ASTM A706 Reinforcing Bars Technical Information - Page 2

Q7: Returning to Question 4, what is the significance of the prescribed or controlled tensile properties?
A: The specification is in effect prescribing "tailored" stress-strain curves for the bars. See Figure 1. In the design of seismic-resistant reinforced concrete structures, the tensile properties of the rebars should be controlled within desired limits, and the bars should have a definite yield plateau and a definite strain hardening region. The lower stress-strain curve in Figure 1 corresponds to the minimum requirements in A706, namely, minimum yield strength fy = 60 ksi, and minimum tensile strength = 80 ksi. The upper stress-strain curve is the upper limit for tensile properties; maximum fy = 78 ksi, and tensile strength at least = 1.25 x 78 = 97.5 ksi.

Fig. 1 - Stress-Strain Curves for A706 Rebars.

WELDABILITY
Q8: The other major requirement in the A 706 specification concerns weldability. How does the specification provide for weldable bars?
A: Unlike the other ASTM specifications for reinforcing bars, A706 includes a statement: "This material is intended for welding". Weldability is accomplished in A706 by limits or controls on chemical composition. There are actually two limits or controls on chemical composition. One limit is on individual chemical elements; the other limit is on "carbon equivalent."

Q9: What is "carbon equivalent?"
A: The term "carbon equivalent," which is abbreviated as C.E., accounts for those chemical elements affecting weldability. C.E. is a numerical value expressed as a percent. The ANSI/AWS D1.4-79 Welding Code⁵ and the ASTM A706 rebar specification have the same formula for C.E.:

Q10: What are the limits on individual chemical elements and carbon equivalent in the A 706 specification?
A: The limits on the major chemical elements affecting weldability (elements in the C.E. formula) are:

The carbon equivalent is limited to 0.55 percent. It should also be noted that the A706 specification requires the producer to report the chemical composition and carbon equivalent to the purchaser.

Q11:How do the above limits tie into weldability and the ANSI/AWS Welding Code?
A: The minimum preheat and interpass temperatures in the ANSI/AWS Welding Code are based upon carbon equivalent and size of rebar. For bars with a carbon equivalent up to 0.55 percent, little or no preheat is required under normal working temperatures for bar size #11 and smaller. For #14 and #18 bars with a carbon equivalent of 0.40 to 0.55 percent, a somewhat higher preheat is required. See Table 1. Whenever a small quantity of weldable reinforcing bars is required for a project, in many cases it will probably be more economical to specify A615 bars and weld according to the more stringent requirements prescribed in ANSI/AWS D1.4-79.

Table 1 - Minimum Preheat and Interpass Temperatures, °F

USAGE
Q12: It was noted in the Answer to Question I that the A706 specification was issued over 15 years ago. Is any information available regarding the demand, or are there any data on the past and current usage of A 706 bars?
A: It is rather difficult to report meaningful information on the demand, and consequently, on the usage and availability of A706 reinforcing bars. Hard data on these important aspects are sparse.

Q13: Please elaborate on usage.
A: Perhaps it is best to begin to describe usage by relating to current practices in the marketplace. Consider cast-in-place concrete construction first. Even on the West Coast, where the campaign for the A706 specification began, Structural Engineers often specify reinforcing bars for seismic-resistant structures as:

"The reinforcing bars shall conform to ASTM A706-XX; or to ASTM A615-XX, Grade 60, except (1) the maximum yield strength shall be 78,000 psi, and (2) the tensile strength shall not be less than 1.25 times the actual yield strength."

    In other words, practicing Structural Engineers permit the use of ASTM A615, Grade 60 bars having the special or "tailored" stress-strain curves (Figure 1) - the controlled tensile properties desired for optimum structural performance under seismic loading.
    Permitting the use of "modified" A6115Grade 60 bars by Structural Engineers is not surprising. Chapter 21 in the current ACI Building Code6 also permits the same option:

"21.2.5.1 - Reinforcement resisting earthquake induced flexural and axial forces in frame members and in wall boundary members shall comply with ASTM A706. ASTM A615 Grades 40 and 60 reinforcement are allowed in these members if (a) the actual yield strength based on mill tests does not exceed the specified yield strength by more than 18,000 psi (retests shall not exceed this value by more than an additional 3000 psi); and (b) the ratio of the actual ultimate tensile strength to the actual tensile yield strength is not less than 1.25."