Most concrete has some embedded steel reinforcement. If the steel is being counted on to provide bending or tensile strength— if the structural element is intended to behave as a composite—we call it reinforced concrete. But even so-called plain concrete, where we are counting on the concrete to provide all of the strength, has some steel—sometimes called temperature steel. This light reinforcement is traditionally welded wire reinforcement and is intended to hold any cracks tightly together.
Steel and concrete work so well together because their thermal characteristics are similar: both expand and contract at similar rates. The biggest problem with steel in concrete is that it corrodes. When it does, the rust formed has more than five times the volume of the original steel, and so the concrete is damaged by the corrosion byproducts.
To counteract this common problem, corrosion-resistant reinforcement has been developed. Several materials are effective for this:
- Fiber-reinforced polymer
- Stainless steel
- Epoxy-coated carbon steel
- Special proprietary steel
The best source of information on reinforcing steel is the Concrete Reinforcing Steel Institute. It publishes information at several levels, from the comprehensive Manual of Standard Practice to the Ready Reference Reinforcing Steel Resource Guide, a very handy spiral-bound book.
Standard reinforcing steel
Selecting standard reinforcing bars is usually as simple as telling your supplier the size and grade. Rebar is sold as a commodity that meets the requirements of an ASTM specification. Grade 40 means the steel has a minimum yield strength of 40,000 psi; Grade 60 means it has yield strength of 60,000 psi. Grade 40 steel is typically only available in bar sizes #3 to #6. Grade 60 is available in bar sizes #3 to #18. Traditional inch-pound bar sizes are the bar's diameter in eighths of an inch; metric bar sizes are the bar's diameter in millimeters. Bars are marked with a series of symbols that indicate many things about them (see diagram).
Most reinforcing steel is placed individually, although a convenient option that has emerged recently is heavy gage structural mesh sheets. These sheets are welded in a shop and shipped to the site, where they can be placed with little tying. They are specified in the same way as more standard welded wire reinforcement, for example 6x12-W16xW8, which means:
- Spacing of the longitudinal wires = 6 inches
- Spacing of the transverse wire = 12 inches.
- Size of longitudinal wire = W16 (smooth wire, cross-section = 0.16 sq. in.)
- Size of transverse wire = W8 (smooth wire, cross-section = 0.08 sq. in.)
The W is for smooth wire; a D would mean deformed wire.
Fiber reinforced polymers
FRP reinforcement is lighter, tougher, and more corrosion-resistant than steel. These bars are made of fiber reinforcement, resin, fillers, and additives. Fibers provide increased stiffness and tensile capacity while the resin has high compressive strength and binds the fibers into a firm matrix. The most common fibers used in structural applications are fiberglass, aramid, and carbon. Fiberglass is the least expensive, and carbon fiber is the most costly. A few bridges have been built using FRP reinforcement.