Reinforcing Steel (Rebar) embedded in concrete provides the necessary resistance to stresses which arise from flexing and bending, allowing reinforced concrete to be used in many applications in the construction industry. Concrete has excellent compressive strength, but it is weak in tension. Reinforcing steel has excellent tensile strength but must be held in place to be effective. Thus, steel reinforced concrete provides a strong structural material or composite that can be aesthetically pleasing, economical, reliable, and durable. Fabricated from carbon steels with high yield strengths, the rebar is also ductile. The coefficients of thermal expansion of concrete and carbon steel are similar, so that internal stresses due to expansion and contraction are minimized. The concrete bonds strongly to the surface of the steel so that stresses are transferred efficiently from the concrete to the rebar. This composite is now widely used for the construction of concrete slabs, concrete walls, concrete beams, concrete columns, elevated concrete floors, and concrete footings and foundations. In the concrete, the rebar is contained in an alkaline environment and a passive film of iron oxide is formed, protecting the steel from corrosion. However, this protection is lost if the concrete structure is exposed to seawater or deicing salts. In these conditions, galvanized (zinc-coated) or epoxy-coated rebar must be used to realize the expected durability for the reinforced concrete.
Methods and Materials
The reinforcing bars (rebar) used in structural and architectural concrete are manufactured from carbon steels that have high yield strengths, e.g., approximately 60,000 PSI. The steels used are also ductile and therefore readily absorb greater amounts of energy when deformed. Rebar is usually round, with the cross-sectional area ranging from 0.1 to 4.0 in2 and it weighs from 0.4 to 14 lbs/foot. Each reinforcing size number represents 1/8” in diameter, e.g., a #3 bar is 3/8” diameter and a #8 bar is 1” diameter.
Rebar is manufactured by pouring molten steel into casters and passing it through a series of stands to shape the steel into bars. Cross-hatching (also referred to as “deformations”), which is formed in the surface of the rebar during the manufacturing process, aids in the transfer of the load from the concrete to the steel.
Reinforcing steel has identification and grade marks between the ribs. The two systems are the “Number System” and the “Line System” and they designate the grade.
ACI ALLOWABLE YIELD AND ULTIMATE STRENGTHS FOR COMMON REBAR
|#3 Thru #18
|S or A
|A-615 or A-617
|#11, #14, & #18
The most commonly used rebar is new billet stock, ASTM A-615, Type S, Grade 60.
The coefficients of thermal expansion of concrete and the carbon steels are similar, so that internal stresses arising from expansions and contractions are minimized. The surface of the rebar is usually roughened or corrugated to enhance the bonding between the concrete and the steel. As the cement hardens it conforms to the topography of the steel surface and the stress is transmitted efficiently between the two materials. The environment in which the steel is contained is normally alkaline. Under these conditions a passive oxide layer is formed at the surface of the rebar, and this oxide layer prevents further corrosion.
A more effective approach to preventing corrosion of the rebar in aggressive environments is the formation of a thin, protective coating at the surface of the steel. The deposition of a layer of zinc metal onto the steel surface is an example of such a coating, and this process is known as galvanizing.
One galvanizing process, hot dip galvanizing, requires the rebar to be dipped into molten zinc to form a surface alloy as a tightly adherent coating. Another approach is to electrochemically deposit zinc onto the steel substrate. The rebar can be cut or bent either before or after galvanizing, with little or no effect upon the tensile strength, elongation, or load requirements of the steel. Galvanized reinforcing steel has proven to be cost effective and provides reliable corrosion protection in a variety of conditions. It is readily manufactured and easily transported, handled, and installed.
An alternative protective coating is obtained using an epoxy resin. To epoxy-coat the rebar, powdered resin is mixed with fillers, pigments, and flow control agents. It is then sprayed onto the steel rebar, which has been cleaned, had its surface roughened, and been heated to approximately 450°F. The particles from the spray gun assume an electrical charge and are attracted to the steel. Here, the particles of the resin mixture melt and bond to the steel, conforming to the surface topography of the bar as a thin, cross-linked polymer film. After application, the coating is allowed to cure for about 30 seconds and then quenched using either air or water. Lengths of 40-60 feet of rebar are usually coated during this process and the epoxy-coated material can then be cut or bent to meet project requirements.
