistance such as thermosetting resin vinyl ester. is there.

  On the other hand, a thin-walled precast formwork and a precast concrete plate used therefor are known (for example, see Patent Document 1). The precast concrete slab shown in Patent Document 1 is obtained by integrally introducing a prestressed reinforcing material therein. As this prestressed reinforcing material, it is shown that carbon fiber and glass fiber can be used in addition to PC steel wire.

JP-A-8-68107

AGC Matex Co., Ltd., “FRP Concrete Reinforcement Nefmac”, [online], [Search May 25, 2009], Internet <URL: http://www.agm.co.jp/doboku/nfm.html>

  By the way, when the above-mentioned conventional FRP reinforcing bar is used for a concrete floor slab such as a runway, there is a possibility that a large crack may be generated in the tensile region, so measures such as introducing pretension on the tensile region side are necessary. It becomes.

  The present invention has been made in view of the above, and an object of the present invention is to provide a hybrid FRP reinforcing bar capable of effectively introducing prestress and a prestress introduction method using the hybrid FRP reinforcing bar. To do.

In order to solve the above-described problems and achieve the object, the hybrid FRP reinforcing bar according to claim 1 of the present invention is a composite of at least carbon fiber and glass fiber having different linear expansion coefficients , and a bundle of carbon fibers. A FRP reinforcing bar having a hybrid structure composed of a rod-like body in which glass fiber bundles are laminated vertically and integrated .

In the prestress introduction method according to claim 2 of the present invention, a hybrid FRP reinforcing bar having a hybrid structure composed of a rod-shaped body by combining at least two kinds of fibers having different linear expansion coefficients is embedded in a hardened cured body. The FRP reinforcing bars are heated and the hardened body is hardened, and then the heating to the FRP reinforcing bars is stopped and prestress is introduced into the hardened body .

Also, the prestress introduction method according to claim 3 of the present invention embeds a hybrid FRP reinforcing bar having a hybrid structure composed of a rod-shaped body composed of at least carbon fiber and glass fiber having different linear expansion coefficients in a hardened body that can be cured. Then, the FRP reinforcing bar is heated and the cured body is cured, and then the heating to the FRP reinforcing bar is stopped and prestress is introduced into the cured body .

Further, the prestress introduction method according to claim 4 of the present invention is a composite comprising at least carbon fiber and glass fiber having different linear expansion coefficients, and a bundle of carbon fibers covered with glass fiber and integrated. A hybrid FRP reinforcing bar having a hybrid structure is embedded in a hardened cured body, the FRP reinforcing bar is heated and the cured body is cured, and then the heating to the FRP reinforcing bar is stopped to prestress the cured body. It is characterized by introducing .

Further, the prestress introduction method according to claim 5 of the present invention is a composite of at least carbon fibers and glass fibers having different linear expansion coefficients, and a stack of carbon fibers and a bundle of glass fibers are vertically stacked and integrated. The hybrid FRP reinforcing bar of the hybrid structure made of the rod-shaped body is embedded in a hardened cured body, the FRP reinforcing bar is heated and the cured body is cured, and then the heating to the FRP reinforcing bar is stopped and cured. It is characterized by introducing prestress into the body.

In addition, the prestress introduction method according to claim 6 of the present invention is characterized in that, in any one of claims 2 to 5 described above, the heating energizes an FRP reinforcing bar.

  According to the hybrid FRP reinforcing bar of the present invention, since it is a FRP reinforcing bar having a hybrid structure composed of a rod-like body by combining at least two kinds of fibers having different linear expansion coefficients, if the rod-like body is subjected to a temperature change, the linear expansion Fibers with a high rate will expand (shrink), while fibers with a low coefficient of linear expansion will not expand (shrink) too much. For this reason, the expansion force (shrinkage force) of the fiber having a high linear expansion coefficient applies tension to the fiber having a low linear expansion coefficient. Therefore, if the temperature change given to the rod-shaped body is removed, the prestress due to the rod-shaped body can be effectively introduced into concrete or the like. Further, when the hybrid FRP reinforcing bar of the present invention is used for frozen soil reinforcement, it is possible to prevent the fixing portion from slipping out due to the shape change of the rod-shaped body.

