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From Wikipedia, the free encyclopedia

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

A rebar, or reinforcing bar, is a common steel bar, and is commonly used in reinforced concrete and reinforced masonry structures. It is usually formed from carbon steel, and is given ridges for better mechanical anchoring into the concrete. It can also be described as reinforcement or reinforcing steel. In Australia, it is colloquially known as reo.

The resulting reinforced concrete or other material is an example of a composite material.

Contents

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 weren't so, it would cause problems through additional longitudinal and perpendicular stresses at temperatures different than the temperature of the setting.[1] 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. Especially epoxy-coated have to be installed with great care, 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.

Rebar sizes and grades

U.S. Imperial sizes

Imperial bar designations represent the bar diameter in fractions of 18 inch, such that #8 = 88 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 18 inch convention.

Imperial

Bar Size

"Soft"

Metric Size

Weight

(lbft)

Weight

(kg/m)

Nominal Diameter

(in)

Nominal Diameter

(mm)

Nominal Area

(in²)

Nominal Area

(mm²)

#3 #10 0.376 0.561 0.375 = 38 9.525 0.11 71
#4 #13 0.668 0.996 0.500 = 48 12.7 0.20 129
#5 #16 1.043 1.556 0.625 = 58 15.875 0.31 200
#6 #19 1.502 2.24 0.750 = 68 19.05 0.44 284
#7 #22 2.044 3.049 0.875 = 78 22.225 0.60 387
#8 #25 2.670 3.982 1.000 = 88 25.4 0.79 509
#9 #29 3.400 5.071 1.128 28.65 1.00 645
#10 #32 4.303 6.418 1.270 32.26 1.27 819
#11 #36 5.313 7.924 1.410 35.81 1.56 1006
#14 #43 7.650 11.41 1.693 43 2.25 1452
#18 #57 13.60 20.284 2.257 57.33 4.00 2581

Canadian metric sizes

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

Metric

Bar Size

Mass

(kg/m)

Nominal Diameter

(mm)

Cross-Sectional

Area (mm²)

10M 0.785 11.3 100
15M 1.570 16.0 200
20M 2.355 19.5 300
25M 3.925 25.2 500
30M 5.495 29.9 700
35M 7.850 35.7 1000
45M 11.775 43.7 1500
55M 19.625 56.4 2500

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).

Metric

Bar Size

Mass

(kg/m)

Nominal Diameter

(mm)

Cross-Sectional

Area (mm²)

6,0 0.222 6 28.3
8,0 0.395 8 50.3
10,0 0.617 10 78.5
12,0 0.888 12 113
14,0 1.21 14 154
16,0 1.579 16 201
20,0 2.467 20 314
25,0 3.855 25 491
28,0 4.83 28 616
32,0 6.316 32 804
40,0 9.868 40 1257
50,0 15.413 50 1963

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 specification are:[2]

  • ASTM A 615 Deformed and plain carbon-steel bars for concrete reinforcement
  • ASTM A 706 Low-alloy steel deformed and plain bars for concrete reinforcement
  • ASTM A 955 Deformed and plain stainless-steel bars for concrete reinforcement
  • ASTM A 996 Rail-steel and axle-steel deformed bars for concrete reinforcement

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

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. Grade ASTM A706 is suitable for welding without damaging the properties of the steel. 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.

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, phosphorous & carbon content. These are not suitable for welding. To weld rebar you must obtain a mill statement that the reinforcing is suitable for welding.

Rebar 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. [3]

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.

Rebar designation

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.

New Zealand

Designation Explanation
HD-16-300, T&B, EW High strength (500 MPa) 16 mm diameter rebars spaced at 300 mm centers (center-to-center distance) on both the top and bottom face and in each way as well (i.e., longitudinal and transverse).
3-D12 Three mild strength (300 MPa) 12 mm diameter rebars
R8 Stirrups @ 225 MAX D grade (300 MPa) smooth bar stirrups, spaced at 225 mm centres. By default in New Zealand practice all stirrups are normally interpreted as being full, closed, loops. This is a detailing requirement for concrete ductility in seismic zones; If a single strand of stirrup with a hook at each end was required, this would typically be both specified and illustrated.

United States

Designation Explanation
#4 @ 12 OC, T&B, EW Number 4 rebars spaced 12 inches on center (center-to-center distance) on both the top and bottom faces and in each way as well, i.e. longitudinal and transverse.
(3) #4 Three number 4 rebars (usually used when the rebar perpendicular to the detail)
#3 ties @ 9 OC, (2) per set Number 3 rebars used as stirrups, spaced at 9 inches on center. Each set consists of two ties, which is usually illustrated.

See also

References

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

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