Carbon steel, element by element — and the three measurements that decide whether a rusty bar still goes in the pour
← Back to the calculatorRusty rebar is not a rejection. ACI 318-19 §26.6.1.2(b) accepts rust and mill scale as long as a hand-wire-brushed specimen still makes its minimum dimensions, its height of deformations and its weight per unit length — three measurements, not a look. The commentary goes further: a normal amount of rust increases bond, and ordinary rough handling removes the rust that is loose enough to hurt it.
Carbon, manganese and sulfur must be determined and printed on the mill certificate, but A615 gives them no maximum. What decides whether the bar can be welded is not the carbon alone but the carbon equivalent: at or under 0.55 it can be welded, above it cannot.
That is the whole answer in five lines, and it is narrower than most explanations of the question, because the standard is narrower than most explanations assume. A615 does not describe a recipe. It describes a bar that must reach a yield strength, survive a bend and carry a set of dimensions — and it polices the chemistry at exactly one point, the element that ruins steel no matter how the bar is used.
Everything below is either quoted from A615/A615M (2020 and 2012 editions, both read for this page) or computed from it. Where a number is something the mill controls rather than the specification, the page says so.
Nine tenths of a rebar is iron by weight and the rest is what makes it useful. The table below gives each element's job and what A615 does about it — and the last column is the part almost nobody states plainly: for three of the four elements A615 requires you to measure and report, the standard sets no ceiling at all.
| Element | What it does in the bar | A615 requirement | Ceiling |
|---|---|---|---|
| Fe | The balance — the metal itself | Not specified | — |
| P | Segregates to grain boundaries, embrittles | Determined (6.1) | 0.06% max |
| C | Raises yield and tensile; hardens; cuts weldability | Determined (6.1) | None |
| Mn | Deoxidises, adds hardenability, multiplies carbon's effect | Determined (6.1) | None |
| S | Forms sulphide inclusions; harms ductility and weldability | Determined (6.1) | None |
| Si | Deoxidation during steelmaking | Not required | — |
| Cu, Cr, Ni, Mo, V, Nb | Residuals from scrap, or deliberate micro-alloying | Allowed if the bar still meets the rest of the spec | — |
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Two sentences from the standard carry the whole table. §6.1: “The percentages of carbon, manganese, phosphorus, and sulfur shall be determined. The phosphorus content thus determined shall not exceed 0.06%.” That is it. Four elements measured, one element capped. A mill can ship a bar at 0.45% carbon or 0.20% carbon and both are A615 bars, provided the yield, the bend and the dimensions come out.
Why phosphorus and not carbon? Because phosphorus is the one that damages the bar in every use. It migrates to the grain boundaries and makes the steel brittle, and no amount of downstream processing puts it back. Carbon is the opposite: carbon is how a mill reaches the yield strength the grade asks for, and its effect on welding is not a simple ceiling but a weighted sum — which is what the carbon equivalent, further down, is for.
There is a second clause worth knowing because it is the one that decides arguments at the gate: §6.2 lets a purchaser's product check for phosphorus run 25% above the 0.06%, i.e. up to 0.075%. A bar that checks at 0.07% is not out of spec; a bar that checks at 0.09% is.
Percentages hide the scale. One 20 ft #4 bar weighs 13.36 lb, which is 6,060 g. Put the usual chemistry on that bar and the ceiling stops being abstract: the entire phosphorus allowance is a few grams.
| Element | Assumed % | Grams in one 20 ft #4 | Grams per lb |
|---|---|---|---|
| Fe | 98.15% | 5,948 | 445.2 |
| Mn | 1.20% | 72.7 | 5.44 |
| C | 0.30% | 18.2 | 1.36 |
| Si | 0.25% | 15.1 | 1.13 |
| P | 0.060% | 3.6 | 0.27 |
| P (product check) | 0.075% | 4.5 | 0.34 |
| S | 0.045% | 2.7 | 0.20 |
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The useful reading is the ratio. One bar carries about 3.6 g of phosphorus and about 18 g of carbon, and the carbon figure is free to move while the phosphorus figure is not. That is why a mill certificate is a chemistry report and not a grade: the grade tells you the yield, the certificate tells you what is actually in the heat, and only one line of it has a hard number attached by the standard.
