JP3383097B2 - Braiding method of PC structure reinforcement - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for braiding a reinforcing body of a prestressed concrete (PC) structure mainly in the civil engineering and construction industry, and in particular, welding of heat-treated PC steel rods and spiral bars constituting the reinforcing body. The present invention relates to a joining method.

[0002]

2. Description of the Related Art As an example of a method for manufacturing a PC structure, PC
The method of manufacturing concrete poles and piles using steel rods is described. As shown in FIG. 2, a mild steel wire 2 is spirally wound around a PC steel rod 1 arranged in parallel on the circumference (hereinafter referred to as a spiral muscle 2), and then an intersection 3 of the PC steel rod and the spiral muscle is joined. Then, a cylindrical basket-shaped reinforcing body (hereinafter abbreviated as a reinforcing body) is manufactured. Next, this reinforcing body is introduced into a form, both ends of the PC steel rod constituting the reinforcing body are fixed, and tension is applied with a stress of about 70% of the tensile strength. After the concrete poured into the mold is solidified, the tension of the PC steel rod is removed, and at the same time a compressive force is applied to the concrete to complete the PC pole and pile. During this manufacturing process, the joining of the PC steel rod and the spiral bar is performed by electric resistance welding in order to automate the assembly of the reinforcing body.

The heat-treated PC steel rod constituting the reinforcing body is manufactured by subjecting a medium-low carbon steel wire rod containing approximately 0.15 to 0.35% by weight of carbon to heat treatment such as quenching and tempering or boiling water quenching. . Compared with PC steel wire produced by cold drawing of piano wire, it has a lower carbon content and can be welded, and is therefore widely used as a reinforcement body braided by electric resistance welding.

By the way, in a PC steel rod used under a high tension force, there is a risk of delayed fracture when hydrogen penetrates inside, and there is a particular concern that delayed fracture occurs in a welded portion. .

FIG. 3 shows a conceptual diagram of welding. Spiral muscle 2
And the PC steel rod 1 are crossed and overlapped, and sandwiched between the upper electrode 4 in contact with the spiral muscle and the lower electrode 5 in contact with the PC steel rod, pressurizing and energizing heats the contact portion between the spiral muscle and the PC steel rod. , Welded to each other.

In the welding for forming a minute heat-affected zone such as electric resistance welding used for braiding a reinforcing body, the heat-affected zone once melted or austenitized is rapidly cooled because the heat input amount is small, and it is harder than the base metal. Will rise significantly. While the hardness of the base material is about Hv450,
The hardness of the heat-affected zone rises to Hv550 to 600, and the delayed fracture susceptibility becomes high.

Further, a similar heat-affected zone called a back spot is formed also in a portion where the PC steel rod 1 contacts the lower electrode 5. These highly hardened heat-affected zones produced by welding are likely to be the starting point of delayed fracture, and there is a strong demand for improvement of delayed fracture characteristics in this area.

In response to the demand for improving the delayed fracture property of the welded portion 3 of the PC steel rod 1, various improving methods have been proposed so far. For example, Japanese Patent Publication No. 3-75325 discloses a method in which a welding machine is provided with a tempering electrode in addition to the welding electrode, and the welded portion is tempered immediately after welding by electric heating. However, it is not widely used because it is difficult to secure a sufficient temperature and time for softening, and it is difficult to control in a temperature range where solution is not re-solutionized.

Delayed fracture is generally considered to occur when hydrogen is present under the action of tensile stress. The above example is an invention from the viewpoint of controlling the structure of the welded portion to improve the delayed fracture characteristics. On the other hand, attempts have been made to control the tensile stress and improve the delayed fracture characteristics. Since the delayed fracture crack is generated from the surface layer and propagates, the delayed fracture characteristics can be improved by reducing the tensile stress acting on the surface layer when the steel material is strained.

In order to reduce the tensile stress acting on the surface layer portion, it is effective to give a compressive residual stress to the surface layer in advance, and the applicant of the present invention has previously proposed JP-A-5-287440. PC with proper shot peening
It is shown that the delayed fracture properties of steel bars can be improved. However, if welding is performed after shot peening, the compressive residual stress introduced into the surface layer in the vicinity of the heat-affected zone of the weld may decrease, and the effect of improving delayed fracture characteristics may also decrease.