FIG. 1 is a view showing an embodiment of a hybrid FRP reinforcing bar according to the present invention, in which (a) is a side view and (b) is a cross-sectional view. FIG. 2 is a cross-sectional view showing another example of the hybrid FRP reinforcing bar according to the present invention. FIG. 3 is a perspective view of step 1 of the prestress introduction method according to the present invention. FIG. 4 is a perspective view of step 2 of the prestress introduction method according to the present invention. FIG. 5 is a perspective view of step 3 of the prestress introduction method according to the present invention. FIG. 6 is a perspective view of step 4 of the prestress introduction method according to the present invention. FIG. 7 is a perspective view of step 5 of the prestress introduction method according to the present invention. FIG. 8 is a perspective view of step 6 of the prestress introduction method according to the present invention. FIG. 9 is a perspective view of step 7 of the prestress introduction method according to the present invention. FIG. 10 is a view showing a bending test when the hybrid FRP reinforcing bar according to the present invention is applied to frozen soil reinforcement. 11A and 11B are diagrams showing a bending test in the case where a conventional FRP reinforcing bar is applied to frozen soil reinforcement. FIG. 11A shows a state at the start of the test, FIG. 11B shows an initial deformation state, and FIG. It is a figure of time.

  Embodiments of a hybrid FRP reinforcing bar and a prestress introduction method according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

  As shown in FIGS. 1A and 1B, a hybrid FRP reinforcing bar 10 according to the present invention is an FRP reinforcing bar having a hybrid structure formed by combining carbon fibers 12 and glass fibers 14 having greatly different linear expansion coefficients. The bundle of fibers 12 and 14 is composed of a rod-like body 10a integrally formed by bonding a bundle of fibers 12 and 14 in a radial direction and an extending direction with a thermosetting resin. A bundle of carbon fibers 12 is spirally wound around a bundle of glass fibers 14. In this form, a temperature change is given to the rod-shaped body 10a to generate a shape change. In the case of FIG. 1, the carbon fiber and the glass fiber may not be bonded and integrated simply by spirally winding the carbon fiber around the glass fiber.

The glass fiber 14 is a non-conductive fiber, has a high linear expansion coefficient, and expands when heated.
The carbon fiber 12 is a conductive fiber, has a low linear expansion coefficient, and hardly expands as compared with the glass fiber 14 even when heated, but generates heat when energized.

  The rod-shaped body 10a is not limited to the spirally wound structure of FIG. 1, and as shown in FIG. 2 (a), a cross-sectional structure in which a bundle of carbon fibers 12 is covered with glass fibers 14 and bonded and integrated, or FIG. As shown in b), a cross-sectional structure in which a bundle of carbon fibers 12, a bundle of mixed fibers 16 of carbon fibers and glass fibers, and a bundle of glass fibers 14 are stacked and bonded together can be formed. . Moreover, it is good also as an integrated cross-section structure as shown in FIG.2 (c). In each form of FIG. 2, when a temperature change is applied to the rod-shaped body 10a, an internal stress and a shape change occur. Note that the cross-sectional structure of FIG. 2B is not preferable for introducing prestress into a concrete slab or the like because it is bent and deformed downward when energized.

[When applied to concrete slabs]
Next, the case where the hybrid FRP reinforcing bar 10 according to the present invention is applied to a concrete floor slab (hardened body) will be described with reference to the manufacturing procedure of FIGS.

  First, as shown in FIG. 3, in step 1, side molds 20 are installed on the bottom steel plate 18 in four directions, and spacers 22 are arranged in the molds 20. A support jig 24 is installed on the outer peripheral side of the mold 20.

  Next, in step 2, as shown in FIG. 4, the hybrid FRP reinforcing bars (lower bars) 10 are arranged in a lattice pattern in the mold 20. In this case, the hybrid FRP reinforcing bar 10 is placed around the support jig 24 and inserted into the plurality of grooves 20a on the upper edge of the mold 20. The hybrid FRP reinforcing bar 10 may be incorporated in the mold 20 in a lattice shape.

  Subsequently, in step 3, as shown in FIG. 5, in order to secure the effective height of the upper and lower hybrid reinforcing bars, the reinforcing frame 26 is installed on the hybrid FRP reinforcing bar 10 in the mold 20.

