Concrete international / april 2010 59

By DaviD p. Gustafson

Raising the Grade Higher yield strength added to astM a706/a706M specification

for low-alloy steel reinforcing bars

ASTM Subcommittee A01.05, Steel Reinforcement, has made a significant revision to A706/A706M “Standard

Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement.”1 The specification was developed in the early 1970s in response to the structural engineering community’s requirements for steel reinforcing bars with controlled tensile properties for use in earth-quake-resistant structures and restricted chemical composition for weldability. For a discussion of the historical and technical aspects of the specification, see Reference 2. From its inception in 1974, the ASTM A706/A706M specification covered only Grade 60 (420) bars, but as of December 2009, it includes requirements for bars with a minimum yield strength of 80 ksi (550 MPa) (designated as Grade 80 [550]). The increased yield strength was especially encouraged by structural engineers, bar producers, bar fabricators, and contractors from seismically active areas because higher strength bars can help reduce congestion of reinforcement and enhance constructibility, especially in earthquake-resistant structures.

PRoduCtion PRoof In August 2006, the first draft of the proposed revision

was developed by Subcommittee A01.05’s Task Group on Reinforcing Bars, chaired by James G. Hutchinson, formerly with Gerdau Ameristeel, Knoxville, TN. During the early vetting of the proposed revision, a major issue arose: was commercial production of Grade 80 (550) bars possible? Four producers, with mills located in California, Oregon, Washington, and South Carolina, volunteered to make trial heats and resolve the issue.

A heat is a batch of steel produced in a single furnace run. The size of a heat, therefore, depends on a mill’s particular equipment and production procedures. Trial heats produced in the four mills ranged from about 35 to 80 tons (32 to 72 tonnes). The trial heats were rolled into several sizes of reinforcing bars, samples were tested, and the results confirmed that

Table 1:Tensile requiremenTs for bars per asTm a706/a706m

Grade 60 (420)

Grade 80 (550)

tensile strength, minimum, psi (Mpa)

80,000 (550)* 100,000 (690)*

yield strength, minimum, psi (Mpa)

60,000 (420) 80,000 (550)

yield strength, maximum, psi (Mpa)

78,000 (540) 98,000 (675)

elongation in 8 in. (200 mm) length, minimum %

no. 3, 4, 5, 6 (10, 13, 16, 19) bars

14 12

no. 7, 8, 9, 10, 11 (22, 25, 29, 32, 36) bars

12 12

no. 14, 18 (43, 57) bars 10 10*Tensile strength shall not be less than 1.25 times actual yield strength

bars could be produced within the proposed limits on tensile properties and chemical composition.

PRoPeRtiestensile and yield strength

For both Grade 60 (420) and 80 (550) bars, ASTM A706/A706M prescribes minimum and maximum yield strengths as well as minimum tensile strengths (Table 1). The latest version of ASTM A706/A706M allows the yield strength to be defined using either the offset method (0.2% offset) or the drop of the beam (alternatively, the halt in the gauge) of the tensile testing machine.

High-strength reinforcing bars typically exhibit a “round-house” stress-strain curve, so it’s possible the offset method will overestimate the value of the yield strength that should be used in the strength design method of ACI 318. For reinforcing bars with a yield strength exceeding

60 april 2010 / Concrete international

60 ksi (420 MPa), ACI 318-08, Section 3.5.3.2, requires that the yield strength is taken as the stress corresponding to a strain of 0.35%.3 Likewise, ASTM A706/A706M requires that the stress corresponding to 0.35% strain is reported as not less than 60 or 80 ksi (420 or 550 MPa) for Grade 60 (420) or Grade 80 (550) bars, respectively.

ASTM A706/A706M also imposes a minimum tensile-yield ratio, requiring tensile strength to be at least 1.25 times the actual yield strength (Table 1). The purpose of the minimum tensile-yield ratio, or minimum strain-hardening requirement, is to ensure that the bending moment increase over the region of plastic hinging can be accommodated (yield stress at one end of the region and, necessarily, a greater stress at the other end) to preclude a premature brittle failure that would result from the steel tensile strength being exceeded.4 There is no corresponding requirement for the minimum tensile-yield ratio of ASTM A615/A615M bars.

