Difference between revisions of "Rebar"

Latest revision as of 09:22, 20 September 2010

A tied rebar beam cage. This will be embedded inside cast concrete to increase the tensile strength of the concrete.

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2008)

A rebar (short for reinforcing bar), also known as reinforcing steel, reinforcement steel, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression. It is usually formed from carbon steel, and is given ridges for better mechanical anchoring into the concrete. In Australia, it is colloquially known as reo.

History

Rebars inside the Leaning Tower of Nevyansk.

Rebars were known in construction well before the era of the modern reinforced concrete. Some 150 years before its invention rebars were used to form the carcass of the Leaning Tower of Nevyansk in Russia, built on the orders of the industrialist Akinfiy Demidov. The purpose of such construction is one of the many mysteries of the tower. The cast iron used for rebars was of very high quality, and there is no corrosion on them up to this day. The carcass of the tower was connected to its cast iron tented roof, crowned with what might have been the first lightning rod in the Western world. This lightning rod was grounded through the carcass, though it is not clear whether the effect was intentional.^[1]

Use in concrete and masonry

Concrete is a material that is very strong in compression, but relatively weak in tension. To compensate for this imbalance in concrete's behavior, rebar is cast into it to carry the tensile loads.

Masonry structures and the mortar holding them together have similar properties to concrete and also have a limited ability to carry tensile loads. Some standard masonry units like blocks and bricks are made with strategically placed voids to accommodate rebar, which is then secured in place with grout. This combination is known as reinforced masonry.

While any material with sufficient tensile strength could conceivably be used to reinforce concrete, steel and concrete have similar coefficients of thermal expansion: a concrete structural member reinforced with steel will experience minimal stress as a result of differential expansions of the two interconnected materials caused by temperature changes.

Physical characteristics

Steel has an expansion coefficient nearly equal to that of modern concrete. If this were not so, it would cause problems through additional longitudinal and perpendicular stresses at temperatures different than the temperature of the setting.^[2] Although rebar has ribs that bind it mechanically to the concrete, it can still be pulled out of the concrete under high stresses, an occurrence that often precedes a larger-scale collapse of the structure. To prevent such a failure, rebar is either deeply embedded into adjacent structural members (40-60 times the diameter), or bent and hooked at the ends to lock it around the concrete and other rebar. This first approach increases the friction locking the bar into place, while the second makes use of the high compressive strength of concrete.

Common rebar is made of unfinished tempered steel, making it susceptible to rusting. Normally the concrete cover is able to provide a pH value higher than 12 avoiding the corrosion reaction. Too little concrete cover can compromise this guard through carbonation from the surface. Too much concrete cover can cause bigger crack widths which also compromises the local guard. As rust takes up greater volume than the steel from which it was formed, it causes severe internal pressure on the surrounding concrete, leading to cracking, spalling, and ultimately, structural failure. This is a particular problem where the concrete is exposed to salt water, as in bridges built in areas where salt is applied to roadways in winter, or in marine applications. Epoxy-coated, galvanized or stainless steel rebars may be employed in these situations at greater initial expense, but significantly lower expense over the service life of the project. Special care must be taken during the installation of epoxy-coated rebar, because even small cracks and failures in the coating can lead to intensified local chemical reactions not visible at the surface.

Fiber-reinforced polymer rebar is now also being used in high-corrosion environments. It is available in many forms, from spirals for reinforcing columns, to the common rod, to meshes and many other forms. Most commercially available rebars are made from unidirectional glassfibre reinforced thermoset resins.

Sizes and grades

U.S. sizes

Imperial bar designations represent the bar diameter in fractions of ⅛ inch, such that #8 = 8⁄8 inch = 1 inch diameter. Area = (bar size/9)² such that area of #8 = (8/9)² = 0.79 in². This applies to #8 bars and smaller. Larger bars have a slightly larger diameter than the one computed using the ⅛ inch convention.

Canadian sizes

Metric bar designations represent the nominal bar diameter in millimeters, rounded to the nearest 5 mm.

European sizes

Metric bar designations represent the nominal bar diameter in millimetres. Bars in Europe will be specified to comply with the standard EN 10080 (awaiting introduction as of early 2007), although various national standards still remain in force (e.g. BS 4449 in the United Kingdom).

Grades

Rebar is available in different grades and specifications that vary in yield strength, ultimate tensile strength, chemical composition, and percentage of elongation.

The grade designation is equal to the minimum yield strength of the bar in ksi (1000 psi) for example grade 60 rebar has a minimum yield strength of 60 ksi. Rebar is typically manufactured in grades 40, 60, and 75.

