High-strength shear reinforcement

Abstract

High-strength shear reinforcement bars can be fabricated at low cost. [Solution] Products with a yield strength of 685 MPa or higher are mainly manufactured by two methods. One method involves bending the material into a loop and then closing it by welding. The other method involves wrapping both ends of the loop around the main reinforcement to effectively close it. In this invention, both ends of the loop are processed into U-shaped hooks at the center of the edges and interlocked to close the loop. Due to its high strength, there is no problem of reduced strength or ductility due to welding. Since the U-shape is bent at 180° or more, it will not open even when subjected to yield strength stress. Since only bending is required, it is advantageous in terms of efficiency and processing cost. The increase in material can be kept to a minimum.

Description

Technical Field

[0001] The present invention relates to high-strength shear reinforcement bars for assisting in the heightening and seismic strengthening of reinforced concrete structures.

Background Art

[0002] Shear reinforcement bars are hoop-shaped auxiliary reinforcing bars provided in multiple stages around vertically arranged main reinforcement bars for the purpose of preventing shear failure in reinforced concrete structures. To heighten and seismic-strengthen concrete structures, it is necessary to strengthen the shear reinforcement bars. Dense winding of the reinforcement bars is effective for this purpose, but it has not been put into practical use because it inhibits the fluidity of concrete. Incorporating high-strength shear reinforcement bars maintains the reinforcement bar spacing as before and solves the inflow problem. For steel materials applied to high-strength reinforcement bars, a yield stress of 800 MPa or more is required as opposed to the conventional 300 MPa. Not only strength, but also ductility such as elongation, bending workability, and weldability, which are feared to decrease due to the high strength, are required to be better than before and are standardized to be more stringent than ordinary reinforcing bars.

[0003] To manufacture shear reinforcement bars, usually, a straight bar or wire rod having predetermined mechanical properties is cut to a predetermined length, bent into a square shape, the two ends are butted and welded, and closed to form a hoop. Another method is to bend a similar wire coil into a square shape through a bending machine, and after cutting, weld and close it in the same manner. If the strength is low, welding is not particularly problematic, but in the case of high strength, the strength and ductility in the vicinity of the weld are lower than those of the normal part. New ideas are required for welding.

[0004] Patent Document 1 discloses an improvement in the welding method. According to this, the strength reduction of the welded part is compensated by expanding the cross section. It is necessary to add not only joining but also forming. Moreover, deterioration of the metal structure due to welding also becomes a problem in order to ensure ductility. For this, appropriate heat treatment must be performed by reheating after welding. The work efficiency decreases. Welding involves reliability issues. Weld quality varies greatly depending on the material and working conditions. Moreover, there is no direct way to guarantee quality, and we have to rely on circumstantial evidence.

[0005] As shown in Figures 3, 4, and 9 of Patent Document 2, another method for forming loops is the bending and wrapping method around the main reinforcement, as shown in Figure 3 of this application, which is available on the market. In this case, since there is no welding, there are no problems with ductility even with high-strength reinforcement. The processing is also excellent in terms of work efficiency as it only involves bending with a bending machine. The problem with this method is that the required length of material increases by approximately 10% compared to the circumference. In the welding method described earlier, the increase is limited to a few percent. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Published Patent Application 2005-074436 [Patent Document 2] Published Patent Application Showa 60-253649 [Overview of the project] [Problems that the invention aims to solve]

[0007] In the manufacture of high-strength shear reinforcement bars, welding closures result in a decrease in strength and toughness. As a countermeasure, the cross-section near the joint is enlarged to an appropriate shape to compensate for the strength, while the elongation is handled by the normal section. Welding is prone to quality fluctuations, and often results in insufficient strength in the vicinity. Work efficiency is also low. The method of forming loops by wrapping the material around the main reinforcement does not involve welding, so there is no deterioration of the joints, and it is also advantageous in terms of manufacturing efficiency, but it has the problem of increasing the length of material used. The present invention follows the latter method, eliminating the problem of quality deterioration at the joint by eliminating welding, and aims to solve the problem of reducing the increase in materials used when maintaining a predetermined 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 having a yield strength of 685 MPa or more, characterized in that the wire is bent into a loop of a predetermined length, 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, the entrance dimension of the U-shaped hook is 0.7 to less than 1.0 times the diameter of the wire, the depth of the U-shaped hook is 1.0 to 1.5 times the diameter of the wire, and both U-shaped hooks are joined by linking them at a 90° angle in the straight section of the corner loop. Shear reinforcement bars typically have a square loop shape, but circular loops also exist, and this invention is applicable to circular loops as well. As a definition of the predicate, 'base diameter' refers to the inner diameter of the base of the U-shaped hook. [Effects of the Invention]

