Rebar (short for reinforcing bar)refers to the steel used for reinforced concrete and prestressed reinforced concrete. Its cross-section is circular and sometimes square with rounded corners. Including plain round reinforcement, ribbed reinforcement, and torsion reinforcement.
Reinforcement for reinforced concrete refers to a straight bar or coiled bar steel used for reinforcement of reinforced concrete. Its shape is divided into smooth round reinforcement and deformed reinforcement, and the delivery status is a straight bar and coiled bar.
Plain round steel bar is actually small round steel and round steel of ordinary low carbon steel. Deformed reinforcement is the reinforcement with ribs on the surface, usually with 2 longitudinal ribs and transverse ribs evenly distributed along the length direction. The shape of the transverse rib is spiral, herringbone, and crescent. Expressed in millimeters of nominal diameter. The nominal diameter of deformed reinforcement is equivalent to the nominal diameter of plain round reinforcement with equal cross-section. The nominal diameter of reinforcement is 8-50mm, and the recommended diameters are 8, 12, 16, 20, 25, 32, and 40mm. Steel grades: 20MnSi, 20mnv, 25mnsi, bs20mnsi. Reinforcement mainly bears tensile stress in concrete. Due to the action of ribs, deformed reinforcement has great bonding capacity with concrete, so it can better bear the action of external forces. Steel bars are widely used in various building structures. Especially large, heavy, light thin-walled, and high-rise building structures.
There are many kinds of reinforcement, which are usually classified according to chemical composition, production process, rolling shape, supply form, diameter, and use in structure:
◆ divided by diameter
Steel wire (diameter 3 ~ 5mm), fine reinforcement (diameter 6 ~ 10mm), thick reinforcement (diameter greater than 22mm).
◆ according to mechanical properties
Grade I reinforcement (grade 300 / 420); Grade II reinforcement (grade 335 / 455); Grade III reinforcement (400 / 540) and grade IV reinforcement (500 / 630)
◆ according to production process
Hot rolled, cold rolled and cold drawn steel bars, as well as heat-treated steel bars made of grade IV steel bars, have higher strength than the former.
◆ according to the function in the structure
Compression reinforcement, tension reinforcement, erection reinforcement, distribution reinforcement, stirrup, etc.
The mechanical properties of reinforcement shall comply with the following table: brand, nominal diameter mm σ s
Total elongation of reinforcement under maximum force δ GT not less than 2.5%. If the supplier can guarantee, the inspection may not be carried out.
According to the requirements of the demander, reinforcement meeting the following conditions can be supplied:
The ratio of measured tensile strength to the measured yield point of reinforcement shall not be less than 1.25;
The ratio of the measured yield point of reinforcement to the minimum yield point specified in the above table shall not be greater than 1.30.
The mechanical properties of steel bars are measured through tests. The mechanical properties of steel bar quality standards include yield point, tensile strength, elongation, cold bending performance, and so on.
◆ yield point (FY)
When the stress of the reinforcement exceeds the yield point, the tensile force does not increase but the deformation increases significantly, which will produce large residual deformation, the tensile value per unit area of the reinforcement obtained by dividing the tensile value by the cross-sectional area of the reinforcement is the yield point σ s°
◆ tensile strength (Fu)
Tensile strength is the tensile value obtained by dividing the maximum tensile force that the reinforcement can bear before being pulled off by the cross-sectional area of the reinforcement. Tensile strength is also known as ultimate strength. It is the largest stress value in the stress-strain curve. Although it has no direct significance in strength calculation, it is an essential guarantee item in the mechanical properties of reinforcement. Because:
Tensile strength is the ultimate capacity of reinforcement to bear the static load. It can indicate how much strength reserve reinforcement has after reaching the yield point. It is an important index to resist plastic failure.
The defects in the melting and rolling process of reinforcement and the instability of the chemical composition content of reinforcement are often reflected in the tensile strength. When the carbon content is too high and the temperature is too low at the end of rolling, the tensile strength may be very high; When the carbon content is low and there are too many non-metallic inclusions in the steel, the tensile strength is low.
