Consulting engineers, government agencies, and other parties frequently query the technical staff at the Concrete Reinforcing Steel Institute (CRSI) on the differences between the ASTM A 706/A 706M specification1 for low-alloy steel reinforcing bars and the ASTM A 615/A 615M specification2 for carbon-steel reinforcing bars. The aim of this article is to: * Compare the th e major requirements r equirements in ASTM A 706/A 706M with provisions provis ions in ASTM A 615/A 615M; * Relate current availability and relative costs of low-alloy steel reinforcing bars (A 706 bars); and * Offer insight into prospective near-term developments and issues. HISTORY OF A 706 BARS The first version of ASTM A 706 was published by ASTM International in 1974. An interesting description of its background and development is included in a 1973 report by ACI Committee 439.3 The specification was developed in response to the engineering community's requirements for reinforcing bars with controlled tensile properties for earthquake-resistant structures and controlled chemical composition for weldability. In 1986, the specification was revised into a combined standard containing both in.-lb and SI units, and the new designation became ASTM A 706/A 706M. In 1990, CRSI published Report No. 34, "ASTM A 706 Reinforcing Bars-Technical Information with Commentary on Usage and Availability."4 Prepared by the CRSI technical staff, this report was a response to questions raised by architects, engineers, contractors, and state and federal government agencies. The 1991 article titled "Questions and Answers on ASTM A 706 Reinforcing Bars"5 was based on this CRSI publication and reported that the demand for weldable bars in castin-place construction had never fully materialized. Starting in January 1995, however, the California Department of Transportation (Caltrans) initiated a policy of requiring A 706 bars for virtually all of their reinforced concrete structures. Carbon-steel reinforcing bars conforming to ASTM A 615/A 615M would only be permitted for use in slope and channel paving, minor structures, sign and signal foundations (pile and spread footing types), roadside rest facilities, certain types of concrete barriers, and temporary temporar y railing. In November 2005, the Illinois Department of Transportation (IDOT) implemented a policy requiring the exclusive use of A 706 bars for all reinforced concrete construction. IDOT's requirements are even broader than those of Caltrans. For example, IDOT requires A 706 bars for continuously reinforced concrete pavement. Other DOTs, including those in Nevada, Alaska, South Carolina, Tennessee, and Virginia, are requiring or currently considering requiring the use of A 706 bars. BAR COMPARISON Because A 706 bars are produced to only one strength level with a minimum yield strength f^sub y^ of 60,000 psi (420 MPa), the following comparisons are made to Grade 60 (420) bars in ASTM A 615/A 615M. In broad terms, ASTM A 706/A 706M is more demanding and includes requirements for controlled tensile properties and restrictions on chemical composition, while ASTM A 615/A 615M does not include comparable requirements. Tensile properties prop erties ASTM A 706/A 706M, like other specifications for non-prestressed steel reinforcement, prescribes a minimum f^sub y^ of 60,000 psi (420 MPa) and a minimum tensile strength f^sub u^ of 80,000 psi (550 MPa). The specification, however, also includes two limits or controls on tensile properties: 1) the actual f^sub y^ cannot exceed 78,000 psi (540 MPa); and 2) f^sub u^ cannot be less than 1.25 times the actual f^sub y^. The latter requirement controls when the actual yield strength obtained in a laboratory tensile test of a bar is greater than 64,000 psi (440 MPa). If laboratory tension tests of reinforcing bars resulted in measured yield strengths ranging from slightly above 64,000 psi (440 MPa) to the maximum permitted value of 78,000 psi (540 MPa), the corresponding tensile strengths would have to range from slightly above 80,000 to 97,500 psi (550 to 675 MPa) to meet the
requirements for A 706 bars. For example, suppose test specimens produced the measured yield strengths shown in the left column of Table 1. The tensile strengths would have to meet or exceed the values of 1.25fy shown in the right column of Table 1. ASTM A 706/A 706M is, in effect, prescribing "tailored" stress-strain curves for the bars (Fig. 1) with the well-defined yield plateaus and strain-hardening regions that are desirable for designing earthquake-resistant reinforced concrete structures. The lower stress-strain curve in Fig. 1 corresponds to the minimum requirements for A 706 bars-minimum f^sub y^ = 60,000 psi (420 MPa) and minimum f^sub u^ = 80,000 psi (550 MPa). The upper stress-strain curve corresponds to the upper limits for A 706 bars-maximum f^sub y^ = 78,000 psi (540 MPa), and