[0011]

SUMMARY OF THE INVENTION The present invention does not control the structure, composition, etc. of the welded portion, but applies prestress by optimizing the heat affected zone generation position by the creation of a new welding method. It is intended to provide a method for manufacturing a PC structure reinforcing body which reduces a tensile stress acting on a welded portion when tensioned and does not deteriorate delayed fracture characteristics of a PC steel rod.

[0012]

In the invention of the present invention, when a reinforcing body braided by welding is tensioned, a PC
Various studies were conducted on means for reducing the tensile stress acting on the welded part of the steel rod. As a result, when joining the helical bar and the PC steel rod, applying tensile stress to the PC steel rod and performing welding under tensile strain, the reinforcement body is inserted into the formwork and re-tensioned, and then acts on the welded portion. It has been found that the tensile stress that occurs is reduced and the delayed fracture characteristics are improved. However, in order to apply a tensile stress to a PC steel rod with a large diameter by a uniaxial tensile load, it is necessary to add a load of several tons to several tens of tons per wire, and it is predicted that the equipment will be large-scale. It

In order to apply a tensile strain to the PC steel rod, a bending stress may be applied by providing an appropriate fulcrum in addition to applying a uniaxial tensile load. By this principle, a tensile force is applied with a small force. it can. However, since the back side becomes a compressive strain, the usual spot welding method in which the intersection point between the spiral bar and the PC steel rod is sandwiched between the upper and lower electrodes as shown in FIG.

In order to solve this problem, not only the helical strain is joined to the tensile strain side, but also the electrode in contact with the PC steel bar is brought into contact with the tensile strain side so that the heat affected zone is not formed on the compression side. The present invention has been completed.

That is, according to the present invention, when a PC steel rod and a spiral bar are joined by electric resistance welding to braid a reinforcement for a PC structure, a tensile strain due to bending stress is applied to the surface of the PC steel rod. A method for braiding a PC structure reinforcing body, characterized in that the helical muscle and the electrode are contacted only on the side where tensile strain is generated, and the electric resistance welding is performed without contacting the electrode on the compression strain side. .

[0016]

According to the present invention, the conventional PC steel rod is used, but the delayed fracture characteristics of the PC steel rod constituting the reinforcement body by the welding method based on a new idea for joining the spiral bar and the PC steel rod when braiding the reinforcement body. Is to improve dramatically. The mechanism is as follows.

It is generally said that tensile residual stress is generated around the welded portion. On the other hand, when welding is performed under tensile strain, if the tensile strain is removed after the welding is completed, the base metal around the welded part that was elastically deformed compresses into the welded part as it tries to return to its original state. Stress acts. Even if a prestress is added to such a PC steel rod as a reinforcing material for the PC structure, the tensile stress acting on the welded portion is reduced and the occurrence of delayed fracture is suppressed.

Further, as a result of diligent examination of the appropriate amount of tensile strain applied to the PC steel rod by bending during welding, no improvement effect was observed if the amount of tensile strain on the surface was less than 0.05%, and 0.7 If the value exceeds%, PC after welding
It was found that the bending of the steel bar became large and hindered the subsequent manufacturing process of the reinforcing body. Therefore, the amount of tensile strain added by bending stress during welding is 0.05% on the surface.
It is desirable to be 0.7% or less.

As described above, a bending stress smaller than the uniaxial tensile load causes a tensile stress only in the joining side portion of the PC steel bar, and further causes a heat-affected zone having a high temperature only in the tensile stress generating side portion. By abutting the electrodes in this manner, the tensile residual stress due to welding is significantly reduced.

[0020]

Example Diameter of steel having the chemical composition shown in Table 1 is 10 m
JIS G manufactured by cold-drawing the m hot-rolled wire and subsequently quenching and tempering by induction heating.
The effect of the present invention was confirmed by using a PC steel rod having a wire diameter of 9.2 mm corresponding to variant D class 3109. The quenching heating temperature was 950 ° C and the tempering temperature was 400 ° C.

Table 2 shows the mechanical properties of this PC steel bar.
The yield stress is 0.2% proof stress, and the value is 1440.
It was MPa. The evaluation distance in the elongation measurement in the tensile test was set to 8 times the wire diameter, and the drawing was expressed as a percentage of the actual cross-sectional area of the fractured material with respect to the nominal cross-sectional area.