Every heat of steel is analysed, and A615 §6.1 requires that analysis to be made on samples taken preferably during the pour. The document that carries the result is the mill certificate, and reading one is a five-line job once you know which lines matter:
| Line on the certificate | What it tells you | What it cannot tell you |
|---|---|---|
| Heat number | Which pour this bar came from — the traceability key | Nothing about the bar's size or length |
| C, Mn, P, S | The chemistry of that heat, as required by §6.1 | Whether the bar welds — that needs the carbon equivalent |
| Yield strength | That the heat met the grade's minimum | Which grade the bar was marked as |
| Tensile strength | The ultimate capacity and the yield ratio | Anything about bend performance |
| Elongation, bend test | That the bar has ductility, not just strength | Whether the bar on site is the bar on the certificate |
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The trap is the last row of that table. A certificate is issued per heat, and a heat becomes thousands of bars in several sizes. What ties a bundle on the ground to the certificate is the heat number stamped on the tag, not the certificate in the office. If a project needs the chemistry, the request is not “send me a mill cert” — it is “send me the cert for this heat number”, and the number comes off the tag.
And when the number is checked, §6.2 is the clause that settles the argument: a purchaser's product check may exceed the §6.1 phosphorus limit by up to 25%, so 0.075% is still conforming. Above that, it is not.
Steel is iron with carbon in it, and the boundary between steel and cast iron is a carbon number: about 2.1% by weight. Published values differ in the second decimal — 2.08%, 2.11%, 2.14% — because the exact point depends on temperature and on which textbook you open, which is why the figure is usually rounded. Rebar sits nowhere near it.
Within steel, carbon does two things at once, and they pull in opposite directions:
There is a reference point in the middle of the range that explains why heat treatment of steel is written the way it is: 0.8% carbon is the eutectoid composition, the point where the microstructure is entirely pearlite. Below it, steel is ferrite plus pearlite; above it, cementite starts appearing at the grain boundaries. Rebar is well below 0.8%, which is exactly what you want in a bar that has to bend around a pin without cracking.
So the honest answer to “what is the carbon content of rebar” is: whatever the mill used to hit the grade, reported on the certificate, with no ceiling in A615. Anyone quoting a single number for all rebar is quoting a habit, not a specification.
Because carbon and manganese both harden steel, and manganese does it several times more strongly per percent, welders do not look at carbon alone. They look at the carbon equivalent, which folds the hardening elements into one number:
The arithmetic is worth doing once by hand, because the manganese term is bigger than most people expect. Take a bar at 0.30% carbon and 1.20% manganese, with no deliberate alloying:
Half of that carbon equivalent is manganese. Raise the manganese to 1.40% and the same carbon lands at 0.533; push carbon to 0.40% as well and it is 0.633 — over the line, with nothing about the bar looking any different. The practical screen in US practice is that material with a carbon equivalent under 0.55 can be welded; above it, welding needs a procedure, preheat and a welder who knows why.
A615 itself does not use the carbon equivalent. Its Note 3 says the opposite of a guarantee: welding “should be approached with caution since no specific provisions have been included to enhance its weldability”, and points to AWS D1.4/D1.4M for filler metal, preheat and procedure qualification. Read that as the specification admitting it did not design for welding — which is exactly why the next section exists.
ASTM A706 is the low-alloy companion specification, and it is a different kind of document. Where A615 says “reach this yield and mind the phosphorus”, A706 controls the chemistry and then puts a window around the strength so the bar behaves predictably:
| ASTM A615 | ASTM A706 | |
|---|---|---|
| Steel type | Carbon steel | Low-alloy |
| Grades | 40 / 60 / 80 / 100 | 60 only |
| Yield | Minimum only | 60 ksi min, 78 ksi max |
| Tensile | Minimum only | 80 ksi min, and ≥ 1.25 × actual yield |
| Chemistry | P capped, rest reported | All controlled, plus carbon equivalent |
| Welding | Note 3: approach with caution | Written to be weldable |
| Marked | S | S and W (SW) where both are met |
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The maximum yield is the part that surprises people. A615 asks only for a floor, so a mill can overshoot; A706 caps the top as well, because a bar that is much stronger than its neighbour in the same plastic hinge changes where the hinge forms. That cap, plus the minimum tensile and the 1.25 ratio, is what makes A706 the seismic and welding specification rather than just a stronger bar.
Practically: if the drawing says A706, you are buying chemistry control. If it says A615, you are buying a strength floor — and any welding on it is a procedure question, not a material guarantee.