minimum f^sub u^ = 97,500 psi (675 MPa). Percent elongation ASTM A 706/A 706M requires larger values of minimum elongation than those in ASTM A 615/A 615M. A comparison of the requirements for the two specifications is shown in Table 2. Bending properties The bend test requirements in ASTM A 706/A 706M are also more restrictive than those in ASTM A 615/A 615M. In addition to the smaller required pin diameters shown in Table 3, 180-degree bend tests are required for all bar sizes. Weldability To enhance weldability, there are limits on the amounts of individual chemical elements and also the "Carbon Equivalent" (C.E.) of A 706 bars. The two major chemical elements affecting weldability are carbon and manganese. The maximum percentage of these two elements is limited to 0.30 and 1.50%, respectively. The C.E. of the steel accounts for those chemical elements affecting weldability. In addition to carbon (C) and manganese (Mn), these elements include copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), and Vanadium (V). It is expressed as a percentage and determined using the following formula: C.E. = % C % Mn/6 % Cu/40 % Ni/20 % Cr/10 - % Mo/50 - % V/10 (1) This is the same as the expression that appears in AWS D1.4/D1.4M-05, "Structural Welding CodeReinforcing Steel."6 The C.E. for A 706 bars is restricted to a maximum of 0.55%. The minimum preheat and interpass temperatures prescribed by AWS D1.4/D1.4M-05 are based on C.E. and the size of the reinforcing bar. As shown in Table 4, No. 11 (No. 36) and smaller bars with C.E. = 0.55% require little or no preheat under normal working temperatures. In contrast, ASTM A 615/A 615M doesn't contain any limits on chemical composition regarding welding. Note 1 in the scope of ASTM A 615/A 615M states: "NOTE 1-Welding of the material in this specification should be approached with caution since no specific provisions have been included to enhance its weldability. When steel is to be welded, a welding procedure suitable for the chemical composition and intended use or service should be used. The use of the latest edition of AWS D1.4 is recommended. This document describes the proper selection of the filler metals, preheat/interpass temperatures, as well as, performance and procedure qualification requirements."2 Marking requirements ASTM specifications for reinforcing bars require identification marks rolled onto the surface of one side of the bar to denote the producer's mill designation, bar size, type of steel, and minimum yield designation. The marking requirements for A 706 bars are shown in Fig. 2. When Grade 60 (420) reinforcing bars are produced to meet the requirements of both ASTM A 706/A 706M and ASTM A 615/A 615M, the mark for the type of steel must include both letters "W" and "S." The practice of producing and marking bars that meet both specifications is called "dual certification." Inclusion of provisions in the specifications for dual certification was intended to make smaller quantities of A
706 bars more readily available. AVAILABILITY AND RELATIVE COSTS Availability At the present time, 16 companies in the U.S. produce steel reinforcing bars and operate a total of 36 steel mills. According to recent CRSI surveys, eight companies (operating 22 steel mills) indicated they are currently making, or are capable of making, A 706 bars. The 22 steel mills are located in 17 states that are well distributed across the country (Fig. 3). These eight companies probably account for more than 90% of the nation's capacity for producing steel reinforcing bars. ow do the results of CRSI's surveys and the industry's production capacity translate into "availability" of A 706 bars? With 22 steel mills distributed across the country that are currently making or are capable of making A 706 bars, the availability of A 706 bars is becoming less of an issue. On the West Coast, for example, the availability of A 706 bars is not an issue. Many fabricators carry A 706 bars along with A 615 bars in their inventory. With the Illinois DOT driving the demand for A 706 bars in the Midwest, the production, and consequently the availability, of A 706 bars is increasing. In the Southeast, the South Carolina DOT is a leading user of A 706 bars, and consequently the industry is responding to fill the needs of this growing market. In other regions of the country, it's apparent that the reinforcing steel industry will respond to any increased demand. Relative costs When it comes to construction materials, the same question is usually raised by government agencies, owners, developers, contractors, engineers, and architects: "What will (or does) it cost?" In this case, what is the cost of A 706 bars versus traditional A 615 bars? Actual costs or prices are challenging to pin down. Probably the only way to secure actual costs is to seek bids for both types of bars on a proposed project. Considering