Rebar

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A tied rebar beam cage. This will be embedded inside of cast concrete to lend it strength. A bit of rust on the rebar actually increases the surface area and bonds well with concrete. The high pH level of the concrete will minimise further rust damage to the rebar. In cases where too much concrete poison is dissolving the cement stone of the concrete, the rebar is then also no longer protected and rust can be seen to creep through cracks in faulty concrete. This becomes particularly apparent in bridge structures, such as the Gardiner Expressway, and parking garages. Common countermeasures include epoxy coating of the rebar, keeping a slight electrical current running through the rebar and the choice of cement types that have greater sulphate resistance.

Rebar, a portmanteau for reinforcing bar, is common steel bar, an important component of reinforced concrete and reinforced masonry structures. It is usually formed from carbon steel, and is given ridges for better frictional adhesion to the concrete.

[edit] Use in concrete and masonry

Concrete is a material that is very strong in compression, but virtually without strength 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.

[edit] Physical characteristics

Although rebar has ridges that bind it mechanically to the concrete with friction, 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, 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 steel, making it susceptible to rusting. As rust takes up greater volume than the iron or 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 rebar or stainless steel rebar may be employed in these situations at greater initial expense, but significantly lower expense over the service life of the project. Fiber-reinforced polymer rebar is now also being used in high-corrosion environments.

A tied rebar beam cage.

Rebars in detail (top) atop angle iron (bottom).

Rebar placement for foundation and walls of a sewage pump station.

Two coils of common rebar.

[edit] Welding

Most grades of steel used in rebar are suitable for welding, which can be used to bind several pieces of rebar together. However, welding can reduce the fatigue life of the rebar, and as a result rebar cages are normally tied together with wire.

[edit] 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.

[edit] Rebar sizes and grades

[edit] US Imperial sizes

Imperial bar designations represent the bar diameter in fractions of ⅛ inch, such that #8 = ⁸⁄₈ inch = 1 inch diameter.

[edit] US Metric sizes

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

[edit] European Metric 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).

[edit] Grades

Historically in Europe, rebar comprised mild steel material with a yield strength of approximately 250 N/mm². Modern rebar comprises 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 against earthquakes for example.

[edit] See also

• Fusion bonded epoxy coating for coated rebars
• Dowel
• Concrete cover
• Reinforced concrete
• Steel fixer

[edit] External links

Wikimedia Commons has media related to:

Reinforcement

• Stainless rebar information
• OSHA Rebar Impalement Protection Measures