Rebar

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

Rebar (short for reinforcing bar), collectively known as reinforcing steel and reinforcement steel,[1] is a steel bar or mesh of steel wires used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and hold the concrete in tension. Rebar's surface is often patterned to form a better bond with the concrete.

History

Rebar inside the Leaning Tower of Nevyansk

Originally, concrete structures were unreinforced. Rebar has been used in construction since at least the 15th century; for example, 2500 m of rebars were used in the Château de Vincennes.[2]

More recently, during the 18th century, rebar was 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 rebar 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.[3] More recently these techniques have been refined by embedding 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

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 divided into primary and secondary reinforcement, but there are other minor uses:

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 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:[4] a concrete structural member reinforced with steel will experience minimal stress as a result of similar expansions of the two interconnected materials caused by temperature changes.

Physical characteristics

Steel has a thermal 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.[5] 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.[6][7] Extra care is 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.[8] 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.[9] These bars are used in over 70,000 bridge decks in the USA.[10]

Fiber-reinforced polymer rebar is also used in high-corrosion environments. It is available in many forms, such as spirals for reinforcing columns, common rods, and meshes. Most commercially available rebar is 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 rebar, can address this type of failure.

Sizes and grades

US sizes

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

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 square inches, and therefore a diameter of 1.128 inches. #10, #11, #14, and #18 sizes correspond to 1 1⁄8 inch, 1 1⁄4, 1 1⁄2, and 2 inch square bars, respectively.[11] #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.[12] 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.

Steel reinforcement bars with color codes indicating the grade
U.S. rebar size chart
Imperial

bar size

 Metric size Linear Mass Density Nominal diameter Nominal area
 lb⁄ft (kg/m) (in) (mm) (in²) (mm²)
#2 No.6 0.167 0.249 0.250 =  1⁄4 6.35 0.05 32
#3 No.10 0.376 0.561 0.375 =  3⁄8 9.525 0.11 71
#4 No.13 0.668 0.996 0.500 =  1⁄2 12.7 0.20 129
#5 No.16 1.043 1.556 0.625 =  5⁄8 15.875 0.31 200
#6 No.19 1.502 2.24 0.750 =  3⁄4 19.05 0.44 284
#7 No.22 2.044 3.049 0.875 =  7⁄8 22.225 0.60 387
#8 No.25 2.670 3.982 1.000 25.4 0.79 509
#9 No.29 3.400 5.071 1.128 28.65 1.00 645
#10 No.32 4.303 6.418 1.270 32.26 1.27 819
#11 No.36 5.313 7.924 1.410 35.81 1.56 1006
#14 No.43 7.650 11.41 1.693 43 2.25 1452
#18 No.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

Corroded concrete and rebar at the bridge of Queen Elizabeth Way crossing the Welland River in Niagara Falls, Ontario, Canada.

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

Metric

bar size

 Linear Mass Density

(kg/m)

 Nominal diameter

(mm)

 Cross-sectional

Area (mm²)

10M 0.785 11.3 100
12M 0.985 12.7 125
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

Metric bar designations represent the nominal bar diameter in millimetres. Preferred bar sizes in Europe are specified to comply with Table 6 of the standard EN 10080,[13] 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.

Steel reinforcement in storage
Metric

bar size

 Linear Mass Density

(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 1.58 16 201
20,0 2.47 20 314
25,0 3.85 25 491
28,0 4.83 28 616
32,0 6.31 32 804
40,0 9.86 40 1257
50,0 15.4 50 1963

Australian sizes

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 apply to testing, welding and galvanising. The designation of reinforcement is defined in AS/NZS4671-2001 using the following formats:

Bars

Shape code Yield strength (MPa) Ductility class Nominal diameter (mm)
D - deformed bar - Grade 300 300 MPa N - normal 10
D - deformed bar - Grade 500 500 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 as "N12"
R250N20 is round bar, 250 MPa strength, normal ductility and 20 mm nominal diameter - also known 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, while round bars are 250 MPa and normal ductility.

India

Rebars are available in the following grades as per IS:1786-2008 FE 415/500/500D. Rebars are quenched with water at a high level pressure so that the outer surface is hardened while the inner core remains soft. Rebars are ribbed so that the concrete can have a better grip. Coastal regions use galvanized rebars to prolong their life.

Weight chart of Indians Rebars with tolerances and sizes.[14]

Mesh

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

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

In US use, 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.

In countries that use the metric system, the grade designation is typically the yield strength in megapascals MPa, for example grade 400 (similar to US grade 60).

