In the United States, the size designations of these mild steel bars used to reinforce concrete are set by ASTM International.1 Distributors usually stock rebar in 20- and 60-foot lengths.
Most bars are “deformed,” that is, a pattern is rolled onto them which helps the concrete get a grip on the bar. The exact patterns are not specified, but the spacing, number and height of the bumps are. Between 1947 and 1968, a separate standard (ASTM A 305) covered the deformations. Since 1968 the deformation requirements have been incorporated into the basic standard. Plain bars are also made, but are used only in special situations in which the bars are expected to slide (for example, crossing expansion joints in highway pavement).
Three grades are defined, with metric equivalents:
| inch-pound grade |
metric grade |
Minimum Yield Strength | |
|---|---|---|---|
| in pounds per square inch |
in megapascals |
||
| Grade 40 | Grade 280 | 40,000 | 280 |
| Grade 60 | Grade 420 | 60,000 | 420 |
| Grade 75 | Grade 520 | 75,000 | 520 |
According to the standard (sec. 20.3.5), “it shall be permissible to substitute a metric size bar of Grade 280 for the corresponding inch-pound size bar of Grade 40, a metric size bar of Grade 420 for the corresponding inch-pound size bar of Grade 60, and a metric size bar of Grade 520 for the corresponding inch-pound size bar of Grade 75.” Nothing is said regarding substituting inch-pound size bars when the specification is metric.
The size designations up through size 8 are the number of eighths of an inch in the diameter of a plain round bar having the same weight per foot as the deformed bar. So, for example, a number 5 bar would have the same mass per foot as a plain bar 5/8 inch in diameter. The metric size is the same dimension expressed to the nearest millimeter.
| Bar designation number | Nominal diameter in inches (not including the deformations) |
Metric designation number |
Weight in pounds per foot |
|---|---|---|---|
| 3 | 0.375 | 10 | 0.376 |
| 4 | 0.500 | 13 | 0.668 |
| 5 | 0.625 | 16 | 1.043 |
| 6 | 0.750 | 19 | 1.502 |
| 7 | 0.875 | 22 | 2.044 |
| 8 | 1.000 | 25 | 2.670 |
| 9 | 1.128 | 29 | 3.400 |
| 10 | 1.270 | 32 | 4.303 |
| 11 | 1.410 | 36 | 5.313 |
| 14 | 1.693 | 43 | 7.650 |
| 18 | 2.257 | 57 | 13.60 |
Specifications require that the producer roll into the bar:
for example, means the bar was rolled from a new billet.| Grade | Metric grade | Continuous line system | Number system number stamped onto bar |
|
|---|---|---|---|---|
| 60 | 420 | 1 line running the length of the bar offset at least five spaces from the center of the bar |
60 | 4 |
| 75 | 520 | 2 lines running the length of the bar offset at least five spaces from the center of the bar |
75 | 5 |
Various laws2 require federally-funded projects to use materials with metric designations. To meet this requirement, in 1979 ASTM issued standard A 615M-79, which described a set of reinforcing bar sizes in whole number SI units. This standard was specified in some contracts.
The cost of producing and stocking two different sets of nearly identical sizes proved onerous. In April 1995, the Concrete Reinforcing Steel Institute and the Steel Manufacturers Association decided to mount a campaign to replace the initial hard metric sizes with soft. In a soft conversion to metric, the original dimensions are simply restated to the nearest number of SI units. In 1996, ASTM changed A 615M to soft metric sizes. For example, a bar with the metric designation “25”, formerly 25 millimeters in diameter, became 25.4 mm in diameter, the same as a size 8 (1-inch) bar.
As a result, the metrically-sized bars are identical to the original inch-sized bars, except for the markings and a small difference in strength (the new metric standard calls for a stronger bar, see the table below).
The grade mark for grade 420 is either a “4” or a single longitudinal grade line. The grade mark for grade 520 is either a “5” or two longitudinal grade lines.
Type of steel mark
| Mark | Meaning | Applicable ASTM Standard by Grade | |||||
|---|---|---|---|---|---|---|---|
| 40 & 50 | 60 | 75 | 300 & 350 | 420 | 520 | ||
| S | billet | A615 | A615 | A615 | A615M | A615M | A615M |
| I | rail | A616 | A616 | — | A996M | A996M | — |
| IR | Rail Meeting Supplementary Requirements S1 |
A616 | A616 | — | — | — | — |
| A | axle | A617 | A617 | — | A996M | A996M | — |
| W | Low-alloy | — | A706 | — | — | A706M | — |
| old US grade |
minimum yield strength |
corresponding current soft metric grade |
minimum yield strength | ||
|---|---|---|---|---|---|
| original hard metric specs |
1996 revisions |
proposal | |||
| 40 | 40,000 psi | 300 | 300 MPa (43,400 psi) |
— | — |
| 60 | 60,000 psi | 420 | 400 MPa (58,000 psi) |
420 MPa (60,900 psi) |
415 MPa (60,100 psi) |
| 75 | 75,000 psi | 520 | 500 MPa (72,500 psi) |
520 MPa (75,400 psi) |
— |
1. ASTM International issues a series of
specifications for rebar:
A615/A615M-05a: Standard Specification for Deformed and Plain Billet-Steel Bars for
Concrete Reinforcement. (covers grades 40 and 60/soft metric grades 420 and 520).
Serves also as Standard M 31 of the American Assn. of State
Highway and Transportation Officials.
A616: Standard Specification for Rail-Steel Deformed Bars for Concrete
Reinforcement. (covers grades 50 and 60).
A617: Standard Specification for Axle-Steel Deformed Bars for Concrete
Reinforcement. (covers grades 40 and 60)
A706/A706M-96b: Standard Specification for Low-Alloy-Steel Deformed and Plain Bars for Concrete
Reinforcement. (grade 60 only)
2. Metric Conversion Act of 1975; Omnibus Trade and Competitiveness Act of 1988 (Public Law 100-418, section 5164); Executive Order 12770, "Metric Usage in Federal Government Programs."
Metric designations of reinforcing bar have the form “K” followed by the mass in kilograms of a 1-meter length of the bar. For example, “K3” rebar weighs 3 kilograms per meter.
| nominal diameter |
|
|---|---|
| 5 | |
| 6 | |
| 8 | |
| 10 | |
| 12 | |
| 14 | |
| 18 | |
| 16 | |
| 20 | |
| 22 | |
| 25 | |
| 28 | |
| 32 | |
| 40 | |
| 50 |
CRSI Manual of Standard Practice. 28th ed.
Schaumburg, IL: Concrete Reinforcing Steel Institute, 2009.
Pocket Guide for Field Inspection of Rebar.
Schaumburg, IL: Concrete Reinforcing Steel Institute, 2008.
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Last revised: 4 October 2008.