The tensile strength has a direct impact on the ability of the reinforced concrete structures to resist the repeated load.
Elongation is the maximum strain in the stress-strain curve when the specimen is broken, also known as elongation. It is an index to measure the plasticity of reinforcement. Like tensile strength, it is also an essential guarantee item in the mechanical properties of reinforcement.
The calculation of elongation is the percentage of the length of the stretched part to the original length when the reinforcement breaks under tension. Put together the two broken sections of the test piece to measure the gauge length L1 after fracture, and subtract the original gauge length l0 to obtain the plastic deformation value. The ratio of this value to the original length is δ Is the elongation δ The higher the value, the better the plasticity of the steel. The elongation is related to the gauge distance. For the gauge distance or hot-rolled reinforcement, the length of 10 times the diameter of the test piece is taken as the measurement standard, and the elongation is expressed in δ 10 indicates. For the steel wire, the gauge length of 100mm shall be taken as the standard for the survey and inspection δ 100 indicates. For strand δ 200。
◆ cold bending performance
Cold bending performance refers to the resistance of steel bars to cracks when plastic deformation occurs after cold processing (i.e. processing at room temperature). A cold bending test is a test to determine the bending deformation capacity of reinforcement at room temperature. During the test, the stress shall not be considered, but the reinforcement specimen with diameter D shall be bent to 180 ° or 90 ° around the bending center with diameter D (1D, 3D, 4D, and 5D are specified in D). Then check whether the reinforcement sample has cracks, scales, fractures, and other phenomena to identify whether its quality meets the requirements. The cold bending test is a more strict inspection, which can reveal the defects such as the uneven internal structure of reinforcement.
◆ bending performance
After the bending center diameter specified in the following table is bent for 180 degrees, there shall be no cracks on the surface of the bending part of the reinforcement. Brand nominal diameter a
◆ reverse bending performance
According to the requirements of the demander, the reinforcement can be tested for reverse bending performance.
The bending center diameter of the reverse bending test is correspondingly increased by one reinforcement diameter compared with the bending test. Bend 45 degrees in the forward direction, then 23 degrees in the reverse direction, and then 23 degrees in the reverse direction. After the reverse bending test, there shall be no crack on the surface of the bending part of the reinforcement.
Reinforcement processing generally goes through four processes: reinforcement derusting; Reinforcement straightening; Reinforcement cutting; Reinforcement forming.
When the reinforcement joint adopts a straight thread or conical thread connection, the reinforcement end upsetting, and thread processing procedures shall be added. Reinforcement batching and replacement
◆ when replacing the reinforcement specified in the design document with another steel grade or diameter, the following provisions shall be observed:
It shall be carried out according to the principle of the equal design value of reinforcement bearing capacity. After reinforcement replacement, it shall meet the specified structural requirements such as reinforcement spacing, anchorage length, and minimum reinforcement diameter.
When replacing the lower reinforcement with the higher reinforcement, the method of changing the diameter of the reinforcement should be adopted instead of changing the number of reinforcement to reduce the cross-sectional area of the reinforcement.
◆ when a diameter reinforcement of the same steel grade is used to replace another diameter reinforcement, the diameter variation range shall not exceed 4mm, and the ratio of the total cross-sectional area of the changed reinforcement to the cross-sectional area specified in the design document shall not be less than 98% or greater than 103%.
◆ when the designed main reinforcement is replaced with reinforcement of the same steel grade, the spacing shall remain unchanged, and two types of reinforcement with a diameter one level larger and one level smaller than the design reinforcement diameter can be used for spacing replacement.
In many countries, after demolishing a concrete structure, workers are asked to remove the rebar. They search the site and extract the metal using wire cutters, welding equipment, sledge hammers and other tools. The metal is partially straightened, bundled and sold.
Rebar, like almost all metal products, can be recycled as scrap. It is usually combined with other steel products, melted and reshaped.