Common ASTM specification are:^[3]

• ASTM A82: Specification for Plain Steel Wire for Concrete Reinforcement
• ASTM A184/A184M: Specification for Fabricated Deformed Steel Bar Mats for Concrete Reinforcement
• ASTM A185: Specification for Welded Plain Steel Wire Fabric for Concrete Reinforcement
• ASTM A496: Specification for Deformed Steel Wire for Concrete Reinforcement
• ASTM A497: Specification for Welded Deformed Steel Wire Fabric for Concrete Reinforcement
• ASTM A615/A615M: Deformed and plain carbon-steel bars for concrete reinforcement
• ASTM A616/A616M: Specification for Rail-Steel Deformed and Plain Bars for Concrete Reinforcement
• ASTM A617/A617M: Specification for Axle-Steel Deformed and Plain Bars for Concrete Reinforcement
• ASTM A706/A706M: Low-alloy steel deformed and plain bars for concrete reinforcement
• ASTM A767/A767M: Specification for Zinc-Coated(Galvanized) Steel Bars for Concrete Reinforcement
• ASTM A775/A775M: Specification for Epoxy-Coated Reinforcing Steel Bars
• ASTM A934/A934M: Specification for Epoxy-Coated Prefabricated Steel Reinforcing Bars
• ASTM A955: Deformed and plain stainless-steel bars for concrete reinforcement
• ASTM A996: Rail-steel and axle-steel deformed bars for concrete reinforcement

ASTM marking designations are:

• 'S' billet A615
• 'I' rail A616
• 'IR' Rail Meeting Supplementary Requirements S1 A616
• 'A' Axle A617
• 'W' Low-alloy — A706

Historically in Europe, rebar is composed of mild steel material with a yield strength of approximately 250 N/mm². Modern rebar is composed of high-yield steel, with a yield strength more typically 500 N/mm². Rebar can be supplied with various grades of ductility, with the more ductile steel capable of absorbing considerably greater energy when deformed - this can be of use in design to resist the forces from earthquakes for example.

Placing rebar

Rebar cages are fabricated either on or off the project site commonly with the help of hydraulic benders and shears, however for small or custom work a tool known as a Hickey - or hand rebar bender, is sufficient. The rebars are placed by rodbusters or concrete reinforcing ironworkers with bar supports separating the rebar from the concrete forms to establish concrete cover and ensure that proper embedment is achieved. The rebars in the cages are connected by welding or tying wires. For epoxy coated or galvanised rebars only the latter is possible.

Welding

The American Welding Society (AWS) D 1.4 sets out the practices for welding rebar in the U.S. Without special consideration the only rebar that is ready to weld is W grade (Low-alloy — A706). In the US, most rebar is not suitable for welding. ASTM A 616 & ASTM A 617 reinforcing are re-rolled rail steel & re-rolled rail axle steel with uncontrolled chemistry, phosphorus & carbon content. To weld rebar you must obtain a mill statement that the reinforcing is suitable for welding.^{[citation needed]}

Welding can reduce the fatigue life of the rebar, and as a result rebar cages are normally tied together with wire. Besides fatigue concerns welding rebar has become less common in developed countries due to the high labor costs of certified welders. Steel for prestressed concrete may absolutely not be welded.

Couplers

When welding or wire-tying rebar is impractical or uneconomical a mechanical connection or rebar coupler can be used to connect two or more bars together. These couplers are popular in precast concrete construction at the joints between members and to reduce rebar congestion in highly reinforced areas.

A full mechanical connection is achieved when the bars connected develop in tension or compression a minimum of 125% of the yield strength of the bar.^[4]

Safety

To prevent workers and / or pedestrians from accidentally impaling themselves, the protruding ends of steel rebar are often bent over or covered with special steel-reinforced plastic "plate" caps. "Mushroom" caps may provide protection from scratches and other minor injuries, but provide little to no protection from impalement.

Designations

For clarity, reinforcement is usually tabulated in a "reinforcement schedule" on construction drawings. This eliminates ambiguity in the various notations used in different parts of the world. The following list provides examples of the different notations used in the architectural, engineering, and construction industry.

See also

References

External links

• Stainless rebar information
• OSHA Rebar Impalement Protection Measures
• Concrete Reinforcing Steel Institute
• American Concrete Institutear:حديد التسليح

bg:Армировка de:Bewehrungsstahl eo:Armaturo fa:میل‌گرد fr:Armature (technique) it:Armatura (edilizia) nl:Wapening ja:鉄筋 no:Armering pl:Zbrojenie ru:Арматура (железобетон) fi:Raudoitus sv:Armeringsstål

1. ↑ The office of the first Russian oligarch (Russian)
2. ↑ GFRP Bar Transverse Coefficient of Thermal Expansion Effects on Concrete Cover
3. ↑ American Concrete Institute: "Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary," ISBN 978-0-87031-264-9
4. ↑ CRSI: 'Manual of Standard Practice', 8-2, 1998.