  Subsequently, in step 4, as shown in FIG. 6, hybrid FRP reinforcing bars (upper bars) 28 are arranged in a grid pattern on the reinforcing frame 26 in the mold frame 20 in the same manner as the lower bars 10. In the support jig 24, a bar-shaped spacer 30 is disposed between the upper bar 28 and the lower bar 10.

  Subsequently, in step 5, as shown in FIG. 7, water is sprayed on the mold 20 and the hybrid FRP reinforcing bars (upper bars 28, lower bars 10) to cool them. A spacer 32 may be disposed above the hybrid FRP reinforcing bar (upper bar) 28.

  Subsequently, in step 6, as shown in FIG. 8, concrete 34 as a hardener is placed in the mold 20. During the curing period until the concrete 34 is completely cured, the electrode 36 is connected to the hybrid FRP reinforcing bar (lower bar) 10 on the pulling region side (lower side in this case) and energized. In addition, the end part of the hybrid FRP reinforcing bar (lower bar) 10 is changed from the support jig 24 to the fixture 38 of the bottom steel plate 18 and is strained.

  When the electrode 36 is connected and energized, the hybrid FRP reinforcing bar (lower bar) 10 is heated via the conductive carbon fiber 12. By this heating, the glass fiber 14 of the hybrid FRP reinforcing bar (lower bar) 10 expands in temperature. On the other hand, the carbon fiber 12 hardly expands. When the hybrid FRP reinforcing bar is in the form shown in FIG. 2, the carbon fiber and the glass fiber are integrated by being aligned in parallel with the extending direction of the reinforcing bar, so that a tensile force (pre-stress) is applied to the carbon fiber by energizing and heating the carbon fiber. ), On the contrary, a compression force (prestress) is applied to the glass fiber. In this way, the hybrid FRP reinforcing bars (lower bars) 10 are energized and heated for curing until the concrete 34 develops strength capable of holding the prestressing force.

  As the heating temperature in this case, for example, the heating temperature can be 60 ° C. with respect to the room temperature of 10 ° C. The concrete 34 is kept at room temperature by suppressing temperature rise by watering curing or the like. And after concrete hardening, electricity supply is stopped and it returns to normal temperature, but the concrete 34 hardly shrinks.

  In this state where the FRP fiber is restrained by the hardened concrete, a tensile force is further applied to the carbon fiber due to a decrease in temperature, and a compressive force is released to the glass fiber to apply a tensile force. Along with this, a compressive force is exerted on the side of the tensile region for the concrete slab. Thereby, the prestressed concrete slab reinforced by the FRP reinforcing bars 10 can be constructed without a tensioning process or a grout process for introducing prestress. Note that if the heating temperature of the carbon fiber 12 is too high, the concrete 34 may harden rapidly, which may adversely affect the quality of the concrete 34. Therefore, the outer periphery of the rod-shaped body 10a of the hybrid FRP reinforcing bar 10 is covered with a resin. This may be prevented.

  Finally, in step 7, as shown in FIG. 9, the mold is removed, and the end of the hybrid FRP reinforcing bar (lower bar) 10 is cut. In this way, it is possible to produce a concrete slab in which prestress is introduced on the tensile region side.

  Incidentally, in the case where the FRP reinforcing bar in which the carbon fiber of FIG. 1 is spirally wound on the glass fiber is applied to the concrete floor slab, the glass fiber is allowed to expand when the carbon fiber is energized, and the temperature is lowered when the energization is stopped. Therefore, the shrinkage of the glass fiber is restricted (restraint effect) by hardening the concrete. Therefore, after the concrete is hardened, a tensile force (prestress) is applied to the glass fiber, and a compressive force (prestress) is applied to the tensile region side of the concrete floor slab accordingly.

  In the present embodiment, the lower floor hybrid FRP reinforcing bar 10 is energized in the concrete floor slab. However, the upper floor hybrid FRP reinforcing bar 28 is also energized to give prestress to the entire concrete floor slab. It may be.

[When applied to frozen soil reinforcement (reference example)]
Next, a case where the hybrid FRP reinforcing bar 100 according to the present invention is applied to frozen soil reinforcement will be described with reference to FIGS. 10 and 2B.