[0022]

[Table 1]

[0023]

[Table 2]

Using the above-mentioned PC steel rod, a state in which tensile strain due to bending stress is added as an example of the present invention (symbols A to
J, L to N) and as a comparative example, the reinforcing body was assembled under normal conditions (symbol K in Table 3) in which no tensile strain was applied. The PC steel rod sampled from this reinforcement was subjected to a delayed fracture fracture test.

[0025]

[Table 3]

Note in Table 3: Note: The delayed fracture test was not conducted because the PC steel bar after welding had a large bend.

A conventional automatic braiding machine was used for assembling the reinforcing body, and the spiral bar 2 was wound around the PC steel bar 1 arranged in a cylindrical shape, and spot welding was performed at the intersection of the PC steel bar and the spiral bar. At this time, in order to bend the PC steel rod, pressure was applied from the support 5 to the PC steel rod 1 at the intersection 9 using the mechanism shown in FIG. 1 to apply bending stress, and the spiral muscle 2 was brought into contact with the tensile strain side. . Further, a rubber sheet 8 is provided between the lower electrode 5 and the PC steel rod 1 to insulate the upper second electrode 7 in place of the lower electrode 5 at a position very close to the upper first electrode 6 of the helical strain 2 on the tensile strain side. Were contacted.

As comparative examples, the insulating rubber sheet was removed and the lower electrode was brought into contact with the compression strain side for welding (symbol N in Table 3), and the ordinary welding shown in FIG. 3 was performed (Table 3). The symbol K) was also evaluated. Welding conditions are selected from the industrially usual range, target value of welding current is 3000 A, energization time is 0.04 seconds, and pressure applied to upper first electrode 6 and second electrode 7 is 410 N each. did. The spiral muscle used is 3.2 mm wire diameter, JIS standard SWRM6
Is a mild steel wire equivalent to. The amount of bending strain was measured by attaching a strain gauge to the surface of the PC steel bar in the portion in contact with the spiral muscle 2, and peeling and welding after the measurement.

The delayed fracture test is carried out by the FIP method at 50 ° C.
In a 20 wt% NH ₄ SCN aqueous solution of 995MP for a PC steel rod with a helical streak remaining about 20 mm.
The tensile stress of a was applied and the breaking time was measured. If the test did not break for 50 hours or more, the test was terminated at that point.
The results are shown in Table 3. All of the examples of the present invention in which tensile strain due to bending stress was added to the PC steel bar during welding, and the amount of tensile strain on the surface was 0.05 to 0.7%, did not break within 50 hours, and compared with the comparative example. The delayed fracture characteristics have been dramatically improved.

In Comparative Examples I and J, the tensile strain due to bending stress was too large, so that the PC steel rod after welding bent and hindered the subsequent steps, so that the delayed fracture test was not carried out. Comparative Examples K to M have insufficient tensile strain due to bending,
The effect was insufficient. Further, in Comparative Example N, since the lower electrode was in contact with the compression side of the PC steel rod, the heat-affected zone was formed on the compression side, and the delayed fracture property was not improved.

[0031]

As is apparent from the above examples, the use of the present invention significantly improves the delayed fracture characteristics of the welded portion, which has been a weak point of the reinforcing body of the PC structure braided by conventional welding. , The reliability and safety of civil engineering buildings will be improved.
As described above, the present invention remarkably improves delayed fracture characteristics by adding an improvement to the welding method while using ordinary industrial materials, and significantly improves the performance of concrete structures used in large amounts in the civil engineering and construction industry. To improve.

[Brief description of drawings]

FIG. 1 is a schematic diagram of tensile strain addition and welding due to bending stress.

FIG. 2 is a schematic diagram of a reinforcement.

FIG. 3 is a conceptual diagram of welding of a PC steel rod and a spiral bar.

[Explanation of symbols]

1 PC steel rod 2 helix muscle 3 welds 4 Upper electrode 5 Lower electrode 6 Upper first electrode 7 Upper second electrode 8 rubber sheets 9 fulcrums

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Claims (2)

(57) [Claims] 1. When braiding a reinforcement for a PC structure by joining a PC steel rod and a spiral bar by electric resistance welding, a tensile strain due to bending stress is applied to the surface of the PC steel rod,
A method for braiding a PC structure reinforcing body, which comprises contacting a spiral muscle and an electrode only on a side where tensile strain is generated, and performing electric resistance welding without contacting an electrode on a compression strain side. 2. The method according to claim 1, wherein the amount of tensile strain due to bending stress applied to the surface of the PC steel rod is 0.05 to 0.7%.