Rebar

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A tied rebar beam cage. This will be embedded inside cast concrete to increase its tensile strength.
Corroded concrete and rebar at bridge of Queen Elizabeth Way crossing Welland River in Niagara Falls, Ontario.

Rebar (short for reinforcing bar), also known as reinforcing steel and reinforcement steel,[1] is a common steel bar or mesh of steel wires commonly used as a tension device in reinforced concrete and reinforced masonry structures, to strengthen and hold the concrete in compression. The surface of the rebar may be patterned to form a better bond with the concrete.

History[edit]

Rebars inside the Leaning Tower of Nevyansk.

Originally concrete structures were unreinforced (e.g. dome of the Pantheon, Rome, still standing since ~126 AD). Rebars have been used in construction since at least the 18th Century; for example, 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 cast iron used for the rebars was of high quality, and there is no corrosion on them to this day. The carcass of the tower was connected to its cast iron tented roof, crowned with one of the first known lightning rods.[2] More recently these techniques have been successively refined by embedding the steel bars in the concrete, and by the introduction of deformed bars to improve bonding, thus producing modern reinforced concrete.

Use in concrete and masonry[edit]

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. Most steel reinforcement is conceptually divided into primary and secondary reinforcement, but there are other minor uses:

  • Primary reinforcement refers to the steel which is employed specifically to guarantee the necessary resistance needed by the structure as a whole to support the design loads.
  • Secondary reinforcement, also known as distribution or thermal reinforcement, is employed for durability and aesthetic reasons, by providing enough localized resistance to limit cracking and resist stresses caused by effects such as temperature changes and shrinkage.
  • It is also employed to confer resistance to concentrated loads by providing enough localized resistance and stiffness for a load to spread through a wider area.
  • Rebar may also be used to hold other steel bars in the correct position to accommodate their loads.
  • External steel tie bars can constrain and reinforce masonry structures, as illustrated by the Nevyansk Tower or several ancient structures in Rome and the Vatican.

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 potentially be used to reinforce concrete (glass and basalt fibers are also common), 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. Steel is also widely available at low cost, and relatively easy to work with.

Physical characteristics[edit]

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 from the temperature of the setting.[3] 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 accompanies 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, and salt penetration. 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 phenomenon is known as oxide jacking. This is a particular problem where the concrete is exposed to salt water, as in bridges where salt is applied to roadways in winter, or in marine applications. Uncoated, corrosion-resistant low carbon/chromium (microcomposite), 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. Care should be taken during the transport, fabrication, handling, installation, and concrete placement process when working with epoxy-coated rebar, because damage will reduce the long-term corrosion resistance of these bars. Users must take appropriate steps to inspect bars after placement and repair any defects found in the epoxy coating.[4] Even damaged bars have shown better performance than uncoated reinforcing bars, though issues from debonding of the epoxy coating from the bars and corrosion under the epoxy film have been reported.[5] These bars are used in over 70,000 bridge decks in the USA. Epoxy Interest Group. "Epoxy Interest Group of CRSI". Epoxy Interest Group of CRSI. Retrieved 24 August 2012. 

Fiber-reinforced polymer rebar is 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 glass fibre reinforced thermoset resins.

Reinforcing steel can also be displaced by impacts such as earthquakes, resulting in structural failure. The prime example of this is the collapse of the Cypress Street Viaduct in Oakland, California as a result of the 1989 Loma Prieta earthquake, causing 42 fatalities. The shaking of the earthquake caused rebars to burst from the concrete and buckle. Updated building designs, including more circumferential rebars, can address this type of failure.

Sizes and grades[edit]

U.S. sizes[edit]

Imperial bar sizes give the diameter in units of ⅛ inch, so that #8 = 88 inch = 1 inch diameter. The cross sectional area, as given by πr², works out to (bar size/9.027)², which is conveniently approximated as (bar size/9)² square inches. For example, the area of #8 bar is (8/9)² = 0.79 sq.inch.

