ASTM A767 coating mass converted to mils, the ψe 1.0 that costs no bar, and the flaking clause epoxy does not have.
← Back to the calculatorThat is half the thickness of an epoxy film, and the same #8 Class B splice is 50 in galvanized and 75 in with epoxy.
Two clauses do the work, and neither of them is quoted by the pages that rank for this query. A767 gives the coating as a mass per area, not a thickness, so the number on the mill certificate is in oz/ft² and nobody converts it. And A767 says flaking and cracking from fabrication after galvanizing are not grounds for rejection — the exact opposite of D3963, where 2% damaged area in a foot is a rejection threshold and every visible holiday gets patched.
So the decision between zinc and epoxy is not “which coating is better”. It is one number on the bar list: the epoxy splice is 25 in longer on a #8, which is 5.56 lb and $2.78 a lap, and the zinc splice is the black bar splice.
ASTM A767 specifies the coating as a minimum mass of zinc per unit of surface area, and it has two classes. Class I is 3.00 oz/ft² on a #3 and 3.50 oz/ft² on #4 and larger; Class II is 2.00 oz/ft² for every size. In metric the same numbers are 915, 1070 and 610 g/m². There is no thickness anywhere in the clause, which is why every galvanizing conversation stalls at “how thick is that in mils”.
| A767 coating | Minimum mass | Same, US units | Thickness | Same |
|---|---|---|---|---|
| Class I, #3 | 915 g/m² | 3.00 oz/ft² | 128 µm | 5.0 mil |
| Class I, #4 and up | 1070 g/m² | 3.50 oz/ft² | 150 µm | 5.9 mil |
| Class II, all sizes | 610 g/m² | 2.00 oz/ft² | 85 µm | 3.4 mil |
The conversion is one division. Zinc weighs 7.14 g/cm³, so a mass per area becomes a thickness by dividing: 1070 g/m² is 0.107 g/cm², and 0.107 ÷ 7.14 is 0.0150 cm, which is 150 µm. In US units the shortcut is that one oz/ft² of zinc is 42.7 µm, or 1.68 mil, so 3.5 oz reads 5.9 mil and the Class II 2.0 oz reads 3.4 mil.
Put those against the epoxy film and the surprise is the direction: A775 asks for 7 to 12 mils, so the epoxy coating is between 1.2 and 2 times thicker than the zinc it competes with. Thickness is not the argument for either product. The epoxy is a barrier that has to stay intact; the zinc is a barrier that is designed to be eaten, which is the subject of the next two sections.
ACI 318-19 Table 25.4.2.5 has one coating factor for zinc-coated bar and a different one for epoxy-coated bar. Zinc-coated takes ψe = 1.0, the same value as uncoated steel. Epoxy-coated takes 1.2 when clear cover is at least 3db and clear spacing is at least 6db, and 1.5 in every other case. ψe enters through ld, not through the lap, and that ordering matters: the factor multiplies the calculated ld, which is then rounded up and floored at 12 in, and only then multiplied by 1.3 and rounded up again. So the epoxy column below is not the black column times 1.2 or 1.5 — it is 1.2 or 1.5 times the calculated ld, rounded, then 1.3 of that, rounded.
| Bar | Black bar | Galvanized | Epoxy ψe 1.2 | Epoxy ψe 1.5 | Epoxy costs |
|---|---|---|---|---|---|
| #3 | 16 in | 16 in | 19 in | 24 in | +8 in |
| #4 | 21 in | 21 in | 25 in | 30 in | +9 in |
| #5 | 25 in | 25 in | 30 in | 38 in | +13 in |
| #6 | 30 in | 30 in | 37 in | 46 in | +16 in |
| #7 | 45 in | 45 in | 52 in | 65 in | +20 in |
| #8 | 50 in | 50 in | 60 in | 75 in | +25 in |
| #9 | 56 in | 56 in | 68 in | 85 in | +29 in |
| #10 | 64 in | 64 in | 76 in | 95 in | +31 in |
| #11 | 71 in | 71 in | 85 in | 106 in | +35 in |
Read the galvanized column against the black column and there is nothing to read: they are the same number in every row. That is the whole cost comparison, and it runs the other way from the way the two products are usually sold. On a #8 at 4000 psi the epoxy splice is 75 in and the galvanized splice is 50 in, so the epoxy bar needs 25 in more steel per lap — 5.56 lb, about $2.78 at $1,000 a ton. On a frame with 96 laps that is 200 ft, 534 lb and $267, which is the same arithmetic that appears on the epoxy page with the sign flipped.