A615 §19.3 requires a specific set of marks rolled onto one side of the bar, in order. Once you know the order you can read a bundle in the yard without a certificate:
| Mark | Meaning | What it does not tell you |
|---|---|---|
| Letter or symbol | The mill | Which heat — that is on the tag |
| Number | Bar size (#4, #5 …) | Length, which is a cut decision |
| S | Made to A615 | — |
| SW | Meets A615 and A706 (Grade 60 and 80 only) | That it is in stock in your length |
| 1 line | Grade 60 (or the number 60) | The actual yield of the heat |
| 3 lines | Grade 80 (or the number 80) | The chemistry |
| 4 lines, or C | Grade 100 (or the number 100) | The chemistry |
| No grade mark | Grade 40 | — |
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The one to look for on a welding job is SW. A bar marked S only is A615; a bar marked SW was produced to meet A706 as well, which means its chemistry was controlled and its yield was capped at the top as well as the bottom. Same shape, same weight, different promise — and the promise is written in two letters you can see without opening a folder.
Grade marks are also how you catch a substitution. A bundle that should be Grade 60 but shows three lines is Grade 80, and putting Grade 80 where Grade 60 was designed is not a free upgrade: it changes the yield ratio and the lap behaviour, and it is worth an RFI before it goes in.
What each grade is required to test at — yield, tensile and the elongation floor by bar size — is in the A615 grade table, which also lists the two-line mark for Grade 75 that this table leaves out.
Carbon steel rebar is ferromagnetic. The reason is structural rather than chemical: at room temperature the iron in the bar is body-centred cubic ferrite, and ferrite responds strongly to a magnetic field. The pearlite mixed into it does not block that, so the bar as a whole is attracted to a magnet and can be found through concrete.
That is the entire principle behind a rebar locator or a cover meter: it generates a field, the steel distorts it, and the instrument reports a bar. It is also why the tool reads depth to steel, not grade — the instrument has no idea whether the bar is Grade 60 or 40, only that something ferromagnetic is at a distance.
Three exceptions matter in practice:
Rusty rebar is not a rejection, and the code settles it in one sentence rather than a paragraph. ACI 318-19 §26.6.1.2(b): “Nonprestressed reinforcement with rust, mill scale, or a combination of both shall be considered satisfactory, provided a hand-wire-brushed representative test specimen complies with the applicable ASTM specification for the minimum dimensions (including height of deformations) and weight per unit length.” A615 §12.2 makes the same statement from the materials side and adds the fourth item the code leaves implicit: tensile properties.
Read either clause as a test rather than a permission. Surface rust is acceptable because it comes off a wire brush and the bar underneath still measures its size, still shows its ribs and still weighs what a bar of that size should weigh. What fails is anything that survives the brush and moves one of those numbers — and the clause names deformation height explicitly, which is the part almost nobody checks.
Rust is not a coating sitting on the bar; it is steel that used to be the bar. Iron oxide occupies roughly twice the volume of the iron it came from, so a rust layer you can measure is about half as much steel off each side. Measure the diameter, subtract it from nominal, and the table gives the share of the cross-section that is gone — which is also the share of the bar's tensile capacity, because capacity is area times stress.
| Diameter under nominal | #3 | #4 | #5 | #6 | #7 | #8 |
|---|---|---|---|---|---|---|
| 0.005 in | 2.6% | 2.0% | 1.6% | 1.3% | 1.1% | 1.0% |
| 0.010 in | 5.3% | 4.0% | 3.2% | 2.6% | 2.3% | 2.0% |
| 0.020 in | 10.4% | 7.8% | 6.3% | 5.3% | 4.5% | 4.0% |
| 0.030 in | 15.4% | 11.6% | 9.4% | 7.8% | 6.7% | 5.9% |
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On a #4 at Grade 60 the highlighted row is 0.008 in² out of 0.20 in², or 480 lb of a 12,000 lb bar. That is the true price of a rust layer you can see: not zero, and not fatal. What does fail bars is not uniform loss at all but a pit — a pit 1/16 in deep is 12.5% of a #4's diameter at one point, and a notch in a bar that is being pulled. Measure the deepest pit you can find, not the average.
ACI 318-19 R26.6.1.2(b) is unusually direct about it: “Research has shown that a normal amount of rust increases bond. Normal rough handling generally removes rust that is loose enough to impair the bond between the concrete and reinforcement.” Two claims, both worth taking literally. A roughened surface grips better, and the material that would debond — the flake, the scale, the dust — is gone by the time the bar has been dragged off a truck and dropped on a stack.