  As shown in FIG. 2B, the hybrid FRP reinforcing bar 100 when applied to frozen soil reinforcement includes a bundle of carbon fibers 12, a mixed bundle 16 of carbon fibers 12 and glass fibers 14, and a bundle of glass fibers 14 from above. It is configured by stacking up and down three layers.

  The hybrid FRP reinforcing bars 100 have a lattice shape, and are arranged above and below the frozen soil specimen 40 as shown in FIG. An FRP stirrup (hoop muscle) 42 and an FRP frame 44 are arranged between the upper and lower sides. Support points A and B are provided at two locations on the lower surface of the frozen soil specimen 40, and load application points C and D are provided at two locations on the upper surface. Pressurization point spacers 46 are provided on the fulcrums A and B in the frozen soil specimen 40 and on the upper surface side.

  As shown in FIG. 10 (b), when the frozen soil specimen 40 is cooled and frozen and hardened, the glass fiber 14 layer of the hybrid FRP reinforcing bar 100 shrinks greatly, while the carbon fiber 12 layer hardly shrinks. . For this reason, the hybrid FRP reinforcing bar 100 changes its shape in the frozen soil specimen 40 so as to warp toward the glass fiber 14 (convex shape upward). In this case, the curvature that curves in a convex shape on the fixing portion 48 at the left and right end portions becomes large.

  As shown in FIG. 10 (c), when an overload is applied to the load application points C and D in this state and the bending test is started, the hybrid FRP reinforcing bar 100 of the present invention is fixed by a convex warp upward. By causing the portion 48 to bite into the frozen soil 40, it is possible to sufficiently resist the loading load and to prevent the fixing portion 48 from coming off. Further, peeling does not occur at the cover portion 50 of the lower hybrid FRP reinforcing bar 100. Furthermore, by restraining the hybrid FRP reinforcing bar 100 in the frozen soil 40 by the FRP frame 44, the spacer 46, and the FRP stirrup 42, a sufficient reinforcing effect can be exhibited. Thus, according to the hybrid FRP reinforcing bar of the present invention, the frozen soil can be effectively reinforced, so that the thickness of the frozen soil in the vertical direction can be significantly reduced.

[Examination of temperature stress]
Next, the examination of the temperature stress using the hybrid FRP reinforcing bar of the present invention will be described below in comparison with the case of reinforced concrete. Table 1 shows the examination conditions, and shows physical property values (thermal expansion coefficient α, cross-sectional area A, Young's modulus E) of concrete, reinforcing steel (steel), carbon fiber bundle, and glass fiber bundle. It is.

The shrinkage strain of concrete is 150 × 10 ⁻⁶ . The long side length of the concrete is 7.875 m. Also, Note 1 in Table 1 is the area _{A s} of the reinforcing bars arranged D19 to two upper and lower stages of the concrete 100mm _intervals, is _{A s = 2 × 2.865 / 0.1} = 57.3cm 2 . Note 2 is the area _Af of the carbon fiber bundle, and the hybrid FRP reinforcing bars are formed in a lattice shape with 100 mm intervals composed of the carbon fibers C13 and the two layers of glass fibers G13. A _f = 2 × 0.65 / 0.1 = 13 cm ^{2 because} it is arranged on the stage. Also, note 3 is the area _{A g} of the glass fiber _bundles, it is _{A g = 4 × 1.31 / 0.1} = 52.4cm 2.

Assuming a temperature change of 50 ° C. due to energization heating under the above examination conditions, the tension σ _T generated in the carbon fiber bundle of the hybrid FRP reinforcing bar is
σ _T = 50 ° C × 52.4 × (11 × 10 ⁻⁶ −0.025 × 10 ⁻⁶ ) × 3.0 × 10 ⁻⁵ /13.0=664 kgf / cm ²
It becomes. This tension remains in the carbon fiber bundle even after the end of the hybrid FRP reinforcing bar protruding from the concrete is cut after the concrete is hardened.