Larger bar sizes are based on the cross-sectional area of square bars that were formerly used. The diameter of the equivalent round shapes is rounded to the nearest ⅛ inch to provide the bar size. For example, #9 bar has a cross section of 1.00 sq. inch, and therefore a diameter of 1.128 inches. #10, #11, #14, and #18 sizes correspond to 118 inch, 114, 112, and 2 inch square bars, respectively.[6] #14 rebar is particularly affected by this approximation; by diameter it would be #13.5.

The tower and sign industries commonly use slightly larger "Jumbo" bars #14J and #18J as anchor rods for large structures.[7] The bars are fabricated from slightly oversized blanks such that threads can be cut at the ends to accept standard 1.75" (#14J) and 2.25" (#18J) anchor nuts.

An example of steel reinforcement bars with color codes indicating the grade.
U.S. rebar size chart
Imperial

Bar Size

"Soft"

Metric Size

Mass per unit length Nominal Diameter Nominal Area
lbft (kg/m) (in) (mm) (in²) (mm²)
#2 #6 0.167 0.249 0.250 = 14 6.35 0.05 32
#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 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 57.3 4.00 2581
#18J 14.60 21.775 2.337 59.4 4.29 2678

Canadian sizes[edit]

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

Metric

Bar Size

Mass per unit length

(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 sizes[edit]

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). In Switzerland some sizes are different from European Standard.

Various sizes and types of steel reinforcement in storage.
Metric

Bar Size

Mass per unit length

(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

Australian sizes[edit]

Reinforcement for use in concrete construction is subject to the requirements of Australian Standards AS3600-2009 (Concrete Structures) and AS/NZS4671-2001 (Steel Reinforcing for Concrete). There are other standards that also apply to testing, welding and galvanising. The designation of reinforcement is defined in AS/NZS4671-2001 using the following formats:

Bars[edit]

Shape Code Yield Strength (MPa) Ductility Class Nominal Diameter (mm)
D - Deformed bar 500 MPa N - Normal 10
R - Round bar 250 MPa L - Low 12
16
20
24
28
32
36
40
Examples:
D500N12 is deformed bar, 500 MPa strength, normal ductility and 12 mm nominal diameter - also known simply as "N12"
R250N20 is round bar, 250 MPa strength, normal ductility and 20 mm nominal diameter - also known simply as "R20"

Bars are typically abbreviated to simply 'N' (hot-rolled deformed bar), 'R' (hot-rolled round bar), 'RW' (cold-drawn ribbed wire) or 'W' (cold-drawn round wire), as the yield strength and ductility class can be implied from the shape. For example, all commercially available wire has a yield strength of 500 MPa and low ductility, round bars are 250 MPa and normal ductility, etc.

Mesh[edit]

Shape Code Ductility Class Nominal Diameter (mm) Nominal Spacing (long direction) Nominal Spacing (short direction)
(only required in rectangular mesh)
S - Square L - Low Ductility 7 mm 1 – 100 mm 1 – 100 mm
R - Rectangular 8 mm 2 – 200 mm 2 – 200 mm
10 mm 8 – 80 mm 8 – 80 mm
12 mm

Grades[edit]

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:American Concrete Institute: "Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary," ISBN 978-0-87031-744-6

  • 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 A1035: Standard Specification for Deformed and Plain, Low-carbon, Chromium, Steel Bars for Concrete Reinforcement

ASTM marking designations are:

Historically in Europe, rebar is composed of mild steel material with a yield strength of approximately 250 MPa (36 ksi). Modern rebar is composed of high-yield steel, with a yield strength more typically 500 MPa (72.5 ksi). Rebar can be supplied with various grades of ductility. The more ductile steel is capable of absorbing considerably more energy when deformed - a behavior that resists earthquake forces and is used in design.

Placing rebar[edit]

A view of steel wire commonly used to secure rebar before concrete is poured over it.