material cost only, A 706 bars cost about $40 to $60 USD/ton ($44 to $66 USD/tonne) more than A 615 bars. A recent issue of ENR magazine reports that the average price (material cost) nationwide for steel reinforcing bars is about $700 USD/ton ($770 USD/tonne),7 which is presumed to be the price for A 615 bars. The premium for A 706 bars translates into a 6 to 9% increase in material cost over A 615 bars. To put the premium in perspective, it should be related to the total in-place cost of the reinforcing bars that would include the cost of the material, detailing, fabrication, accessories, transportation, and placing (installing) the bars in the forms. According to the price index issued by the California Department of Transportation,8 the total in-place cost of reinforcing steel is as high as $2000 USD/ton ($2200 USD/tonne). For the sake of this discussion, let's assume the in-place cost is $1500 USD/ton ($1650 USD/tonne). The premium for using A 706 bars instead of A 615 bars would be only about 3 to 4%. When discussing the cost of A 706 bars, a brief discussion of the chemical element vanadium is necessary. Vanadium is the principal alloying element currently being used in the production of A 706 bars. The concerns about vanadium are supply and cost. Vanadium is found in commercial quantities in only a few places in the world, and the current market price is on the order of $20 USD/lb ($44 USD/kg). In recent years, however, the price has spiked to over $50 USD/lb ($110 USD/kg). Thus, the concerns about a reliable supply and the cost of vanadium cast a cloud over competitively producing A 706 bars in the future using current steel-making practices. NEAR-TERM DEVELOPMENTS Increased use The use of A 706 bars is expected to continue to increase in the near-term due to actions taken by agencies and other parties regarding their reinforcement requirements. When more agencies and other parties realize that the additional cost for A 706 bars is relatively minor compared to the overall cost of the steel reinforcement for a structure, specifying A 706 bars should become more
attractive. Growth in other areas that require welded reinforcing bar assemblies will also increase the use of A 706 bars. One example is the increasingly common practice of preassembling reinforcing bar cages by welding in the fabrication shop. Another example is the growth of the precast concrete industry where many connections use reinforcing bars welded to steel embedments for anchorage. Higher strength level As noted earlier, ASTM A 706/A 706M currently covers only Grade 60 (420) bars. The reinforcing steel industry, with input from the engineering community, is exploring the feasibility of adding a higher strength level with a minimum yield strength of 80,000 psi (550 MPa). References 1. ASTM A 706/A 706M-06a, "Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement," ASTM International, West Conshohocken, PA, 2006, 6 pp. 2. ASTM A 615/A 615M-06a, "Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement," ASTM International, West Conshohocken, PA, 2006, 6 pp. 3. ACI Committee 439, "Uses and Limitations of High Strength Steel Reinforcement f^sub y^ = 60 ksi (44.2 kgf/mm^sup 2^)," ACI Journal , Proceedings V. 70, No. 2, Feb. 1973, pp. 77-104. 4. "ASTM A 706 Reinforcing Bars-Technical Information with Commentary on Usage and Availability," Report No. 34, Concrete Reinforcing Steel Institute, Schaumburg, IL, 1990, 4 pp. 5. Gustafson, D.P., and Felder, A.L., "Questions and Answers on ASTM A 706 Reinforcing Bars," Concrete International, V. 13, No. 7, July 1991, pp. 54-57. 6. AWS D1 Structural Welding Committee, "Structural Welding Code-Reinforcing Steel (AWS D1.4/D1.4M-05)," American Welding Society, Miami, FL, 2005, 70 pp. 7. "Construction Economics," ENR, V. 257, No. 4, July 24, 2006, p. 23. 8. "Price Index for Selected Highway Construction Items, 2nd Quarter Ending June 30, 2006," California Department of Transportation, Division of Engineering Services-Office Engineer, Aug. 2006. Selected for reader interest by the editors. David P. Gustafson, FA CI, is Vice President of Engineering at the Concrete Reinforcing Steel Institute (CRSI) in Schaumburg, IL. He is a member of ACI Committees 301, Specifications for Concrete, and 318, Structural Concrete Building Code. He is also the Chair of AST M Subcommittee A01.05, Steel Reinforcement, and has been on the CRSI technical staff for more than 32 years. Copyright American Concrete Institute Jan 2007 Provided by ProQuest Information and Learning Company. All rights Reserved Gustafson, David P "Revisiting Low-Alloy Steel Reinforcing Bars". Concrete International. FindArticles.com. 26 Aug, 2010. http://findarticles.com/p/articles/mi_qa5363/is_200701/ai_n21290713/ Copyright American Concrete Institute Jan 2007 Provided by ProQuest Information and Learning Company. All rights Reserved