[For winter construction]
First, Table 2 and Table 3 show the results of a comparison between the case of reinforced concrete and the case of hybrid FRP reinforced concrete regarding the degree of stress generated in the concrete in the winter construction (long-side direction) as a study example of temperature stress. This assumes January as the concrete placement time. For example, the average temperature in January is 5.6 ° C in Yokohama. Table 2 shows that the concrete temperature changes from 5.6 ° C to 50 ° C (when the highest temperature occurs), and Table 3 shows that the concrete temperature changes from 5.6 ° C to -10 ° C (when the lowest temperature occurs). Shows the case.

Here, the stress sigma _s occur rebar in concrete, the stress sigma _sc that calculated by the strain × Young's modulus E, occur concrete, are calculated by _{σ s × A s / A c} . Further, the stress sigma _f generated in the hybrid FRP reinforcement in a hybrid FRP reinforcement concrete, the stress sigma _hc that calculated by the strain × Young's modulus E, generated in the concrete is calculated in σ _f × _{A f} / _{A c} is there.

[For summer construction]
Next, Table 4 and Table 5 show the results of comparison between the case of reinforced concrete and the case of hybrid FRP reinforced concrete regarding the degree of stress generated in the concrete in the summer construction (long side direction) as a study example of temperature stress. This assumes August as the concrete placement time. The average temperature in August is 26.4 ° C in Yokohama, for example. Table 4 shows that the concrete temperature changes from 26.4 ° C to 50 ° C (when the highest temperature occurs), and Table 5 shows that the concrete temperature changes from 26.4 ° C to -10 ° C (when the lowest temperature occurs). Shows the case.

[In case of mass concrete]
Next, Table 6 and Table 7 show the results of a comparison between the case of reinforced concrete and the case of hybrid FRP reinforced concrete regarding the degree of stress generated in the concrete (long-side direction) in mass concrete construction as a study example of temperature stress. This assumes that the internal temperature of the concrete at the time of placing is 50 ° C. Table 6 shows that when the concrete temperature changes from 50 ° C. to 50 ° C. (when the highest temperature occurs) (that is, no change), Table 7 shows that the concrete temperature changes from 50 ° C. to −10 ° C. (when the lowest temperature occurs) The case where it changed is shown.

[In the case of concrete using the hybrid FRP reinforcing bar of the present invention]
Next, Table 8 and Table 9 show the results of comparison between the case of reinforced concrete and the case of hybrid FRP reinforced concrete regarding the degree of stress generated in the concrete (long side direction) using the hybrid FRP reinforcing bar of the present invention. This assumes that the temperature at the time of fixation of the hybrid FRP reinforcement before concrete placement is 20 degreeC. Table 8 shows the case where the concrete temperature changes from 20 ° C. to 50 ° C. (when the highest temperature occurs), and Table 9 shows the case where the concrete temperature changes from 20 ° C. to −10 ° C. (when the lowest temperature occurs). ing.

Tables 10 and 11 summarize the results of the examination of the degree of stress generated in concrete in Tables 2 to 9 above. In Table 10, Case 1 is based on Tables 2 and 3, Case 2 is based on Tables 4 and 5, and Case 3 is based on Tables 6 and 7. In Table 11, Case 4 is based on Tables 8 and 9. Case 4a shows the result of tensioning the FRP reinforcement of Case 4 at 450 kgf / cm ² .

  As shown in Table 10 and Table 11, when reinforced concrete is used as the original design proposal and concrete using the hybrid FRP reinforcing bar of the present invention is used as the modified design proposal, tensile stress is generated in the reinforced concrete in most cases. I understand that. On the other hand, in the concrete using the hybrid FRP reinforcing bar of the present invention, it is expected that tensile stress is generated in the concrete when the temperature is lowered.

In this case, when the increase stress during fiber expansion during energization heating is −293 kgf / cm ² at the maximum and the stress when cracks of about 0.5 mm occur is −2630 kgf / cm ² , the initial tension The possible stress may be about 3600−2630−293 = 677 kgf / cm ² . Therefore, in this case, it is understood that prestressing of about 650 kgf / cm ² is necessary for the hybrid FRP reinforcing bar.

