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.
Almost all 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:
|Minimum Yield Strength|
per square inch
|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 about 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
|Weight in pounds per foot|
Specifications require that the producer roll into the bar:
|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
|75||520||2 lines running the length of the bar
offset at least five spaces from the center of the bar
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|
|IR||Rail Meeting Supplementary
|minimum yield strength|
|40||40,000 psi||300||300 MPa
|60||60,000 psi||420||400 MPa
|75||75,000 psi||520||500 MPa
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.
United States Department of Commerce.
Simplified Practice Recommendation No. 26.
Steel Reinforcing Bars.
Issued by the Bureau of Standards. Original Draft, September 9, 1924.
Washington: Gov't Printing Office, 1925.
In accordance with the unanimous action of the joint conference of representatives of manufacturers, distributors, and users named on page 7 the United States Department of Commerce, through the Bureau of Standards, recommends that the areas of steel reinforcing bars conform to the following simplified list:
in square inches
square and round bars
It is further recommended that this simplified list of areas become effective as applying to new production January 1, 1925, subject to regular annual revision by similar conference, and that every effort be made to clear current orders and existing stocks of the eliminated areas before March 1, 1925.
Prior to 1917 dealers in new billet reinforcing bars carried a minimum of 15 sizes in two grades of steel. Under the pressure of war conditions the variety in sizes and grades was reduced in number, with consequent relief all along the line. The War Industries Board was instrumental in the first application of simplification to the reinforcing bar industry, just as it initiated similar practice in many other industries.
However, with the exigencies of war removed, there has been a tendency to reinstate the discarded sizes, and the former confusion has returned to disconcert producers, distributors, and users. Furthermore, there are to-day very definitely three grades of reinforcing bars on the market—“structural grade,” “intermediate grade,” and “hard grade.” This means that the dealers have now more costly inventories with which to cope, and it means that they have been hampered in the matter of giving minimum quotations on public works.
The dealers are convinced that a simplified list of sizes is desirable.
Representatives of mills producing more than 80 per cent of the annual tonnage of the steel used for reinforcing bars attended the subsequent meeting of producers, distributors, and users. At that time it was the opinion of the conferees that “square” and “round” are merely inexact descriptive terms, and that the fundamentally important characteristic of steel reinforcing bars is the cross-sectional area. It was therefore unanimously moved that this recommendation be expressed in terms of area, and that the sizes be mentioned only for the purpose of providing equivalents for the information and guidance of those who have been accustomed to the use of that nomenclature.
After one item had been added to the list originally submitted the simplified practice recommendation was unanimously accepted.
Whether or not a single grade of steel for reinforcing bars would satisfy all requirements is a question that is technical in character and one which has been referred to the Association of American Steel Manufacturers and to the American Society for Testing Materials.
At some future time a recommendation covering this subject of grades will be presented for the consideration of a conference similar to the one under review. If it proves practicable to fabricate reinforcing bars of a single grade of steel only, the inventory problem of distributors will be decidedly lightened. The present recommendation reduces the number of piles of stock from 96 to 33. The establishment of a single grade of steel for this commodity will place distributors in a position to further reduce from 33 piles to 11, If dealers can concentrate upon 11 piles of stock, they will be free to use released investment to cut costs to consumers and to strengthen their own business organizations generally.
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.
ACI Committee 439.
ACI 439.4R-09. Report on Steel Reinforcement—Material Properties and U.S. Availability.
ACI Manual of Concrete Practice, Part 5—2010.
Farmington Hills, MI: American Concrete Institute, 2010.
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Last revised: 28 March 2012.