Reinforcement
What Is Rebar?
The steel bar that carries the tension concrete cannot.
Reviewed July 2026
Rebar, short for reinforcing bar, is the steel embedded in concrete to carry tension. Concrete is very strong when squeezed and weak when pulled, so rebar goes exactly where the pulling happens. The two materials divide the work: concrete takes compression, steel takes tension, and together they form reinforced concrete, the material behind nearly every slab, footing, column, and bridge deck around you.
Why concrete needs steel at all
Concrete has high compressive strength and relatively low tensile strength. Load a concrete beam and its top face is squeezed while its bottom face is stretched, and it is the stretched face that fails first. Adding steel bars along that face gives the assembly something that handles tension well. This is the whole idea of reinforced concrete, and it is why you see rebar mats in a slab and bar cages inside a column rather than a solid block of plain concrete.
The pairing works for a second reason that is easy to overlook. Steel and concrete have similar coefficients of thermal expansion, so a reinforced member sees minimal differential stress as the temperature swings. If the two materials moved at very different rates, ordinary weather would tear the bond apart.
What the ribs are for
Look at any bar on a jobsite and you will see a continuous series of ribs, lugs, or indentations rolled into its surface. Those deformations are not decoration. They mechanically lock the bar into the surrounding concrete, improving the bond and reducing the risk of the bar slipping when the member is loaded. Requirements for those deformations were standardized in the late 1940s, and current bar specifications, ASTM A615 and ASTM A706 among them, still carry the same geometry rules.
What rebar is made of
The most common rebar is carbon steel: hot-rolled round bars with the deformation pattern embossed into the surface. Bars are described by a size number and a grade, which we cover in rebar sizes explained. Other bars exist for harder environments. Stainless steel is used where corrosion resistance justifies its cost. Composite bars are made from glass fiber, carbon fiber, or basalt fiber; the glass-fiber version is common enough to have its own comparison in fiberglass rebar vs steel. Plain carbon-steel bars can also be coated in zinc or in an epoxy resin to resist corrosion, which matters in saltwater environments.
Epoxy coating comes with a handling rule. Damage to the coating during transport, fabrication, or concrete placement reduces the long-term corrosion resistance of the bar, so crews treat coated bars carefully. Even damaged epoxy-coated bars have still shown better performance than uncoated ones.
Cover is what protects the steel
Ordinary rebar is unfinished steel and will happily rust. What keeps it from rusting is the concrete around it. Sound concrete cover normally maintains a pH above 12, and in that alkaline environment the corrosion reaction does not proceed. This is the quiet reason cover depth appears on every structural drawing.
Take that protection away and the failure sequence is predictable. Too little cover, or cover breached by carbonation, chlorides, or cracks, lowers the pH at the bar. The steel corrodes. Rust occupies more volume than the steel it replaced, so it pushes outward, and the concrete cracks and spalls from the inside. That mechanism is behind most crumbling bridge decks and parking structures, and we cover it further in why concrete cracks.
What this means on a real job
Rebar is not something you scatter into a pour and hope for the best. The bar size, the grade, the spacing, and above all the amount of concrete cover are engineering decisions tied to the loads a structure carries and to the local building code. Get the bar right and put it in the wrong place, too close to the surface, and you have built in the corrosion problem you were trying to prevent. Reinforcement is a system, not a shopping list.
Reference: the Rebar overview, which collects the standards history and material data cited above.
Frequently asked questions
What is rebar and what does it do?
Rebar, short for reinforcing bar, is a tension device embedded in concrete. Concrete is strong in compression but has low tensile strength, so steel bars are placed where the concrete would be pulled apart. The rebar carries that tension while the concrete carries the compression.
Why does rebar have ribs on it?
The ribs, lugs, or indentations rolled into the bar surface improve the bond with the surrounding concrete and reduce the risk of the bar slipping under load. Their shape and spacing are set by standards such as ASTM A615 and ASTM A706.
Why does rebar not rust inside concrete?
Sound concrete cover around the bar normally holds a pH above 12, and that alkaline environment prevents the corrosion reaction. Too little cover lets carbonation lower the pH, and the steel begins to rust, which cracks the concrete from inside.
More on reinforcement
Materials Review is an independent educational resource. It is not affiliated with Pittsburg State University or the former Kansas Polymer Research Center, and it is not a substitute for a licensed engineer. Confirm structural, safety, and code questions with a qualified professional before acting.