  As described above, according to the hybrid FRP reinforcing bar of the present invention, since it is a FRP reinforcing bar having a hybrid structure composed of a rod-shaped body by combining at least two types of fibers having different linear expansion coefficients, the temperature change in the rod-shaped body , Fibers having a high linear expansion coefficient expand (shrink), while fibers having a low linear expansion coefficient do not expand (shrink) so much. For this reason, the expansion force (shrinkage force) of the fiber having a high linear expansion coefficient applies tension to the fiber having a low linear expansion coefficient. Therefore, if the temperature change given to the rod-shaped body is removed, the prestress due to the rod-shaped body can be effectively introduced into concrete or the like. Further, when the hybrid FRP reinforcing bar of the present invention is used for frozen soil reinforcement, it is possible to prevent the fixing portion from slipping out due to the shape change of the rod-shaped body.

  As described above, according to the present invention, a carbon fiber and glass fiber having different linear expansion coefficients are combined to form a rod-like body, and a temperature change is applied to the rod-like body to generate internal stress in the rod-like body. The shape can be changed. For this reason, prestress can be introduced into the FRP reinforcing bars by electric heating in concrete reinforcement, and a structure having excellent durability can be constructed.

  Moreover, it can also be used for the structure which uses the hybrid FRP reinforcement of this invention only for a short period. In this case, the tension is temporarily generated by energizing and heating the hybrid FRP reinforcing bar, and if it is used as a structure for a short period of time and then the energization is stopped and the temperature is lowered, the tension is released, so it is easily destroyed. Can be removed. For this reason, there is a possibility of application to a temporary structure or the like that requires early removal and requires weight reduction.

  As described above, the hybrid FRP reinforcing bar and the prestress introduction method according to the present invention are useful for concrete reinforcement and frozen soil reinforcement, and are particularly suitable for introducing prestress into concrete using the FRP reinforcing bar. .

10 Hybrid FRP reinforcement (lower)
DESCRIPTION OF SYMBOLS 10a Bar-shaped body 12 Carbon fiber 14 Glass fiber 16 Mixed bundle 18 Bottom steel plate 20 Side formwork 20a Groove 22,32 Spacer 24 Support jig 26 Reinforcement frame 28 Hybrid FRP reinforcement (upper)
30 Bar-shaped spacer 34 Concrete 36 Electrode 38 Fixture 40 Frozen earth specimen 42 FRP stirrup 44 FRP frame 46 Pressure point spacer 48 Fixing part 50 Covering part 100 Hybrid FRP reinforcement

Claims (6)

A hybrid FRP reinforcing bar having a hybrid structure composed of a rod-like body formed by combining at least carbon fibers and glass fibers having different linear expansion coefficients, and stacking and integrating a bundle of carbon fibers and a bundle of glass fibers .   A hybrid FRP reinforcing bar having a hybrid structure made of a rod-shaped body by combining at least two types of fibers having different linear expansion coefficients is embedded in a hardened cured body, and the cured body is cured while heating the FRP reinforcing bar, The prestress introduction method characterized by stopping the heating to the FRP reinforcing bars and introducing prestress into the hardened body.   A hybrid FRP reinforcing bar having a hybrid structure composed of a rod-shaped body composed of at least carbon fibers and glass fibers having different linear expansion coefficients is embedded in a hardened cured body, and the cured body is cured while heating the FRP reinforcing bar, Thereafter, heating to the FRP reinforcing bars is stopped and prestress is introduced into the cured body.   A hybrid FRP reinforcing bar of a hybrid structure consisting of a rod-shaped body composed of a composite of at least carbon fiber and glass fiber having different linear expansion coefficients and covered with a glass fiber bundle is embedded in a hardened cured body. The pre-stress introduction method is characterized in that the FRP reinforcing bars are heated and the hardened body is hardened, and then heating to the FRP reinforcing bars is stopped and prestress is introduced into the hardened body. A hybrid FRP reinforcing bar of a hybrid structure composed of a rod-like body composed of a composite of at least carbon fibers and glass fibers having different linear expansion coefficients, and a stack of carbon fibers and glass fibers stacked one above the other can be cured. A prestress introduction method characterized by being embedded in a hardened body, heating the FRP reinforcing bar and hardening the hardened body, and then stopping heating to the FRP reinforcing bar and introducing prestress into the hardened body . The prestress introduction method according to any one of claims 2 to 5 , wherein the heating energizes an FRP reinforcing bar .

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