As shown in Table 1, ASTM A706/A706M also requires larger values of minimum elongation than ASTM A615/A615M. The more restrictive requirements for minimum elongation are intended to ensure ductile behavior in earthquake-resistant structures. For example, while ASTM A706/A706M requires 12% elongation for No. 8 (No. 25) bars in either Grade 60 (420) or Grade 80 (550), ASTM A615/A615M requires only 8 and 7% elongation for No. 8 (No. 25) bars in the respective grades.

Chemical compositionTo accommodate the addition of Grade 80 (550) to the

specification, no modifications were made to the requirements for restricted chemical composition. The same limits on individual chemical elements and carbon equivalent that apply to Grade 60 (420) also apply to Grade 80 (550).

Why GRade 80 (550)?ACI 318-08 requirements for specified yield strengths

of reinforcement and transverse reinforcement ( fy and fyt, respectively) dictated the yield strength limits in ASTM A706/A706M. Section 9.4 limits the values of fy and fyt used in calculations to 80 ksi (550 MPa). Sections 10.9.3 and 21.1.5.4 do, however, allow exceptions for spiral and confinement reinforcement. For these applications, fyt can be up to 100 ksi (690 MPa).

Many code sections also limit fy to 60 ksi (420MPa): ■ Section 11.4.2 for shear reinforcement (stirrups fabricated from reinforcing bars);

■ Section 11.5.3.4 for torsion reinforcement; ■ Section 11.6.6 for shear-friction reinforcement; ■ Section 18.9.3.2 for bonded reinforcement in prestressed

concrete members; ■ Section 19.3.2 for reinforcement in shells and folded plates; and

■ Section 21.1.5.2 for reinforcement in special moment frames and special structural walls.

GRade histoRyWorking stress to strength design

ASTM A431, which was issued in 1959, covered Grade 75 reinforcing bars.5 ASTM A432, which was also issued in 1959, covered Grade 60 bars.6 In 1968, Grade 60 and 75, along with Grade 40, were melded into ASTM A615.7 When the ASTM subcommittee began exploring the feasibility of adding Grade 80 (550) to ASTM A706/A706M, committee members questioned why their predecessors had selected Grade 75 rather than Grade 80, as the logical choice would be to maintain a progression with Grades 40, 60, and 80 rather than Grades 40, 60, and 75. I believe the rationale for the subcommittee’s choice of Grade 75 can be traced back to the requirements in ACI 318-51.8 A provision in that edition limited the nominal allowable compressive stress in vertical column reinforcement to 40% of fy, but not to exceed 30,000 psi (207 MPa). Although Grade 75 was not cited per se, the 1951 Code, in effect, permitted the use of column reinforcement with a yield strength of up to 30 ksi/0.40 = 75 ksi.

ACI 318-56 included the same limit on allowable compressive stress for working stress design, but it also included the strength design method (via an appendix).9 Provisions in the appendix limited the stress in tensile and compressive reinforcement at “ultimate load” to the minimum of fy or 60 ksi. Whereas ACI 318-6310 carried over the 40% of fy limit for working stress design from ACI 318-56, the strength design provisions changed. When reinforcement was used with fy in excess of 60 ksi (420 MPa), the yield strength used in design was required to be reduced to 0.85 fy or 60 ksi (420 MPa), whichever was greater, unless tension tests showed that the strain in the bars would be limited to 0.003 at the specified yield strength. ACI 318-63 also permitted the use of Grade 75 bars in tension if tests on the full-size typical structural members showed satisfactory performance with regard to cracking at service load.

