High-strength shear reinforcement

Abstract

To process high-strength shear reinforcement bars at low cost. SOLUTION: The product, which has a yield strength of 685 MPa or more, is mainly manufactured by two methods. One is to bend it into a loop shape and then close it by welding. The other is to wrap both ends of the loop around the main reinforcement and essentially close it. In the present invention, both ends of the loop are processed into U-shaped hooks at the center of the sides, interlinked, and closed. Because of its high strength, there is no problem with reduced strength and ductility due to welding. Because the U-shape is bent 180 degrees or more, it will not open even if the stress of the yield strength acts on it. Since it is only a bending process, it is advantageous in terms of efficiency and processing costs. The increase in material is kept to a minimum.

Description

[Technical field]

[0001] The present invention relates to high-strength shear reinforcement bars that assist in building high-rise and earthquake-resistant reinforced concrete structures. [Background technology]

[0002] Shear reinforcement is a type of auxiliary steel bar in the shape of a hoop that is arranged in multiple stages around a forest of main reinforcement bars in order to prevent shear failure in reinforced concrete structures. Strengthening of shear reinforcement is necessary for making concrete structures taller and more earthquake-resistant. Densely packed winding of reinforcement is effective for this purpose, but this is not in practical use because it inhibits the flowability of concrete. By incorporating high-strength shear reinforcement, the spacing between the reinforcement bars can be maintained as before, eliminating the inflow problem. Steel materials used for high-strength reinforcing bars are now required to have a yield stress of at least 800 MPa, up from the conventional 300 MPa. Not only strength, but also ductility and toughness such as elongation, bending workability, and weldability, which are at risk of being reduced due to high strength, are required more than ever before, and are standardized to stricter standards than ordinary reinforcing bars.

[0003] To manufacture shear reinforcement, usually straight rod or wire with the required mechanical properties is cut to length, bent into an angle, and butt-welded at both ends to form a hoop. Another method is to bend a similar coil of wire into an angle through a bending machine, cut it, and weld it closed in the same way. If the strength is low, welding does not pose any particular problems, but if the strength is high, the strength and ductility of the area near the weld will be lower than normal areas, and new welding techniques will be required.

[0004] Patent Document 1 discloses an improvement to the welding method. According to this, the decrease in strength of the welded part is compensated for by enlarging the cross section. It becomes necessary to add forming to the joining process. Furthermore, deterioration of the metal structure due to welding, which also requires maintaining ductility and toughness, is also an issue. To deal with this, appropriate heat treatment must be performed by reheating after welding. This leads to a decrease in work efficiency. Welding involves the issue of reliability. Weld quality varies greatly depending on the material and working conditions. Moreover, there is no direct method of quality assurance, so one must rely on circumstantial evidence.

[0005] As shown in Figures 3, 4, and 9 of Patent Document 2, another method of forming loops is available on the market, which is a method of bending and winding the main reinforcing bars as shown in Figure 3 of the present application. In this case, since there is no welding, there is no problem with ductility and toughness even in high-strength reinforcing bars. Since the processing is only bending with a bending machine, the work efficiency is also excellent. The problem with this method is that the required length of material is about 10% longer than the circumference, whereas with the previously mentioned welding method the increase is only a few percent. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Published Patent Publication 2005-074436 [Patent Document 2] Published Patent Publication No. 60-253649 Summary of the Invention [Problem to be solved by the invention]

[0007] When manufacturing high-strength shear reinforcement bars, closing by welding results in a decrease in strength and toughness, so as a countermeasure, the cross section near the joint is expanded to an appropriate shape to compensate for the strength, and the normal part bears the elongation. Welding is prone to quality fluctuations, and there is often a problem of insufficient strength in the vicinity. Work efficiency is also low. The method of forming loops by wrapping around the main reinforcement bars does not require welding, so there is no deterioration of the joints and it is also advantageous in terms of manufacturing efficiency, but there is a problem that the length of material used increases. The present invention follows the latter method, and aims to eliminate the problem of quality deterioration at the joint by eliminating welding, while reducing the increase in material used when maintaining a specified quality through mechanical joining. [Means for solving the problem]