The weather conditions at the construction site will not affect the reinforcing steel, although prior to use the material should be stored in a clean, dry area. Soil, oil, or grease can alter the bonding of the concrete to the reinforcing. Thus, reinforcing should be kept as clean as is practical.
The design and size of the reinforcing structure should comply with local building codes and should properly utilize the strengths of the two materials, i.e., the steel and the concrete. The structure must be constructible and cost effective. Labor costs on-site can be significant, so it is important to plan the details of reinforcing installation. The necessary supports, ties, overlaps, and other accessories should be included in the plan. Reinforcing materials are more expensive than concrete and there can be a trade-off between the quantity of the rebar and the volume of concrete to be used. However, it is important to note that mechanical failure of the concrete element can occur if there is insufficient reinforcing material or if the spacing of the rebar is too wide.
The designation of the reinforcing steel is usually tabulated in a Reinforcing Schedule on the construction drawings to eliminate any ambiguity in notation. For example, a notation of #4 at 12" O.C., T&B, EW refers to the use of Number 4 rebar, spaced 12" on center, on both the top and bottom faces, and oriented in both the longitudinal and transverse directions.
A concrete cover can protect the rebar from aggressive environments, in addition to providing sufficient embedment to prevent slipping under stress. The depth of this concrete cover is dependent upon the environment to which the structure is exposed. In the United States, the ACI recommends various concrete cover depths for protection of reinforcement depending on the structure and exposure. The following are guidelines (from the Ohio Building Code) for concrete protection of reinforcing:
MINIMUM CONCRETE COVER
|Concrete cast against and permanently
exposed to earth.
|Concrete exposed to earth or weather.
#6 thru #18 bar
#5 bar, W31 or D31 wire, and smaller
|Concrete not exposed to weather or in contact with ground,
slabs, walls, and joists.
#14 and #18 bars
#11 bar and smaller
Beams and Columns: Primary reinforcement, ties,
stirrups, and spirals
Shells, folded plate members:
#6 bar and larger
#5 bar, W31 or D31 wire, and smaller
Bending the rebar to form a 90° or 180° hook may increase the anchoring strength of the reinforcing steel in a concrete element.
The choice of reinforcing materials and the design of the structure will be stated in the construction drawings, together with details of the installation. If already on-site, the rebar should be clearly labeled and stored in a clean, dry area.
Field inspections of adjacent surfaces such as form liners and vapor barriers should be completed prior to placement of the reinforcing steel.
Applications and Installations
As a rule of thumb, splices of reinforcing should have overlaps of 30 diameters (15” for a #4 rebar). Consult with a design professional for specific requirements for splicing. Slab reinforcing steel is normally set on chairs and wired together. Vertical reinforcing is normally secured to the ties and may form a grid with the concrete.
Reinforced concrete that is exposed to moisture and freeze/thaw cycles may crack and spall, exposing the reinforcing bars. This weakens the structure and over time may render the structure unusable. Damaged concrete should be repaired immediately with the appropriate products, and the source of the moisture should be remedied prior to the repairs.
Standards and Codes
- The International Building Code Chapter 19 by ICC states the minimum design standards for reinforcement in concrete.
- Reinforcing steel and reinforced concrete shall comply with ACI 318 and ACI 318 Section 3.5., which is published by the ACI as Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R-05) and can be purchased from the ICC.
- The welding of reinforcing bars shall comply with AWS D 1.4.
- ASTM standards for Cold finished, carbon steel bars (A108-99)
- Standard fabricated, deformed steel bar materials for concrete reinforcement (A184/A184M-05)
- Epoxy-coated rebar (A775/A775M)
- Zinc-coated (galvanized) rebar (A767)