With the acknowledged benefit of hindsight, I believe Grade 80 should have replaced Grade 75 in the A615 specification at about the time ACI 318-7111 was issued. Perhaps the subcommittee considered such a replacement, but the modest 5000 psi (35 MPa) increase in minimum yield strength may not have been compelling. Shortly thereafter, there was a step backward as Grade 75 was deleted from the 1974 edition of the A61512 specification because a conflict existed between the yield strength definitions in ASTM A615 and ACI 318-71. While ASTM A615 required the minimum yield strength of 75,000 psi (518 MPa) to correspond to a tensile strain of 0.6%, ACI 318-71 prescribed that the yield strength correspond to a strain of 0.35%. Grade 75 was reinstated in the 1987 edition of ASTM A615, with the provisions for yield strength compatible with those in the ACI Code.

Working beyond astM a706 to astM a615In August 2006, when the ASTM Task Group on

Concrete international / april 2010 61

Reinforcing Bars decided to pursue the addition of a higher yield strength to the ASTM A706/A706M specification, they had the benefit of starting with a “clean sheet of paper.” Hence, the task group’s focus was immediately on Grade 80 (550).

It’s important to note, however, that while Grade 80 (550) has also recently been added to ASTM A615/A615M, Grade 75 (520) has been maintained in the specification for at least the near-term.13 This will accommodate downstream users of Grade 75 (520) reinforcing bars that use bars for such things as roof bolts for the mining industry. Thus, the current edition of the specification, ASTM A615/A615M-09b, includes Grades 40 (280), 60 (420), 75 (520), and 80 (550). ASTM A615/A615M also permits the use of the offset method for determining the yield strength for Grades 60 (420) and 80 (550), and it requires that the stress corresponding to 0.35% strain is reported and is not less than the minimum yield strength for either grade. As with those in ASTM A706/A706M, the tensile requirements in ASTM A615/A615M-09b are in harmony with Section 3.5.3.2 of ACI 318-08.

Although both ASTM A615/A615M and ASTM A706/A706M now cover Grade 80 (550) bars, it’s important to note that ASTM A706/A706M continues to place upper limits on yield strength—a fundamental requirement for satisfactory use in reinforced concrete structures designed to resist earthquakes. According to McDermott,3 the purpose of the maximum limitation on yield strength is to ensure that the reinforcing bars will yield before the concrete crushes, and so that excessive bending moment resistance is not induced within the region of plastic hinging, which could induce high shear resulting in shear and/or bond failures. Whereas ASTM A706/A706M limits the maximum respective yield strengths for Grade 60 (420) and 80 (550) bars to 78 and 98 ksi (540 and 675 MPa), there are no corresponding limits for ASTM A615/A615M bars.

Revising ASTM A615/A615M to add Grade 80 (550) was considerably less challenging than revising ASTM A706/A706M. The Task Group on Reinforcing Bars reviewed the test results of several hundred heats of Grade 75 (520) bars produced by many steel mills. The review showed that a large percentage of the Grade 75 heats would meet the requirements of the proposed Grade 80 (550). Hence, the only aspect of the proposed revision that was intensely debated was the marking requirement for the minimum yield strength designation.

Grade marksTo distinguish the new bars, ASTM A706/A706M and

ASTM A615/A615M require identification marks rolled onto the surface of a deformed bar to denote the producer’s mill designation, the size of the bar, the type of steel, and the minimum yield strength designation. The “minimum yield strength designation” part of the marking scheme for Grade 80 (550) bars underwent an intense debate

during the final stages of the balloting process of the proposed revision. It was finally agreed that the marking required for yield strength would be either the number 80 for Grade 80, 6 for Grade 550, or three continuous longitudinal lines through at least five deformation spaces (Fig. 1). The type of steel is indicated by a letter W for bars meeting ASTM A706/A706M.

hoGnestad’s Vision aChieVedAs a “force” in the industry with regard to his

involvement and contributions to ACI 318 and to other codes and standards, the late Eivind Hognestad established a legendary career at the Portland Cement Association. In October 1967, Hognestad delivered an invited presentation at the Concrete Reinforcing Steel Institute’s (CRSI) Fall Business Meeting. The title of his presentation was

Fig. 1: Identification marks for bars meeting aSTM a706/a706M specifications: (a) Grade 80; and (b) Grade 550