Two things keep this honest. First, the 1.0 is not a gift for good behaviour: the code charges the factor for bond, and zinc-coated bar is the one case where the coating does not cost bond — section 04 is the reason. Second, the galvanized bar still pays for its coating somewhere, at the galvanizer, per ton, on a quote that moves with the kettle and the quantity. It does not pay in inches of bar, and inches of bar are what show up on the drawing.
Stainless steel rebar is the third route and it lands on the same number: uncoated, ψe = 1.0, a #8 Class B lap of 50 in. The difference is where the money goes — zinc is a coating the galvanizer prices per ton, stainless is an alloy priced per pound of bar — and both cost zero inches of lap, which is the one line item epoxy cannot match.
A third route stops being steel altogether. GFRP bar has no yield point, a modulus 3.9 to 5.7 times lower than steel, and a design strength that is the guaranteed value times 0.7 wherever the concrete is exposed to earth and weather — a different set of numbers, not a different coating.
This is the clause that separates the two products more sharply than any thickness number. ASTM A767/A767M sets minimum bend diameters, and in the same clause states that flaking and cracking caused by fabrication after hot-dip galvanizing are not grounds for rejection. The wording here is taken from the Galvanizers Association’s A767 summary at galvanizeit.org, which quotes the clause and prints the bend table; it is the one claim on this page that rests on a single source, so it is named rather than paraphrased. Bend the bar in the field, crack the zinc on the outside of the bend, and the bar still conforms.
D3963 does the opposite for epoxy. Damage is counted per 1 ft section, 2% of the surface area is the rejection threshold, the patched area may not exceed 5%, and every visible holiday has to be repaired whether or not the count passes. The reason for the difference is electrochemical rather than administrative: zinc is sacrificial, so a crack in the coating is a small bare area with a donor beside it, and the steel at the bottom of the crack is still protected. An epoxy holiday is a bare area with nothing beside it.
The bend numbers are worth carrying to the job. A767's recommended finished bend diameters are 6db for #3 through #6, 6db for #7 and #8 at Grade 40 and 50 and 8db at Grade 60 and 75, 8db for #9 through #11, and 10db for #14 and #18. Bend tighter than that and the bar has to be stress relieved at 900 to 1050 °F for one hour per inch of diameter, because the cold work is released when the bar goes into an 850 °F zinc bath. Coating that is damaged by welding, cutting or handling is repaired to ASTM A780, which A767 already references.
A767 requires something no other rebar coating specification asks for: the bars come out of the zinc bath and go straight into a heated, dilute sodium dichromate solution. It is not decoration. It does two jobs that both matter at the job site.
The first is wet storage stain — the white zinc corrosion product that forms on tightly bundled, damp bars before they are ever placed. The second is the reaction between freshly galvanized zinc and wet concrete, which evolves hydrogen at the bar surface; the gas travels out through the paste and can leave voids against the bar, and voids are lost bond. The chromate passivates the surface enough to suppress that reaction, which is the reason a galvanized bar can be poured against without a bonding agent and without a coating factor.
That is the second half of the ψe story, and the two halves are usually told separately. A767 passivates the zinc so the concrete can grip it; ACI then charges it no coating factor. Epoxy has no such mechanism, which is exactly why A775's film has to be intact, why D3963 polices the holidays, and why the code multiplies the lap by 1.2 or 1.5.