Bond in reinforced concrete is not adhesion anyway. It is rib bearing: the deformations bear on the concrete cast between them, which is why development length in ACI 318-19 25.4 is built from bar size, cover and spacing rather than surface finish. Which is also why the reject case is rust that fills the rib spaces rather than rust that colours them.
Rebar that is properly placed corrodes at roughly 0.1 µm a year off its diameter — the figure engineers quote for bars inside sound concrete, where the pore solution holds the pH above 12 and the steel stays passive. Over a 75-year design life that is 7.5 µm, or 0.0003 in, or about 0.1% of a #5's area. The 0.010 in rust layer in the table above is thirty-four times that.
Which is the argument for the wire brush in one line: the rust that decides whether a bar goes in the pour is the rust you can take off in thirty seconds, and the rust the concrete has to live with afterwards is thirty-four times smaller than the layer in your hand.
It does not keep rusting inside the slab. What protects the steel is the concrete, not the surface. The pore solution in healthy concrete holds a pH above 12, and at that pH steel passivates behind a thin oxide film. Two things break that film — carbonation reaching the bar because cover is too thin, and chlorides from deicing salt or seawater. That is why the cover tables and the chair heights on this site are worth taking seriously: they are the corrosion design, and the bar's own surface is not.
It does not stain a slab from underneath. Rust bleeding through a floor comes from a bar within about half an inch of the surface, from tie wire ends left proud, or from a crack that reaches the steel. A bar at 1½ to 2 in of cover in sound concrete has no path to the top.
It does not give you tetanus. Tetanus is caused by Clostridium tetani, a bacterium that lives in soil and manure, not by iron oxide. A rusty bar that has been lying in dirt is a plausible way to meet the bacterium — through a puncture, not through the rust. If you have been punctured, that is a doctor question, and it has nothing to do with how brown the bar was.
Three cases, and only three: rust that survives a wire brush and leaves the bar under its nominal dimensions; ribs the rust has filled flush; a pit deeper than about 1/16 in on a #3 or #4. In those cases the bar is not a structural bar — it is a chair, a stake or a form pin, where none of the three measurements is being asked to carry anything. Anything else goes in the pour, and the phrase to use with an inspector is not “it looks fine” but the clause: ACI 318-19 26.6.1.2(b), hand wire brushed, dimensions and weight per unit length.
What this section does not cover. It is a test on loose bar at the point of placement, not an assessment of steel already inside a structure — that is a cover survey, a half-cell potential map and a chloride test, and it needs an engineer. It also does not carry over unchanged to coated or alloy bar: galvanised (A767), epoxy-coated (A775) and stainless (A955) each have their own surface and damage rules, and epoxy fails differently, because a breach in the coating concentrates corrosion at the breach instead of spreading it. And no clause tells you what to do with a bar that has lost 20% of its section: 26.6.1.2(b) is pass or fail on a specimen, not a capacity calculation, so a heavily pitted bar is an engineering question rather than a page on a calculator site.
Specifications move, and the grade list is where it shows. The 2012 edition of A615 lists four levels — 40, 60, 75 and 80 — while the 2020 edition lists 40, 60, 80 and 100. Grade 75 was dropped and Grade 100 was added.
| Edition | Grades listed | Chemistry clause |
|---|---|---|
| A615/A615M-12 | 40 [280] · 60 [420] · 75 [520] · 80 [550] | §6.1, P ≤ 0.06% |
| A615/A615M-20 | 40 [280] · 60 [420] · 80 [550] · 100 [690] | §6.1, P ≤ 0.06% |
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Two practical consequences. First, an old drawing calling for Grade 75 is calling for a bar that is no longer in the current grade list — that is an RFI, not a substitution you make quietly. Second, the chemistry clause did not move: §6.1 reads the same in both editions, which is why the 0.06% figure is safe to quote against either.
The other change worth knowing is in §5.1: the 2012 edition named the steelmaking processes (electric furnace, basic oxygen, open hearth), and the 2020 edition says the steel shall be made by any commercially accepted process. The chemistry requirement did not get stricter to compensate — it is the same one line.
Carbon steel: iron with carbon, manganese, a little silicon and residuals. It is bought to ASTM A615 in the US, which caps only phosphorus at 0.06% and requires carbon, manganese and sulphur to be measured and reported without a maximum. One 20 ft #4 bar is 13.36 lb (6,060 g) and may hold 3.6 g of phosphorus at the ceiling.