(a)

(b)

62 april 2010 / Concrete international

David P. Gustafson, faCi, is a Consultant based in Winthrop Harbor, il. after receiving his phD in civil engineering from tulane university and serving as an assistant professor of civil engineering at Michigan technological university, he served on the Crsi technical staff for more than 32 years. He is a member of aCi Committees 222, Corrosion of Metals in Concrete; 301,

specifications for Concrete; and 318, structural Concrete Building Code; and he chairs astM subcommittee a01.05, steel reinforce-ment. He is a licensed structural engineer in illinois and a licensed professional engineer in illinois, Michigan, and Wisconsin.

“Trends in Consumer Demands for New Grades of Reinforcing Steel.”14 Based on what he perceived as the needs in the design of reinforced concrete structural members and for the construction of reinforced concrete structures, Hognestad presented the rationale for the grades of steel reinforcing bars that should be produced and available in the marketplace in the 1970s and beyond. He proposed three grades: Grades 60, 60W, and 80W—the “W” designating reinforcing bars with restricted chemical composition.

When the revision of ASTM A706/A706M received final approval, it occurred to me that Hognestad’s visionary proposition had been finally realized, albeit some 40 years after his eloquent presentation to the reinforcing steel industry.

References1. ASTM A706/A706M-09b, “Standard Specification for Low-Alloy

Steel Deformed and Plain Bars for Concrete Reinforcement,” ASTM International, West Conshohocken, PA, 2009, 7 pp.

2. Gustafson, D.P., “Re-Visiting Low-Alloy Steel Reinforcing Bars,” Concrete International, V. 29, No. 1, Jan. 2007, pp. 55-59.

3. ACI Committee 318, “Building Code Requirements for Structural

Concrete (ACI 318-08) and Commentary,” American Concrete Institute, Farmington Hills, MI, 2008, 473 pp.

4. McDermott, J.F., “Interrelationships between Reinforcing Bar Physical Properties and Seismic Demands,” ACI Structural Journal, V. 95, No. 2, Mar.-Apr. 1998, pp. 175-182.

5. ASTM A431, “Specifications for High Strength Deformed Billet-Steel Bars for Concrete Reinforcement with 75,000 psi Minimum Yield Strength,” ASTM International, West Conshohocken, PA, 1959. (withdrawn)

6. ASTM A432, “Specifications for Deformed Billet-Steel Bars for Concrete Reinforcement with 60,000 psi Minimum Yield Strength,” ASTM International, West Conshohocken, PA, 1959. (withdrawn)

7. ASTM A615, “Standard Specification for Deformed Billet-Steel Bars for Concrete Reinforcement,” ASTM International, West Conshohocken, PA, 1968.

8. ACI Committee 318, “Building Code Requirements for Reinforced Concrete (ACI 318-51),” ACI Journal, Proceedings V. 22, No. 8, Apr. 1951, pp. 589-652.

9. ACI Committee 318, “Building Code Requirements for Reinforced Concrete (ACI 318-56),” ACI Journal, Proceedings V. 27, No. 9, May 1956, pp. 913-986.

10. ACI Committee 318, “Building Code Requirements for Reinforced Concrete (ACI 318-63),” American Concrete Institute, Farmington Hills, MI, 1963, 144 pp.

11. ACI Committee 318, “Building Code Requirements for Reinforced Concrete (ACI 318-71),” American Concrete Institute, Farmington Hills, MI, 1971, 78 pp.

12. ASTM A615, “Standard Specification for Deformed and Plain Billet-Steel Bars for Concrete Reinforcement,” ASTM International, West Conshohocken, PA, 1974.

13. ASTM A615/A615M-09b, “Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement,” ASTM International, West Conshohocken, PA, 2009, 6 pp.

14. Hognestad, E., “Trends in Consumer Demands for New Grades of Reinforcing Steel,” Proceedings, Fall Business Meeting, Concrete Reinforcing Steel Institute, 1965, pp. 22-32, published as PCA Development Department Bulletin D130.

Selected for reader interest by the editors.

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