Common US specifications, published by ACI and ASTM, are:

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. These high yield strength ductile steels are usually produced using the TEMPCORE process,[15] a method of thermomechanical processing. The manufacture of reinforcing steel by re-rolling finished products (e.g. sheets or rails) is not allowed.[16] In contrast to structural steel, rebar steel grades are not harmonized yet across Europe, each country having their own national standards. However some standardization of specification and testing methods exist under EN 10080 and EN ISO 15630:

Placing rebar

Steel wire used to secure rebar before concrete is poured over it. A scale marked in centimetres and millimetres is shown for size reference.

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 iron workers, 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 by spot welding, tying steel wire, sometimes using an electric rebar tier, or with mechanical connections. For tying epoxy coated or galvanised rebars, epoxy coated or galvanized wire is normally used, respectively.

Stirrups

Stirrups form the outer part of a rebar cage. Stirrups are usually rectangular, and are placed at regular intervals along a column or beam to prevent shear failure.[17]

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). 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.

ASTM A 616 & ASTM A 617 (now replaced by the combined standard A996) reinforcing bars are re-rolled rail steel and re-rolled rail axle steel with uncontrolled chemistry, phosphorus and 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 United States. High strength steels for prestressed concrete may absolutely not be welded.

Mechanical connections

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 between 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.[18]

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").[19]

Safety

To prevent injury, 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.[20]

Designations

Reinforcement is usually tabulated in a "reinforcement schedule" on construction drawings. This eliminates ambiguity in the notations used around the world. The following list provides examples of the 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

Workers extracting rebar from demolition rubble

In China and many other countries, after the demolition of a concrete structure, 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.

References

  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. ↑ "Le donjon de Vincennes livre son histoire".
  3. ↑ The office of the first Russian oligarch (in Russian)
  4. ↑ "Coefficients of Linear Thermal Expansion". The Engineering ToolBox. Retrieved 2015-07-06.
  5. ↑ "GFRP Bar Transverse Coefficient of Thermal Expansion Effects on Concrete Cover" (PDF). Retrieved 2012-08-24.
  6. ↑ O’Reilly, Matthew; Darwin, David; Browning, JoAnn; Locke Jr, Carl E. (2011-01-01). "Evaluation of multiple corrosion protection systems for reinforced concrete bridge decks".
  7. ↑ "Cost-Effective Corrosion Protection Systems for Reinforced Concrete - Epoxy Interest Group (based on the KU study)". epoxyinterestgroup.org. Retrieved 2017-04-15.
  8. ↑ Recommended Field Handling of Expoy-Coated Reinforcing Bars, Concrete Reinforcing Steel Institute
  9. ↑ Ramniceanu, Andrei Parameters Governing the Corrosion Protection Efficiency of Fusion-Bonded Epoxy Coatings on Reinforcing Steel, Virginia Transportation Research Council, January 2008
  10. ↑ Epoxy Interest Group. "Epoxy Interest Group of CRSI". Epoxy Interest Group of CRSI. Retrieved 24 August 2012.
  11. ↑ Wang, Chu-Kia; Salmon, Charles; Pincheira, Jose (2007). Reinforced Concrete Design. Hoboken, NJ: John Wiley & Sons. p. 20. ISBN 978-0-471-26286-2.
  12. ↑ Threaded Rebar Bolts
  13. ↑ "BS EN 10080: Steel for the reinforcement of concrete. Weldable reinforcing steel. General.", pp. 19 (2005).
  14. ↑ "Reinforcement Bars/Re-barsAs/TMT as Per IS.1786". Indana. Retrieved June 21, 2016.
  15. ↑ Noville, J.F. (June 2015). TEMPCORE, the most convenient process to produce low cost high strength rebars from 8 to 75 mm (PDF). 2nd ESTAD - METEC. Düsseldorf.
  16. ↑ "BS EN 10080: Steel for the reinforcement of concrete. Weldable reinforcing steel. General.", clause 6.4, pp. 13 (2005).
  17. ↑ Jesse (January 29, 2013). "Reinforced Concrete Beam Design: Concrete Beam Stirrups? What are they and why are they important?". Retrieved 2015-02-04.
  18. ↑ ACI committee 318 (2014). ACI 318-14 Building Code Requirements for Structural Concrete and Commentary. American Concrete Institute (ACI). ISBN 978-0870319303.
  19. ↑ California Dept. of Transportation. "METHOD OF TESTS FOR MECHANICAL AND WELDED REINFORCING STEEL SPLICES" (PDF). Caltrans. Retrieved February 28, 2011.
  20. ↑ Occupational Safety and Health Administration. "Mushroom Style Plastic Rebar Covers Used For Impalement Protection". OSHA. Retrieved February 28, 2015.
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