Rebar is made of carbon steel. The vast majority of rebar used in construction is a low-carbon steel alloy containing controlled amounts of carbon, manganese, phosphorus, and sulfur. These elements are carefully balanced to give the bars enough strength to reinforce concrete while remaining flexible enough to bend without snapping. Beyond standard carbon steel, rebar also comes in stainless steel, galvanized steel, epoxy-coated steel, and newer non-metal composites made from glass or basalt fibers.
Carbon Steel: The Standard Material
Standard rebar is governed by ASTM A615, which specifies four strength grades based on minimum yield strength: Grade 40 (40,000 psi), Grade 60 (60,000 psi), Grade 80 (80,000 psi), and Grade 100 (100,000 psi). Grade 60 is the most common in everyday construction. The higher the grade number, the more force the bar can withstand before it starts to deform permanently.
The steel’s composition is what determines its grade. Carbon makes steel harder and stronger but also more brittle, so the percentage is kept relatively low. Manganese improves strength and hardness while helping the steel resist wear. Phosphorus and sulfur are kept to minimum levels because too much of either makes the steel crack-prone. Every batch of rebar undergoes a heat analysis to confirm these elements fall within required ranges, along with tensile, bend, and deformation tests.
How the Manufacturing Process Shapes the Steel
Most modern rebar is produced through a process called Quenching and Self-Tempering (QST), sometimes referred to as Thermo-Mechanical Treatment (TMT). After the steel is hot-rolled into bar shape, it passes through a rapid water-cooling system right on the production line. This creates two distinct zones within a single bar: a hard outer ring and a softer, more flexible core.
The rapid cooling transforms the bar’s outer layer into a structure called tempered martensite, which is extremely strong but brittle on its own. The core, which cools more slowly using residual heat from inside the bar, develops a different structure called ferrite-pearlite, which is weaker but very ductile. Together, these two layers give QST rebar an ideal combination: the outer shell resists high loads while the inner core allows the bar to flex and absorb energy without fracturing. This is why modern rebar can handle both the compressive forces of heavy structures and the dynamic stresses of earthquakes or wind loads.
Recycled Steel Content
One detail that surprises many people is just how much recycled material goes into rebar. Most rebar is produced in electric arc furnaces that melt down scrap steel rather than starting from raw iron ore. Standard reinforcing bars (ASTM A615 and A706) typically contain greater than 98% recycled steel scrap. Even specialty reinforcing products use at least 75% recycled content. This makes rebar one of the most recycled building materials in the construction industry, with the electric arc furnace process using over 90% scrap steel as its feedstock.
Stainless Steel Rebar
When corrosion is a serious concern, stainless steel rebar replaces standard carbon steel. The key difference is chromium, which forms an invisible protective layer on the steel’s surface that resists rust. Stainless rebar typically falls into two families:
- Type 304 contains 17.5 to 19.5% chromium and 8 to 11.5% nickel. It works well in most environments where chloride exposure is a concern, such as parking garages treated with road salt.
- Type 316 variants contain similar chromium levels (16.5 to 18.5%) but add 2 to 3% molybdenum, an element that significantly boosts resistance to chloride attack. This makes 316 the preferred choice for marine structures, bridges, and coastal buildings.
Stainless rebar costs significantly more than carbon steel, so it’s typically reserved for structures with long design lives or harsh exposure conditions where replacing corroded rebar would be impractical or dangerous.
Coated Carbon Steel Rebar
Rather than switching to an entirely different alloy, builders sometimes protect standard carbon steel rebar with a coating. The two most common approaches are epoxy coating and hot-dip galvanizing.
Epoxy-coated rebar, covered under ASTM A775, uses a protective layer of epoxy resin applied through electrostatic spraying. The powdered epoxy is given an electrical charge and sprayed onto the heated bar, where it melts and bonds into a continuous film. This barrier physically separates the steel from moisture and chlorides. It’s widely used in bridge decks and other structures exposed to deicing salts, though any damage to the coating during handling or installation creates a weak point where corrosion can begin.
Galvanized rebar gets its protection from a zinc coating applied through hot-dip galvanizing. The steel bars are submerged in molten zinc, which bonds metallurgically to the surface. Zinc protects steel in two ways: it acts as a physical barrier, and even if scratched, it corrodes preferentially before the underlying steel does, sacrificing itself to keep the rebar intact. Coating thickness and weight requirements are standardized under ASTM A767.
Glass Fiber Reinforced Polymer (GFRP) Rebar
GFRP rebar is a completely non-metallic alternative. It’s made of continuous glass filaments embedded in a polymer resin, typically vinyl ester for structural applications or polyester for non-structural ones. The glass fibers provide tensile strength while the resin binds them together and transfers loads between fibers.
Several types of glass fiber are used depending on the application. E-glass (low-alkali glass) is the most common due to its good strength and low cost. E-CR glass offers better acid and heat resistance. S-glass provides higher tensile strength for demanding applications. AR-glass, which contains at least 18% zirconium oxide, is specifically designed to resist the alkaline environment inside concrete, which can degrade other glass types over time.
The main advantage of GFRP rebar is that it cannot corrode, making it attractive for marine structures, water treatment plants, and buildings where electromagnetic neutrality matters (since it doesn’t conduct electricity or interfere with signals). It also weighs roughly a quarter of what steel rebar does, which simplifies handling on job sites. The tradeoff is that GFRP bars don’t bend like steel, they have lower stiffness, and they can’t be welded or reshaped in the field.
Basalt Fiber Rebar
Basalt rebar is a newer composite option made from fibers of volcanic basalt rock bound together with a polymer resin, often epoxy or vinyl ester. The fibers themselves are composed of minerals including pyroxene, olivine, and plagioclase. Like GFRP, basalt rebar doesn’t corrode and is lightweight. It offers slightly better heat resistance than glass fiber and is made from a naturally abundant raw material. It remains less common than either steel or GFRP rebar but is gaining use in specialized applications where both corrosion resistance and thermal performance matter.