Rebar is steel bar embedded in concrete to keep it from cracking and breaking apart under tension. Concrete handles compression extremely well, but it’s weak when pulled or bent. Rebar compensates for that weakness, turning ordinary concrete into reinforced concrete that can support bridges, buildings, foundations, and highways.
Why Concrete Needs Rebar
Concrete resists crushing forces beautifully. Stack weight on top of it and it holds firm. But pull it, bend it, or subject it to vibration, and it cracks. This is because concrete has high compressive strength but very low tensile strength, sometimes only about one-tenth of its ability to resist compression.
Rebar solves this by handling the tension that concrete can’t. When a concrete beam spans a gap, the bottom of that beam stretches slightly under load. Without reinforcement, that stretching produces cracks that quickly lead to failure. Steel rebar embedded in the tension zone absorbs those pulling forces and holds the concrete together. The result is a composite material where each component does what it’s best at: concrete resists compression, steel resists tension.
This pairing works because steel and concrete expand and contract at nearly identical rates when temperatures change. Both materials have a thermal expansion coefficient of roughly 10 millionths per degree Celsius, according to Penn State engineering data. If the two materials expanded at different rates, temperature swings would eventually break the bond between them and the whole system would fail.
How Rebar Sizes Work
Rebar is sized by number, and the number corresponds roughly to its diameter in eighths of an inch. A #4 bar is 4/8 of an inch (half an inch) in diameter. A #8 bar is a full inch across. Here are common sizes:
- #3: 0.375 inches diameter, 0.376 lbs per foot
- #4: 0.500 inches, 0.668 lbs per foot
- #5: 0.625 inches, 1.043 lbs per foot
- #6: 0.750 inches, 1.502 lbs per foot
- #8: 1.000 inches, 2.670 lbs per foot
- #10: 1.270 inches, 4.303 lbs per foot
- #14: 1.693 inches, 7.65 lbs per foot
- #18: 2.257 inches, 13.60 lbs per foot
For residential work like footings, slabs, and retaining walls, #3 through #5 bars are most common. Larger sizes like #8 through #18 show up in commercial buildings, bridges, and heavy infrastructure. The ridges running along the bar’s surface aren’t decorative. They’re deformations that grip the surrounding concrete, creating a mechanical bond so the bar can’t simply slide through when forces are applied.
Grades and Strength Ratings
Beyond size, rebar comes in different grades that indicate how much force it can handle before it permanently bends. The grade number refers to the bar’s minimum yield strength in thousands of pounds per square inch (psi):
- Grade 40: 40,000 psi yield strength
- Grade 60: 60,000 psi yield strength
- Grade 80: 80,000 psi yield strength
- Grade 100: 100,000 psi yield strength
Grade 60 is the standard for most construction in the United States. It’s strong enough for the vast majority of structural applications and widely available. Higher grades like 80 and 100 allow engineers to use less steel in a given design, which can reduce congestion in heavily reinforced sections where fitting bars close together becomes a challenge.
Reading the Marks on Rebar
Every piece of rebar is stamped with a series of raised symbols that tell you exactly what you’re looking at. Reading from one end, the markings typically identify four things in order: the producing mill, the bar size, the type of steel, and the grade.
The mill is identified by a letter or symbol. The bar size appears as a number (a “5” means it’s a #5 bar). The steel type is indicated by a letter, with “S” being the most common designation for standard carbon steel meeting ASTM A615 specifications. The grade appears as a number, often “60” for Grade 60. Some bars use a shorthand where “4” represents Grade 60 rather than spelling it out. If you see a “W” instead of “S” for the steel type, that indicates a weldable bar made to a different specification, which matters when bars need to be welded together on the job site rather than simply tied with wire.
Types of Rebar Material
Standard carbon steel rebar is by far the most widely used type, but it has one significant vulnerability: corrosion. When moisture and salt reach the steel, rust forms. Rust takes up more volume than the original steel, so as corrosion products expand, they crack and push off the surrounding concrete in a process called spalling. This is the damage you see on aging bridges and parking garages where chunks of concrete have broken away to reveal rusty bars underneath.
To combat this, several corrosion-resistant options have been developed over the past 50 years:
- Epoxy-coated rebar has a thin layer of epoxy applied to the surface that acts as a barrier against moisture. It’s common in bridge decks and structures exposed to road salt, though any nick in the coating can become a corrosion entry point.
- Galvanized rebar is coated with zinc, which corrodes sacrificially to protect the underlying steel. It offers better protection than epoxy if the coating gets scratched.
- Stainless steel rebar resists corrosion inherently without relying on a coating. It costs significantly more but is used in structures with 75- to 100-year design lives, particularly in marine environments.
- Low-carbon chromium rebar contains enough chromium in the steel itself to resist corrosion, offering a middle ground between standard carbon steel and full stainless.
Glass Fiber Rebar (GFRP)
Glass Fiber Reinforced Polymer, or GFRP, is a non-metallic alternative that eliminates the corrosion problem entirely. These bars are made from glass fibers embedded in a polymer resin, so there’s no steel to rust. They’re also roughly one-quarter the weight of steel, which makes handling easier on the job site.
GFRP offers high tensile strength and strong fatigue resistance, making it attractive for aggressive environments like coastal structures, water treatment plants, and areas heavily treated with de-icing chemicals. However, it behaves differently from steel under load. Steel bends before it breaks, giving warning signs of overload. GFRP tends to fail more suddenly, which changes how engineers need to design around it. Steel remains the dominant choice for general construction, but GFRP has carved out a growing role where long-term durability in corrosive conditions is the priority.
How Concrete Cover Protects Rebar
The concrete surrounding the rebar isn’t just structural. It’s also protective. Fresh concrete is highly alkaline, with a pH around 13, and this chemistry creates a passive layer on the steel surface that prevents rust from forming. As long as this alkaline environment stays intact and moisture can’t reach the bar, the steel remains protected indefinitely.
The thickness of concrete between the rebar and the outside surface is called “concrete cover.” A minimum of 2 inches of properly placed concrete will inhibit corrosion under normal conditions, according to U.S. Army Corps of Engineers research. Structures in harsher environments, like those exposed to seawater or de-icing salts, often require thicker cover or corrosion-resistant bar types. When cover is too thin, or when the concrete cracks and lets chlorides penetrate inward, the protective chemistry breaks down and corrosion begins. That’s why proper placement of rebar at the correct depth within the concrete is one of the most critical steps during construction.