What Is Rebar Made Of? How Reinforcing Steel Is Produced

Rebar is made of carbon steel: iron alloyed with carbon, manganese and small amounts of other elements. Most rebar in North America starts as recycled scrap that is melted in an electric arc furnace, refined, cast into square billets, then reheated and hot rolled into round bar with ribs (deformations) rolled onto the surface. In Canada the finished bar has to meet CSA G30.18, which sets its sizes, grades, strength, ductility and markings.

This post explains what goes into the bar, how a mill turns it into rebar, and answers the questions people ask about the material: is it iron, is it high carbon, is it magnetic, and can it be recycled. If you are looking for which mills make rebar in Canada, our post on rebar manufacturers in Canada covers the producers.

What rebar is made of

Steel is iron with a controlled amount of carbon and other elements. In rebar, each element does a specific job:

The chemistry limits depend on the grade. CSA G30.18 has R grades (400R, 500R) and W grades (400W, 500W). According to the RSIC Manual of Standard Practice, which reproduces the requirements of G30.18-09 (R2019):

• W grades (the weldable grades most Canadian drawings call for) are limited to 0.30% carbon, 1.60% manganese, 0.035% phosphorus and 0.045% sulphur, with silicon under 0.50% and a carbon equivalent of 0.55% or less.
• R grades only carry a phosphorus limit. Their carbon is not specified; the bar just has to pass the strength, elongation and bend tests.

Those limits are why W grade bar can be welded with a qualified procedure and R grade generally should not be. Our post on weldable rebar in Canada explains what that means on site.

How rebar is made, step by step

Mills differ in the details, but the route from scrap to bundled bar follows the same broad steps.

1. Scrap collection and sorting. Old cars, appliances, structural steel, demolition rebar and offcuts from fabrication shops are sorted by grade and chemistry. Some mills also add direct reduced iron or pig iron to dilute residual elements.
2. Melting. The charge goes into an electric arc furnace, where graphite electrodes melt it with an electric arc. Lime and other fluxes form a slag that carries away impurities.
3. Refining. The liquid steel is tapped into a ladle, where alloys such as manganese, silicon and, for some grades, vanadium are added and the chemistry is trimmed to target. Samples are taken for analysis; each batch is a "heat" with its own heat number.
4. Continuous casting. The steel is poured into a caster that forms long square billets, which are cut to length as they solidify.
5. Reheating and rolling. Billets are reheated in a furnace and run through a series of roll stands. Each pass makes the section smaller and longer until it reaches the bar size.
6. Forming the ribs and markings. The final rolls have grooves cut into them. These press the deformations onto the bar and roll in the identification marks: the mill symbol, the bar size and the grade marking.
7. Strength control. Strength comes from the chemistry, the alloys and the way the bar is cooled. Many mills around the world use a controlled water quench right after the last roll stand, which hardens the outer layer while the hot core tempers it; others rely more on alloying. Either way, the bar has to meet the same minimum properties.
8. Cooling, cutting and bundling. The bar cools on a cooling bed, is sheared to stock lengths and is bundled and tagged with the heat number. Smaller sizes may instead be wound into coils.
9. Testing. Samples from each heat are tension tested (yield strength, tensile strength and elongation) and bend tested. The results and chemical analysis go on the mill test report.

From there, straight bar goes to fabricators and suppliers. A fabricator like us cuts and bends it to the bar list for a specific job; our post on what rebar fabrication is covers that half of the process.

What the standard checks

CSA G30.18 is the reason a 15M bar from one mill can replace a 15M bar from another. It sets:

• Sizes from 10M to 55M, by nominal area and mass per metre.
• Minimum yield strength of 400 MPa or 500 MPa, which is where the grade names come from. The W grades also have a maximum yield, so the bar is not unexpectedly stronger than the designer assumed.
• Minimum tensile strength and elongation, so the bar stretches and gives warning before it breaks.
• A bend test: the bar must bend 180 degrees around a pin of a set diameter without cracking on the outside of the bend.
• Deformations, so the bar grips the concrete.
• Markings rolled onto the bar: the mill symbol and bar size on all grades, then the grade shown either as a number or as offset lines, with the letter W added for weldable grades. The letter R is not rolled on.