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 steel fixers "rodbusters" or concrete reinforcing ironworkers, with bar supports and concrete or plastic rebar spacers separating the rebar from the concrete formwork to establish concrete cover and ensure that proper embedment is achieved. The rebars in the cages are connected either by spot welding, tying steel wire, sometimes using an electric rebar tier,[8] or with mechanical connections. For tying epoxy coated or galvanised rebars, epoxy coated or galvanized wire is normally used.

Welding[edit]

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). Rebar that is not produced to the ASTM A706 specification is generally not suitable for welding without calculating the "carbon-equivalent". Material with a carbon-equivalent of less than 0.55 can be welded. (AWS D1.4)

ASTM A 616 & ASTM A 617 reinforcing are re-rolled rail steel & re-rolled rail axle steel with uncontrolled chemistry, phosphorus & carbon content. These materials are not common.

Rebar cages are normally tied together with wire, although spot welding of cages has been the norm in Europe for many years, and is becoming more common in the US. High strength steels for prestressed concrete may absolutely not be welded.[citation needed]

Mechanical connections[edit]

Also known as "mechanical couplers" or "mechanical splices", mechanical connections are used to connect reinforcing bars together. Mechanical couplers are an effective means to reduce rebar congestion in highly reinforced areas for cast-in-place concrete construction. These couplers are also used in precast concrete construction at the joints between members.

The structural performance criteria for mechanical connections varies considerably between different countries, codes, and industries. As a minimum requirement, codes typically specify that the rebar to splice connection meets or exceeds 125% of the specified yield strength of the rebar. More stringent criteria also requires the development of the specified ultimate strength of the rebar. As an example, ACI 318 specifies either Type 1 (125% Fy) or Type 2 (125% Fy and 100% Fu) performance criteria.[9]

Rebars with safety caps installed before being covered in concrete.

For concrete structures designed with ductility in mind, it is recommended that the mechanical connections are also capable of failing in a ductile manner, typically known in the reinforcing steel industry as achieving "bar-break". As an example, Caltrans specifies a required mode of failure (i.e., "necking of the bar").[10]

Safety[edit]

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[edit]

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.
#7 @ 12" EW, EF Number 7 rebar spaced 12 inches apart, placed in each direction (each way) and on each face.

Reuse and recycling[edit]

Workers extracting rebar from demolition rubble.

In China and many other countries, after the demolition of a building, workers are called in to remove the rebar. They scour the site, extracting the metal using bolt cutters, welding equipment, sledgehammers, 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 down, and re-formed.

See also[edit]

References[edit]

  1. ^ Merritt, Frederic S., M. Kent Loftin and Jonathan T. Ricketts, Standard Handbook for Civil Engineers, Fourth Edition, McGraw-Hill Book Company, 1995, p. 8.17
  2. ^ The office of the first Russian oligarch (Russian)
  3. ^ "GFRP Bar Transverse Coefficient of Thermal Expansion Effects on Concrete Cover" (PDF). Retrieved 2012-08-24. 
  4. ^ Recommended Field Handling of Expoy-Coated Reinforcing Bars, Concrete Reinforcing Steel Institute
  5. ^ Ramniceanu, Andrei [1] Parameters Governing the Corrosion Protection Efficiency of Fusion-Bonded Epoxy Coatings on Reinforcing Steel, Virginia Transportation Research Council, January 2008
  6. ^ Wang, Chu-Kia; Salmon, Charles; Pincheira, Jose (2007). Reinforced Concrete Design. Hoboken, NJ: John Wiley & Sons. p. 20. ISBN 978-0-471-26286-2. 
  7. ^ Threaded Rebar Bolts
  8. ^ google.com
  9. ^ ACI. "ACI 318-11 Building Code Requirements for Structural Concrete and Commentary". ACI (American Concrete Institute). 
  10. ^ California Dept. of Transportation. "METHOD OF TESTS FOR MECHANICAL AND WELDED REINFORCING STEEL SPLICES". Caltrans. Retrieved Feb 2011. 

External links[edit]