[0008] The present invention relates to a high-strength shear reinforcement formed from a hot-rolled wire rod having a yield stress of 685 MPa or more, characterized in that the wire rod is bent into a loop of a predetermined length, and U-shaped hooks are formed at both ends of the loop by bending, the bottom diameter of the U-shaped hook is 1.0 to 1.5 times the diameter of the wire rod, the inlet dimension of the U-shaped hook is 0.7 to less than 1.0 times the diameter of the wire rod, and the depth of the U-shaped hook is 1.0 to 1.5 times the diameter of the wire rod, and both U-shaped hooks are linked together by interlinking at an angle of 90° at the straight portions of the square loops. The shape of shear reinforcement is usually a square loop, but circular loops are also available, and the present invention is also applicable to circular loops. By definition, "base diameter" is the inside diameter of the base of the U-shaped hook. Effect of the Invention

[0009] The high-strength shear reinforcement of this invention is made by bending wire into a square loop, forming U-shaped hooks at both ends, and linking them together to form a loop. The connecting edge maintains the same yield strength and elongation as the straight edge. Since it is not welded, there is no deterioration in the mechanical properties of the joint. With welding, the mechanical properties of the joint tend to vary greatly due to variations in the material and welding conditions.

[0010] The increase in material used due to the formation of hooks at both ends is smaller than in the method of joining the main reinforcement by bending the hoop corners, because the U-shaped dimensions are kept to a minimum, and the yield loss is reduced from approximately 10% to approximately 5%. The yield loss is somewhat inferior to the welding method. The welding method also requires a right-angle cut in advance to form an accurate weld shape, and the loss in strength due to welding is compensated for by expanding the cross-sectional area, so the required length of the material is longer than the circumference, resulting in a yield loss of several percent. The yield loss of the present invention is somewhat inferior to the welding method, but it is competitive due to its advantages in processing cost and efficiency.

[0011] The hook of the present invention is close to its own diameter and is bent at 180°. The bending standard for high-strength shear reinforcement is 1.5D, which is stricter than 180° or 2.0D. Inappropriate metal structure makes it easy to break. This bending condition supports the quality assurance of each product regarding the ductility and toughness. [Brief description of the drawings]

[0012] [Figure 1] 1 shows the shape of the shear reinforcement bar that is closed by the hook of the present invention. [Diagram 2] The shape of the shear reinforcement bar closed by welding is shown. [Diagram 3] The shape of the closed shear reinforcement by bending wrapping is shown. [Figure 4] The basis for the minimum dimensions of the hook opening is shown below. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The steel type used in the high-strength shear reinforcement of the present invention is basically in the same range of components as the current product, and in mass%, The basic component C is between 0.10% and 0.20%. Strengthening and hardenability auxiliary alloy (Si+Mn+Cr) is 2.0% or more and 3.5% or less. Hardenability and toughening alloy (V+Nb+Mo) is 0.05% or more and 0.50% or less, The balance consists of Fe and unavoidable impurities. The C content must be at least 0.1% for strength, and preferably not more than 0.2%. If it exceeds 0.2%, problems are likely to occur during welding not only during manufacturing but also at the construction site. The amount of hardenability and toughening alloy required depends on the cooling rate after hot rolling, with more being required for air cooling and less being required for accelerated cooling. Semi-high strength steel of 685MPa has a small amount of alloy and is easy to manufacture, and is also available in large quantities on the market. The present invention is directed to steels of 685MPa or more.

[0014] The billets of the above steel types are processed into bars and wires of the specified dimensions by hot rolling in the same manner as reinforcing steel bars or wire rods. The resulting metal structure is mainly bainite, with ferrite and pearlite mixed in, and the crystal grains are finely refined. The yield strength is over 785MPa as specified, and the high tensile strength is over 850MPa. If the metal structure is appropriate, it can withstand 1D180° bending (180° bending of the same diameter as the wire diameter).

[0015] The shape of the high-strength shear reinforcement of the present invention will be described with reference to Figure 1. The shear reinforcement 2 fulfills its function by surrounding and closing the main reinforcement 1. There are various closing methods, each with its own advantages and disadvantages. In the present invention, as shown in Figure 1, both ends of an unclosed loop in the middle of the sides of a square loop are bent into a U-shaped hook 3 in a direction twisted 90 degrees from each other, and the ends are interlinked 4. The optimal shape of the loop just before interlinking is one in which the outer peripheries of both hooks are in contact with each other, i.e., slightly open. Immediately after bending, both ends are grasped by a robot arm and interlinked. After interlinking, the shape is stable due to residual stress that tries to open outward.