The mill test report ties a bundle back to its heat. If your engineer or inspector asks for certificates, request them when you order. Our guide to Canadian rebar grades goes through the grade differences in more detail, and how to identify rebar size shows how to read the markings.

Is rebar iron, high carbon or mild steel?

These three questions come up a lot, usually from people who have found old bar on a renovation or are comparing products.

Is rebar steel or iron? It is steel. Iron is the main ingredient, but cast iron has far more carbon and is brittle, and wrought iron has not been used for reinforcement in a very long time. Rebar's controlled carbon content and hot rolling give it the combination of strength and ductility concrete needs.

Is rebar high carbon steel? No. Rebar is a carbon steel, and the W grades cap carbon at 0.30%, well below what metallurgists call high-carbon steel (the kind used for springs and cutting tools). High carbon would make the bar hard but brittle, which is the opposite of what reinforcement needs: bars must bend tightly in the shop without cracking and stretch before breaking in a structure.

Is rebar mild steel? Not in the usual sense. "Mild steel" normally means low-carbon, lower-strength steel such as general plate and bar stock. Canadian rebar has a minimum yield of 400 MPa or 500 MPa, which is higher than common structural steel grades. In some countries plain round "mild steel" bars are still used for light work, but deformed bar is the standard reinforcement in Canada.

Is rebar magnetic?

Yes, ordinary steel rebar is magnetic. It is a carbon steel, and carbon steels are strongly attracted to a magnet. That is useful in practice:

• Cover meters and rebar locators use magnetic and electromagnetic fields to find bars and estimate their depth in existing concrete. See our guide on how to find rebar in existing concrete.
• A strong magnet can tell you whether a bar in a demolition pile is steel or something else.

Some structures need reinforcement that does not interfere with magnetic fields or sensitive equipment. The RSIC manual notes that stainless steel reinforcing is used in highly corrosive areas and non-magnetic applications. GFRP bar is not steel at all; it is glass fibre in a resin, so it is non-magnetic and non-conductive. Our post on GFRP vs steel rebar compares the two.

Can rebar be recycled?

Yes. Steel can be melted and rolled again without losing its properties, and rebar is a big part of that loop:

• Most rebar is already recycled steel. Electric arc furnace mills run mainly on scrap. In Canada, AltaSteel states that its steel comes from 100% recycled material, and other producers also describe scrap-based production.
• Fabrication offcuts from cutting and bending are collected and sold back into the scrap stream.
• Demolition bar is separated from crushed concrete, often with magnets, and sent to scrap yards. The crushed concrete can then be reused as aggregate.

For a builder, the practical point is that rebar's recycled origin does not change what you order or how it performs. The bar still has to meet G30.18 in the grade on the drawings, and the mill test report is what proves it.

Types of rebar steel

Plain black bar is the standard, but the same steel can be coated, or swapped for a different material, where corrosion is a concern:

• Black (uncoated) bar in R or W grades: the default for footings, walls, slabs and most building work.
• Epoxy-coated bar: black bar with a fusion-bonded epoxy coating, used on bridges and parking structures.
• Galvanized bar: black bar with a zinc coating.
• Stainless bar: a different alloy, used in the most aggressive exposures.
• GFRP bar: glass fibre reinforced polymer, made by a different process altogether; see how GFRP rebar is made.

Our overview of types of rebar in Canada compares them side by side.

Getting rebar for your job

Whatever mill rolled it, the bar on your site should match the size and grade on the drawings and arrive cut, bent and tagged so your crew can place it without sorting. We detail, cut and bend 10M to 55M bar, supply GFRP rebar, and deliver across the GTA and Ontario. Send us your drawings for a free take-off and quote, or see our rebar products.