Steel. Iron becomes steel when carbon is added, and the steel/cast-iron boundary is about 2.1% carbon (published values run 2.08–2.14%). Rebar sits far below that, in the low-carbon range where a bar can yield and bend instead of cracking. By weight a bar is roughly 98% iron; the other 2% is doing all the work.
Yes. Ferrite and pearlite make carbon steel strongly ferromagnetic, which is how a rebar locator or cover meter finds it through concrete. Austenitic stainless rebar has low permeability and FRP rebar is non-magnetic, so both defeat those tools; heating steel past about 770 °C also removes the magnetism.
Only with a qualified procedure. A615 Note 3 says welding should be approached with caution because the specification includes no provisions to enhance weldability, and refers to AWS D1.4/D1.4M. The screen is the carbon equivalent: under 0.55 it can be welded. Bars made to A706 are the weldable ones, and they are marked SW when they meet both specifications.
There is no single number, because A615 sets no carbon maximum. The figure is on the mill certificate for the heat. What matters downstream is the carbon equivalent, since manganese multiplies carbon's hardening effect: 0.30% C with 1.20% Mn already reaches 0.50, only 0.05 under the weld line.
As much as comes off with a wire brush. ACI 318-19 §26.6.1.2(b) makes rust and mill scale satisfactory provided a hand-wire-brushed specimen still meets the minimum dimensions (including height of deformations) and the weight per unit length; A615 §12.2 adds cross-sectional area and tensile. Rust that survives the brush and leaves the bar undersized, ribs filled flush, or a pit over about 1/16 in deep on a #3 or #4 is the rejection case.
Yes, and on most sites you will. Brown surface rust is what a bar looks like after a month in the yard, and the code treats it as satisfactory. What decides it is not the colour but three measurements after brushing: the diameter against nominal (0.500 in on a #4), the ribs still standing proud, and 13.36 lb for a 20 ft #4. A 0.010 in layer of rust costs a #4 about 4% of its area, which is 480 lb of a 12,000 lb bar.
Normal rust does not, and the code commentary says it increases bond: R26.6.1.2(b) notes that research shows a normal amount of rust improves it, and that rough handling removes the rust loose enough to impair it. What weakens a bar is section loss, and it takes a visible, measurable layer to matter — 0.020 in off a #4's diameter is 7.8% of its area. Pitting is the exception: a single deep pit is a notch, not a percentage.
No. Tetanus comes from Clostridium tetani, a bacterium that lives in soil and manure, so the risk is dirt on the bar rather than the rust on it — and the route is a puncture wound, not contact. A bar that has been lying on the ground is a plausible way to meet the bacterium; treat any puncture seriously and let a doctor decide about a booster.
Mechanically, and only as far as the code asks: a wire brush, a knotted wire cup on a grinder, or just dropping and dragging the bundle, which is what R26.6.1.2(b) means by normal rough handling. Do not acid-wash it in the field — hydrochloric acid leaves chlorides on the surface, and chlorides are the thing that destroys the passive film once the bar is cast in. Do not oil or paint it either: the bar needs to bond, and the concrete is the protection.
A615 §19.3: mill symbol, bar size, the letter S for A615, then the grade. Grade 60 carries one continuous longitudinal line (or 60), Grade 80 three lines (or 80), Grade 100 four lines (or 100) or the letter C, and Grade 40 needs no grade mark. SW means the bar meets both A615 and A706 — the bar to specify for welding.
Because phosphorus damages the bar in every use: it segregates to grain boundaries and embrittles the steel, which is why §6.1 holds it to 0.06% and §6.2 still permits a purchaser's product check 25% above that (0.075%). Carbon is left open because it is how a mill reaches the grade's yield, and its effect on welding is handled by the carbon equivalent instead of a flat ceiling.
If the question behind the question is “which bar do I buy”, the numbers live on three other pages: the rebar size chart for diameters, areas and weights by bar number; rebar measurement for how the bar number, the inch size and the metric size line up; and the weight calculator for turning a takeoff into pounds.
If the question is about using the bar, development length covers how far it must be embedded, rebar dowels covers the bar that crosses a joint, and epoxy coated rebar covers the bar that gets a film instead — A775 thickness, D3963 repair and the lap the coating adds — and hook and bend covers what happens to the length when the bar is bent — which is also where the ductility that carbon controls stops being theory.