[0016] The condition for hook processing is that the diameter of the bottom of the U-shape must be between 1.0D and 1.5D of the wire rod diameter D. If it is less than D, it will naturally not reach the bottom, and if it exceeds 1.5D, the amount of material used will increase. The gap at the entrance of the U-shape should be 0.7D or more and less than 1.0D. Even if it is less than 1.0D, the hooks are perpendicular to each other, so they can interlock, and once interlocked, they are difficult to separate. This is the most basic application of the "puzzle ring." The reason for making it 0.7 or more is that, as shown in Figure 4, interlocking is possible if it is geometrically √2 / 2 (≒0.7) or more. The reason why the depth of the U-shape is set to 1.0 to 1.5 times the diameter of the wire is that if it is less than 1.0, the hook dimensions will be insufficient and there is a risk of insufficient strength, while if it exceeds 1.5 times, material will be wasted.

[0017] When comparing the tensile test of the linkage with the straight section, the yield strength is at the same level for both. The breaking strength of the linkage varies depending on the conditions, but is not significantly different from the straight section. It is presumed that work hardening of the bent section contributes to this. There seems to be an optimal shape for the hook section, but in reinforced concrete, the yield strength is specified, and although the breaking strength is tested for quality control, it is not actually required for use, so it is not included in the main standards. The above bending conditions correspond to a bending test of 180° or more, which is the winding diameter (1.0 to 1.5) x D. This induces two effects. First, because the bending curvature is large and the winding is more than half a turn, there is no unbending below the yield stress against the tensile load, i.e., sufficient resistance is maintained. The other conditions are strict for ordinary reinforcing bars, so that the bending workability of the material used in the present invention is subject to strict and frequent validation testing, thus enhancing quality assurance.

[0018] The length of material required increases due to the formation of the hooks. It is about 5% of the hoop circumference. It never exceeds 10%. It is more advantageous than the wrapping method, but slightly inferior to the welding method. However, the cost reduction effect of eliminating the need for welding is advantageous. The material cost itself is advantageous in that the amount of expensive alloy is small because controlled cooling is applied when wire is used. Since straight bars are air-cooled after hot rolling, the amount of alloy V added increases, resulting in a significant increase in cost. The only processing cost is the bending machine. The efficiency is also far superior to the welding method.

[0019] Figure 2 shows the shape of shear reinforcement that is most commonly available on the market. Welding is applied to close the hoop, and 5 is the welded part. Since it is a high-strength material, the reduction in strength due to welding is an issue. To compensate for the strength, the area near the joint requires an appropriate cross-sectional expansion. The strength of the area between the normal part and the expanded part is minimal due to the heat effect. It is necessary to set appropriate heat treatment conditions after welding. Even with this welded hoop, the length of the required material increases by several percent. In order to enlarge the cross section and form the shape accurately, it is necessary to cut at right angles in advance, which results in loss. The problem with the welding method is that although it is used in large quantities, the quality assurance of the actual product is somewhat weak because it is welded. It is assured by random sampling tests. Another problem is that the welding work time per piece is several times longer than bending work, so the number of welding machines required increases, resulting in a decrease in efficiency.

[0020] Figure 3 shows a method of closing the hoop by wrapping it around the main reinforcement, which is also available on the market. 6 is the wrapping part. Since it only requires bending, it is advantageous in terms of processing cost and efficiency, and there is no problem of quality deterioration at the joint, but the required material length increases by about 10% of the circumference. EXAMPLES

[0021] Materials used: Steel type: 0.15% C, (Si+Mn+Cr) 2.5%, hardenability and toughening alloy (V+Nb+Mo) 0.20% Steel material: 13.0mm diameter wire (air blast type controlled cooling applied) Yield stress: 820MPa Breaking stress: 910MPa Processing: 400mm x 500mm loop U-shaped bending on both ends Bottom diameter 15mm Entrance 12mm Depth 16mm There are no problems with the materials or processing. [Industrial Applicability]

[0022] The present invention helps reduce the cost of high strength shear reinforcement, which is in growing demand. [Explanation of symbols]

[0023] 1;Main bar 2;Shear reinforcement bar 3;Hook 4;Linkage 5;Welded part 6;Wrapped part

Claims

[Claim 1] A high-strength shear reinforcement formed from a hot-rolled wire rod having a yield stress of 685 MPa or more, characterized in that the wire rod is bent into a loop of a predetermined length, and U-shaped hooks are formed at both ends of the loop by bending, the bottom diameter of the U-shaped hook is 1.0 to 1.5 times the diameter of the wire rod, the inlet dimension of the U-shaped hook is 0.7 to less than 1.0 times the diameter of the wire rod, and the depth of the U-shaped hook is 1.0 to 1.5 times the diameter of the wire rod, and both U-shaped hooks are linked together by interlinking at an angle of 90° at the straight portion of the square loop.

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