The coating is thin, but it is metal, and it shows up on a bundle. Take the surface of the bar per foot, π × db × 12, multiply by the coating mass, and convert ounces to pounds:
| Bar | Surface per ft | Zinc per ft | Same in lb | Of bar weight |
|---|---|---|---|---|
| #4 | 0.131 | 0.458 | 0.029 | 4.3% |
| #6 | 0.196 | 0.687 | 0.043 | 2.9% |
| #8 | 0.262 | 0.916 | 0.057 | 2.1% |
| #11 | 0.369 | 1.292 | 0.081 | 1.5% |
On a #8 the zinc is 0.057 lb per foot, which is 2.1% of the 2.670 lb/ft nominal bar. The share falls as the bar grows, because surface area goes up with the diameter and weight goes up with the square: 4.3% on a #4, 2.1% on a #8, 1.5% on a #11. A hundred-bar bundle of 20 ft #8 carries about 114 lb of zinc, and the bar order does not change by one pound, because A615 and A767 both weigh and price the nominal uncoated section.
For scale, the epoxy film on the same #8 weighs 0.023 to 0.026 lb/ft — 0.9 to 1.0% of the bar, and about 2.3 times lighter than the zinc. That is the trade in one line: the zinc puts twice the mass on the bar and takes none of the lap; the epoxy puts half the mass on and takes 25 in.
ASTM A767 sets it as a mass, not a thickness: Class I requires 3.00 oz/ft² on a #3 and 3.50 oz/ft² on #4 and larger, and Class II requires 2.00 oz/ft². Divided by the density of zinc, 7.14 g/cm³, that is 128 µm (5.0 mil), 150 µm (5.9 mil) and 85 µm (3.4 mil). The epoxy film on the same bar is 7 to 12 mil, so the zinc is roughly half as thick and works by a different mechanism.
No. ACI 318-19 Table 25.4.2.5 gives zinc-coated bar a coating factor ψe of 1.0, the same as uncoated bar, where epoxy-coated bar takes 1.2 or 1.5. A #8 Class B lap at 4000 psi is 50 in galvanized and 75 in with epoxy, so the coating costs the zinc bar nothing and the epoxy bar 25 in.
Not per ASTM A767/A767M. Its bend-diameter clause says flaking and cracking caused by fabrication after hot-dip galvanizing are not grounds for rejection, which is the opposite of the epoxy rule: D3963 limits damaged coating to 2% of a 1 ft section before repair and 5% patched. Zinc protects the steel beside a crack galvanically, so a crack is a smaller event on a galvanized bar than a holiday is on a coated one.
ASTM A767 requires the bars to be quenched in a heated dilute sodium dichromate solution straight out of the zinc bath. It does two jobs: it stops wet storage stain, the white rust that forms on bundled bars, and it suppresses the reaction between freshly galvanized zinc and wet concrete, which evolves hydrogen at the bar surface and can leave voids that cost bond.
Epoxy. The coating factor is 1.0 for zinc and 1.2 or 1.5 for epoxy, so on a #8 at 4000 psi the epoxy lap is 75 in against 50 in — 25 in, 5.56 lb and about $2.78 a lap at $1,000 a ton. On 96 laps that is 200 ft of extra bar, 534 lb and $267. What galvanizing costs is the galvanizing, which is quoted per ton by the plant.
On a #8 with Class I coating it is 0.057 lb per foot, which is 2.1% of the 2.670 lb/ft nominal bar. The share falls as the bar grows because surface area grows slower than section: 4.3% on a #4, 1.5% on a #11. A 100-bar bundle of 20 ft #8 gains about 114 lb of zinc, and the bar is still weighed and priced on its nominal uncoated section.
ASTM A767 points welded assemblies at AWS D1.4, and the zinc has to come off the weld zone before welding because the coating produces fumes and the weld will not be sound through it. The damaged or burnt coating is then repaired to ASTM A780, which is the repair specification A767 already references. If the bar is welded into a fabricated assembly rather than shipped as bar, ASTM A123 is the coating specification, not A767.
Yes. A767 is the hot-dip zinc coating specification for concrete reinforcement, and it applies to plain and deformed bar but not to wire. It is not the same product as zinc-rich paint, electroplated zinc, or a zinc and epoxy dual coat, and it is not A153, which covers hardware and castings.