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

[0010] The increase in material usage due to the formation of hooks at both ends is small compared to the method of joining to the main reinforcement by bending at the corners of the hoop, because the U-shaped dimension is kept to the minimum necessary, and the yield loss is reduced from approximately 10% to approximately 5%. The yield loss is slightly inferior compared to the welding method. Even with the welding method, precise right-angle cutting is required beforehand to form the correct weld shape, and the reduction in strength due to welding is compensated for by increasing the cross-sectional area. As a result, the required length of material becomes longer than the circumference, resulting in a yield loss of several percent. Although the yield loss of the present invention is slightly inferior to the welding method, it is competitive due to its advantages in processing costs and efficiency.

[0011] The hook part of the present invention is close to its own diameter and has undergone a 180° bending process. The bending standard of the high-strength shear reinforcing bar is more stringent than 180° or 2.0D at 1.5D. It is prone to breakage with an inappropriate metal structure. This bending processing condition supports the quality assurance for each individual product regarding the ductility of the product.

Brief Description of the Drawings

[0012] [Figure 1] Shows the shape of the shear reinforcing bar closed by the hook of the present invention. [Figure 2] Shows the shape of the shear reinforcing bar closed by welding. [Figure 3] Shows the shape of the closed shear reinforcing bar by bending and winding. [Figure 4] Shows the basis for the minimum entrance dimension of the hook part.

Modes for Carrying Out the Invention

[0013] The steel grade of the steel material used for the high-strength shear reinforcing bar of the present invention is basically within the same component range as the current product, in mass %, C, which is the basic component, is 0.10% or more and 0.20% or less, The strengthening and hardenability auxiliary alloy (Si + Mn + Cr) is 2.0% or more and 3.5% or less, <00:00085>The hardenability and toughening alloy (V + Nb + Mo) is 0.05% or more and 0.50% or less, The balance consists of Fe and inevitable impurities. The content of C needs to be 0.1% or more for strength, and 0.2% or less is desirable. If it exceeds 0.2%, welding problems are likely to occur not only during manufacturing but also at the construction site. The required addition amount of the hardenability and toughening alloy depends on the cooling rate after hot rolling, being more in air cooling and less in accelerated cooling. The quasi-high-strength bar of 685 MPa has a small alloy content and is easy to manufacture, and there is also a large amount of it on the market. The present invention targets 685 MPa or more.

[0014] The steel slab of the above steel grade is processed into bars or wires of a predetermined size by hot rolling similar to bars for reinforcing bars or wire rods. The resulting metallographic structure is mainly bainite, with ferrite and pearlite mixed in, and the crystal grains are refined. A yield strength of 785 MPa or more and a high tensile strength of 850 MPa or more can be obtained. If the metallographic structure is appropriate, it can withstand 1D 180° (180° bending with a diameter equal to the wire diameter) bending.

[0015] The shape of the high-strength shear reinforcement of the present invention will be described according to FIG. 1. The shear reinforcement 2 functions 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 FIG. 1, at the middle of the side of the square loop, both ends of the unclosed loop are bent into a U-shaped hook 3 in a direction twisted 90° with respect to each other, and the two ends are crossed 4. The shape of the loop immediately before crossing is preferably such that the outer peripheral surfaces of both hooks are in contact with each other, that is, in a slightly open state. After bending, the two ends are immediately gripped by a robot arm and crossed. After crossing, the shape is stable due to the residual stress tending to open outward.

