Metrication of Reinforcing Steel in the U.S.

You may not realize it, but virtually all the reinforcing bar currently produced in the United States is a “soft metric” bar. This not-so-well-known fact conjures up several important questions: Is there a difference in size between a soft metric rebar and an inch-pound rebar? What about the design strengths of reinforcing steel with soft metric and inch-pound units of measure? More importantly, do your employees understand that there is no physical difference between a #3 inch-pound rebar and a #10 soft metric rebar?

In the late 1990s, producers of rebar (the steel mills) implemented a phasing-in of soft metric bars. The implementation of the soft metric designation allows the steel industry to produce the same reinforcing steel bars for construction projects that require and specify inch-pound rebar as those that call for metric rebar. This eliminates the need for the steel mills, fabricators and other stakeholders – including precast concrete producers – to maintain dual inventories of the various sizes of reinforcing steel, thus avoiding additional expense and all of the other headaches that might result from such a need.

So what does “soft metric” mean? The soft metric terminology is simply a designation, because in reality, the rebar has not changed. Soft metric rebar is actually just a way to describe the physical or dimensional parameters of inch-pound bars in the International System of Units (SI). The only physical difference results from final processing of the rebar at the mills, where the marking scheme is “rolled in.” The marking scheme identifies the mill, the bar size, the material type and grade, which is distinct for the soft metric rebar. An illustration of the physical equivalence is shown in the table by bar size. For example, we see that a #10 soft metric bar has a nominal diameter of 9.5 mm and that this size corresponds to a three-eighths inch nominal diameter for the equivalent #3 inch-pound rebar.

We notice further evidence of a soft conversion when we compare inch-pound and metric data taken from the ASTM International specifications for the minimum tensile and yield strength requirements of reinforcing steel. Independent of whether we examine data taken from Table 2 of ASTM A615/A615M, “Standard Specification for Deformed and Plain Carbon Steel Bars for Concrete Reinforcement,” or from Table 2 of ASTM A706/A706M, “Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement,” an equivalence is established between the minimum strength values of bars with inch-pound units in psi (lbf/in2) and the corresponding metric units of MPa. Early on, the minimum yield strengths of metric rebar were established and then increased slightly in the late ’90s as a result of a revision. The current values reflect this revision and bring the minimum yield strength for metric rebar much closer to the corresponding inch-pound levels. As an example for the A615/A615M Grade 60 (60,000 psi minimum yield strength) inch-pound bar, a corresponding metric strength value is shown at 420 MPa. An exact conversion from 420 MPa to psi is 60,915 psi.

In order to complete equivalence, both of the ASTM specifications mentioned above contain a provision for substitution of metric rebar for inch-pound rebar based upon strength level. The current ASTM A615/A615M specification in Section 20.3.5 states, “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 bar of Grade 75.” Similarly, the current ASTM A706/A706M Specification in Section 16.3.5 states, “It shall be permissible to substitute a metric size bar of Grade 420 for the corresponding inch-pound size bar of Grade 60.”

For additional information on reinforcing steel, see the ASTM A615/A615M and ASTM A706/A706M specifications at www.astm.org.