[0016] The conditions for hook processing are such that the diameter of the bottom of the U-shape is 1.0D or more and 1.5D or less of the wire diameter D. If it is less than D, it will not reach the bottom naturally, and if it exceeds 1.5D, the material usage will increase. The gap at the entrance of the U-shape is 0.7D or more and less than 1.0D. Even if it is less than 1.0D, the two hooks are orthogonal and can be crossed, and once crossed, it is difficult to separate. This is the most basic application of the "wheel of wisdom". The reason for setting it to 0.7 or more is that, as shown in FIG. 4, geometrically, if it is √2 / 2 (≈0.7) or more, crossing is possible. The reason for setting the depth of the U-shape to 1.0 times or more and 1.5 times or less of the diameter of the wire is that if it is less than 1.0, there is a risk of insufficient hook dimensions and insufficient strength, and if it exceeds 1.5 times, there will be waste of the material used.

[0017] When comparing the tensile strength of the interlocking section with that of the straight section, the yield strength is at the same level for both. The fracture strength of the interlocking section varies depending on the conditions, but it is not significantly different from that of 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, yield strength is specified and fracture strength is tested for quality control purposes, but it is not actually necessary for use, so it is not included in the main standards. The bending conditions described above correspond to a bending test of 180° or more with a winding diameter (1.0~1.5) × D. This induces two effects. First, because the bending curvature is large and the material is wound over more than half a turn, bending reversal does not occur below the yield stress under tensile load, meaning that sufficient resistance is maintained. Other than the above, the conditions are harsh for ordinary reinforcing bars, and in this invention, the bendability of the materials used is subjected to rigorous and frequent reliability testing. Therefore, quality assurance is strengthened.

[0018] The formation of the hook increases the required material length, which is approximately 5% of the hoop circumference and never exceeds 10%. While advantageous compared to the wrap-around method, it is slightly inferior to the welding method. However, the cost reduction effect of eliminating welding is a significant advantage. The material cost itself is advantageous when using wire rods because controlled cooling is applied, resulting in a lower amount of expensive alloys. Since straight bars are air-cooled after hot-rolling, the amount of alloy V added increases, resulting in a considerably higher cost. The processing cost is solely due to the bending machine. Efficiency is also significantly better compared to welding.

[0019] Figure 2 shows the shape of the most common shear reinforcement bars on the market. Welding is applied to close the hoop, and 5 is the welded joint. Since it is a high-strength material, the reduction in strength due to welding is a problem. To compensate for the strength, appropriate cross-sectional enlargement is required near the joint. The strength is minimized due to heat-induced changes between the normal and enlarged sections. Appropriate setting of heat treatment conditions after welding is necessary. In this welding hoop, the required material length increases by several percent. Pre-cutting at right angles is necessary to enlarge the cross-section and form an accurate shape, resulting in material loss. The problem with welding methods is that, although they are used in large quantities, the quality assurance of the actual product is somewhat weak due to the welding process. It is guaranteed by sampling tests. Another problem is that the welding time per piece is several times longer than that of bending, which increases the number of welding machines required. This results in a disadvantage in terms of 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 section. Since it only involves bending, it is advantageous in terms of processing costs and efficiency, and there is no problem of quality degradation 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) is 2.5%, hardenability and toughening alloy (V+Nb+Mo) is 0.20% Steel material; 13.0 mm diameter wire rod (applying blast-type controlled cooling) Yield stress: 820 MPa Breaking stress: 910 MPa Processing: 400mm x 500mm loop shape Both ends are bent into a U shape. Bottom diameter 15mm 12mm opening Depth: 16mm There are no problems whatsoever with the materials or processing. [Industrial applicability]

[0022] This invention helps reduce the cost of high-strength shear reinforcement bars, for which demand is growing. [Explanation of Symbols]

[0023] 1; Main reinforcement 2; Shear reinforcement 3; Hook 4; Crossbar 5; Welded joint 6; Wrapped joint

Claims

[Claim 1] A high-strength shear reinforcement bar formed from a hot-rolled wire with a yield strength of 685 MPa or more, characterized in that the wire is bent into a loop of a predetermined length, 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, the entrance dimension of the U-shaped hook is 0.7 to less than 1.0 times the diameter of the wire, the depth of the U-shaped hook is 1.0 to 1.5 times the diameter of the wire, and both U-shaped hooks are joined by linking them at a 90° angle in the straight section of the corner loop.

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